research report 369 - health and safety executive · hse health & safety executive research to...

HSE Health & Safety

Executive

Research to improve guidance on separation distance for the multi-energy method (RIGOS)

Prepared by TNO Prins Maurits Laboratory for the Health and Safety Executive 2005

RESEARCH REPORT 369

HSE Health & Safety

Executive

Research to improve guidance on separationdistance for the multi-energy method (RIGOS)

A.C. van den BergA.L. Mos

TNO Prins Maurits Laboratory Lange Kleiweg 137

P.O. Box 452280 AA Rijswijk

The Netherlands

This report describes the RIGOS-research program. The primary objective of this program was to develop practical guidance with regard to the Critical Separation Distance. The Critical Separation Distance is a basic element in the application of the TNO Multi-Energy method for vapour cloud explosion blast modelling. To this end, a series of gas explosion experiments have been performed in a donor acceptor configuration. The blast was recorded at several positions around while the separation distance between donor and acceptor was gradually diminished. When the blast was observed to consist of just one single instead of two separate waves, the separation distance was assumed critical. The experimental program resulted in a limited number of concrete indications with respect to the Critical Separation Distance, in particular in the low explosion overpressure range. On the basis of this limited information concrete guidance was drawn up based on safety and conservatism. This guidance was extrapolated to the high explosion overpressure range on the basis of common sense.

Apart from this primary objective, the RIGOS program led to a substantial amount of additional and interesting information. The data show, for instance, that for separation distances somewhat larger than critical, the donor explosion may largely suppress the acceptor explosion. The gas dynamics of the donor’s negative phase could explain this surprising observation. The experimental data also gave the opportunity to validate current simple blast modelling methodologies on the acceptor gas explosion data. Because of the directional flame propagation mode in the acceptor, the acceptor gas explosions differ substantially from the spherically developing gas explosions, the simple blast modelling methodologies were derived from. The conclusion is that the current simple blast modelling methodologies tend to overestimate the blast effects from the acceptor explosions by more than an order in magnitude, in particular in the low explosion overpressure range.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

HSE BOOKS

© Crown copyright 2005

First published 2005

ISBN 0 7176 6146 6

All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmitted inany form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.

Applications for reproduction should be made in writing to:Licensing Division, Her Majesty's Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ or by e-mail to [email protected]

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COMPANY CONFIDENTIAL

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Contents

1 Introduction................................................................................................1

2 Background ................................................................................................3

2.1 History ........................................................................................3

2.2 The Critical Separation Distance ................................................4

3 Research program .................................................................................... 7

3.1 Objectives ................................................................................. 7

3.2 Approach .................................................................................. 7

3.3 Test location ............................................................................. 8

3.4 The obstacle configurations...................................................... 8

3.5 The test set-up........................................................................... 9

4 Test program ............................................................................................12

4.1 Variation of Separation Distance (SD) .....................................12

4.2 Tests with a connection between donor and acceptorrepresenting a pipe rack............................................................15

4.3 Combination of large and small obstacles ................................16

5 Results and Analysis ................................................................................18

5.1 Donor-acceptor explosion behaviour........................................18

5.2 The Critical Separation Distance ..............................................25

5.3 Influence of a Connecting Obstacle Configuration ..................30

5.4 Influence of the Obstacle Diameter ..........................................37

5.5 Practical Guidance on the critical separation distance .............39

6 Additional information from experiments ...............................................42

6.1 Donor-acceptor explosion behaviour........................................42

6.2 Validation of blast modelling methods.....................................46

7 Conclusions..............................................................................................67

8 References................................................................................................69

9 Authentication..........................................................................................72

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1 Introduction

This report describes the results of a research project with the primary objective to

develop practical guidance with respect to the Critical Separation Distance. The

Critical Separation Distance is a basic element in the application of the TNO Multi-

Energy Method. The TNO Multi-Energy Method is a simple method for vapour

cloud explosion blast modelling, which is based on the Multi-Energy concept. The

Multi-Energy concept is the basic feature of vapour cloud deflagration that over-

pressure/blast develops only in parts of the cloud that are located in partially con-

fined or congested areas. An important consequence of the Multi-Energy concept is

that if one single extended vapour cloud of flammable composition comprises more

than one partially confined or congested areas that are separated by open spaces of

sufficient extent, the vapour cloud explosion on ignition develops the same number

of separate blasts. If, on the other hand, open spaces between partially confined or

congested areas are insufficient, the blast of the vapour cloud explosion should be

modelled as one single blast of summed energy content. The blasts are modelled by

the application of blast charts compiled for an equivalent hemispherical fuel-air

charge.

The Critical Separation Distance between partially confined/congested areas is the

criterion that enables to discriminate between the modelling by one single blast or

more than one blast, i.e. to be able to discriminate between open spaces of suffi-

cient or insufficient extent. The critical separation distance is, therefore, a basic

element in the Multi-Energy method and of paramount importance for its applica-

tion.

This report describes an experimental program to develop quantitative guidance

with regard to the Critical Separation Distance. To this end, explosions of small-

scale vapour clouds containing two separate configurations of obstacles (repre-

senting separate process units on a chemical plant) were produced.

Blast effects at various distances were recorded while the separation distance

between the configurations of obstacles was varied. The critical separation distance

is defined as the minimum separation distance between two congested areas still

resulting in two separate blast waves. In addition to the separation distance, the size

and obstacle density of the donor as well as the fuel in the gas cloud were varied.

These series of experiments constituted the bulk of the research program.

In addition, it was investigated through a limited number of tests if the critical

separation distance is also dependent on the obstacle diameter in the donor. Also, it

was studied whether the critical separation distance would possibly be influenced

by a connection of limited cross-sectional area between donor and acceptor, repre-

senting for instance a pipe rack between two process units.

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This research program resulted in a substantial body of experimental data on do-

nor-acceptor explosions. This data set offered the opportunity to validate the meth-

odologies for simple vapour cloud explosion blast modelling on gas explosions

substantially different from those these methods were derived from.

This report describes the research program. After a brief introduction in Chapter 1,

Chapter 2 describes the background of the Multi-Energy blast modelling as well as

the guidance for its application, developed over the years in the multi-sponsor

research programs GAME and GAMES. The Chapters 3 and 4 describe the objec-

tives, the general approach in this research program as well as a detailed definition

of the various series of experiments. Finally, the experimental results and their

analysis have been reported in the Chapters 5 and 6.

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2 Background

2.1 History

The Multi-Energy Method (Van den Berg, 1985), developed as a more reasonable

alternative for TNT-equivalency, is a simple method to model the blast from va-

pour cloud explosions. The method recognises a characteristic feature of vapour

cloud explosions by assuming that the overpressure and blast are generated only in

the parts that are located inside partially confined and congested areas. The implicit

assumption is that any detonative flame propagation will immediately fail outside

the partially confined and congested areas because of the inhomogeneity of the

flammable mixture. Generally speaking, vapour clouds dispersing in the atmos-

phere are too inhomogeneously mixed to maintain a detonation. The vapour cloud

explosion record shows that this assumption holds for a vast majority of cases.

Initially, the Multi-Energy Method could only be applied in a rather global manner

by making safe and conservative assumptions. Basic potentials of the methodology

had not yet been explored. Nevertheless, the Multi-Energy Method was selected for

inclusion in the latest fully revised version of the Yellow Book (CPR-14E, 1997).

An important potential of the fuel-air blast charts used in the Multi-Energy Method

is the possible discrimination in explosion strength or the explosion overpressure

DP0. The first initiative to develop guidance for the determination of the explosion

overpressure has been the project GAME (Eggen, 1995 and Van den Berg and

Eggen, 1996). The main effort in this project consisted of the compilation of a

correlation of the explosion overpressure with a set of parameters characterising

the size and obstacle density of a congested area as well as the reactivity of the

flammable mixture in the cloud. The correlation was compiled from the results of

extensive experimental research programmes performed over the years in the

projects MERGE and EMERGE (Mercx, et al., 1994a and b, 1996, 1997). The

correlation is based on the observation that the overpressure in a gas explosion is

predominantly determined by:

· the number of obstacles passed by the flame during propagation away from the

ignition to the edge of the obstacle configuration;

· the type of fuel;

· the geometric scale.

To parameterise an obstacle configuration, three parameters were introduced:

· The Volume Blockage Ratio (VBR) which is the ratio of the summed volume of

the obstacles in an obstructed region and the volume of that region, assuming

the obstacles consist of cylinders;

· The distance a flame can propagate within an obstructed region (Lf);

· The average obstacle diameter (D).

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The influence of the scale and the fuel is taken into account by a theory, developed

by Taylor and Hirst (1988), Catlin (1991) and Catlin and Johnson (1992), based on

Karlovitz number similarity. Because the origin of scale effects in gas explosions is

predominantly in the scale of the turbulence, the average obstacle diameter D was

chosen as the scale parameter while the laminar burning velocity Sl was adopted as

the parameter characteristic for the reactivity of the flammable mixture.

1

10

Hj

0.001

0.01

0.1

100

Over

pre

ssu

re (

Bar)

Harrison

ertager

MERGE

-2.50 -2.00 -1.50 -1.00 -0.50 0.00 0.50 1.00

b 2.7 0.7LOG ((VBR*Lf/D) *Sl *D )

Figure 1: Observed overpressures and correlation line for MERGE experiments.

The correlation, consisting of a best fit of the experimental data in Figure 1, is

expressed as:

75 , 2 .L VBR f öæ 7.2 7 .0 P ç

ç

è 84.0 .S D÷

÷ D = l . o D

ø

The correlation makes it possible to make an estimate of an explosion overpressure

in realistic problems such as, for instance, a chemical or refinery plant.

The follow-up project GAMES (Mercx, 1998) was meant to investigate the practi-

cal difficulties encountered when the correlation was applied to a realistic plant.

The exercises performed in GAMES resulted in practical guidance for the determi-

nation of the volume blockage ratio VBR, the flame path length Lf and the average

obstacle diameter D.

2.2 The Critical Separation Distance

The Multi-Energy method recognises that in gas deflagration, turbulence genera-

tive boundary conditions are the predominant factor in the development of over-

pressure and blast. The mechanism of a gas deflagration implicates that as soon as

the appropriate turbulence generative boundary conditions are lacking, the burning

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speed and the pressure build-up in the process of flame propagation drop. The

implicit assumption is that the flammable mixture in the cloud is too inhomogene-

ously mixed to maintain a detonation.

The direct consequence of the Multi-Energy concept is that an extended vapour

cloud containing several obstructed areas, separated by open spaces of sufficient

extent, will produce the same number of separate blast waves on ignition. In the

modelling of blast effects, therefore, the individual obstructed areas should be

separately considered.

The problem has been visualised in Figure 2. A large flammable vapour cloud has

covered two densely obstructed areas of explosive potential: a chemical plant and

closely parked boxcars at a railway shunting yard. The space in between the two

regions is open and unobstructed. If the distance between the two is sufficient, the

ensuing gas explosion on ignition will develop two separate blasts.

Figure 2: Two obstructed regions in a large cloud. One big or two smaller explosions?

The Yellow Book (CPR-14E, 1997) intuitively defines a congested area as an area

in which obstacles are positioned within a 10 obstacle diameters distance from one

another with an upper limit of 25 m. This statement would suggest a Critical Sepa-

ration Distance equal to 10 obstacle diameters with an upper bound of 25 m.

In the final report of the GAMES-project, a preliminary safe and conservative

guideline for the quantification of the term “open spaces of sufficient extent” has

been developed on the basis of global theoretical considerations. This preliminary

guideline runs as follows:

· The Critical Separation Distance around a potential blast source area is equal to

half its linear dimension in each direction.

· If the distance between potential sources is larger, the sources should be mod-

elled as separate blasts.

· If not, they should be modelled as one single blast of summed energy content.

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This preliminary guidance was based on the observation that the combustion proc-

ess in a gas explosion is driven by turbulence. As the turbulence is generated in the

expanding medium in interaction with the obstacles, the influence of the turbulent

combustion cannot extend beyond the fuel-air mixture initially present within the

obstructed area. As the expansion factor of stoichiometric fuel-air mixtures is

approximately equal to 8, the turbulent combustion in a threedimensionally ex-

panding gas explosion will be anyway limited to a volume of a linear dimension

equal to twice that of the obstructed region.

It is to be expected that the critical separation distance can be expressed as a certain

portion of the size of the obstructed donor volume, indeed, but can probably be

taken smaller than the preliminary estimate.

The obstacles in an obstructed area determine the scale of the turbulence and

thereby the turbulence decay outside the obstructed region. Because turbulence is

the predominant combustion-driving phenomenon in gas explosions, it may be

expected that the critical separation distance is also dependent on the diameter of

the obstacles.

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3 Research program

3.1 Objectives

The primary objective of the project is to develop practical guidance with regard to

the Critical Separation Distance, a basic element in the application of the Multi-

Energy method. This overall objective can be split into several related sub-

objectives, which can be designated as follows:

· To generate a database on donor-acceptor gas explosions.

· To generate understanding of the process of flame propagation from donor to

acceptor in relation to the blast produced.

· To measure the Critical Separation Distance dependent on the donor size, the

donor obstacle density and fuel reactivity.

· To investigate whether the Critical Separation Distance is influenced by the

diameter of the obstacles in the donor obstacle configuration.

· To investigate whether the Critical Separation Distance is influenced by a

connection of small cross-sectional dimension between two obstacle configura-

tions, representing a pipe rack between process units of a chemical plant.

· To develop practical guidance on the Critical Separation Distance in the appli-

cation of the Multi-Energy method.

The availability of the substantial body of experimental data generated in this

project offered the opportunity to extend the program with an additional objective,

namely:

· To evaluate the performance of simple vapour cloud explosion blast modelling

methods on explosions substantially differing from those, these methods were

derived from.

3.2 Approach

To determine the Critical Separation Distance, two configurations of obstacles

were placed at a certain distance (Separation Distance) from one another. The two

configurations including the open space in between were enclosed in plastic sheet

to contain a flammable gas mixture. One obstacle configuration, in whose centre

the flammable cloud was ignited, was referred to as the ‘donor’ and the other as the

‘acceptor’. In a series of tests the Separation Distance was varied and the blast was

measured at various locations around. The maximum separation distance at which

the blast waves from donor and acceptor were found to coincide, was designated as

the Critical Separation Distance.

The size and obstacle density of the acceptor were kept constant all over the pro-

gram as they were considered not to influence the Critical Separation Distance. The

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composition and dimensions of the donor, on the other hand, were expected to havea substantial influence. Therefore, the following parameters were varied:· The dimensions of the Donor (DD);· The Volume Blockage Ratio of the donor (VBR);· The Fuel (F);· The Separation Distance between donor and acceptor (SD).

