1. Comparison of NFPA and ISO Approaches for Developing Separation Distances
Jeffrey L. LaChance, Bobby Middleton, & Katrina Groth
Sandia National Laboratories
Albuquerque, NM
Presented at the
4th International Conference on Hydrogen Safety
San Francisco, CA
September 12-14, 2011
11/12/2018

2. 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
11/12/2018

3. 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
11/12/2018

4. 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
11/12/2018

5. Separation Distance Table Format
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)
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 ≤ psig
>51,711 to £103,421 kPa
Internal Pipe Diameter (ID)
d mm
d = 52.5 mm
d = mm
d = 7.31 mm
d = 7.16 mm
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
Category 1
Category 2
Category 3
(<= 55 Mpa &<= 100kg)
(> 55 Mpa & <= 3000L)
(>100 kg & >3000L VS S C A
Both standards have methods to modify separation distances in tables to account for differences in pressures and maximum component diameter.
11/12/2018

6. ISO System Classification
11/12/2018

7. Comparison of Representative Facilities
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.
11/12/2018

8. 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
11/12/2018

9. Risk Evaluation Model Used in Both Approaches
Risk curve is discretized to evaluate separation distances
11/12/2018

10. 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
11/12/2018

11. Consequence Evaluation
• 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
11.3 m
Nighttime photograph of 413 bar (6000 psig)
large-scale H2 jet-flame test (dj = 5.08mm,
Lvis = 10.6 m) from Sandia/SRI tests.
Reference: Houf and Schefer, “Predicting Radiative Heat Fluxes and Flammability Envelopes from Unintended Releases of Hydrogen,” IJHE Paper GI-353

12. 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).

13. Example Results - Joints
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
11/12/2018

14. Example Results - Valves
Hydrogen data provides similar frequencies as generic data. Generic data influences shape and magnitude of hydrogen leak frequencies.
11/12/2018

15. 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

16. Example of Modification of Leak Frequencies for Use in ISO QRA
ISO curve is conservative over a large range compared to hydrogen mean from Bayesian analysis
11/12/2018

17. Ignition Probabilities
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
ISO QRA used probability of 0.04 for all leak sizes and did not differentiate between immediate and delayed ignition
11/12/2018 17

18. Comparison of NFPA and ISO Leak Sizes

19. Sensitivity Results- Joints
Shifting generic data order of magnitude has little effect on hydrogen frequencies. No justification for shifting frequencies based on this prior distribution.
11/12/2018 19

20. Sensitivity Results- Valves
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.
11/12/2018 20

21. Risk Results Using ISO Systems and NFPA Data
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
11/12/2018

22. 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
11/12/2018

23. Additional Slides
11/12/2018

24. Mean Component Leakage Frequencies from Bayesian Analysis
11/12/2018

25. System Leak Frequency Results From NFPA Analysis
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
11/12/2018

26. Risk Results From NFPA QRA
Total Risk 20.7 MPa (3000 psig) System
Total Risk MPa (15000 psig) System
J. LaChance et al., “Analyses to Support Development of Risk-Informed Separation Distances for Hydrogen Codes and Standards”, SANDIA REPORT, SAND , Printed March 2009
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
11/12/2018

27. ISO QRA Results
Risk Criteria
11/12/2018

28. Effect of ISO Leak Frequency Modification
11/12/2018 28

29. 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
11/12/2018 29

30. Alternative Prior - Joints
Generic leak frequencies for flanges were used as an alternative prior distribution.
11/12/2018 30

31. Sensitivity Results- Valves
Shifting generic frequencies changed magnitude and shape of curves
11/12/2018 31

32. Sensitivity Results- Hoses
Shifted ISO curve provides reasonable fit if data is re-binned.
11/12/2018 32

33. Sensitivity Results- Compressors
11/12/2018 33

34. Sensitivity Results- Compressors
ISO shifted curve is below revised (new) hydrogen mean curve. Moving ISO shifted curve upwards would provide better fit.
11/12/2018 34

35. 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
11/12/2018 35

36. 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:
11/12/2018 36

37. Ignition Probability Sensitivity Study on ISO Separation Distances
Use of constant ignition probability does not necessarily result in conservative separation distances in a risk-based approach
11/12/2018 37