gap.8.0.1.1. oil and chemical properties loss potential estimation guide

GAP Guidelines A Publication of Global Asset Protection Services

GAP.8.0.1.1June 2, 2003

20 Security Drive, Avon, Connecticut 06001-4226 Copyright© 2003, Global Asset Protection Services

Global Asset Protection Services and its affiliated organizations provide loss prevention surveys and other risk management, business continuity and facility asset management services. Unless otherwise stated in writing, our personnel, publications, services, and surveys do not address life safety or third party liability issues. The

OIL AND CHEMICAL PROPERTIES LOSS POTENTIAL ESTIMATION GUIDE

INTRODUCTION

Global Asset Protection Services (GAP Services) develops a Probable Maximum Loss (PML) for each risk. This PML is estimated considering “reasonably adverse conditions” which might exist at the time of the loss with due regard to the size, location of the plant, construction, partial cutoffs, occupancy, protection of hazards, exposure protection installations, public protection, and any other factors pertinent to the risk involved.

Within the scope of this definition, PMLs can be established for oil and chemical properties utilizing one of four types of postulated incidents:

• fires

• building explosions

• vapor cloud explosions

• vessel explosions

Of course, any risk may possess the potential for one or more of these incident types.

This GAP Guideline describes a method used by GAP Services for estimating the loss potential in oil and chemical properties that can be used in establishing PMLs. A method for estimating the catastrophic loss potential from a vapor cloud explosion is also given.

This GAP Guideline should be used for property loss prevention purposes only and should not be used for designing or siting blast resistant buildings or for specifying personnel protection.

FIRES AND INTERNAL BUILDING EXPLOSIONS

Loss estimates based on vessel or vapor cloud explosions lend themselves to a rigorous analysis method. Fires, however, must be analyzed more subjectively. The guidance contained in this section is therefore limited to a listing of the factors which must be considered in arriving at a description of the postulated fire or building explosion incident, and the loss estimate based on that incident. Some of the factors which must be considered are:

• The amount of material which might be spilled and the nature of the failure which would cause this spill.

• The physical properties of the material and the operating conditions of the process, such as temperature and pressure.

provision of any service is not meant to imply that every possible hazard has been identified at a facility or that no other hazards exist. Global Asset Protection Services and its affiliated organizations do not assume, and shall have no liability for the control, correction, continuation or modification of any existing conditions or operations. We specifically disclaim any warranty or representation that compliance with any advice or recommendation in any document or other communication will make a facility or operation safe or healthful, or put it in compliance with any law, rule or regulation. If there are any questions concerning any recommendations, or if you have alternative solutions, please contact us.

GAP.8.0.1.1 June 2, 2003

• The reactivity of process materials.

• The protection features in the effected area, such as:

° Fireproofing of structural steel and vessel supports.

° Drainage and diking in the area.

° Automatic or manual water spray.

° Monitor nozzles or hand hose protection.

° Foam protection.

° Fire brigade.

° Public fire department or mutual aid assistance.

• Spacing between units or provision of blast-resistant walls.

• Past experience with similar units.

All of the foregoing factors should be considered, and it should be kept in mind that the loss estimated must be based on “reasonably adverse conditions.” The estimate should consider the effect of the impairment of one form of active protection, such as water spray, foam, or drainage. Obviously, passive protection features, such as spacing, fireproofing, and diking are not subject to impairment simultaneous with a loss.

Generally speaking, where good drainage (more than one direction) or diversionary diking is utilized to prevent the spread of flammables, and the spacing is in accordance with GAP.2.5.2 the loss estimate should be based on major damage to the production units within the drainage/diking zone. Some damage would be expected to surrounding units from radiation. Missile damage from exploding vessels is generally not to be considered as a PML incident.

As with analysis of a postulated fire incident, internal building explosions cannot be rigorously analyzed. The release of flammable liquids or gases into a building, with subsequent ignition and explosion, will result in destruction of the building. Possible damage to nearby buildings should be considered. Subsequent fires from broken piping would also be a factor. Automatic sprinkler protection would most likely be rendered inoperative from an internal building explosion. The degree of damage anticipated from a building fire with one or more sprinkler systems out-of- service (reasonably adverse conditions) and an internal building explosion with subsequent damage to the sprinkler system would be similar.