In some additional tests, it was investigated if the obstacle diameter in the donorhas some influence on the Critical Separation Distance. Besides, a very limitednumber of experiments have been performed in which the donor and acceptor wereconnected by an obstacle configuration of small cross-sectional dimensions, repre-senting a pipe rack between separate process units of a chemical plant. Both, thesize of the connecting obstacle configuration as well as the fuel were varied.

3.3 Test location

The experiments have been performed at the so-called FAST facility of TNO Prins

Maurits Laboratory. The facility consists of a concrete pad in which various cable

trays are present for the protection of cables and other measuring equipment. The

concrete pad is situated in an open flat terrain, large enough to prevent reflections

from influencing the measurements.

3.4 The obstacle configurations

The composition of the obstacle configurations are identical to those used in the

MERGE and EMERGE projects. They consist of a number of tubes of circular

cross-section. The tubes are orientated in a fully regular way in three perpendicular

directions. Figure 3 shows a part of a typical MERGE obstacle configuration. This

particular part has 5 horizontal layers of cylindrical obstacles. Each layer consists

of 5 by 5 obstacles orientated in two perpendicular directions. There are 25 vertical

obstacles; each of these connects the knots in the horizontal layers at corresponding

horizontal co-ordinates. This particular configuration is denoted as a 5 ´ 5 ´ 5

obstacle configuration.

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Figure 3: Typical MERGE obstacle configuration as used in the RIGOS-project.

The composition of an obstacle configuration is characterised by:· the Diameter of the cylinders (D);· the axial spacing (Pitch) between adjacent tubes (P);· the Number of cylinders in a row (N).The length L and the width W of the configurations were taken equal, while theheight H was taken as 0.5L. The ignition location was in the centre of the donor atground level.A tube diameter of D=19.1 mm in arrays of pitches of P = 4.65D and P = 7Dresulted in obstacle configurations of volume blockage ratios of VBR = 10.1% andVBR = 4.6% respectively. The obstacle configurations were indicated as type Aand type B, respectively.The configuration of the acceptor is characterised by N = 16, P = 4.65D, giving aVBR = 10.1% and was kept constant all over the program.

3.5 The test set-up

3.5.1 Layout

Roughly speaking, all experiments were of a similar set-up as represented in

Figures 4a and 4b.

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F9

P9

F1

P8P7P6P5P4P3P2P1B1

3m from

centre of donor

B2, B3 and B4

3,6 and 12 m from

centre of acceptor

Figure 4a: Test set-up in the AE, AM, BE and BM test series.

Experiments were performed on a concrete pad in which a cable tray has been cut

away. The cable tray was covered with a steel lid in which measuring instrumenta-

tion can be mounted. The donor and acceptor obstacle arrays were placed on the

pad centred on the cable tray (Figure 4a and 4b). Together they were enclosed in a

tent of plastic sheet to contain the flammable gas mixture. The position of the

acceptor was fixed all over the experimental program. The position of the donor

varied with the variation of the separation distance and the donor size.

Pressure was measured in 9 stations positioned at more or less regular distances

along the axis within the donor-acceptor configurations (P1 – P9). The pressure

gauges P4 to P9 were mounted in and flush to the cable tray lid. The pressure

gauges P1 to P3 were fixed to the donor obstacle array. At nearly the same loca-

tions, thermocouples were mounted to measure flame arrival times (F1-F9).

In the first 4 tests AE01 – AE04, blast overpressures were recorded at 1 station at

3-m distance from the donor centre (B1), at 2 stations at 3 and 6-m distance from

the acceptor centre in the donor-acceptor direction and at 1 station at 3-m distance

from the acceptor in cross direction. In the rest of the AE, AM, BE and BM test

series, the blast overpressures were measured at 1 station at 3-m distance from the

donor centre (B1) and 3 stations at 3, 6 and 12 m distance respectively from the

acceptor centre (B2-B4).

To study directionality effects in the blast, in a later stage in the program the num-

ber of blast overpressure gauges was extended. As indicated in Figure 4b, the

number of pressure gauges near the donor was extended to 3, positioned at 3, 6 and

12 m distance from the donor centre. The number of pressure gauges near the

acceptor was extended with 3, positioned at 3, 6 and 12 m distance from the ac-

ceptor centre perpendicular to the direction of flame propagation (Figure 4b).

B1,B2 and B3

3,6 and 12 m from

centre of donor F9

P9

F1

P8P7P6P5P4P3P2P1 B4, B5 and B6

3,6 and 12 m from

centre of acceptor

B7, B8 and B9

3,6 and 12 m from

centre of acceptor

Figure 4b: Test set-up in the AP, CP, DP and the DM test series.

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3.5.2 Instrumentation

The signals from the pressure gauges and thermocouples have been transmitted to

the SCADAS II Signal Conditioning and Data Acquisition System. Pressures

inside the donor-acceptor configuration are measured by means of piezo-resistive

transducers, Druck type PDCR 200 (FS 600 kPa). The membranes are covered with

2 mm black and 1 mm white silicon grease to reduce flashlight sensitivity and drift

caused by the heat of the flame.

Two different kinds of blast gauges have been be used: blast pencils (Kulite trans-

ducers) as well as pressure gauges mounted in skimmer plates (Druck PDCR 10/F

transducers).

To detect the flame position during the experiments Chromel/Alumel thermocou-

ples have been used. Thermocouples record a temperature increase and thereby the

passage of a flame front (interface between unburned and burnt gases).

The ignition consisted of a spark capacitor circuit and a spark plug. The energy

released to the spark plug has a maximum gross content of 2 Joule. A combination

of electromagnetic and optic techniques determines the moment of ignition.

3.5.3 Mixture preparation and control

The flammable gas is taken from a gas cylinder and piped into the array through a

gas inlet. Fans at the perimeter of the obstacle array create a flow inside the array

to promote adequate mixing of gas and air. The fans are switched off about one

minute prior to ignition.

The gas mixture is pumped to the gas analyser via suction lines. One sample point

is a few cm above ground level and a second sample point is located near the top of

the array. Samples are taken continuously at the two locations until one minute

prior to ignition. The samples are inserted to an Infrared Analysis System. The

concentration can be measured with an accuracy of approximately 0.1% by vol-

ume.

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4 Test program

The primary objective of the experimental program was to determine the Critical

Separation Distance (CSD). Because only donor properties were expected to influ-

ence the Critical Separation Distance, the acceptor has been kept constant all over

the experimental program. In addition to the separation distance, of course, the

Donor Dimension (DD), the donor Volume Blockage Ratio (VBR) as well as the

fuel reactivity were varied. Besides, it was investigated through a limited number

of experiments whether the Critical Separation Distance is dependent on the obsta-

cle diameter in the donor.

The test program has been largely completed in conformity with the original proj-

ect proposal, although some straightforward adaptations appeared necessary during

the project. Many separation distances tested, for instance, have been taken sub-

stantially smaller than those originally proposed.

After a progress meeting with sponsors halfway through the project, the program

was adjusted to some specific sponsor wishes. In addition, some changes appeared

to be necessary to avoid damaging high explosion pressures. It appeared that if the

separation distance was near critical, sometimes overpressures were produced that

destroyed the obstacle arrays and produced blast that damaged surrounding build-

ings. This compelled to test only separation distances larger than critical for ethyl-

ene-air.

In the next Chapters, all experiments of the program will be defined and provided

with a number that corresponds to the data in the RIGOS measuring report (De

Bruijn and Van Ierschot, 2002).

4.1 Variation of Separation Distance (SD)

The acceptor obstacle configuration had dimensions of 1408 ´ 1408 ´ 704 mm3, anobstacle diameter of D = 19.1 mm and a Volume Blockage Ratio of VBR = 10.1%(type A) and N = 16 and was kept constant all over the program.The experiment number was composed from a letter combination and a number.The letter combination indicated the obstacle configuration of the donor and thefuel used.

· AE - type A donor and ethylene as fuel

· AM - type A donor and methane as fuel

· BE - type B donor and ethylene as fuel

· BM - type B donor and methane as fuel

For each value of the Donor Dimensions (DD), the separation distance was varied.

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· Test series AE:

Fuel = Ethylene (stoichiometric mixture with air)Donor array:D = 19.1 mmP = 4.65D = 89 mm hence : VBR = 10.1%

DD1 = 1060 mm, N = 12DD2 = 1408 mm, N = 16DD3 = 1760 mm, N = 20

Table 1: Definition of test series AE.

Test number

SD/DD 0.25 0.5 1 1.5 2

DD1 AE05 AE04 AE01 AE02 AE03

DD2 AE09 AE07 AE06 AE08

DD3 - Cancelled Cancelled Cancelled

Because donor explosion overpressures with medium donor size DD2 were already

substantial and damaging overpressures were anticipated with larger donor size, it

was decided to cancel the AE experiments with the large donor dimension DD3

originally proposed. Instead, these experiments were performed with methane as

part of the AM-series.

· Series AM:

In the AM-series, the donor obstacle configuration was of type A again while

stoichiometric methane-air was used as the flammable mixture.

Four tests of Series AE have been repeated using methane as the fuel, while the

experiments with the largest donor size (DD3) replaced the tests in the AE-series,

cancelled because of the expected high explosion pressures. In Table 2 the various

tests in the AM-series have been defined and numbered.

Table 2: Definition of test series AM.

Test number

SD/DD 0.125 0.25 0.50 1.00 1.50 2.00

DD2 - AM05 AM04 AM02/03 AM01 -

DD3 AM12 AM11 AM10 AM09 - -

· Series AM extended

During the performance of the AE and AM-series, it was observed that, although

the separation distance was much larger than critical, the donor explosion had a

substantial influence on the strength of the acceptor explosion. Therefore, it

seemed appropriate to know the strength of the acceptor explosion without the

presence of a donor. To this end, 3 additional experiments AM6 – AM8 were

defined as indicated in Figure 5.

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A

A

A

A

Plastic sheet

AM02 / AM03

AM06

AM07

AM08

Figure 5: Definition of experiments AM06 – AM08.

The donor obstacle configuration was removed. In experiment AM06 and AM07

the original plastic sheet tent configuration was maintained while the ignition

location was varied from the original (centre donor) to the edge of the acceptor. In

experiment AM08, the size of the tent was reduced to the size of the acceptor.

· Series BE:

In the B-series, the donor obstacle configuration was of type B, i.e. a volume

blockage ratio VBR of 4.6%. Parameters, characterising the type B configuration,

are:

Obstacle diameter D = 19.1 mm and pitch Z = 7 D = 134 mm

Fuel = Ethylene (stoichiometric mixture with air)

Donor size:

DD1 = 1064 mm, N = 8 rows

DD2 = 1330 mm, N = 10 rows

DD3 = 1596 mm, N = 12 rows

The experiments in the BE-series were defined and numbered as tabulated below in

Table 3.

Table 3: Definition and coding of the BE-tests.

Test number

SD/DD 0.125 0.25 0.50 1.00 1.50 2.00

DD1 - - BE02 BE01 - BE03

DD2 - BE07 BE06 BE05 - -

DD3 - BE10 BE09 BE08 - -

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· Series BM:

Only two tests with maximum donor size DD3 have been performed with methane

as a fuel in the B-series. The tests have been defined and numbered in Table 4

below.

Table 4: Definition and coding of the BM-tests.

Test number

SD/DD 0.125 0.25

DD3 BM02 BM01

4.2 Tests with a connection between donor and acceptor repre-

senting a pipe rack

· Series CM, CE, CP and AP:

The A and B-series so far, consisted of donor-acceptor experiments representing

two chemical plants separated by a fully open and unobstructed area, an empty lane

for instance. In practice, however, separate plants are often mutually connected by,

for instance, a pipe rack. To investigate whether the critical separation distance is

influenced by the presence of some connection like a pipe rack between separate

plants, 6 experiments were originally projected. The donor configuration was of

type A (VBR=10.1%) and separation distance was 0.5DD. The connection between

donor and acceptor consisted of obstacle configuration type A and its cross-

sectional area was varied. A schematic view of this test set-up is represented in

Figure 6.

Figure 6: Schematic view of set-up in the test series C.

During the progress meeting halfway through the project, sponsors proposed to

start with the tests with the maximum rack cross-section. Because the presence of

the rack was not observed to substantially influence the flame propagation from

donor to acceptor, the other tests in the CM series were cancelled.

The experiments in the C-test series were defined and numbered in the Tables 5a.

Table 5a: Definition and numbering of C test series (SD=0.5DD).

CM01 As AM04 plus rack 8 x 8 x 4 tubes

CE01 As AE07 plus rack 8 x 8 x 4 tubes

CP01 As AP02 plus rack 8 x 2 x 1 tubes


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Because the result of the CE01 test was disastrous for the experimental rig, the rest

of the C test series was performed with propane as fuel. However, to be able to

observe the influence of a connecting obstacle configuration on the development of

a donor-acceptor explosion, identical experiments with propane but without a

connection had to be performed. To this end, the experiments AP01 and AP02

were defined and numbered as in Table 5c.

Table 5c: Definition and numbering of the AP series.

AP01 SD = DD

AP02 SD = 0.5DD

4.3 Combination of large and small obstacles

So far, the experiments were performed with obstacle configurations of one single

obstacle diameter. The diameter of the obstacles determines the scale of the large

eddies in the spectrum of turbulent motion and the rate of turbulence decay and

thereby the distance over which the turbulent burning in the wake of obstacles may

decrease. Therefore, it seems likely that the Critical Separation Distance is depend-

ent on the obstacle diameter in the donor. To investigate this aspect, the D test

series was defined.

To this end, a B type obstacle configuration (P = 7D, VBR=4.6%, N=12) was

provided with a regular vertical configuration of 24 PVC tubes of 114 mm diame-

ter (Figure 7). The tubes contribute 9.6% to the volume blockage ratio. The result

is configuration of obstacles of mixed diameters of a total VBR of 14.2%.

Figure 7: Horizontal cross-section of B-type obstacle configuration provided with a

regular pattern of 114 mm diameter vertical tubes, used as donor in the D

test series

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Originally some tests with ethylene-air were planned. However, to preclude any

chance of damaging high overpressure and blast, ethylene was replaced by propane

and for the smallest separation distances by methane. Test definition and number-

ing has been tabulated in Table 6.

Table 6: Definition and numbering of D- test series.

Test number

SD/DD SD = 0.05 m 0.125 0.25 0.375 0.50

DD3 DP04 DP03 DP02 DP01

DD3 DM02 DM01

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5 Results and Analysis

5.1 Donor-acceptor explosion behaviour

5.1.1 Some preliminary theoretical considerations

A gas explosion as a process of two competing phenomena

Sometimes, a simplified conception of complicated phenomena may be helpful in

the understanding and interpretation of behaviour observed. A gas explosion, for

instance, can be circumscribed as a process of flame propagation through a flam-

mable mixture. The explosion overpressure developed can be considered the result

of two competing effects, namely:

· Pressure build-up by the combustion rate, which is governed by the flame speed

as a characteristic velocity.

