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Comparison of NFPA and ISO

Approaches for Developing Separation Distances

Jeffrey L. LaChance, Bobby Middleton, & Katrina GrothSandia National Laboratories

Albuquerque, NM

Presented at the 4th International Conference on Hydrogen Safety

San Francisco, CASeptember 12-14, 2011

Presentation Outline• Harmonization of NFPA and ISO Separation

Distance Approaches• Separation Distance Table Format• Representative Facilities• Approaches for Developing Separation

Distances• Criteria, Models, and Data Utilized• Comparison of Leak Sizes Used to Determine

Separation Distances

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Separation Distances• Separation distances for small leaks – not major ruptures

– Desirable to cover events that may occur during facility life time– Risk from larger events not covered by separation distances should be

acceptable

• Quantitative Risk Analysis (QRA) was used to help establish many of NFPA and ISO separation distances

• QRA requires information for possible accidents:– Component leak frequencies (e.g., hoses, valves, and joints)– Ignition probabilities– Consequence models– Harm and risk criteria

• Under U.S. DOE sponsorship, Sandia provided methods, data, models, and manpower to support both efforts

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Harmonization• Desirable to harmonize NFPA and ISO approaches and

separation distances• Commonalities in approaches:

– Both use same QRA approach (limited scope QRA)

– Same consequence models and component leak data

• Differences that challenge harmonization of separation distances:– Evaluated for different types of facilities: Bulk storage (NFPA)

versus refueling facility (ISO)

– Different separation table format

– Different risk criteria

– Application of data is different

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Separation Distance Table Format

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Pressure > 15 to ≤ 250 psig>103.4 to 1724

kPa

> 250 to ≤ 3000 psig

>1724 to 20,684 kPa

> 3000 to ≤ 7500 psig

>20,684 to 51,711 kPa

> 7500 to ≤ 15000 psig

>51,711 to 103,421 kPa

Internal Pipe Diameter (ID)

d mm d = 52.5 mm d = 18.97 mm d = 7.31 mm d = 7.16 mm

NFPA bulk storage:•Typical bulk storage facility defined for each pressure range•All facility components/modules assumed to be co-located•Gas volume not a variable in table format (also not a factor in QRA)

Category 1 Category 2 Category 3

(<= 55 Mpa &<= 100kg) (> 55 Mpa & <= 3000L) (>100 kg & >3000L

VS S C A VS S C A S C

                   

ISO refueling station:•Six different subsystems ranging from very simple and limited volume to complex and high

volume•Risk criteria applied to each subsystem (2.5 m separation between systems is required) •Gas volume included in system categorization but not a factor in QRA

Both standards have methods to modify separation distances in tables to account for differences in pressures and maximum component diameter.

ISO System Classification

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Storage classification for determination of clearance distances

10

100

100 1000 10000 100000

Water volume (L)

Se

rvic

e p

res

su

re (

MP

a)

3

1

2

P <= 55 MPa

P > 55 Mpa

Stored quantity

> 100 kg

3000

55

1 kg

Storage classification for determination of clearance distances

10

100

100 1000 10000 100000

Water volume (L)

Se

rvic

e p

res

su

re (

MP

a)

3

1

2

P <= 55 MPa

P > 55 Mpa

Stored quantity

> 100 kg

3000

55

1 kg

Comparison of Representative Facilities

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System Pressure

(MPa) Diameter

(mm) Number of Risk-Significant Components LPI1

Valves Joints Hoses Compressor ISO

Very Simple Gas System (VS)- (e.g., pressure regulator station)

55 or 110 8 2 7 0 0 ≤15 (15)

Simple Gas System (S) – (e.g., cylinder pack)

55 or 110 8 5 32 0.33 0 ≤60 (60)

Complex Gas System (C) – (e.g., buffer storage)

55 or 110 8 20 55 0 0 >60 (135)

Simple Large Storage System2 (SL) – (e.g., larges storage system)

55 or 110 8 NA NA NA NA ≤45 (NA)

Complex Large Storage System2 (CL) – (e.g., tube trailer)

55 or 110 8 NA NA NA NA >45 (NA)

Process System (A) (e.g., compressor with connections )

55 or 110 8 20 55 0 1 NA (>135)

NFPA Bulk Storage System:

Tube Trailer Stanchion

Pressure Regulator Module

Compressor Buffer Storage

36 93 17 1 - 20.7 12.7 13 38 10 0 (330) 20.7 18.9 4 6 1 0 (46)

20.7, 51.7, or 103.4

18.97, 7.8, or 7.2

20 28 0 0 (108)

20.7 7.8, or 7.2 0 0 0 1 (NA) 51.7, or 103.4

7.8, or 7.2 9 21 6 0 (201)

1 Leak Probability Indicator (LPI) determined based on number of joints, valves, and hoses each multiplied by a Joint-Euivalency Ratio (JER). JER (joints)=1, JER (valves)=4, JER(hoses)=24. 2 Simple and Complex Large Storage System leak sizes are not based on risk. Leak sizes were subjectively selected.