VAPOR CLOUD EXPLOSIONS

Background

For many years, it was believed that in order for the combustion of flammable gases or vapors to create pressure, the combustion reaction must be confined. Therefore, it was considered only a fire problem to release quantities of flammable gases or hot flammable liquids in open areas. The potential for explosion was not considered even though a number of open-air vapor cloud explosions had occurred as early as 1948.

GAP Services recognized, in the early 1960’s, that the spillage of large quantities of flammable gases could result in an open-air vapor cloud explosion that could cause damage due to overpressure to wide areas of plant properties. A method of calculating the approximate damage potential was formulated and used to establish PML estimates based on spills of flammable gases. Other losses

2

GAP GuidelinesA Publication of Global Asset Protection Services

GAP.8.0.1.1 June 2, 2003

have shown a need to include hot flammable liquid spills and a method of establishing the catastrophic loss potential of a vapor cloud explosion. The initial incident is assumed to be the spilling of a large quantity of flammable materials. It is highly probable that a large spill will become ignited. The “reasonably adverse condition” assumed is that the ignition of the cloud will propagate to an unconfined vapor cloud explosion. Thus, it is not necessary to assume that any fire protection facilities will be impaired prior to the ignition. These facilities may be damaged and impaired, however, due to the explosion.

Assumptions

GAP Services uses a reasonably simple vapor cloud explosion potential model. A number of papers have been written in recent years that define the vapor cloud explosion phenomenon to a finer degree than is used in this calculation method. These papers provide methods of analyzing losses after they occur by considering variables such as spill rate, wind velocity, direction, other atmospheric conditions, reactivity of the spilled material, amount of obstructions and partial confinement within the cloud.

In predicting the loss potential, these variables are generally unknown and a conservative and practical approach must be chosen which will reduce these variables to little or no consequence. Therefore, the following assumptions are made in this calculation method:

• The spill is instantaneous and leak rate is not considered. The one exception to this is a spill from a pipeline fed by large capacity remote storage facilities.

• The spilled material is instantaneously vaporized and a cloud is immediately formed based on the thermodynamic conditions of the flammable liquid or gas prior to release. For example, spills of liquefied gases are assumed to fully vaporize instantaneously with no autorefrigeration of the liquid pool.

• The cloud formed is cylindrically shaped with a vertical axis as the cloud height. Wind distortion and distortion due to the presence of buildings or structures are not considered.

• The cloud composition is assumed to be of uniform composition with the vapor-air mixture being at the midpoint of the explosive range.

• A heat of combustion of 2000 Btu/lb (4648 kJ/kg) for TNT is used to convert the heat of combustion of the material to an equivalent weight of TNT.

• An ambient temperature of 70°F (21°C) is assumed.

It is recognized that explosion of a confined vapor-air mixture in a building will result in greater explosive efficiency than the unconfined explosion of the same volume of vapor-air mixture. However, in most cases, the volume occupied by the open-air cloud from most credible spills will be much greater than the volume of most buildings. Therefore, a spill which originates inside of a building should be assumed to form the same size and shape cloud as if the spill occurred in the open.

Factors Determining The Need For A Vapor Cloud Analysis

Only the following materials are to be considered as having a vapor cloud potential for the purposes of this calculation:

• Flammable gases existing as liquid because of refrigeration. Spills of liquefied anhydrous ammonia, when confined by natural or artificial dikes such that the spilled liquid will “pool,” are not considered.

• Flammable gases existing as a liquid because of the application of pressure.

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GAP.8.0.1.1 June 2, 2003

• Flammable gases existing under a pressure of 500 psig (34.5 bar) or higher.

• Flammable or combustible liquids existing above the atmospheric boiling point and maintained as a liquid because of the application of pressure. Materials which have a viscosity greater than 1 10× 6 centipoises or a melting point above 212°F (100°C) are not considered to be liquids within the scope of this calculation method. Liquids in the process at temperatures above their autoignition temperatures should not be considered as having a vapor cloud potential since they will ignite upon contact with air.

CALCULATION METHOD

Throughout these calculations, units from both the English system and the SI (International System of Units) are used in the same equation. This is due to the availability of data from the sources indicated in the references.