· Pressure relief by the expansion, which is governed by the speed of sound as a

characteristic velocity.

If the flame speed is much lower than the speed of sound, the pressure relief domi-

nates the pressure development and consequently the explosion overpressure is

low. Then, phenomena like side relief of combustion products play an important

role. If, on the other hand, the flame speed is much higher than the speed of sound,

the pressure relief cannot keep pace with the pressure build-up and the explosion

pressure is high. When the flame speed increases up to the order of the speed of

sound, side relief is less and less important and flame propagation in an obstacle

environment seems to develop more and more independently of the expansion.

When the flame speed is of the order of the speed of sound, the pressure build-up

and relief balance more or less. Then, the explosion overpressure is typically of the

order of 100 kPa.

A gas explosion as an acoustic volume source

Another simplified conception of a gas explosion that greatly enhances the feel for

and the understanding of blast generation by flame propagation in gas explosions is

the acoustic volume source analogue according to Auton and Pickles (1978a and

1978b) and Strehlow (1981).

Because combustion is accompanied with a large density drop of the gaseous

material, a gas explosion can be considered a source of volume. The overpressure

generated at a distance r from an acoustic volume source in half space can be

calculated from a potential function (Lighthill, 1978) and is equal to (Strehlow,

1981):

0 DP =

r0 . dV

= (a - )1 r

. d

( A .Sb )f2pr dt 2pr dt

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where: DP = overpressure (Pa)

a = expansion factor

ro = ambient density (kg/m3)

r = distance from source (m)

V = volume source strength (m3/s)

t = time (s)

Af = flame surface area (m2)

Sb = burning speed (m/s)

This simple expression demonstrates that a volume source generates an overpres-

sure only if the volume source strength grows with time. Consequently, a gas

explosion will develop blast only if it increases its volume source strength, i.e. the

product of its flame surface area and its burning speed. This explains why a spheri-

cally developing gas explosion, even if its flame speed is constant, generates a

pressure effect (blast). The reason is that its flame surface area continually grows

during the flame propagation. It also explains why an elongated flammable vapour

cloud of more or less constant cross-sectional area, consumed by a constant veloc-

ity flame, develops hardly any blast. Substantial blast is produced only by accel-

eration of the flame propagation process.

Very similarly, this acoustic analogue may explain the directionality found in the

blast of many explosions where a flammable cloud is consumed by a flame travel-

ling from one end to the other. If a propagating flame is considered a moving

volume source, the developed overpressure piles up in the direction of flame

propagation while the pressure rarefies in the opposite direction. In this way, the

directionality in the blast effect can be explained by some sort of acoustic Doppler-

effect.

The Shchelkin effect

In a gas explosion developing in an obstacle configuration, the (turbulent) flow

structure developing in the flow in front of flame controls the burning. Because the

combustion products expand, the flame bubble acts as an expanding piston that

generates a flow field around. Such a flow field (blast) consists of a particle veloc-

ity distribution as well as a pressure distribution. A spatial obstacle configuration

triggers a positive feedback mechanism by which the flame propagation process

continuously accelerates. This feedback mechanism, called the Shchelkin effect,

can be summarised as follows. Turbulence enhances combustion. An increasing

burning speed intensifies the expansion flow. Increasing flow velocities go hand in

hand with intensifying turbulence, which again enhances combustion etc. etc.

Within a centrally ignited obstacle configuration, the flame is coupled to and drives

a spherical flow field consisting of a continuously intensifying overpressure, flow

velocity and turbulence field. The flame acts as an expanding piston. Because of

the symmetry, a quantity of relatively stagnant, pressurised product gases develops

within the flame bubble.

As soon as the flame leaves the obstacle configuration, the flame front runs into a

decaying turbulence field, by which the burning speed starts to decrease. The rate

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at which the turbulence decays outside the obstacle configuration is dependent on

the scale of the large eddies in the turbulence spectrum and, consequently, on the

obstacle size.

As soon as the burning speed decreases, the overpressure drops at the location of

the flame and rarefaction waves run backward into the pressurised bubble of com-

bustion products and forward after the blastwave. The blast wave propagates at the

local speed of sound and the flame front, transported by the particle velocity field

of the expanding bubble of combustion products, tends to lag behind. The flame

and the blast wave uncouple.

Donor-acceptor flame propagation

As soon as the flame leaves the donor obstacle configuration, the flame will tend to

lag behind the donor blastwave. The conclusion is that, in principle, any separation

distance between donor and acceptor will lead to some form of interruption in the

flame propagation process and an uncoupling of the donor and acceptor blasts to

some extent. Coincidence of donor and acceptor blasts for larger separation dis-

tances is only possible if the time difference between the passage of the donor blast

in the acceptor and the generation of the acceptor blast is sufficiently small to

enable the acceptor blast to overtake the donor’s at short distance. Acceptor blast

can overtake the donor blast only if the acceptor blast is substantially stronger than

the donor’s. The consequence is that coincidence of donor and acceptor blast can

only be observed in the donor-acceptor direction. In the opposite acceptor-donor

direction two separate blasts will be observed.

When the spherical donor explosion runs out of fuel, the spherical symmetry is lost

the flame front area abruptly reduces and the explosion progresses on as a direc-

tional process of flame propagation from the donor to and into the acceptor. The

flame speed reduces to low, sometimes near-laminar flame speed level by back and

side relief of the combustion products. The more or less abrupt transition of the

flame propagation from a spherically symmetric process in the donor to a donor-

acceptor-directed process results in a temporal distortion of the gas dynamic equi-

librium, which may be accompanied with pressure oscillations. Because a flame is

an interface between high-density flammable mixture and low-density combustion

products, it is susceptible to Taylor instability. Dependent on the direction of the

pressure gradient, a flame surface is stable or unstable. Pressure oscillations may

couple to and be amplified by such flame instabilities dependent on the fuel and

composition of the mixture. The development of donor-acceptor flame propagation

has been visualised in the Figures 8a (high-speed movie shots) and 8b (schema-

tised).

By the time the acceptor has been initiated, the flame speed development in the

acceptor may be substantially suppressed by the lift off of the bubble of donor

product gases. The buoyancy and consequently the ascent of the large cloud of

combustion products entrains surrounding mixture and generates a backflow in the

acceptor by which the development of the flame front propagation process in the

acceptor may be substantially influenced.

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Figure 8a: Donor-acceptor flame propagation.

Figure 8b: Schematised donor-acceptor flame propagation.

5.1.2 Interpretation of typical donor-acceptor pressure-time records

The Figures 9a, 9b, 10a and 10b show some typical results of the RIGOS donor-

acceptor tests, measured during the RIGOS program. Figures 9a and 9b show the

experimental results of test AE8.

Test AE8 is defined as follows:

· 10.1% VBR donor of medium size DD2;

· SD = 1.5DD;

· Stoichiometric ethylene-air.

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0

20

40

60

80

P1

P2

P3

P4

P5

P6

P7

P8

P9

-20

RIGOS AE08 SD=1.5DD PressureAE08, SD1.5 p=4.65d, 16+ 014.10375

06/10/2000 PEB/PEI

Pre

ssure

[kP

a]

0 20 40 60 80 100 120 140 160 180

Time [ms]

Figure 9a: Pressure-time signals P1- P9 recorded in test number AE8.

Figure 9a shows 9 overpressure-time histories recorded by the gauges P1 – P9,

positioned at various locations along the centre line of the donor-acceptor configu-

ration from the point of ignition in the donor centre towards and into the acceptor

(Figures 4a and 4b). Figure 9b shows the blast overpressure-time histories recorded

at 3m from the donor centre in acceptor-donor direction (B1) and at 3, 6 and 12 m

from the acceptor centre in the donor-acceptor direction (B2, B3 and B4).

All the pressure-time records in the Figures 9a and 9b show a double-peaked path,

indicating the development of two separate explosions. Because the donor develops

a more or less spherically symmetric flame propagation, the pressure inside the

flame bubble is more or less homogeneously distributed and the maximum over-

pressure developed is about equal all over the full donor volume (P1, P2 and P3).

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-5

0

5

10

15

20

25

30

35 B1

B2

B3

B4

RIGOS AE08 SD=1.5DD BlastAE8 014.10375

16/08/2000 PEB/PEI

Pre

ssure

[kP

a]

0 25 50 75 100 125 150 175 200 225

Time [ms]

Figure 9b: Blast overpressure-time signals B1- P4 recorded in test number AE8.

After the flame left the donor obstacle configuration, the flame speed dropped and

the donor explosion developed a negative phase. Subsequently, the spherical donor

explosion ran out of fuel and the flame traveled on in a directional mode crossing

the separation distance. Here, the burning velocity reduced to near-laminar values,

witness the zero pressure developed. The drop of the flame speed and the transition

from the spherical into the directional mode of flame propagation was accompa-

nied with a temporal loss of gas dynamic equilibrium by which pressure oscilla-

tions develop. These pressure oscillations may couple to flame instabilities by

which, dependent on the fuel, the pressure oscillations may be amplified.

The overpressure in the acceptor gradually developed from P6, where the flame

enters the acceptor to a maximum in P9, where the flame left the acceptor.

The explosion overpressures developed by donor and acceptor are 80 kPa and

55 kPa respectively. The time difference between the donor and acceptor explosion

is about 100 ms. This time gap is also observed in the blast signals in Figure 9b,

which clearly show that the blasts in acceptor-donor direction (B1) as well as in

donor-acceptor direction (B2, B3 and B4) consisted of 2 waves separated by about

100 ms.

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0

200

400 P1

P2

P3

P4

P5

P6

P7

P8

P9

RIGOS AE09 SD=0.25DD PressureAE9 014.10375

17/08/2000 PEB/PEI

Pre

ssure

[kP

a]

0 20 40 60 80 100

Time [ms]

Figure 10a: Pressure-time signals P1- P9 recorded in test number AE9.

0

20

40

60

80

100

120

RIGOS AE09 SD=0.25DD Blast

B2 3m

B3 6m

B4 12m

AE9 014.10375

17/08/2000 PEB/PEI

Pre

ssure

[kP

a]

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Time [ms]

Figure 10b: Blast overpressure-time signals B1- P4 recorded in test number AE9.

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Figures 10a and 10b show the comparable overpressure-time signals recorded in

test AE9. In comparison to test AE8, in AE9 only the separation distance has been

reduced to 0.25 DD.

Figure 10a shows that, despite the small separation distance of 0.25DD, the flame

propagation process and pressure build-up were clearly interrupted by the separa-

tion distance. The overpressure in the acceptor developed 4 to 6 ms after the donor.

Now the acceptor overpressure (more than 400 kPa) was substantially higher than

the donor’s. Pressure waves of that strength readily shock up and propagate at

supersonic velocity. The consequence was that the acceptor blast overtook the

donor blast in the donor-acceptor direction. This is clearly shown in the blast

overpressure signals in Figure 10b. In the donor-acceptor direction the signals B2,

B3 and B4 clearly show one single shocked blast wave. In the opposite direction,

however, the B1-signal (black) shows that the blast consisted of two separate

waves. The coincidence of the donor and acceptor blasts in the donor-acceptor

direction indicates that the separation distance of 0.25DD in this test has been

smaller than critial.

The timing of the blast signals B2, B3 and B4 accurately corresponds with the

respective gauge positions relative to the blast source and the blast wave propaga-

tion velocity.

5.2 The Critical Separation Distance

5.2.1 Approach

The primary objective of the RIGOS project is the development of guidance re-

garding the critical separation distance. To this end, concrete data with regard to

the critical separation distance dependent on various donor parameters is required.

Initially, the intended approach in measuring the critical separation distance was

very straightforward. The critical separation distance was to be determined by

recording the blast effects produced by donor-acceptor obstacle configurations

while the separation distance was gradually reduced. At the separation distance

where just one single instead of two separate blast waves were observed in donor-

acceptor direction, the distance between donor and acceptor was assumed to be

critical. However, during the performance of the program, it became clear that

various practical difficulties stood in the way of such a simple approach or ob-

scured clear observations.

· Due to damaging explosion overpressures that developed in the AE9 experiment

when the separation distance was below critical, this straighforward approach

appeared no longer possible. Necessarily, we had to restrict ourselves to low

explosion overpressure experiments in the rest of the program.

· Low explosion overpressures were obtained by the application of methane and

propane as fuels instead of ethylene. The overpressures, developed by the ac-

ceptor explosion with methane and propane as fuels, were low, just a few kPa.

In addition, the effective energy contributing to the blast is very low. The con-

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sequence is that in many experiments with the smaller separation distances, the

acceptor blast overpressure more or less coincided and was obscured by the

secondary donor gas dynamics. This made the experimental results often hard to

interprete.

Nevertheless, in a limited number of tests the experimental results gave the indica-

tion that the separation distance was near-critical. In the next Chapter the results of

these tests will be briefly summarised and commented. The full pressure-time

records concerned can be found in the accompanying RIGOS experimental results

report (De Bruijn and Van Ierschot, 2002).

5.2.2 Results

Test no. AE09

Test definition:· Intermediate donor size of obstacle density A;· Fuel ethylene;· Separation distance: SD/DD2=0.25;· Donor overpressure = 73 kPa;· Acceptor overpressure = > 400 kPa.Although the pressure development in donor and acceptor are clearly separated, theacceptor overpressure is so high that the acceptor blast overtakes the donor blast atshort distance. One single blast was observed in the donor-acceptor direction. Theconclusion is that the separation distance is less than critical.

Test no. AM12

Test definition:

· Intermediate donor size of obstacle density A;

· Fuel methane;

· Separation distance: SD/DD2=0.125;

· Donor overpressure = 26.5 kPa;

· Acceptor overpressure = ? kPa.

Acceptor explosion overpressure seems to be obscured by negative phase and

secondary wave of the donor.

Test no. BE10

Test definition:· Large donor size of obstacle density B;· Fuel ethylene;· Separation distance: SD/DD3=0.25;· Donor overpressure = 19 kPa;· Acceptor overpressure = 106 kPa.The pressure development in donor and acceptor are clearly separated. The accep-tor blast is much stronger than the donor’s but is not strong enough to overtake thedonor’s. Two separate blasts observed in either direction. In the donor-acceptordirection, the blasts are coupled, i.e. they are not separated by a negative phase.

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Blasts may further overtake in the far field. The conclusion is that the separation

distance is near but just more than critical.

Test no. BM02

Test definition:

· Large donor size of obstacle density B;

· Fuel methane;

· Separation distance: SD/DD3=0.125;

· Donor overpressure = 3.6 kPa;

· Acceptor overpressure = 2.2 kPa.