First number is limit for LPI for the system. Value in parenthesis is actual LPI for example system used to evaluate separation distance in table.

Comparison of Approaches• NFPA approach:

– Most separation distances based primarily on expected frequency of leakage events

– Cumulative risk from larger leaks reviewed• Risk to person at facility lot line used to establish leak size for four facility

configurations (3% leak sizes chosen for all)

– Other factors considered, safety margin added to address uncertainties and limited scope of analysis

• ISO approach:– Separation distances for six systems based only on limited risk

evaluation or subjective judgment• Risk to a person used to establish leak size for four Category 1 and 2

systems (variable leak sizes chosen)• Leak sizes for two Category 3 systems based on subjective judgment

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Risk Evaluation Model Used in Both Approaches

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1.0E-08

1.0E-07

1.0E-06

1.0E-05

0.00 5.00 10.00 15.00

Separation Distance (m)

Cu

mm

ula

tiv

e R

isk

(/y

r)

Risk Criteria

Separation Distance

Cummulative frequency of accidents requiring this separation distance

Increasing leak diameter

Risk curve is discretized to evaluate separation distances

Harm and Risk Criteria• Both NFPA and ISO assumed exposure to hydrogen

flame would result in fatality • NFPA used single fatality risk criteria of 2E-5/yr to

maximum-exposed individual based on:– Fatality risk at gasoline stations– 10% of risk from other accidental causes– Risk criteria used in several countries

• ISO used two risk criteria:– Normal exposures – 1E-5/yr (International Energy Agency Task

19 (Hydrogen Safety) recommended value for fatality risk)– Critical exposures (propagation potential, potential for multiple

people being harmed) – 4E-6/yr

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• Sandia hydrogen leak models were used to evaluate safety distances in both NFPA and ISO standards• Objects exposed to a hydrogen plume can encounter

• Heating from radiation (ignited jet)• Flame impingement (ignited jet)• Combustible cloud contact (unignited jet)

• Flame impingement and presence in 4% combustible cloud after ignition assumed to result in high probability of fatality

• Experimental measurements• Flame shape and flame impingement distances for different flow rates• Hydrogen flame radiation values• Lean ignition limit for hydrogen/air mixtures

• Computational models with validation• Jet flame radiation model• Unignited jet flammability limit contour model• Predictions outside the range of available data

• Models and experiments published in peer reviewed journal articles

Consequence Evaluation

Nighttime photograph of 413 bar (6000 psig)

large-scale H2 jet-flame test (dj = 5.08mm,

Lvis = 10.6 m) from Sandia/SRI tests.

11.3 m

Reference: Houf and Schefer, “Predicting Radiative Heat Fluxes and Flammability Envelopes from Unintended Releases of Hydrogen,” IJHE Paper GI-353

Component Leak Frequencies

• Currently there is insufficient hydrogen data to generate hydrogen leak frequencies using traditional statistical methods

• Thus, a Bayesian approach was used by SNL to generate hydrogen component leak data– Multiple sources of generic data (non-hydrogen) used to

generate a “guess” for each hydrogen component leak frequency (prior distribution)

– Uncertainty in assignment of generic data to specific leak sizes– Available hydrogen data used to update the prior distribution for

a component to obtain a hydrogen-specific leak frequency estimate (posterior distribution)

– In some cases, hydrogen data did not always match the prior distribution shape or magnitude

Reference: “Handbook of Parameter Estimation for Probabilistic Risk Assessment,” NUREG/CR-6823, U.S. Nuclear Regulatory Commission, Washington, D.C. (2003).

Example Results - Joints

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Amount of hydrogen data (number of failures and component years of operation) is large. Generic data has little influence on shape and magnitude of hydrogen leak frequency curve.

Available data suggests leak frequencies are similar over a large range of leak sizes

Example Results - Valves

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Hydrogen data provides similar frequencies as generic data. Generic data influences shape and magnitude of hydrogen leak frequencies.