Choice Of Credible Spill For Probable Maximum Loss

For purpose of PML analysis of a plant with a vapor cloud hazard, the following criteria for estimating the size of a spill should be used:

• The size of a spill is based on the contents of the largest process vessel or train of process vessels connected together and not readily isolated. Shut-off valves which are actuated both automatically and manually from a remote location may be considered in reducing the size of the estimated spill. Automatic dump or flare systems, if safely arranged, may be considered in reducing the size of the spill. Shut-off valves and dump and flare systems may only be considered in limiting the spill from vessels connected to the largest process vessel. The minimum spill source to be used is the largest process vessel. The largest spill does not always present the largest vapor cloud potential, smaller spills of light products can create a larger vapor cloud.

• The existence of ignition sources may not be used in reducing the cloud size. The total amount which might be spilled must be used in estimating the cloud size. Loss experience has shown that large clouds may be formed without ignition by nearby ignition sources.

• Gases or liquids used as fuels are not considered, based on loss experience.

• The failure of a major storage tank is not considered, based on loss experience.

• The failure of a major pipeline is not considered.

Choice Of Credible Spill For Catastrophic Loss Potential

For purposes of estimating the catastrophic loss potential in a plant having a vapor cloud hazard, the following criteria for estimating the size of a spill should be used:

• The size of a spill should be based on the contents vessels or train of vessels connected together and having the largest vapor cloud potential. The existence of shut-off valves should not be considered.

• The catastrophic failure of major storage tanks should be considered.

• Leaks in pipelines carrying materials of concern from large capacity, off-site, remote, storage facilities owned by the insured or others must be considered. For this purpose, assume that the pipeline is completely severed and that the spill will run for 30 min. This 30 min limit is based on the assumed existence of multiple, widely separated, shutoff valves.

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GAP.8.0.1.1 June 2, 2003

• The existence of ignition sources may not be used in reducing the cloud size. The total amount which might be spilled must be used in estimating the cloud size. Loss experience has shown that large clouds may be formed without ignition by nearby ignition sources.

• Gases or liquids used as fuels should be considered.

Calculation Of Weight Of Material In System

If the material exists in the system as a gas at a pressure of 500 psig (34.5 bar) or greater, the weight of material may be calculated as follows:

MnWG = (1)

where:

WG = weight of gas discharged

n = number of moles of gas

M = molecular weight of gas calculated from atomic weights

According to the Gay-Lussac “perfect gases law”:

RTPVn = (2)

where:

P = pressure of gas in process

V = volume of gas in process

R = gas constant (10.73 psi-ft3/°R-lb-mole) (0.08314 bar-m3/°K-kg-mole)

T = temperature of gas in process

Therefore:

RTMPVWG = (3)

English Units, the following units should be used in equation 3:

)460)((73.10)()()(

3

+=

FTftVpsiaMPlbW

G o (3E)

SI Units, the following units should be used in equation 3:

))273((08314.0)()()(

3

+=

CTmVbarMPkgWG o

(3S)

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GAP.8.0.1.1 June 2, 2003

If the material exists in the system as a liquid, the weight of the liquid may be calculated as follows:

WL = ρVL (4)

where:

WL = weight of liquid spilled

ρ = liquid density of material at process temperature (Gallant1)

VL = volume of liquid spilled

English Units, the following units should be used in equation 4:

WL(lb) = ρ(g/mL)VL(gal) (4E)

SI Units, the following units should be used in equation 4:

1000)()(

)(3mVmL

kgW Lg

Lρ

= (4S)

Calculation Of Liquid Vaporized

For liquefied gases having an atmospheric boiling point less than 70°F (21.1°C), assume that 100% will be vaporized and use either WG or WL calculated in equations 3 and 4 as W in equations 9, 10, and 11.

For liquids having an atmospheric boiling point above 70°F (21.1°C), the following relationship may be used to determine the amount vaporized:

v

pmLV H

TTCWW

∆

−=

)( 21 (5)

where:

Wv = weight of vapor formed following the spill

WL = weight of liquid spilled

Cpm = geometric mean of specific heats over the range T1 to T2. The geometric mean of the specific heats over the range T1 to T2 may be calculated from:

Cpm = (C1 x C2 x … x Cpn)!/n

where:

C1, C2, C3, ….Cpn are values of Cp taken at equal intervals between T1 and T2 (Gallant)

T1 = temperature of the liquid in the process or boiling point of the liquid at process pressure, whichever is less (Gallant)