Explosion overpressure development in donor and acceptor is coupled. Two peaks

of comparable height (0.5 kPa) without an underpressure in between. This picture

is maintained in the blast. As a consequence of low energy, acceptor blast seems to

decay faster than donor’s and to lag behind increasingly. Separation distance seems

to be slightly more than critical.

Test no. DP04

Test definition:· Large donor size of mixed obstacle density (B with big tubes);· Fuel propane;· Separation distance: SD/DD3=0.125;· Donor overpressure = 8 kPa;· Acceptor overpressure = 19 kPa.The process of pressure development in donor and acceptor is hardly interrupted bythe separation gap. Separation distance clearly less than critical.

Test no. DM01

Test definition:· Large donor size of mixed obstacle density (B with big tubes);· Fuel methane;· Separation distance: SD/DD3=0.125;· Donor overpressure = 5.2 kPa;· Acceptor overpressure = 3 kPa.Acceptor overpressure seems to develop in the secondary wave of the donor. Thispicture is maintained in blast signals. Separation distance seems to be just morethan critical.

Test no. DM02

Test definition:· Large donor size of mixed obstacle density (B with big tubes);· Fuel methane;· Separation distance: SD = 5 cm, i.e. SD/DD3=0.03;· Donor overpressure = 8 kPa;· Acceptor overpressure = 12 kPa.

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One process of overpressure development in donor and acceptor at this small

separation gap. Separation distance clearly less than critical.

The above-listed tests, where the separation distance was near-critical, have been

graphically represented in Figure 11. The less than critical and near-critical ex-se

pa

ratio

n d

ista

nce

/do

no

r siz

e (

-)

periments have been indicated with red and purple dots respectively.

0.6

0.4

0.2

0.0

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

j

more than critical

ust more than critical

less than critical

DM01

BM02

BE10

DM02

DP04 AM12

AE09

donor explosion overpressure (kPa)

Figure 11: Critical separation distance dependent on donor overpressure.

5.2.3 Analysis

Despite the practical difficulties in the direct observation of critical separation

distances, the results of the entire test series greatly increased the understanding of

the phenomena observed. The general trend coming forward is that the critical

separation distance seems to increase up to more than 0.25DD as the donor explo-

sion overpressure is higher than 90kPa (test AE9). On the other hand, the critical

separation distance seems to tend to below 0.125DD for lower and lower donor

explosion overpressures. A larger critical separation distance seems to apply for

large obstacle diameter (test DP04). These tendencies can be made plausible by

simple theoretical reasoning.

In the theoretical considerations in Chapter 5.1.1, it was already explained that as

soon as the flame leaves the donor, it propagates into a decaying turbulence field.

The consequence is that its burning speed decreases and flame front and donor

blast wave uncouple. The flame propagates at a speed determined by the local

decaying turbulence level while the blast wave travels away with the speed of

sound. Any separation distance between donor and acceptor will result in some

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form of uncoupling of flame and donor blasts. Coincidence of donor and acceptor

blasts is only possible if the time difference between donor and acceptor pressure

build-up is sufficiently small while, in addition, the acceptor blast is sufficiently

strong and energetic to be able to overtake the donor blast at short distance.

The time difference between donor and acceptor pressure build-up can only be

small if the separation gap is sufficiently small. Larger critical separation distances

are only possible if the respective velocities, at which the flame and donor blast

cross the separation gap, are of similar magnitude. For low donor explosion over-

pressure, donor flame speed and blast propagation speed (sound speed) differ too

much to enable coincidence of donor and acceptor blasts. If, on the other hand, the

maximum donor flame speed is of the order of the speed of sound, the time differ-

ence between donor and acceptor pressure build-up is small. If, in addition, the

acceptor blast is much stronger than the donor blast, the acceptor blast will over-

take the donor’s.

The consequence is that larger critical separation distances will only be observed

for higher explosion strengths. The donor overpressure must be of the order of

100 kPa (flame speed of the order of sound speed) while the acceptor blast must be

sufficiently energetic and substantially stronger than the donor’s.

Practical problems compelled to limit this experimental program to the relative low

overpressure range. With respect to the question of practical guidance, it is inter-

esting to philosophise on how the findings of this program could be extrapolated

into the high overpressure range.

In the limit of donor detonation, the environment of the donor is fully quiescent at

the moment the donor fuel is completely burnt and the flame leaves the donor.

According to the basic assumptions of the Multi-Energy concept, the detonation

will fail at that instant. The flame surface will cross the separation gap riding on the

expanding bubble of combustion products, which is a very fast process. As the

expansion factor for stoichiometric hydrocarbon-air combustion is approximately

8, the bubble of donor combustion products will not expand beyond a distance of

half the donor dimension (0.5DD). Therefore, a critical separation distance of

0.5DD seems a safe and conservative limit in the high explosion pressure range.

This statement is in accordance with the preliminary guideline drafted during the

GAMES-project (Mercx et al., 1998).

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5.3 Influence of a Connecting Obstacle Configuration

5.3.1 Approach

In practice, different process units at (petro-)chemical plant are often mutually

connected through pipe racks. To investigate whether the critical separation dis-

tance between two process units is influenced by the presence of a connecting pipe

rack, the C test series was designed. The objective was to represent a realistic

situation by connecting the donor and acceptor by an obstacle configuration of the

same obstacle density but of limited cross-section. The influence of the connection

can be observed by comparison with the results from an identical test without a

connection.

5.3.2 Results

In Figure 12a and 12b, the pressure-time signals P1 – P9 and B1 – B4 recorded intest number AM04 have been represented. This experiment is defined as follows:· Donor obstacle density A;· Methane;· Medium donor size DD2;· Separation distance 0.5DD.In Figures 13a and 13b, the pressure-time signals P1 – P9 and B4– B6 recorded intest number CM01 have been represented. Experiment CM01 differs from AM04only in the presence of a connecting obstacle configuration of 8´4 obstacles cross-section.

Like all pressure records in the AM test series, the pressure records P1 – P9 inFigure 12a (test no. AM04) are characterised by substantial high-frequency pres-sure oscillations, which arise after the donor explosion in particular near the sta-tions P4 – P6 between donor and acceptor.Probably, the pressure oscillations are the consequence of flame instabilities,triggered by the temporal loss of the gas dynamic equilibrium at the moment theflame propagation process progresses from the spherically symmetric mode (do-nor) into the (donor-acceptor) directional mode. A striking difference with Figure13a (test no. CM01) is that here, the pressure oscillations are absent. Apparently,the presence of an obstacle connection between donor and acceptor completelysuppresses the oscillations.

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-8

-6

-4

-2

0

2

4

6

8

10

12

14

P1

P2

P3

P4

P5

P6

P7

P8

P9

-10

RIGOS AM04 SD=0.5DD PressureAM4, SD=0.5 014.10375

28/08/2000 PEB/PEI

Pre

ssure

[kP

a]

0 25 50 75 100 125 150 175 200

Time [ms]

Figure 12a: Pressure-time signals P1- P9 recorded in test number AM04, without an

obstacle connection.

PEB/

RIGOS AM04 SD=0.5DD BlastAM4, SD=0.5 014.10375

28/08/2000 PEI

Pre

ssure

[kP

a]

4

2

0

-2

B1

B2

B3

B4

0 20 40 60 80 100 120 140 160 180 200 220

Time [ms]

Figure 12b: Blast overpressure-time signals B1(3m from donor) and B2 - B4 (3,6 and

12 m from acceptor) recorded in test number AM04 without an obstacle

connection.

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/PEB/

RIGOS CM01, SD=0.5DD, bridge 8x8x4, pressureCM01 014.10375

15 05/2001 PEI

P1

P2

P3

P4

P5

P6

P7

P8

P9

-10.0k

-5.0k

0.0

5.0k

10.0k

15.0k

Pre

ssure

[P

a]

0.08 0.10 0.12 0.14 0.16

Time [s]

Figure 13a: Pressure-time signals P1- P9 recorded in test number CM01 with an obstacle

connection.

-2k

-1k

0

1k

2k

3k

4k

/

Blast CM01; 3, 6 and 12 m from centre ACCEPTOR, bridge 8x8x4

B4 3m

B5 6m

B6 12m

CM01 014.10375

09 03/2001 PEB/PEI

Pre

ssure

[P

a]

0.08 0.10 0.12 0.14 0.16 0.18

Time [s]

Figure 13b: Blast overpressure-time signals B4 – B6 (3,6 and 12 m from acceptor)

recorded in test number AM04 with an obstacle connection.

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The acceptor’s explosion overpressure develops in test CM01 much earlier after

the donor’s than in test AM04. The presence of a connecting obstacle configuration

does hardly amplify the acceptor explosion overpressure in this experiment but

does substantially accelerate the moment of initiation of the acceptor. Where the

average flame speed over the separation distance in test AM04 reduces down to

near laminar values, in the connecting obstacle configuration in test CM01 it is of

the order of several tens of m/s. Nevertheless, the connecting obstacle configura-

tion is not able to maintain the flame speed and overpressure developed in the

donor. The flame speed is too low and the pressure development is dominated by

sideward and backward pressure relief.

Figure 13b shows that despite the earlier initiation of the acceptor in test CM01, the

blast clearly consists of two separate waves, which indicates that the separation

distance has been larger than critical. The conclusion is that the presence of a

connecting obstacle configuration substantially influences the flame propagation

process between donor and acceptor and, consequently, will influence the critical

separation distance.

This is further demonstrated by comparing the results of the tests number AE07

and CE01. The only difference with the tests AM04 and CM01 is that now the fuel

is ethylene, a lot more reactive than methane. The results are shown in the

Figures 14 (AE07) and 15a, 15b and 15c (CE01).

Because here the fuel is ethylene, the donor explosion develops a substantially

higher overpressure, i.e. more than 70 kPa. Nevertheless, the separation distance

(0.5DD) is amply sufficient to separate the donor and acceptor explosions (30 ms)

and to completely separate their blasts.

The Figures 15a, 15b and 15c show that, here, the presence of a connecting obsta-

cle configuration of 8´4 obstacles cross-section has dramatic consequences.

Figure 15a shows that the flame speed and overpressure developed in the donor

(P1 – P3) is sufficient to maintain and even further amplify in the connecting

obstacle configuration (P6) and develop a detonation in the acceptor (P7 - P9). The

pressure gauges in the acceptor got overloaded and the obstacle configurations

were severely damaged. Probably, the anomalous behaviour observed by pressure

gauge P2 is the the consequence of having been struck by some debris.

Figure 15b shows the blastpressure signals at 3, 6 and 12 m distance from the

donor centre in acceptor-donor direction. The signals show a clear double-peaked

character indicating the interruption in the flame propagation by the transition from

spherically symmetric mode (donor) to the directional mode (connection and

acceptor). The blast pressure signals in the donor-acceptor direction (Figure 15c),

on the other hand, show clearly that donor and acceptor blasts coincided, indicating

that the separation distance was less than critical.

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16/08/2000 PEB/PEI

0

10

20

30

40

50

60

70 P1

P2

P3

P4

P5

P6

P7

P8

P9

-20

-10

Pre

ssure

[kP

a]

0 10 20 30 40 50 60 70 80 90 100

Time [ms]

Figure 14: Pressure-time signals P1- P9 recorded in test number AE07 without an

obstacle connection.

0

100k

200k

300k

400k

P1

P2

P3

P4

P5

P6

P7

P8

P9

/05/PEB/PEI

RIGOS Pressure CE01, bridge 8x8x4CE01 014.10375

15 2001

Pre

ssure

[P

a]

0.02 0.03 0.04 0.05 0.06 0.07 0.08

Time [s]

Figure 15a: Pressure-time signals P1- P9 recorded in test numberCE01 witht an obstacle

connection.

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PML 2002-C50

-10k

0

10k

20k

30k

40k

50k B1 3m

B2 6m

B3 12m

/

Blast CE01; 3, 6 and 12 m from centre DONOR, bridge 8x8x4CE01 014.10375

09 05/2001 PEB/PEI

Pre

ssure

[P

a]

0.030 0.035 0.040 0.045 0.050 0.055 0.060 0.065 0.070 0.075 0.080 0.085 0.090

Time [s]

Figure 15b: Blast overpressure-time signals B1 – B3 (3,6 and 12 m from donor) recorded

in test number CE01 with an obstacle connection.

-10k

0

10k

20k

30k

40k

50k

60k

/05/

defect

Blast CE01; 3, 6 and 12 m from centre ACCEPTOR, bridge 8x8x4

B4 3m

B5 6m

B6 12m

CE01 014.10375

09 2001 PEB/PEI

Pre

ssure

[P

a]

0.040 0.045 0.050 0.055 0.060 0.065 0.070 0.075 0.080 0.085 0.090

Time [s]

Figure 15c: Blast overpressure-time signals B4 – B6 (3,6 and 12 m from acceptor)

recorded in test number CE01 with an obstacle connection.

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Experiment number CE01 strongly suggests that the connecting obstacle configu-

ration of 8´4 obstacles cross-section, initiated by a donor explosion of sufficient

strength could propagate and further amplify a gas explosion over separation

distances of any length.

It would be interesting to gradually diminish the connecting obstacle configuration

cross-section to see whether smaller cross-sections are also able to propagate an

explosion in a similar way. However, because we ran out of obstacle parts, after

this test any chance of damaging overpressures had to be precluded. Therefore, the

rest of the series with other connecting obstacle configurations were performed

with propane as fuel. An identical test with propane but without an obstacle con-

nection served for comparison. The tests, all performed with an A type donor,

medium donor size and a separation distance equal to half the donor size, were

defined and numbered as in Table 7 below.

Table 7: Test series to study the influence of connecting obstacle configurations.

AM04 No rack

CM01 As AM04 plus rack 8 x 8 x 4 tubes

AE07 No rack

CE01 As AE07 plus rack 8 x 8 x 4 tubes

AP02 No rack



With propane as fuel, the donor explosion develops about 30 kPa instead of the

70 kPa with ethylene. Comparing test number CP01 and CP02 with AP02, the

conclusion is that an obstacle connection of a 2 x 1 obstacle cross-section has no

observable influence on the flame propagation process. A 4 x 2 obstacle cross-

section only slightly accelerated the flame propagation process over the separation

distance.

5.3.3 Analysis

The highly simplified conceptions of a gas explosion, described in Chapter 5.1.1,

may be helpful in making the results of the C-test series plausible. At low speed,

the flame propagation process is largely dominated by expansion. Consequently,

slow flame propagation is very sensitive for effects like side and back relief. The

flame speed will reduce down to low values when the flame propagation mode

progresses from spherically symmetric (donor) to directional (acceptor) and the

flame surface area is substantially reduced. A slow flame, propagating in a con-

necting obstacle configuration, will hardly speed up and consequently hardly

develop any overpressure.

