Comparison of NFPA and ISO Approaches for Developing

Comparison of NFPA and ISO Approaches for Developing Separation Distances Jeffrey L. La. Chance, Bobby Middleton, & Katrina Groth Sandia National Laboratories Albuquerque, NM Presented at the 4 th International Conference on Hydrogen Safety San Francisco, CA September 12 -14, 2011 10/24/2020 1

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 10/24/2020 2

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 10/24/2020 3

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 10/24/2020 4

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 k. Pa > 250 to ≤ 3000 psig >1724 to £ 20, 684 k. Pa > 3000 to ≤ 7500 psig >20, 684 to £ 51, 711 k. Pa > 7500 to ≤ 15000 psig >51, 711 to £ 103, 421 k. Pa d = 52. 5 mm d = 18. 97 mm d = 7. 31 mm d = 7. 16 mm Internal Pipe Diameter (ID) d 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 &<= 100 kg) (> 55 Mpa & <= 3000 L) (>100 kg & >3000 L VS S C A S C Both standards have methods to modify separation distances in tables to 10/24/2020 account for differences in pressures and maximum component diameter. 5

ISO System Classification 10/24/2020 6

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. 10/24/2020 7

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 10/24/2020 • 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 8 judgment

Risk Evaluation Model Used in Both Approaches Risk curve is discretized to evaluate separation distances 10/24/2020 9

Harm and Risk Criteria • Both NFPA and ISO assumed exposure to hydrogen flame would result in fatality • NFPA used single fatality risk criteria of 2 E-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: 10/24/2020 – Normal exposures – 1 E-5/yr (International Energy Agency Task 19 (Hydrogen Safety) recommended value for fatality risk) – Critical exposures (propagation potential, potential for 10 multiple people being harmed) – 4 E-6/yr

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) 11. 3 m • 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 Nighttime photograph of 413 bar (6000 psig) large-scale H 2 jet-flame test (dj = 5. 08 mm, 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 • 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

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 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. Leakage Frequency (/yr) 1. 0 E+00 1. 0 E-01 Generic Mean 1. 0 E-02 Generic Median 1. 0 E-03 Hydrogen Mean 1. 0 E-04 Hydrogen Median 1. 0 E-05 Available data suggests leak frequencies are similar over a large range of leak sizes 1. 0 E-06 1. 0 E-07 0. 01% 0. 10% 1. 00% 10. 00% Leakage Area (% Flow Area) 10/24/2020 13

Example Results - Valves Hydrogen data provides similar frequencies as generic data. Generic data influences shape and magnitude of hydrogen leak frequencies. Leakage Frequency (/yr) 1. 0 E-02 Generic Mean 1. 0 E-03 Generic Median 1. 0 E-04 Hydrogen Mean 1. 0 E-05 Hydrogen Median 1. 0 E-06 1. 0 E-07 0. 01% 0. 10% 1. 00% 10. 00% Leakage Area (% Flow Area) 10/24/2020 14

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 ISO curve is conservative over a large range compared to hydrogen mean from Bayesian analysis 10/24/2020 16

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. 12 0. 45 0. 23 ISO QRA used probability of 0. 04 for all leak sizes and did not differentiate between immediate and delayed ignition 10/24/2020 17

Comparison of NFPA and ISO Leak Sizes

Sensitivity Results- Joints Leakage Frequency (/yr) Generic Mean 1. 0 E+00 Generic Median 1. 0 E-01 New Generic Median 1. 0 E-02 Hydrogen Mean New Hydrogen Mean 1. 0 E-03 ISO Mean 1. 0 E-04 ISO Mean - Shifted 1. 0 E-05 1. 0 E-06 1. 0 E-07 0. 01% 10/24/2020 0. 10% 1. 00% 10. 00% Leakage Area (% Flow Area) Shifting generic data order of magnitude has little effect on hydrogen frequencies. No justification for shifting frequencies based on this prior distribution. 19