T2 = atmospheric boiling point of the liquid (Gallant)

∆Hv = heat of vaporization of the liquid at temperature T2 (Gallant)

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GAP.8.0.1.1 June 2, 2003

English units:

( ) ( ) ( )( )gcal

v

gcal

pmLV H

CTTCClbWlbW

∆

−−=

oo21)(

)( (5E)

( ) ( ) ( )( )gcal

v

gcal

pmLV H

FTTCClbWlbW

∆

−−×=

oo21)(

95)(

(5E')

SI Units:

)()()()()(

)( 21

kgkJ

v

gkJ

pmLV

CTTKCkgWkgW

∆Η

−−=

oo

(5S)

)2389.0( CgcalKkgkJl oo −=−

Calculation Of Energy Released

The energy released in an explosion of the vapor cloud may now be estimated. This energy release is normally expressed as a TNT equivalent. Thus, based on a heat of combustion for TNT of 2000 Btu/lb (4648 kJ/kg), the following may be used to calculate a TNT equivalent for a vapor cloud containing a known weight of flammable gas of vapor:

ce

ce H

HWW∆

ƒ∆= (6)

where:

We = Weight of TNT which will produce an explosive force equivalent to the force produced by explosion of the vapor cloud.

∆Hce = Heat of combustion of TNT (2000 Btu/lb)(4648 kJ/kg)

W = Weight of vapor in cloud

∆Hc = Heat of combustion of vapor (Perry,2 NFPA Handbook3) Use the values for gaseous products of CO2 and H2O. Other heats of combustion may be calculated from available heat of formation data.

f = Explosive yield factor. Published information indicates that the explosive yield factor f, for spillage explosions of rocket propellants and liquid oxygen, initiated at no time delay, is about 0.1. Analysis of actual chemical plant vapor cloud explosions indicates f values in the range of 0.01 to 0.05. In establishing probable maximum loss estimates (PMLs) and catastrophic loss estimates, a f value of 0.02 should be used.

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GAP.8.0.1.1 June 2, 2003

English Units:

( )6104

)()(

×

ƒ∆Η= lb

Btuc

elbWtonW

(6E)

SI Units:

( )4648

HkgWkgW kg

kJc

eƒ∆

=)(

)(

( )4,216,573

)()ton(

ƒ∆= kg

kJc

eHkgW

W (6S)

The equivalent weight of TNT is expressed in tons as explosion damage tables are given in tons (1 ton = 2000 lb).

Calculation Of Diameter Of Overpressure Circles

From the scaling law, overpressure distances are calculated:

31

31

)()(

11 WW

dd e= (7)

31)(1 eWdd= (8)

where:

d = distance from the explosion of W kilotons

d1 = distance from explosion of 1 kiloton of TNT (2 × 106 lb)

W1 = equivalent weight of 1 kiloton of TNT

We = equivalent weight of TNT (kilotons TNT)

This relationship has been reduced to a graph (Figure 1) which may be used to determine the overpressure circle diameters. In Figure 1, a conversion has been made from kilotons TNT to tons TNT. No metricated graph is available.

For establishing the PML, the maximum overpressure to be used is 5 psi (0.35 bar). Current theory suggests that overpressures up to 15 psi (1 bar) should be establishing catastrophic loss potential estimates.

Determination Of Damage

Tables 1A, 1B, 1C, 1D and 2 are to be used in determining the extent of damage from explosion of a vapor cloud. These tables are based on overpressure damage only and do not consider the possibility of major damage from ensuing fire. If there is a possibility of an ensuing fire of long duration because of large volumes of flammable liquid holdup in processes, the ensuing fire must be considered. Within the 5 psi (0.35 bar) circle blast damage will probably result in total destruction of most processing units. Damage within this area should be considered as 100%.

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GAP.8.0.1.1 June 2, 2003

Figu

re 1

. Dia

met

er O

f Ove

rpre

ssur

e C

ircle

s Fo

r Var

ious

Yie

lds.

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GAP.8.0.1.1 June 2, 2003

TABLE 1A Summary of Blast Damage To Structures

Controlhouses Crude Units Atmos./Vacuum Towers Fractionator Towers Over-

pressures (psi)

Steel Roof Decking and

No Frame

Precast Concrete Roof

and Steel Frame

Steel Frame bet. Vessels Rectangular

Conc. Frame Octagonal

Conc. FrameRectangular Conc. Frame

Mounted on Conc.