With increasing speed, on the other hand, flame propagation is less and less sensi-

tive to expansion. As flame propagation becomes faster and faster, it is less af-

fected by the backward expansion of combustion products.

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5.4 Influence of the Obstacle Diameter

5.4.1 Approach

To observe whether the donor obstacle diameter has an influence on the Critical

Separation Distance, the B type obstacle configuration of maximum size DD3 was

provided with a regular pattern of vertical tubes of 114 mm diameter (Figure 7). A

series of 6 donor-acceptor tests was performed in which the separation distance

was gradually diminished according to Table 8. In the first 4 tests the fuel was

propane. Where the separation distance in the propane tests tends to be smaller than

critical, the propane was replaced by methane.

Table 8: Definition and numbering of D- test series.

Test number

SD/DD SD = 0.05 m 0.125 0.25 0.375 0.50

DD3 DP04 DP03 DP02 DP01

DD3 DM02 DM01

The full pressure-time records have been represented in the RIGOS experimental

results report (De Bruijn and Van Ierschot, 2002) under the tabulated test numbers.

The test series AE, AM, BE and BM have shown that the critical separation dis-

tance is primarily dependent on the explosion strength or the explosion overpres-

sure in the donor. When the explosion overpressure was about 100 kPa, the critical

separation distance appeared to be larger than 0.25 times the donor size. For de-

creasing donor strength, the critical separation distance seemed to tend to nearly

zero.

To be able to observe whether the obstacle diameter in the donor has an influence

on the critical separation distance, strictly speaking, two identical experiments that

only differ in obstacle diameter in the donor should be compared. Because the

donor explosion overpressure and final flame speed is a significant factor in the

critical separation distance, for a fair comparison the donor explosion strengths and

consequently the maximum donor flame speed should be equal in both experi-

ments. This is an impossibility with the available obstacle parts.

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RIGOS AM12 SD=0.125DD PressureAM12, SD0.125, 20+ 014.10375

06/09/2000 PEB/PEI

-5

0

5

10

15

20

25

30

P1

P2

P3

P4

P5

P6

P7

P8

P9

-10

Pre

ssure

[kP

a]

0 20 40 60 80 100 120 140 160 180 200 220

Time [ms]

Figure 16a: Pressure time records in experimentin test AM12, showing that the acceptor

overpressure is fully obscured by the donor’s secondary wave.

RIGOS,

P1

P2

P3

P4

P5

P6

P7

P8

P9

/

-4.0k

-2.0k

0.0

2.0k

4.0k

6.0k

DM01, SD=0.125DD, PresssureDM01, SD=0.125 014.10375

01/06 2001 PEB/PEI

Pre

ssure

[P

a]

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

Time [s]

Figure 16b: Pressure time records in experimentin test DM01.

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5.4.2 Results

Nevertheless, comparison of the results of the tests AM12 and DM01 suggests

there is some influence of the donor obstacle size. Both tests are with methane.

Both tests are characterised by a large donor size and a separation distance of

0.125 times the donor size. Although the donor overpressure in test AM12 was

substantially higher than in test DM01, the pressure time records P1 – P9 of the test

with the larger obstacles suggests a closer coupling of donor and acceptor explo-

sions. Where the donor develops a pronounced negative phase in test AM12 and

the acceptor overpressure is obscured by the donor’s secondary pressure wave

(Figure 16a), the development in test DM01 seems to be more closely coupled

(Figure 16b). This result strongly suggests that a larger obstacle size in the donor

has a tendency to increase the critical separation distance.

5.4.3 Analysis

Although the experimental results are not very convincing for the time being, there

is good physical reason for a relation between donor obstacle diameter and critical

separation distance. When the flame leaves the donor, it propagates into a decaying

turbulence field. The decay rate of turbulence is closely related to the scale of the

large eddies in the turbulence spectrum. The larger the scale, the slower the decay.

The scale of the large eddies is directly related to the size of the obstacles. Conse-

quently, larger obstacles will result in a relatively lower drop rate of the flame’s

speed on leaving the donor. Higher flame speeds during the separation gap’s

crossing will tend to increase the critical separation distance.

On the basis of the indication above, it would be expected that the Critical Separa-

tion Distance in the low explosion overpressure range could be expressed in a

number of the obstacle diameters rather than in a number of donor dimensions.

In test DP04, a separation distance of 0.125DD (which is equal to 1.75 times the

obstacle diameter) was clearly less than critical. A separation distance of 0.25DD

(3.5 times obstacle diameter) was clearly more than critical (test DP03). Data too

limited for closer conclusions.

5.5 Practical Guidance on the critical separation distance

In many of the tests where the separation distance was near-critical, the donor gas

dynamics partly suppressed the acceptor explosion overpressure or the acceptor

overpressure was largely obscured by the secondary donor gas dynamics. This

made the pressure-time records often hard to interpret. Nevertheless, the Critical

Separation Distance can be expressed as a portion of the donor dimension, at least

as long as the donor is centrally ignited. In addition to the donor explosion over-

pressure, the donor obstacle size appeared to be a possible determining parameter,

at least in the low donor explosion overpressure range. Looking at Figure 11, we

may conclude that in 3 tests the separation distance has been definitely less than

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critical. In 4 tests, it was concluded that the separation distance was near but just

more than critical.

On the basis of Figure 11, some concrete guidance may be drawn up for the Criti-

cal Separation Distance in the range of phenomena covered in the experimental

program. This guidance may be extrapolated outside this range on the basis of the

understanding developed.

Any practical guidance on the Critical Separation Distance, drawn up on the basis

of this limited information, should be safe and conservative. Therefore, sufficiently

safe margins, a factor of about 2 for instance, should be observed in the drafting of

this guidance. On the basis of these considerations, the guidance with regard to the

Critical Separation Distance may be as follows:

· A Critical Separation Distance between obstructed areas equal to ½ of the donor

dimension should be observed for any donor explosion overpressure of high

strength (> 100 kPa).

· A Critical Separation Distance between obstructed areas equal to ¼ of the donor

dimension should be observed for donor explosion overpressures of low

strength (< 10kPa). Possibly, the donor obstacle diameter is a governing pa-

rameter in this range.

· A linear interpolation between these two data is proposed and has been indi-

cated in Figure 17 together with the experimental observations.

· A connecting obstacle configuration of sufficient cross-sectional area between

two obstructed areas, such as a pipe rack for instance, may substantially in-

crease the Critical Separation Distance.

se

pa

ratio

n d

ista

nce

/do

no

r siz

e (

-)

0.6

0.4

0.2

0.0

less than critical

BM02

BE10

AE09 more than critic

j

al

DM01 DP04 AM12

DM02

ust more than critical

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

donor explosion overpressure (kPa)

Figure 17: Proposed guidance for the Critical Separation Distance.

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The upper limit of the Critical Separation Distance equal to half the donor size is

based on the observation that a stoichiometric hydrocarbon-air mixture does not

expand beyond twice its original linear dimension on combustion, at least when the

flame propagation process is spherically symmetric.

However, it is uncertain whether this statement may be extrapolated to donor

explosions with a directional flame propagation mode. Such gas explosions are

characterised by a directional expansion process and consequently by blast effects

with a preferential direction. This directionality is stronger for increasing flame

speeds. In the limit of a gas detonation, almost all of the expansion is generated in

the direction of flame propagation.

It is interesting to note that the Critical Separation Distance that results from the

intuitive definition of a congested area in the The Yellow Book (CPR-14E, 1997),

being equal to 10 obstacle diameters, is not safe and conservative, in particular in

the higher explosion overpressure range.

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6 Additional information from experiments

The primary objective of the RIGOS research program was to develop guidance on

the Critical Separation Distance. To this end, quite an extensive experimental

program of donor-acceptor explosions has been run. In addition to the primary

information on the critical separation distance, the program resulted in an interest-

ing data set on donor-acceptor explosion behaviour from which additional infor-

mation can be deduced.

It has been observed, for instance, that the donor explosion may have a big and

sometimes unexpected influence on the development of the acceptor explosion.

The data also offers an opportunity to evaluate the performance of simple methods

for vapour cloud explosion blast modelling under conditions substantially deviating

from the conditions the methods were derived from.

6.1 Donor-acceptor explosion behaviour

6.1.1 Results

The test set-up, maintained in almost the entire RIGOS program, consisted of two

obstacle configurations separated by an empty space of varying length. Because the

donor was centrally ignited, the donor explosion always developed approximately

spherically symmetric. When the donor explosion ran out of fuel, the spherical

symmetry was lost and the flame propagated across the separation gap towards the

acceptor. In the acceptor, the flame propagated from one side to the other. In all

cases the acceptor consisted of an obstacle configuration of exactly the same size

and obstacle density.

Because the donor explosion influences the conditions in the acceptor at the time of

initiation, it can be expected that the donor explosion will influence the develop-

ment and the strength of the acceptor explosion. To investigate this, the maximum

explosion overpressures observed in the acceptor have been graphically repre-

sented as a function of the relative separation distance in Figure 18a for test series

AE and AM and in Figure 18b for test series BE and BM.

Figure 17a shows a general and surprising tendency that when the separation

distance is diminished from large down to about half the donor size, the overpres-

sure in the acceptor explosion reduces. The larger the donor size the bigger the

reduction. In the AE and AM test series, a maximum reduction of the acceptor

explosion overpressure was attained if the separation distance was approximately

half the donor size, the range up to which the donor combustion products expand.

A further reduction of the separation distance, on the other hand, resulted in a

strong amplification of the acceptor explosion.

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Maximum Pressure Acceptor

at opposite site of the donor, P9

series AE & AM

1000

ov

erp

res

su

re (k

Pa

)

100

10

1

0 0.5 1 1.5 2 2.5

separation distance SD/DD

AE, donor size DD1 AE, donor size DD2

AM, donor size DD2 AM, donor size DD3

Figure 18a: Overpressure in the acceptor as function of the relative separation distance

SD/DD for test series AE and AM.

1

10

100

1000

0 1 2

RIGOS series BE and BM

maximum source pressure in Acceptor

0.5 1.5 2.5

Separation distance SD/DD

Ov

erp

res

su

re (

kP

a)

BE, donor size DD1 BE, donor size DD2

BE, donor size DD3 BM, donor size DD3

Figure 18b: Overpressure in the acceptor as function of the separation distance SD for

test series BE and BM.

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A similar trend but less systematic and pronounced comes forward from the results

from the tests in the series BE in Figure 18b. Only the BE-tests with the small

donor size DD1 seem to show a deviating behaviour of a continually increasing

acceptor overpressure with reducing separation distance.

6.1.2 Analysis

To be able to analyse the influence of the donor on the acceptor explosion, it is

interesting to know what overpressure the acceptor would develop in the absence

of a donor obstacle configuration. To this end, three additional tests without a

donor, numbered as AM06, AM07 and AM08, were designed. The respective test

set-ups are sketched in Figure 19 below.

In test AM06, just the donor obstacle configuration was removed. The plastic sheet

tent and ignition location were kept exactly identical to the tests AM02/03. Subse-

quently, in test AM07, only the ignition was replaced to the edge of the acceptor

obstacle configuration. In test AM08, finally, the plastic sheet tent was reduced to

the acceptor size.

A

A

A

A

Plastic sheet

AM02 / AM03

AM06

AM07

AM08

Figure 19: Design and numbering of the tests AM06, AM07 and AM08.

Table 9: Overpressures developed in the test AM06, AM07 and AM08.

Overpressure (kPa)

AM06 2.4

AM07 5.2

AM08 6.7

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The results of the test AM06, AM07 and AM08 in terms of acceptor explosion

overpressures have been tabulated in Table 9. The figures show that the slow

combustion of a substantial amount of gas before initiating the acceptor substan-

tially reduced the overpressure development in the acceptor. In test AM06, the

entire contents of the plastic sheet tent was burnt before the acceptor initiated.

Apparently, the buoyancy and consequently the ascent of the large cloud of com-

bustion products entrained surrounding mixture and generated some backflow in

the acceptor that hampered the development of the flame propagation in the ac-

ceptor. In both, the AM07 and AM08 test, the acceptor was directly initiated by

which the described phenomena could only occur to a lesser extent. It is to be

expected that particularly in the tests with the large separation distances the accep-

tor overpressures have been influenced by the phenomena described above. Be-

cause lift off by buoyancy is a relatively slow process, it can be expected that the

acceptor overpressure reduction is most substantial in the methane tests with low

flame velocities. In the ethylene tests on the other hand, the overpressure reduction

by this effect is expected to be limited.

However, the most substantial reductions of the acceptor explosion overpressure

have been observed in the ethylene tests and for separation distances near half the

donor size. These reductions must have a different origin.

In this respect, it is important to realise that a gas explosion when the flame speed

reduces or when it runs out of fuel develops an underpressure (a negative phase).

The inertia of the expanding material causes overexpansion, underpressure and

subsequently a backflow into the explosion centre. In particular when the explosion

develops sphere-symmetrically like in the donor, the underpressure in the centre

can be quite substantial.

Flame propagation development is determined by the structure of the flow field in

front of the flame. If this flow field is reduced or even reversed temporarily, it is

well imaginable that the flame propagation development in the acceptor is ham-

pered by the negative phase of the donor blast. This can only be the case, of course,

when negative donor blast passage and flame propagation into the acceptor more or

less coincide, i.e. for the small separation distances. A separation distance equal to

half the donor size, which is the range up to which the donor expands upon com-

bustion, assures a close connection between the gas dynamics of the donor and the

initiation of the acceptor. This is demonstrated in Figure 20, which shows the

overpressure development observed in 9 pressure gauges in test number AE7, the

test in which the most substantial acceptor overpressure reduction was observed.

Without the donor, the acceptor would have developed an overpressure of about

100 kPa where it is now only 19 kPa.

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16/08/2000 PEB/PEI

70

Pre

ssure

[kP

a]

60

50

40

30

20

10

0

-10

-20

0 10 20 30 40 50 60 70 80 90 100

P1

P2

P3

P4

P5

P6

P7

P8

P9

Time [ms]

Figure 20: Pressure development recorded by 9 gauges in experiment AE07 where the

most substanstial acceptor overpressure reduction was observed.

It would be expected that an even bigger acceptor overpressure reduction effect

could be obtained for separation distances slightly smaller than half the donor size.

It is obvious that the larger the donor size relative to the acceptor size and the

higher the donor overpressure, the bigger the influence of the donor explosion on

the acceptor explosion.

The considerations above would possibly enable a second definition of the Critical

Separation Distance as being the separation distance within which the acceptor

explosion is influenced by the donor. Concrete figures in this respect, however, are

hard to give because within the scope of this experimental program in all tests the

acceptor was influenced by the donor. Even in the tests a donor obstacle configu-

ration was absent, the acceptor explosion was influenced by the ascent of the

combustion products of the unobstructed mixture.