Sensitivity Results- Valves – Hydrogen Frequencies Leakage Frequency (/yr) Hydrogen Mean Hydrogen Median 1. 0 E-03 New Hydrogen Mean New Hydrogen Median ISO Mean - Shifted 1. 0 E-04 1. 0 E-05 1. 0 E-06 0. 01% ISO shifted curve is below the revised (new) hydrogen mean curve. 0. 10% 1. 00% 10. 00% Shifting generic frequencies had minor effect on hydrogen frequencies. Shifting hydrogen curve an order of magnitude is not justified. Leakage Area (% Flow Area) 10/24/2020 20

Risk Results Using ISO Systems and NFPA Data Cumulative Frequency of Fatality (/yr) 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 CGS-NFPA Data CGS 1. 0 E-05 SGS-NFPA Data SGS VSGS-NFPA Data VSGS AS-NFPA Data 1. 0 E-06 AS 3%A 1. 0 E-07 1. 0 100. 0 Distance (m) 10/24/2020 21

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 22 10/24/2020

Additional Slides 10/24/2020 23

Mean Component Leakage Frequencies from Bayesian Analysis 10/24/2020

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 10/24/2020

Risk Results From NFPA QRA Total Risk 20. 7 MPa (3000 psig) System Total Risk 103. 4 MPa (15000 psig) System J. La. Chance et al. , “Analyses to Support Development of Risk-Informed Separation Distances for Hydrogen Codes and Standards”, SANDIA REPORT, SAND 2009 -0874, Printed March 2009 • • 10/24/2020 Risk close to the “guideline” of 2 E-5 fatalities/yr selected by NFPA Task Group 6 Risk from leaks greater than 3% of flow area were deemed acceptable

ISO QRA Results Risk Criteria 10/24/2020 27

Effect of ISO Leak Frequency Modification 10/24/2020 28

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 29 area) leak size bins 10/24/2020

Alternative Prior - Joints Generic leak frequencies for flanges were used as an alternative prior distribution. 10/24/2020 30

Sensitivity Results- Valves – Generic Frequencies Leakage Frequency (/yr) 1. 0 E-02 1. 0 E-03 Generic Mean 1. 0 E-04 1. 0 E-05 Generic Median Shifting generic frequencies changed magnitude and shape of curves New Generic Mean New Generic Median 1. 0 E-06 1. 0 E-07 0. 01% 0. 10% 1. 00% 10. 00% Leakage Area (% Flow Area) 10/24/2020 31

Sensitivity Results- Hoses Leakage Frequency (/yr) 1. 0 E+02 1. 0 E+01 Generic Mean 1. 0 E+00 Generic Median New Generic Median 1. 0 E-01 Hydrogen Mean 1. 0 E-02 New Hydrogen Mean 1. 0 E-03 ISO Mean 1. 0 E-04 1. 0 E-05 1. 0 E-06 0. 01% 10/24/2020 ISO Mean - Shifted ISO curve provides reasonable fit if data is re-binned. 0. 10% 1. 00% Leakage Area (% Flow Area) 10. 00% 32

Sensitivity Results- Compressors – Generic Frequencies 1. 0 E+00 Leakage Frequency (/yr) 1. 0 E-01 Generic Mean 1. 0 E-02 Generic Median 1. 0 E-03 New Generic Mean 1. 0 E-04 New Generic Median 1. 0 E-05 1. 0 E-06 0. 01% 10/24/2020 0. 10% 1. 00% Leakage Area 10. 00% 33

Sensitivity Results- Compressors – Hydrogen Frequencies Leakage Frequency (/yr) 1. 0 E-01 1. 0 E-02 Hydrogen Median New Hydrogen Mean 1. 0 E-03 New Hydrogen Median 1. 0 E-04 1. 0 E-05 1. 0 E-06 0. 01% 10/24/2020 Hydrogen Mean ISO shifted curve is below revised (new) hydrogen mean curve. Moving ISO shifted curve upwards would provide better fit. 0. 10% ISO Mean - Shifted 1. 00% Leakage Area 10. 00% 34

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 10/24/2020 35

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: 10/24/2020 36

Ignition Probability Sensitivity Study on ISO Separation Distances Cumulative Frequency (/yr) Use of constant ignition probability does not necessarily result in conservative separation distances in a riskbased approach 1. 0 E-05 CGS - ISO CGS SGS VSGS -ISO VSGS 1. 0 E-06 1. 0 E-07 1 10/24/2020 10 Separation Distance (m) 100 37