Pedestal 0.5 Windows shatter Windows

shatter

1.0 Roof collapse (switchgear room)

Frame deformation

1.5 Roof collapse (control room)

Roof collapse (all rooms)

West BlastPartial roof collapse (control room) North and South blast

NOTE: Atmospheric Vacuum Towers

NOTE: Vacuum Towers only

3.5 Conc. block walls fail

Conc. block walls fail

Conc. brackets fail causing frame collapse

4.5 Anchor bolts yielding

5.0 5.5 Conc. frame

cracking Conc. frame

cracking

7.0 Conc. frame collapse

Conc. frame cracking

Conc. frame collapse

7.5 Vessel anchor bolts fail causing frame collapse

Vessel & foundation overturn

8.0 8.5 10.0 Steel frame

collapse

12.0 16.0

Source: Minimize Damage to Refineries From Nuclear Attack, National And Other Disasters, The Office Of Oil & Gas, U.S. Dept. Of The Interior; February 1970

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GAP.8.0.1.1 June 2, 2003

TABLE 1B Summary Of Blast Damage To Structures

Fluid Catalytic Cracking Units (FCCU) Regenerator Tower Reactor Tower Fractionator Tower

Over- pressures

(psi) Rectangular Steel Frame

Rectangular Conc. Frame

Rectangular SteelFrame

Rectangular Conc. Frame

Mounted on Conc. Pedestal

00.5 01.0 01.5 NOTE:

Reactor & Fractionator supported by same frame

03.5 04.5 05.0 Leeward

columns buckle East or West blast

Anchor bolts yielding

05.5 07.0 Overturns

Easterly blast Leeward cols.

buckle. East blast

Overturns Anchor bolts fail

07.5 Leeward cols. buckle. West blast

08.0 Overturns Westerly blast

Concrete frame cracking

08.5 Conc. frame cracking

10.0 12.0 Steel frame

overturns Conc. frame collapse

16.0 Conc. frame collapse


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GAP.8.0.1.1 June 2, 2003

TABLE 1C Summary Of Blast Damage To Structures

Light Ends Units Deisobutanizer Vapor

Recovery Unit

Furnaces - Pipe Still Flares Over-

pressure (psi)

Mounted on Pedestal

and Large Footing

Rectangular Steel

Frame

Atmospheric Vacuum

MaintenanceBuilding

Water Cooling Tower

Tower Supported

Guyed

00.3 Corrugated Asbestos Siding fails

Corrugated Asbestos Louvers fail

01.5 Moves slightly from original position

Moves slightly from original position

02.0 03.0 Steel frame

deformation Steel

frame overturns Blast diagonally< oriented.

03.5 Tower collapses

04.0 Steel frame overturns. Blast squarely oriented

05.0 Brick walls collapse. Severe frame deformation.

06.0 Steel frame collapse.

Stacks collapse

Stacks collapse

Steel frame collapses

06.5 Steel frame collapse

Steel frame collapse

07.0 07.5 09.0 09.5 Vessel

overturns

10.0 10.5 11.0 Collapse

above middle collar

15.0 Complete collapse

20.0


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GAP.8.0.1.1 June 2, 2003

TABLE 1D Summary Of Blast Damage To Structures

Pipe Bands Storage Tanks Over- pressure

(psi) Steel Frame Concrete

Frame

Boiler StackF.C.C. Unit

TEL Building

Bulk Terminal Cone Roof Floating Roof Spherical

00.3 01.5 Tile walls

fail Roof of Admin. Bldg. collapses. Cone roofs of tanks collapse

Empty tank uplift

Empty tank uplift

02.0 Bulk Terminal 03.0

03.5 Steel frame deformation

Concrete frame cracking

04.0 05.0 Concrete

frame collapse


06.5 Stack and foundation overturn

Tanks uplift (0.5 to 0.9) filled)

Ta

nks

0.5

& 0

.9 fi

lled)

upl

ift

ov

er th

is ra

nge

depe

ndin

g on

heig

ht-to

-dia

met

er ra

tio a

nd

di

amet

er o

f tan

k.