6.2 Validation of blast modelling methods

6.2.1 Approach

The body of data on explosion overpressures and blast overpressures offers an

opportunity to assess the performance of the simple methods for the modelling of

vapour cloud explosion blast. The TNO fuel-air blast charts are used for the repre-

sentation of blast effects. The TNO blast charts represent the blast wave overpres-

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sure and the positive phase duration dependent on the distance from an equivalent

hemi-spherical fuel-air charge on the earth’s surface. The TNO blast charts are the

result of numerical compilation of hemispherical explosion of an average stoichi-

ometric hydrocarbon-air mixture for various constant flame speeds. Application of

the TNO blast charts requires two parameters for input:

· A charge strength or an explosion overpressure;

· A charge energy.

The charge strength or the explosion overpressure can be estimated through appli-

cation of the GAME-correlation. The GAME-correlation relates the explosion

overpressure to parameters, such as: size and density of the obstacle configuration,

the fuel reactivity and the geometric scale of the experiment. The charge energy is

determined according to the TNO recommendation by taking the combustion

energy present within the obstructed area, assuming stoichiometry, i.e. the nominal

energy.

6.2.2 The performance of the GAME-correlation

6.2.2.1 Results

In order to assess the performance of the GAME-correlation on the experimentally

observed explosion overpressures, the donor explosion overpressure as well as the

acceptor explosion overpressures have been tabulated in the Tables 10a and 10b

respectively together with the overpressures calculated with the GAME-

correlation.

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Table 10a: Experimentally observed donor explosion overpressures compared to

estimates on the basis of the GAME correlation.

Exp. no. VBR (%) Lf (m) D (m) Sl (m/s) DPGAME DPexp (kPa) (kPa)

AE01 10.1 0.53 0.019 0.66 29 26.0

AE02 10.1 0.53 0.019 0.66 29 25.9

AE03 10.1 0.53 0.019 0.66 29 23.0

AE04 10.1 0.53 0.019 0.66 29 27.0

AE05 10.1 0.53 0.019 0.66 29 23.2

AE06 10.1 0.704 0.019 0.66 64 74.5

AE07 10.1 0.704 0.019 0.66 64 71.8

AE08 10.1 0.704 0.019 0.66 64 79.2

AE09 10.1 0.704 0.019 0.66 64 72.9

AM01 10.1 0.704 0.019 0.40 17 14.8

AM02 10.1 0.704 0.019 0.40 17 14.3

AM03 10.1 0.704 0.019 0.40 17 14.0

AM04 10.1 0.704 0.019 0.40 17 11.2

AM05 10.1 0.704 0.019 0.40 17 14.2

AM09 10.1 0.88 0.019 0.40 30 25.9

AM10 10.1 0.88 0.019 0.40 30 23.2

AM11 10.1 0.88 0.019 0.40 30 29.5

AM12 10.1 0.88 0.019 0.40 30 26.5

BE01 4.6 0.532 0.019 0.66 3.4 3.3

BE02 4.6 0.532 0.019 0.66 3.4 3.2

BE03 4.6 0.532 0.019 0.66 3.4 2.5

BE05 4.6 0.665 0.019 0.66 6.3 9.6

BE06 4.6 0.665 0.019 0.66 6.3 8.2

BE07 4.6 0.665 0.019 0.66 6.3 6.8

BE08 4.6 0.798 0.019 0.66 10.4 18.6

BE09 4.6 0.798 0.019 0.66 10.4 19.2

BE10 4.6 0.798 0.019 0.66 10.4 19.4

BM01 4.6 0.798 0.019 0.40 2.7 3.1

BM02 4.6 0.798 0.019 0.40 2.7 3.6

CM01 10.1 0.704 0.019 0.40 17 16

CE01 10.1 0.704 0.019 0.66 64 80

CP01 10.1 0.704 0.019 0.50 30 31

CP02 10.1 0.704 0.019 0.50 30 30

AP01 10.1 0.704 0.019 0.50 30 33

AP02 10.1 0.704 0.019 0.50 30 33

DP01 14 0.798 0.043 0.50 20 9.4

DP02 14 0.798 0.043 0.50 20 10

DP03 14 0.798 0.043 0.50 20 10

DP04 14 0.798 0.043 0.50 20 one blast

DM01 14 0.798 0.043 0.40 11 5.2

DM02 14 0.798 0.043 0.40 11 one blast

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100

DP

(kP

a)

GA

ME

10

11 10 100

AE

AM

BE

BM

AP

DP

DM

DP (kPa) exp

Figure 21a: Performance of the GAME-correlation on the donor explosion overpressures.

In Figure 21a the calculated donor overpressures have been graphically represented

dependent on the experimentally observed donor explosion overpressures. Because

the GAME correlation was compiled from the MERGE-experiments, which are

mostly identical and sometimes very similar to the RIGOS donor explosions, it is

no surprise that the scatter of the data around the DPGAME = DPexp line is very

similar to the scatter in the GAME-correlation (Figure 1).

The only data of more interest in this respect are the data from the test series DP

and DM. In the DP and DM test series the donor consisted of a regular B-type

obstacle configuration of maximum size DD3, completed with a regular vertical

configuration of 24 tubes of 114 mm diameter (Figure 7). The volume blockage of

such a mix of obstacles in the donor is equal to 14%. The average obstacle diame-

ter of this obstacle mix was calculated from the average hydraulic diameter and

equal to 0.043 m. The average hydraulic obstacle diameter has been defined as

4 times the ratio of the summed volume and the summed surface area of the obsta-

cles, assuming they merely consist of tubes, i.e.:

Vå tubesD = 4H

Aå tubes

The use of the average hydraulic diameter for a mix of obstacles of various size

was recommended in the GAMES final report (Mercx, 1998) as a result of practical

applications of the GAME-correlation to realistic plants.

It appears that the GAME correlation, applied according to the GAMES recom-

mendations on configurations of obstacles of various size in the DP and DM series,

gives a conservative result. This finding is in line with results from the GAME-

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project (Van den Berg and Eggen, 1996), which suggested that any lack of homo-

geneity in a configuration of obstacles tends to lower explosion overpressures.

The acceptor explosion is characterised by a process of directional flame

propagation from one end of the configuration to the other. This is far from the

centrally ignited experiments, the GAME–correlation was compiled from. In

addition, it has been shown that the acceptor explosion overpressures are

substantially influenced by the donor explosions. It is to be expected, therefore,

that the performance of the GAME-correlation on the acceptor explosion

overpressures will be much worse than on the donor explosions. It shows in

Table 10b and its graphical representation in Figure 21b.

Only for some of the ethylene tests, the correlation correctly predicts the order of

magnitude of explosion overpressure. The less reactive the fuel, the bigger the

overestimation of the overpressure by the GAME-correlation.

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Table 10b: Acceptor explosion overpressures compared to GAME correlation.

Exp. no. VBR (%) Lf (m) D (m) Sl (m/s) DPGAME DPexp (kPa) (kPa)

AE01 10.1 0.87 0.019 0.66 115 75

AE02 10.1 0.87 0.019 0.66 115 95

AE03 10.1 0.87 0.019 0.66 115 109

AE04 10.1 0.87 0.019 0.66 115 84

AE05 10.1 0.87 0.019 0.66 115 >178

AE06 10.1 0.87 0.019 0.66 115 33

AE07 10.1 0.87 0.019 0.66 115 20

AE08 10.1 0.87 0.019 0.66 115 56

AE09 10.1 0.87 0.019 0.66 115 430

AM01 10.1 0.87 0.019 0.40 30 3.2

AM02 10.1 0.87 0.019 0.40 30 3.3

AM03 10.1 0.87 0.019 0.40 30 2.9

AM04 10.1 0.87 0.019 0.40 30 1.9

AM05 10.1 0.87 0.019 0.40 30 4.7

AM06 10.1 0.87 0.019 0.40 30 2.4

AM07 10.1 0.87 0.019 0.40 30 5.2

AM08 10.1 0.87 0.019 0.40 30 5.8

AM09 10.1 0.87 0.019 0.40 30 1.8

AM10 10.1 0.87 0.019 0.40 30 1.6

AM11 10.1 0.87 0.019 0.40 30 5.5

AM12 10.1 0.87 0.019 0.40 30 8.5

BE01 10.1 0.87 0.019 0.66 115 110

BE02 10.1 0.87 0.019 0.66 115 144

BE03 10.1 0.87 0.019 0.66 115 40

BE05 10.1 0.87 0.019 0.66 115 94

BE06 10.1 0.87 0.019 0.66 115 89

BE07 10.1 0.87 0.019 0.66 115 128

BE08 10.1 0.87 0.019 0.66 115 135

BE09 10.1 0.87 0.019 0.66 115 48

BE10 10.1 0.87 0.019 0.66 115 106

BM01 10.1 0.87 0.019 0.40 30 2.4

BM02 10.1 0.87 0.019 0.40 30 2.4

CM01 10.1 0.87 0.019 0.40 30 4

CE01 10.1 0.87 0.019 0.66 115 >430

CP01 10.1 0.87 0.019 0.50 54 6

CP02 10.1 0.87 0.019 0.50 54 10

AP01 10.1 0.87 0.019 0.50 54 5

AP02 10.1 0.87 0.019 0.50 54 8

DP01 10.1 0.87 0.019 0.50 54 9

DP02 10.1 0.87 0.019 0.50 54 6.2

DP03 10.1 0.87 0.019 0.50 54 8.8

DP04 10.1 0.87 0.019 0.50 54 One blast

DM01 10.1 0.87 0.019 0.40 30 3

DM02 10.1 0.87 0.019 0.40 30 One blast

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1

1

10

DP

10 100

100

methane

propane

ethylene

GA

ME (

kP

a)

DP (kPa) exp

Figure 21b: Performance of the GAME-correlation on the acceptor explosion

overpressures.

The GAME-correlation was applied to the acceptor explosions according to the

recommendations developed during the GAMES-program (Mercx, 1998). The

acceptor consisted of an A-type obstacle configuration of medium size DD2 and

kept constant throughout the experimental program. The flamepath length Lf was

calculated from the radius of a hemisphere of a volume equal to the volume of the

acceptor obstacle configuration.

6.2.2.2 Analysis

The development of a deflagrative gas explosion is largely determined by the

feedback coupling in the interaction of the flame with the structure of its self-

generated expansion flow field ahead of the flame. The feedback is triggered by

turbulence generative boundary conditions. The feedback coupling and conse-

quently the self-amplification of the flame propagation process is optimum when a

maximum velocity in the gas (and consequently a maximum turbulence intensity)

is generated by the burning. In other words: when the ratio of the velocity in the

gas ahead of the flame and the flame front’s burning speed is maximum, the feed-

back coupling is optimum and the self-amplification capability of the process is

maximum.

An optimum feedback coupling is obtained when the combustion products behind

the flame front cannot expand backward. This is the case in a spherically symmet-

ric process such as in the experiments the GAME-correlation was derived from.

Then the combustion products in the flame bubble are stagnant and the expansion

is fully utilised to generate flow ahead of the flame.

When the mixture in an obstructed area is not ignited in the centre but at an edge

like in the acceptor explosions, the flame finds turbulence generative conditions

only inside the obstructed area. Then there is no gas dynamic symmetry and the

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combustion products are not stagnant but expand backward away from the obstacle

configuration. Then the feedback coupling is not optimum and the self-

amplification capability of the process is substantially less than in the spheresym-

metric case. It will be obvious, therefore, that a correlation compiled from centrally

ignited explosion cannot perform satisfactorily on edge-ignited gas explosions.

In addition, Figure 21b shows that despite the constancy of the acceptor during the

program, the experimentally observed acceptor explosion overpressures varied

over almost an order of magnitude. The acceptor explosion overpressure is -

dependent on the separation distance - strongly influenced by the donor explosion.

The data in Figure 21b show that the estimation of acceptor explosion

overpressures on the basis of a correlation compiled from spherically developing

explosions completely fails. In view of the theoretical considerations above and in

Chapter 5.1.1, it must be no surprise that the estimation of the overpressure of a

process of edge-ignited, directional flame propagation requires a correlation of

experimental data specific to that geometry.

6.2.3 The charge energy

The original TNO recommendation for the estimation of the explosion energy was

to take the nominal energy, which is the full combustion energy originally present

within the obstructed region and assuming stoichiometry. The experimental data,

generated in the RIGOS-program, enable validation of this recommendation.

If both the explosion overpressures and blast overpressures are measured in an

experiment, a fit of the respective blast decay curve of the TNO blast chart enables

a calculation to be made of the amount of energy required. If this amount of effec-

tive energy is divided by the full amount of energy present within the congested

area (the nominal energy), the result is an energy ratio or an energy efficiency. As

part of the EMERGE-work (Mercx, et al., 1996), such an exercise has been per-

formed on the MERGE experimental data. The result has been reproduced in

Figure 22a.

Figure 22a shows the effective energy ratio observed in almost all of the small (S),

medium (M) and large-scale (L) MERGE-experiments as a function of the explo-

sion overpressure. The data exhibit quite some scatter. This is due to the calcula-

tion of the effective charge energies from the observed blast overpressures, as-

suming perfect spherical symmetry. The blast overpressure is roughly proportional

to the cube root of the charge energy. Therefore, a small scatter in blast overpres-

sures observed translates into a big scatter in effective energies computed.

Nevertheless, Figure 22a shows a clear trend. When the explosion overpressure is

high, the amount of energy required to model the blast observed, is approximately

100% of the amount of energy present within the congested area. If the explosion

overpressure is low, on the other hand, the energy required is no more than 10 to

20% of the amount of energy present within the congested area.

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Figure 22a: Portion of available energy required to model the blasts observed in the

MERGE-experiments, calculated on the basis of the TNO-blast charts

(Mercx, et al., 1996).

The origin of this finding is in the nature of the blast charts and can be simply

explained. The blast charts have been compiled under the assumption that the full

charge is consumed at the same constant flame speed. The gas explosions in the

MERGE-obstacle configurations show a different behaviour as already described

in Chapter 5.1.1. The flow structure (turbulence) developed in the flow in front of

flame controls the burning. Turbulence enhances combustion. An increasing burn-

ing speed intensifies the expansion flow. Increasing flow velocities go hand in

hand with intensifying turbulence, which again enhances combustion again etc. etc.

Within a centrally ignited obstacle configuration, therefore, the flame propagates in

a spherical flow field consisting of a continuously intensifying overpressure, flow

velocity and turbulence field.

As soon as the flame leaves the obstacle configuration, the flame runs into a de-

caying turbulence field, by which the burning speed and the overpressure drop.