ISO Leak Frequencies• SNL hydrogen component leak frequencies were

modified for use in ISO QRA:– Linearized (on log-log scale)

• Steep slopes selected for all components(not justified by SNL data results) to facilitate selection of risk-based safety distances - can result in under shorter separation distances

• Similar slopes for each component allows establishing “Leak Probability Indicator “(LPI) which allows modification of tabular safety distances for plant-specific configurations

– Shifted an order of magnitude lower based on selected rebinning of a fraction of the generic leak frequencies into alternate bins

• No hydrogen data was reviewed• Bayesian analysis was not performed• Shifted curves provides safety distances that are a factor of 2 to 3

shorter when leak frequencies are not shifted

• ISO leak frequencies results in shorter safety distances than if SNL leak frequencies were used directly

Example of Modification of Leak Frequencies for Use in ISO QRA

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ISO curve is conservative over a large range compared to hydrogen mean from Bayesian analysis

Ignition Probabilities

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Values used in NFPA QRA

Hydrogen Release Rate (kg/s)

Immediate Ignition Probability

Delayed Ignition Probability

Total

Ignition Probability

<0.125 0.008 0.004 0.012

0.125 – 6.25

0.053 0.027 0.08

>6.25 0.23 0.12 0.45

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ISO QRA used probability of 0.04 for all leak sizes and did not differentiate between immediate and delayed ignition

Comparison of NFPA and ISO Leak Sizes

Leak Size (% of Flow Area)

System Type

Example Systems Regular Exposure

Critical Exposure

ISO Very Simple Gas System (VS)

Pressure regulation module 0.03% 0.09%

Simple Gas system (S) Cylinder pack 0.16% 0.48% Complex Gas System (C) Cascaded buffer storage system 0.42% 1.30% Simple Large Storage System1 (SL)

Large hydrogen storage (e.g., 100 m3) 0.38% 1.50%

Complex Large Storage System1 (CL)

Hydrogen tube trailer 0.75% 3.00%

Process System (A) Compressor plus connections 0.65% 1.81% NFPA Bulk storage system with a hydrogen

tube trailer, pressure regulator module, compressor, and buffer storage area

3.00%2

1 The leak sizes for these systems were not evaluated using the ISO risk model. They were subjectively selected. 2 The NFPA risk assessment used a single risk guideline of 2E-5/yr to evaluate leak sizes and resulting separation distances. This risk guideline is comparable to the regular exposure criteria of 1E-5/yr in the ISO risk assessment.

Sensitivity Results- Joints

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Shifting generic data order of magnitude has little effect on hydrogen frequencies. No justification for shifting frequencies based on this prior distribution.

Sensitivity Results- Valves

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ISO shifted curve is below the revised (new) hydrogen mean curve. Shifting generic

frequencies had minor effect on hydrogen frequencies. Shifting hydrogen curve an order of magnitude is not justified.

Risk Results Using ISO Systems and NFPA Data

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Risk profile is flatter when NFPA data is utilized due primarily to variable ignition probability. Risk is acceptable but not as low as predicted with ISO data.

3% of flow area

Summary • NFPA and ISO approaches for determining

separation distances are very similar– Both use QRA, but with different levels of

emphasis and complexity– Selected leak frequency distributions and ignition

probabilities can significantly affect separation distances

• Differences between reference systems used in QRA evaluations result in differences in separation distances

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Additional Slides

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Mean Component Leakage Frequencies from Bayesian Analysis

Hydrogen Leakage Frequencies

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

0.10% 1.00% 10.00% 100.00%

Leak Area (% Flow Area)

Mea

n L

eaka

g F

req

uen

cy (

/yr)

Compressors

Cylinders

Hoses

Joints

Valves

Pipes

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System Leak Frequency Results From NFPA Analysis

0.85

0.90

0.95

1.00

0.01% 0.10% 1.00% 10.00% 100.00%

Leak Size (% Flow Area)

Cu

mu

lati

ve

Pro

ba

bili

ty o

f S

ys

tem

Le

ak

ag

e

20.7 MPa

103.4 MPa

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0.01% 0.10% 1.00% 10.00% 100.00%

Leak Size (% Flow Area)

Sys

tem

Lea

kag

e F

req

uen

cy (

/yr)

20.7 MPa

103.4 MPa

Expert opinion used to select 3% of system flow area• captures >95% percent of the leaks • covers leaks expected during facility life time• the resulting separation distances protect up to the 3% leak size• QRA performed to determine if associated risk from leaks greater than this

is acceptable

Risk Results From NFPA QRA

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• Risk close to the “guideline” of 2E-5 fatalities/yr selected by NFPA Task Group 6