07.0 Steel frame deformation

Support deformation (full) support

07.5 Support deformation (empty)

09.0 Overturns (full)


Overturns (empty)

10.0 10.5 11.0 15.0 20.0 Roof

collapse


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GAP.8.0.1.1 June 2, 2003

TABLE 2 Blast Overpressure Effects On Vulnerable Refinery Parts

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Code

A. Windows and gauges break B. Louvers fall at 0.3-0.5 psi C. Switchgear is damaged from roof collapse D. Roof collapses E. Instruments are damaged F. Inner parts are damaged G. Brick cracks H. Debris-missile damage occurs I. Unit moves and pipes break J. Bracing fails K. Unit uplifts (half-filled)

L. Power lines are severed M. Controls are damaged N. Block walls fail O. Frame collapses P. Frame deforms Q. Case is damaged R. Frame cracks S. Piping breaks T. Unit overturns or is destroyed U. Unit uplifts (0.9 filled) V. Unit moves on foundation

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GAP.8.0.1.1 June 2, 2003

Outside of the 5 psi (0.35 bar) circle (but within the 3 psi (0.21 bar) circle) blast damage should be considered to be a minimum of 70%. There is also a definite chance of broken piping leading to ensuing fire. Sprinkler and waterspray systems will be damaged if not of proper explosion resistant design which should include the following:

• Welded flange piping

• Barricading of control valves

• Location of main risers and feed mains behind structural members

Outside of the 3 psi (0.21 bar) circle (but within the 1 psi (0.07 bar) circle), the blast damage will be a minimum of 40%. There is less chance of broken piping and ensuing fire in modern plants, but the possibility should be considered.

Calculation Of Cloud Size

The volume occupied by the cloud, assuming that the cloud has a cylindrical shape, can be calculated as follows:

4

2h

cD

Vc

π= (9)

where:

Vc = volume of vapor cloud

Dc = diameter of vapor cloud

h = height of vapor cloud

If the cloud is assumed to be at the midpoint of the flammable range and at atmospheric conditions:

atm

catmTR

VPMuW

= (10)

where:

W = weight of vapor in cloud

Patm = atmospheric pressure (14.7 psia)(1.013 bar)

Tatm = atmospheric temperature (70°F + 460 = 530°R) (21.1°C + 273 = 294.1°K)

R = gas constant (10.73 psi-ft3/°R-lb-mole) (0.08314 bar-m3/°K-kg-mole)

u = fraction of the cloud represented by vapor when the entire cloud is at the midpoint of the flammable range. u is calculated from an average of the lower flammable limit (LFL) and the upper flammable limit (UFL):

%1002)(%)(%

×+

=volUFLvolLFLu (11)

By substituting equation 10 in equation 9 and solving for Dc, the approximate size of the resulting vapor cloud may be calculated as follows (assuming that the vapor is heavier than air at its atmospheric boiling point):

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GAP.8.0.1.1 June 2, 2003

huMW

PTR

catm

atmDπ

4 (12)

For gases that are lighter than air at their atmospheric boiling point, the size of the vapor cloud has no meaning within the scope of this guide and should not be calculated.

English Units:

( )fthMlbW

c ftD υ)(19.22)( = (12E)

SI Units:

)()(544.5)( mhM

kgWc mD υ= (12S)

Many cases have shown that a leak at low pressure will form a drifting vapor cloud about 10 ft (3.05 m) high or deep.

Equations 12E and 12S can be reduced to:

English Units:

υMlbW

c ftD )(017.7)( = (13E)

SI Units:

υMkgW

c mD )(174.3)( = (13S)

If there is a reason to believe that a different cloud height is likely, a different height value can be used. For example, discharge at a high point or a high pressure, directed upwards, may result in a deeper cloud.

A sample calculation of a vapor cloud explosion loss estimate is shown in GAP.8.0.1.1.A.

VESSEL EXPLOSIONS

Background

Explosion of a vessel in a process unit can cause extensive damage to the process unit and surrounding buildings, structures, and equipment. The intensity of the explosion is dependent upon the size and ultimate failure pressure of the vessel. The driving force for the explosion is not important. The cause of the overpressure can be runaway chemical reaction, overheating, or plugged or inadequate vent facilities.

The process conditions or fire conditions which lead to the vessel failure are considered to be the initial incident. The fact that a major process vessel ruptures explosively at its failure point is considered to be a “reasonably adverse condition.” Thus, it is not necessary to assume that any fire

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GAP.8.0.1.1 June 2, 2003

protection facilities will be impaired prior to ignition. Damage and impairment, however, may occur due to the explosion.