Immediately after the flame leaves the congested area, it has developed its maxi-

mum overpressure. At that moment, the amount of combustion products present

within the hemispherical flame is more or less homogeneously compressed.

If the maximum overpressure is low (nearly atmospheric) at the moment the flame

leaves the obstacle configuration, only 10 to 20% of the mixture originally inside

the obstructed area has been consumed. The rest has been expanded out of the

obstructed area before it is consumed at lower and lower burning speeds and does

not contribute to the maximum explosion overpressure. As the expansion ratio of

stoichiometric hydrocarbon-air mixture is about 7 to 8, at low (near-atmospheric)

explosion overpressure an energy ratio of about 12 to 15% should be the lower

limit. This is in line with the experimental observations.

If on the other hand, the explosion overpressures increase, a growing portion of the

mixture originally inside is burned inside the obstructed area. Therefore, it is to be

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expected that the contributing energy ratio is growing for increasing explosion

overpressure, ending up to 100%.

The above considerations hold only when the cloud extends over the full obstructed

area. If, on the other hand, the obstructed area is larger than the cloud, the cloud

may fully expand and burn within the obstructed area regardless the explosion

overpressure. In that case always 100% of the energy will effectively contribute to

the blast regardless the explosion overpressure.

The same procedure applied to the donor explosions in the RIGOS-project will

undoubtedly result in very similar conclusions. It is more interesting to perform

such an exercise on the acceptor explosion overpressure data.

In the acceptor explosions, the flame propagates from one end of the congested

area to the other. Here, the flame is not spherical and not self confined. Then, there

is no homogeneously pressurised bubble of combustion products inside a spherical

flame that starts to depressurise as soon as the flame leaves the congested area.

Here only a region near the flame is pressurised to an extent determined by the

competing effects of pressure production the burning rate and pressure loss by the

expansion. Therefore, it is to be expected that the energy, required for the model-

ling of blast of the acceptor explosions, would be substantially lower than for the

donor, at least in the lower explosion overpressure range.

In the Tables 11a to 11e, the acceptor explosion overpressures as well as the ac-

ceptor blast overpressures observed in various directions and at various distances,

have been tabulated. In addition, each experimentally observed blast overpressure

is accompanied by the portion (%) of the nominal energy required for its modelling

through application of the TNO-blast charts. The nominal acceptor energy is cal-

culated as the volume of the A-type obstacle configuration of medium size (DD2)

times the heat of combustion of an average stoichiometric hydrocarbon-air mixture

(3.5 MJ/m3) and is equal to 4.88 MJ.

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Table 11a: Acceptor explosion overpressures and blast overpressures observed in the

AE-test series as well as the % of the nominal energ required for modelling.

Test DP0

(kPa)

Distance Acceptor

(m)

Acceptor blast overpressure (kPa) and energy ratio (%)

Donor-acceptor Direction

Cross direction

Acceptor-donor Direction

AE01 3 35 43% 13 4.3%

75 5.29 5 2.2%

6 15.6 51%

AE02 3 48.5 73% 34 32%

95 5.82 8 10%

6 15 46%

AE03 3 43 48% 11.5 3.2%

110 6 15 45

6.35 6.2 6.8%

AE04 3 40 54% 36.5 43%

84 4.76 9.5 8.4%

6 13.1 35%

AE05 3 71 100%

250 4.499 25.3 56%

6 18 65%

12 5.8 38%

AE06 3 10.4 8.4%

34 5.816 2.5 1.1%

6 4.8 7.8%

12 2.2 6.9%

AE07 3 4 2.2%

19.5 5.112 1.1 0.2%

6 2 2.2%

12 1 2.2%

AE08 3 21.8 15%

55 6 10.2 21%

6.52 2.6 0.7%

12 3.7 12%

AE09 3 ##

550 4.76 ##

6 ##

12 ##

## AE09 separation distance less than critical; no separate acceptor blast

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Table 11b: Acceptor explosion overpressures and blast overpressures observed in the

AM-test series as well as the % of the nominal energ required for modelling.

Test DP0

(kPa)

Distance Acceptor

(m)



Cross direction


AM01 3 1.05 6.2%

3.2 6 0.52 6.1%

6.52 0.44 4.7%

12 0.3 9.3%

AM02 3 1.1 7.2% AM03 5.82 0.3 1.1%

3.2 6 0.46 4.2%

12 0.26 6.1%

AM04 3 0.5 3.2%

1.9 5.11 0.18 0.8%

6 0.21 2.0%

12 0.05 0.2%

AM05 3 1 1.8%

4.7 4.76 1.4 20%

6 0.7 4.8%

12 0.4 7%

AM06 3 0.69 4.1%

2.4 5.82 0.38 5.1%

6 0.3 2.8%

12 0.17 4.1%

AM07 3 1.8 8%

5.2 5.82 0.9 7%

6 0.95 9%

12 0.45 7.4%

AM08 3 1.95 6.9%

5.9 5.82 1.15 10%

6 1.05 8.4%

12 0.55 9.4%

AM09 3 0.5 3.7%

1.8 6 0.3 6.6%

6.344 0.3 7.8%

12 0.1 2.1%

AM10 3 ##

1.6 5.464 ##

6 ##

12 ##

AM11 3 ##

5.5 5.024 ##

6 ##

12 ##

AM12 3 ##

8.5 4.804 ##

6 ##

12 ##

## - AM10 acceptor blast not measurable.

- AM11 and AM12 acceptor overpressure and blast obscured by donor secondary wave.

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Table 11c: Acceptor explosion overpressures and blast overpressures observed in the

BE-test series as well as the % of the nominal energ required for modelling.

Test DP0

(kPa)

Distance Acceptor

(m)



Cross direction


BE01 3 40 41%

110 5.3 8 7.5%

6 14 39%

12 4.8 23%

BE02 3 55 74%

144 4.768 15 23%

6 16.5 55%

12 5 26%

BE03 3 9.7 4.6%

39.6 6 4.2 3.7%

6.364 3.5 2.7%

12 2 3.8%

BE05 3 42 52%

94 5.699 8.4 11%

6 14.8 44%

12 4 14%

BE06 3 38.5 45%

89 5.034 6 3.1%

6 12.8 33%

12 4.5 20%

BE07 3 55 77%

128 4.702 14 19%

6 16 51%

12 5.2 29%

BE08 3 49 60%

135 6 18 66%

6.098 8 12%

12 5.8 39%

BE09 3 16.2 9.1%

48 5.3 3.8 1.2%

6 6.6 6.1%

12 3.2 9%

BE10 3 37.5 36%

106 4.901 11.2 13%

6 14.8 44%

12 5 26%

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Table 11d: Acceptor explosion overpressures and blast overpressures observed in the

BM-test series as well as the % of the nominal energ required for modelling.

Test DP0

(kPa)

Distance Acceptor

(m)



Cross direction


BM01 3 0.74 4.9%

2.44 4.9 0.3 1.4%

6 0.32 3.2%

12 0.18 4.7%

BM02 3 0.54 2%

2.4 4.7 0.33 1.8%

6 0.2 0.8%

12 0.11 1.1%

Table 11e: Acceptor explosion overpressures and blast overpressures observed in the

AP, CP, DP and the DM-test series as well as the % of the nominal energ re-

quired for modelling.

Test DP0

(kPa)

Distance Acceptor

(m)



Cross direction


AP01

5

3

5.816

1.2 2.6 0.8 0.8%

0.4 0.7%

6 0.5 1.4% 0.4 0.7%

8.816 0.2 0.3%

12 0.3 2.4% 0.2 0.7%

14.816 0.1 0.2%

AP02

8

3

5.112

2.2 3.9% 1.2 0.6%

0.8 0.9%

6 1.0 2.9% 0.7 1.0%

8.112 0.6 1.5%

12 0.5 2.9% 0.4 1.5%

14.112 0.4 2.4%

CP01

6

3

5.112

1.2 1.5% 0.8 0.4%

0.5 0.5%

6 0.5 0.8% 0.4 0.4%

8.112 0.2 0.1%

12 0.2 0.4% 0.2 0.4%

14.112 0.1 0.08%

CP02

10

3

5.112

2.6 3.3% 1.8 1.1%

1.2 1.6%

6 1.2 2.6% 1.1 2.0%

8.112 0.8 1.9%

12 0.8 6.1% 0.6 2.6%

14.112 0.6 4.2%

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Table 11e: Acceptor explosion overpressures and blast overpressures observed in the

AP, CP, DP and the DM-test series as well as the % of the nominal energ re-

quired for modelling (continued).

Test DP0

(kPa)

Distance Acceptor

(m)



Cross direction


DP01 3 2.4 3.5% 1.3 0.5%

9 5.3 0.8 0.7%

6 1.2 3.5% 0.7 0.7%

8.3

12

0.5 0.7%

0.6 3.5% 0.4 1%

14.3 0.3 0.7%

DP02

6.2

3

5.1

1.5 2.7% 0.5 0.1%

Defect

6 0.7 2.1% 0.3 0.2%

8.1 ?

12 0.3 1.3% 0.2 0.4%

14.1 ?

DP03

8.8

3

4.9

2.4 3.8% 1.7 1.4%

1.2 2%

6 1.2 3.8% 1.0 2.2%

7.9 0.7 1.7%

12 0.6 3.8% 0.5 2.2%

13.9 0.3 0.7%

In Figure 22b, the calculated energy efficiencies for the acceptor explosions have

been represented dependent on the explosion overpressures.

ene

rgy r

atio

(%

)

100

80

60

40

20

0

0 50 100 150 200 250

explosion overpressure (kPa)

Figure 22b: Portion of nominal acceptor energy (4.88 MJ), required to model the accep-

tor blast overpressures, dependent on the acceptor explosion overpressure

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The scatter in the effective energy ratios in Figure 22b for the acceptor explosions

is even much bigger than in Figure 22a for the donor explosions. A reason for the

bigger scatter may be that here the acceptor explosion overpressures are strongly

influenced by the donor explosions. In addition, because the flame propagation in

the acceptor is edge-initiated, the acceptor blast is far from spherically symmetric.

Substantial differences in blast overpressures have been observed in the different

directions translating into even bigger differences in effective energy.

Nevertheless, the Figures 22a and 22b show a very similar and clear trend. The

portion of the available energy required to model the blast grows for increasing

explosion overpressure. Generally speaking, the effective portion of the nominal

energy required to model the acceptor blasts is lower than that for the spherically

developing donor explosions. Where the lower limit for the donor explosions is

12 to 15%, theoretically speaking, the lower limit for the acceptor explosions is

lower still. Values of a fraction of 1% have been observed in many experiments.

6.2.4 Modelling of the tests with Separation Distance less than critical

The results of the tests with a separation distance clearly less than critical deserve a

special attention because here a separate treatment of donor and acceptor is not

justified.

Test AE09

This test is defined by an A-type donor of medium size DD2 and a Separation

distance equal to 0.25DD

The damage to the acceptor observed in this test is a strong indication that the

ethylene-air mixture detonated locally at the end of the flame propagation through

the obstacle configuration. The same holds for experiment CE01, in which as a

consequence of the presence of a connecting obstacle configuration detonation

transition occurred at a very early stage in the acceptor.

Although a separated pressure development was observed in experiment AE09

witness the pressure-time records P1 – P7, the acceptor blast overtook the the

donor’s in the donor-acceptor direction. In test CE01, the overpressure seemed to

amplify continuously in the donor, the connecting configuration and the acceptor.

Nevertheless, separate blasts were observed in the acceptor-donor direction. This

suggests application of the Multi-Energy concept dependent on the direction of

flame propagation. By doing so, the blasts in the donor-acceptor direction should

be modelled with a summed energy and an equivalent charge position in the centre

between donor and acceptor. The two separate blasts in acceptor-donor direction,

on the other hand, may be modelled by separate charges positioned at the respec-

tive locations of the donor and the acceptor centres. The calculated and the experi-

mentally observed overpressures in the tests AE09 and CE01 have been tabulated

in the Tables 12a and 12b. The respective charge energies are:

· Donor energy = 4.88 MJ;

· Acceptor energy = 4.88 MJ;

· Summed energy = 11 MJ (donor+acceptor+interspace).

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Table 12a: Calculated and experimentally observed blast overpressures in test AE09.

AE09 blast overpressures (kPa) in donor-acceptor direction (B2, B3 and B4) E=11 MJ, DPo > 550 kPa

Distance from centre

Donor + acceptor (m)

MEM Experiment

3.87 71 136

6.87 27 32

12.87 11 12

AE09 blast overpressures (kPa) in acceptor-donor direction (B1)

Donor blast

E=4.88 MJ, DPo = 73 kPa

Acceptor blast

E=4.88 MJ, DPo > 550 kPa


Donor/acceptor (m)

MEM Experiment MEM Experiment

3 49 32

4.76 31 52

Table 12b: Calculated and experimentally observed blast overpressures in test CM01.

CM01 blast overpressures (kPa) in donor-acceptor direction (B2, B3 and B4) E=12.2 MJ, DPo > 550 kPa


Donor + acceptor (m)

MEM Experiment

4.05 70 > 60

7.05 27 24

13.05 11 Defect

CE01 blast overpressures (kPa) in acceptor-donor direction (B1)

Donor blast

E=4.88 MJ, DPo = 80 kPa

Acceptor blast

E=4.88 MJ, DPo > 550 kPa


Donor/acceptor (m)

MEM Experiment MEM Experiment

3 52 40

5.1 28 47

A reason for the substantial underestimation of the blast overpressures in the sta-

tions B2, B3 and B4 is the substantial directionality in the near field in donor-

acceptor direction. The directionality appears to vanish quickly with increasing

distance.

In the modelling of blast overpressures in the acceptor-donor direction, on the other

hand, the substantial underestimation of the acceptor blast is a striking result be-

cause, due to directionality effects, an overestimation was expected. A plausible

explanation may be that, probably, transition to detonation in the acceptor took

place before the donor explosion ran out of fuel. The consequence may be then that

after transition the detonation wave quickly consumed all remaining flammable

mixture including the mixture originating from the donor. Transition to detonation

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may lead to an irregular behaviour that leads to extremely anomalous blast over-

pressure distribution in the near-field.

A basic assumption in the Multi-Energy concept is that the unobstructed mixture in

an atmospherically dispersing vapour cloud is too inhomogeneously mixed to

maintain a detonation. In this respect the conditions in a realistic scenario differ

substantially from the experimental conditions in the RIGOS-test series, where the

mixture is deliberately homogenised.

Another experiment in which the separation distance was clearly less than critical

is tes DP04. A test with a donor of type A and large size, completed with 114 mm

diameter tubes. The fuel was propane and the separation distance was equal to

0.125DD. In the Table 12c the calculated as well as the experimentally observed

blast overpressures have been tabulated. The calculation was performed on the

basis of the full summed nominal energy and the equivalent fuel-air charge was

positioned in the centre between donor and acceptor.