• Risk from leaks greater than 3% of flow area were deemed acceptable

Total Risk 20.7 MPa (3000 psig) System Total Risk 103.4 MPa (15000 psig) System

Total Risk - 3000 psig System

5.0E-06

1.0E-05

1.5E-05

2.0E-05

2.5E-05

3.0E-05

3.5E-05

4.0E-05

4.5E-05

5.0E-05

0 5 10 15 20 25 30

Separation Distance (m)

Cu

mu

lati

ve F

req

uen

cy o

f F

ata

lity

(/y

r)

Sum

Flame Length

4% H2

Lot Line Separation

Distance for 1%A to 10%A

Leak Sizes

Total Risk - 15000 psig System

1.0E-05

3.0E-05

5.0E-05

7.0E-05

9.0E-05

0 5 10 15 20 25 30

Separation Distance (m)

Fre

qu

en

cy o

f F

ata

lity

(/y

r)

Sum

Flame Length

4% Hydrogen

Lot Line Separation

Distance for 1%A to 10%A

Leak Size

J. LaChance et al., “Analyses to Support Development of Risk-Informed Separation Distances for Hydrogen Codes and Standards”, SANDIA REPORT, SAND2009-0874, Printed March 2009

ISO QRA Results

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Risk Criteria

Effect of ISO Leak Frequency Modification

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Exposure Type Increase in Safety Distance Compared to ISO

Category 1 (<= 55 MPa) Category 2 (> 55 MPa) VSGS SGS CGS PRS VSGS SGS CGS PRS

ISO Frequencies Not Shifted 4% H2 Critical 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 4% H2 Regular 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 Thermal Effect Critical 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 Thermal Effect Regular 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2

SNL Frequencies 4% H2 Critical 1.6 3.3 5.1 4.6 1.6 3.3 5.1 4.6 4% H2 Regular 1.0 1.7 3.2 3.1 1.0 1.7 3.2 3.1 Thermal Effect Critical 1.6 3.3 5.1 4.6 1.6 3.3 5.1 4.6 Thermal Effect Regular 1.0 1.7 3.2 3.1 1.0 1.7 3.2 3.1

Data Sensitivity Studies• Modification of SNL leak frequency data was not

based on rigorous statistical methods– A change in generic frequencies does not

necessarily result in an equivalent change in hydrogen frequencies

• To evaluate the potential effect of generic leak frequency-size assignments, sensitivity evaluations have recently been performed– Generic leak frequencies and hydrogen information

re-binned into 0.01%-0.1%, 0.1%-1%, 1%-10%, and 10%-100% (fraction of flow area) leak size bins

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Alternative Prior - Joints

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Generic leak frequencies for flanges were used as an alternative prior distribution.

Sensitivity Results- Valves

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Shifting generic frequencies changed magnitude and shape of curves

Sensitivity Results- Hoses

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Shifted ISO curve provides reasonable fit if data is re-binned.

Sensitivity Results- Compressors

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Sensitivity Results- Compressors

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ISO shifted curve is below revised (new) hydrogen mean curve. Moving ISO shifted curve upwards would provide better fit.

Summary of Data Sensitivity Study

• Shifted ISO leak frequencies for valves and compressors are not consistent with results of sensitivity studies where generic and hydrogen data was re-binned to lower leak sizes (i.e., leak intervals)

• There is justification for the shifted ISO leak frequencies for hoses and joints if generic leak frequencies are modified

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Impact on Separation Distances

• Based on results of sensitivity studies, use of shifted ISO leak frequencies for hoses and joints and non-shifted frequencies for valves and compressors results in following increase in ISO separation distances:

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Exposure Type Increase in Safety Distance Compared to ISO

Category 1 (<= 55 MPa) Category 2 (> 55 MPa)

VSGS SGS CGS PRS VSGS SGS CGS PRS Only ISO Hose and Joints Leak Frequencies Shifted

4% H2 Critical 2.2 1.8 2.3 2.6 2.2 1.8 2.3 2.6 4% H2 Regular 2.2 1.8 2.3 2.6 2.2 1.8 2.3 2.6 Thermal Effect Critical 2.2 1.8 2.3 2.6 2.2 1.8 2.3 2.6 Thermal Effect Regular 2.2 1.8 2.3 2.6 2.2 1.8 2.3 2.6

Ignition Probability Sensitivity Study on ISO Separation Distances

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Use of constant ignition probability does not necessarily result in conservative separation distances in a risk-based approach

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