Selection Of Vessel Giving Maximum Loss

Since the size, ultimate failure and pressure, and location of the exploding vessel are variables in the calculated loss estimate, it is possible that a number of calculations must be made to determine the maximum loss potential. This method cannot be used to evaluate the rupture of a long tubular or a “pipeline” reactor.

Calculation Method

The TNT equivalent of the potential energy of a gas above ambient temperature and stored at pressure can be calculated by the textbook formula for isothermal expansion:

2

1lnPΡTRnE = (14)

where:

E = energy of isothermal expansion

n = number of moles of gas

R = gas constant

T = gas temperature

P1 = initial pressure

P2 = final pressure after expansion

The number of moles of gas contained in a given vessel can be determined using the “Ideal Gas Law”:

RTPV = (15)

Therefore, equation 14 can be written as follows:

2

11 ln

PP

VPE = (16)

where:

E = energy of isothermal expansion

P1 = maximum pressure of vessel at failure

V = volume of gas in vessel

P2 = final pressure after expansion

The energy released in a vessel explosion may now be estimated. This energy release is normally expressed as a TNT equivalent. Thus, based on the heat of combustion for TNT of 2000 Btu/lb (4648 kJ/kg), the following equation may be used to calculate a TNT equivalent for a vessel explosion:

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GAP.8.0.1.1 June 2, 2003

2

11 lnPPVPEW

cecee ∆Η

=∆Η

= (17)

where:

We = Weight of TNT which will produce an explosive force equivalent to the force produced by explosion of the vessel.

∆Hce = Heat of combustion of TNT (2000 Btu/lb)(4648 kJ/kg)

A simplified form of equation 17 can be used since the expansion of the gas is into the open air (P2 = Patm):

atmcecee P

PVPEW 11 ln∆Η

=∆Η

= (18)

English Units:

7.14)(

ln)()(185.0)( 131

psiaPftVpsiaPBtuE = (17E)

7.14)(

ln)()(1063.4)( 131

8 psiaPftVpsiaPtonsWe

−×= (18E)

SI Units:

013.1)(

ln)()(101)( 131

5 barPmVbarPbarE ×= (17S)

013.1)(

ln)()(101

)( 13

15 barP4,648,000

mVbarPkgWe

×=

013.1)(

ln)()(1037.2)( 131

5 barPmVbarPtonsWe

−×= (19S)

The maximum pressure of the vessel at failure P1 should be, in most cases, the maximum failure pressure of the vessel for a runaway reaction where vent facilities are plugged or overtaxed. The pressure at which the vessel will fail is often approximately four times the maximum working pressure of the vessel. However, if lower or higher design safety factors were used and can be documented, the actual multiplier should be used.

It is conceded that vessels also fail for reasons other than simple overpressure. Mechanical failure due to overheating or corrosion/erosion thinning can result in failure at pressures at or below the normal working pressure of the vessel. However, the use of the maximum yield pressure of the vessel will give a conservative result.

The value of We obtained is then used with the graph in Figure 1 to determine the diameter of the overpressure circles. As with the vapor cloud calculation, the maximum overpressure assumed is

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GAP.8.0.1.1 June 2, 2003

5 psi (0.35 bar). Determination of damage from explosion and ensuing fire should be done utilizing the minimum damage amounts outlined for vapor cloud explosions.

A sample calculation is shown in GAP.8.0.1.1.B.

GAP Guidelines Referenced

GAP.2.5.2 Plant Layout And Spacing For Oil And Chemical Plants.

REFERENCES

1. R. W. Gallant, Physical Properties Of Hydrocarbons, Volumes 1 & 2, Gulf Publishing Company, Houston, TX.

2. R. H. Perry, D. Green, Perry’s Chemical Engineers’ Handbook, 6th edition, McGraw-Hill Book Company, New York, NY.

3. NFPA Handbook, 16th edition, Section 5, Chapter H, National Fire Protection Association, Quincy, MA.

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gap.8.0.1.1. oil and chemical properties loss potential estimation guide

Documents

protection of hazards

personnel protection

public protection

catastrophic loss potential

loss prevention surveys

gap guidelines

probable maximum loss

risk management