Table 12c: Calculated and experimentally observed blast overpressures in test DP04.

DP04 blast overpressures (kPa) in donor-acceptor direction (B4, B5 and B6) E=12.8 MJ, DPo = 19 kPa

Distance from centre Donor + acceptor (m)

MEM Experiment

3.8 15 5

6.8 8 2.8

12.8 4.5 1.6

DP04 blast overpressures (kPa) in acceptor-donor direction (B1, B2 and B3)

E=12.8 MJ, DPo = 19 kPa

Distance from centre Donor + acceptor (m)

MEM Experiment

3.9 15 5

6.9 8 2.6

12.9 4.5 1.2

The Table shows a substantial overestimation of the blastoverpressures and hardly

any directionality. Application of an energy efficiency of 4% would reproduce the

experimentally observed pressures very well.

6.2.5 Directionality in the acceptor blast

6.2.5.1 General

Blast effects from accidental vapour cloud explosions are mostly highly direc-

tional. In a realistic scenario, a vapour cloud mostly ignites where the dispersing

and drifting flammable mixture happens to find an open fire, a spark or a hot spot.

In such a scenario, the cloud is consumed by a flame that propagates from one end

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of the cloud to the other. When the flame propagation process has a preferential

direction, the resulting blast effects are directional. Here, the acoustic volume

source analogue, described in Chapter 5.1.1, may contribute in making things

comprehensible. If a propagating flame is considered a moving volume source, the

overpressure piles up in the direction of flame propagation while the overpressure

rarefies in the opposite direction. In this way, the directionality in the blast effects

can be considered the consequence of an acoustic Doppler-effect. The acoustic

analogue also explains that effects of directionality in the blast must be stronger as

the flame speed, i.e. the overpressure in the gas explosion is higher. For gas explo-

sions of very high strength or even detonation, when the flame speed is supersonic

relative to the pressure gauges, the major portion of the blast is directed in the

direction of the flame propagation. In the opposite direction hardly any blast effects

are to be expected, at least in the immediate vicinity of the explosion process.

With increasing distance to the blast source, however, the directionality in the blast

will gradually vanish. It is well known that blast wave propagation in free space

has a natural tendency to a spherical symmetry. A highly directional blast wave

has, therefore, the tendency to equalise its strength over it surface. The equalizing

tendency is stronger as the blast wave is stronger. Blast from a high-explosive

detonation, for instance, will attain a nearly spherical shape within a few charac-

teristic charge dimensions (Baker, 1973). Blast from a gas detonation will have a

nearly sphere-symmetric strength within about 10 equivalent charge radii (Moen

et al., 1983).

Simple blast modelling methods make use of blast charts. Implicit to the use of

blast charts is the assumption that the blast effects are fully symmetric around a

blast centre. This is, generally speaking, a drastic simplification, at least for gas

explosions of high strength and in the near-field. Generally speaking, therefore, the

performance of simple blast modelling methods is much better in the far field than

in the near field. The more so considering the fuel-air blast charts, which show that

in the far field the blast parameters nearly are independent of the explosion over-

pressure if it is higher than 50 kPa.

Because acceptor explosions in the RIGOS-program are characterised by a process

of directional flame propagation, the data offer an opportunity to study the asym-

metry in the blast and to evaluate the performance of symmetrical blast modelling

on the basis of blast charts.

6.2.5.2 Results

The idea to study the directionality of the blast effects in more detail came forward

during the sponsor meeting halfway the project. Consequently, the test series AE,

AM, BE and BM, that were already finished by that time, were not very well

equipped for the recording of directionality in the blast. Only the test series per-

formed from that sponsor meeting on, namely: the series AP, CP, DP and DM were

appropriately equipped to this end. Nine blast pressure gauges were positioned

around the donor-acceptor test layout according to Figure 4b in three directions:

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· At 3, 6 and 12 m distance from the donor centre in the acceptor-donor direction.

· At 3, 6 and 12 m distance from the acceptor centre in the acceptor-donor direc-

tion.

· At 3, 6 and 12 m distance from the acceptor centre perpendicular to the donor-

acceptor axis.

Unfortunately, the explosion overpressures observed in the well-equipped tests

were all relatively low and, consequently, so are the directionality effects. The

most pronounced effects of directionality, that occurred for the higher acceptor

explosion overpressures in the AE and BE test series, have only poorly been re-

corded.

The acceptor explosion overpressures as well as the acceptor blast overpressures as

far as observed have been summarised in the Tables 9a - 9e. In many of the ex-

periments, the separation distance was relatively small. The consequence is that

often the acceptor overpressure is obscured by the donor gas dynamics. Such a mix

of symmetric (donor) and directional (acceptor) effects is hard to interprete. There-

fore, directionality effects in the blast do not come forward in a systematic way that

can be analysed quantitatively. In addition the scatter in the data is substantial.

Nevertheless, on the average the data in the Tables 9a-9e show the following

trends.

A direct indication for the directionality in the acceptor blast is, of course, that in

the direction of flame propagation substantially higher blast overpressures are

observed than in the opposite and cross direction. An indirect indication is in the

calculated effective energy ratio’s, which are higher in the direction of flame

propagation than in the other directions.

6.2.6 Conclusions validation blast modelling

Vapour cloud explosion blast prediction by simple methods is based on highlyidealised modelling and experimentation. The highly idealised blast model (charts)assuming sphere-symmetry requires input for two parameters:· an explosion strength (overpressure);· an amount of contributing energy.Methods for estimating these input parameters have been derived from highlyidealised experiments that can be characterised as follows:· Threedimensional regularly spaced obstacle configuration;

· Central ignition;

· Spherically symmetric flame propagation.

If a problem exhibits the highly idealised conditions the methods were derived

from, the explosion strength can be estimated within a factor of about 2

(Figure 20a). If the amount of contributing energy is appropriately corrected for the

extent of the obstructed area relative to the cloud, the blast overpressures at a

distance of the explosion can be predicted within about a similar accuracy.

However, generally speaking realistic vapour cloud explosion scenario’s do not at

all satisfy the idealisations the modelling was based on. Vapour cloud explosions in

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realistic scenario’s mostly develop in irregular obstacle configurations (plant) and

are mostly ignited where the dispersing and drifting cloud finds some open fire or

hot object. The flame propgation process mostly has a preferential direction, from

one end of the cloud to the other. The consequence is that the resulting blast effects

are highly directional, at least in the near-field.

Assessment of the explosion strength prediction for less idealised explosions in the

acceptor (Figure 20b) shows that, generally speaking, the predicted explosion

overpressures are an order of magnitude higher than experimentally observed.

Assessment of the amount of effectively contributing energy in the acceptor explo-

sions shows that the portion of the nominal energy that effectively contributes to

the blast in edge-ignited vapour cloud explosions can be substantially lower than

that in centrally ignited explosions. Energy ratio’s of a fraction of a percent have

been observed in the lower explosion overpressures range. This observation impli-

cates that blast prediction for low-strength gas explosions on the basis of the nomi-

nal energy may result in an another order of magnitude too high blast overpres-

sures.

The conclusion can only be that a proper prediction of blast from vapour cloud

explosions with a directional mode of flame propagation requires methods spe-

cially tailored to that explosion type.

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7 Conclusions

The main objective of the RIGOS-program was to develop practical guidance to be

used in application of the Multi-Energy Method in realistic problems. Looking

back over the experimental program one may conclude that the AE, BE, AM and

BM-test series were orientational. Subsequently, the AP, CP, CE, DP and DM test

series addressed more specific questions.

Unfortunately, not the full range of conditions of concern could be covered. The

range of explosion overpressures higher than 100 kPa, where the most substantial

critical separation distances are to be expected, had to be left out of consideration.

Nevertheless, the experimental program resulted in a good understanding of the

phenomena and some concrete indications for the Critical Separation Distance in

the low explosion overpressure range. On the basis of this information, concrete

guidance concerning the Critical Separation Distance was proposed. The guidance

was extrapolated to the high explosion overpressure range on the basis of common

sense. The applicability of this guidance to donor explosions with a directional

mode of flame propagation is uncertain.

Compared to this new guidance, the Critical Separation Distance intuitively sug-

gested in the Yellow Book (CPR-14E,1997), being equal to 10 obstacle diameters,

is not always safe and conservative in particular in the high explosion overpressure

range.

The experiments in the C-test series showed that a connecting obstacle configura-

tion of sufficient cross-sectional dimensions - such as a pipe rack between units of

a chemical plant - may substantially increase the critical separation distance in the

low explosion overpressure range. In the high explosion overpressure range, a

connecting obstacle configuration may transmit the flame propagation process

without decay over any separation distance from donor to acceptor.

The experiments in the D-test series gave an indication that larger obstacles in the

donor tend to a larger critical separation distance. This can be explained as follows:

Larger obstacles induce eddies of larger scale in their wakes and thereby a slower

turbulence decay. The consequence is that the burning speed of the flame will

reduce less quickly on leaving a donor with larger obstacles.

The experiments showed that, even when the separation distances were larger than

critical, the donor explosions substantially influenced the development of the

acceptor explosions. At separation distances only slightly larger than critical, a

substantial suppression of the acceptor explosion was observed. The suppression

effect appeared bigger as the donor size was larger and the donor explosion over-

pressure was higher. The explanation for this surprising phenomenon is that the

underpressure and the backflow during the negative phase of the donor hampered

the development of the flame propagation process in the acceptor.

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The donor-acceptor experiments in the RIGOS-program allowed validation of the

simple vapour cloud explosion blast prediction methodology on data from gas

explosions substantially different from those they were derived from. The blast

prediction methodology requires input of two parameters: an explosion overpres-

sure and an effective energy.

The experimental data show that the GAME-correlation applied to the acceptor

explosions substantially overestimates the explosion overpressures, in particular in

the low overpressure range. The effective energies, calculated from the observed

acceptor blast overpressures, show that the portion of the nominal acceptor energy

effectively contributing to the blast is low, in particular for the low explosion

overpressure range.

Evaluation of the results shows that if the input parameters for prediction are

determined according to the presently recommended guidelines, the blast overpres-

sures from the acceptor explosions may be overestimated by more than an order in

magnitude.

In addition, the simple blast modelling methodology cannot take any effects of

directionality of the acceptor blast into account. The experimental data show that

the directionality in the acceptor blast is substantial, in particular in the high explo-

sion overpressure range.

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8 References

Auton, T.R. and Pickles, J.H. (1978a),A mathematical model for unconfined vapour cloud explosions,General Electricity Research Laboratories,Laboratory note no. RD/L/N 5/78.

Auton, T.R. and Pickles, J.H. (1978b),The calculation of blast waves from the explosion of pancake-shaped vapourclouds,General Electricity Research Laboratories,Laboratory note no. RD/L/N 210/78.

Baker, W.E. (1973),Explosions in air,Iniversity of Texas Press,Austin, London, 1973.

Catlin, C.A. (1991),Scale effects on the external combustion caused by venting of a confined explo-sion,Combustion and Flame,Vol.83,(1991),pp.399-411.

Catlin, C.A. and Johnson, D.M. (1992),Experimental scaling of the flame acceleration phase of an explosion by changingfuel gas reactivity,Combustion and Flame,Vol.88,(1992),pp.15-27.

CPR-14E (1997),Methods for the calculation of the physical effects due to releases of hazardousmaterials, Committee for the Prevention of Disasters, The Hague, The Netherlands,1997 (Yellow Book).

De Bruijn, P.C.J. and Van Ierschot, P.G.A. (2002),RIGOS Experimental results,TNO report, PML 2002-C51,Rijswijk, The Netherlands, 2002.

Eggen, J.B.M.M. (1995),GAME: development of Guidance for the Application of the Multi-Energy method,TNO report, PML 1995-C44,Rijswijk, The Netherlands, August 1995.

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Lighthill, J. (1978),Waves in Fluids,Cambridge University Press,Cambridge, 1978.

Mercx, W.P.M., editor (1994a),Modelling and Experimental Research into Gas Explosions,Overall final report of the project MERGE, CEC contract STEP-CT-0111(SSMA),European Commission, Directorate General XII, Brussels, Belgium, 1994.

Mercx, W.P.M.; Van den Berg, A.C. and Mouilleau, Y. (1994b),Modelling and experimental research into gas explosions,Contribution of TNO-PML to the MERGE project,TNO report, PML 1993-C137, Rijswijk, The Netherlands, 1994.

Mercx, W.P.M.; Van den Berg, A.C.and Van Dongen, Ph. (1996),Extended Modelling and experimental research into gas explosions,Contribution of TNO-PML to the MERGE project,TNO report, PML 1996-C16, Rijswijk, The Netherlands, 1996.

Mercx, W.P.M., editor (1997),Extended Modelling and Experimental Research into Gas Explosions,Final summary report of the project EMERGE, CEC contract EV5VCT930274European Commission, Directorate General XII, Brussels, Belgium, 1997.

Mercx, W.P.M.; Van den Berg, A.C. and Leeuwen, D. (1998),Application of correlations to quantify the source strength of vapour cloud explo-sions in realistic situations, Final report for the project GAMES,TNO report, PML 1998-C53, Rijswijk, The Netherlands, 1998.

Moen, I.O., et al., (1983),Blast from non-spherical fuel-air explosions,8th Int.Symp.on Military Appl.of Blast Simulation,Spiez, Switzerland, 20-24 June, 1983.

Strehlow, R.A. (1981),Blast wave from deflagrative explosions: an acoustic approach,13th AIChE Loss Prevention Symposium,Philadelphia (PA), 1981.

Taylor, P.H. and Hirst, W.J.S. (1988),The scaling of vapour cloud explosions: a fractal model for size and fuel type,Poster presented at the 22nd Symp.(Int.) on Combustion, Seattle (WA), USA, 1988.

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Van den Berg, A.C. (1985),The Multi-Energy Method, a framework for vapour cloud explosion blast predic-tion, Journal of Hazardous Materials, volume 12, pp 1 - 10, 1985.

Van den Berg, A.C. and Eggen, J.B.M.M. (1996),GAME, Guidance for the Application of the Multi-Energy method,The second International Specialists Meeting on Fuel-Air Explosions, Bergen,Norway, June 27-28, 1996.

Wingerden, C.J.M. van (1988),Investigation into the blast produced by vapour cloud explosions in partially con-fined areas, TNO report, PML 1988-C195, Rijswijk, The Netherlands,December 1988 (DISCOE project).

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9 Authentication

A.C. van den Berg A.L. Mos

Author/Project leader Author

Dr. J. Weerheijm Dr. L.H.J. Absil

Research Co-ordinator Group leader

72

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73

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