Quantitative Risk Assessment Safety Studies and Risk Mitigation
- Slides: 49
Quantitative Risk Assessment, Safety Studies and Risk Mitigation Jeffrey La. Chance Sandia National Laboratories Presented at the 2 nd International Conference on Hydrogen Safety San Sebastian, Spain 12 September 2007 3/11/2021 Preliminary Data
Outline • • 3/11/2021 Role of QRA in Hydrogen Safety Applications of QRA Risk-Informed Codes and Standards Separation Distances Application to an Example Facility Input Data and Assumptions for QRA Models Results Summary Preliminary Data
Project Background • Work performed under U. S. DOE Hydrogen, Fuel Cells & Infrastructure Technologies Program, Multi-Year Research, Development and Demonstration Plan – Hydrogen Safety, Codes & Standard R&D • Sandia National Laboratories is developing the scientific basis for assessing credible safety scenarios and providing the technical data for use in the development of codes and standards – Includes experimentation and modeling to understand behavior of hydrogen for different release scenarios – Use of Quantitative Risk Assessment (QRA) methods to help establish separation (setback, safety) distances at hydrogen facilities and to identify accident prevention and mitigation strategies for key risk drivers 3/11/2021 Preliminary Data
Hydrogen Safety • Public perception of hydrogen safety has been skewed by major incidents such as the Hindenburg accident • In reality, the use of hydrogen has an excellent safety record • The expanded use of hydrogen will include new challenges (e. g. , very high pressures) that will require design features and operational requirements to manage the risk to acceptable levels Risk assessment (both qualitative and quantitative) provides a means to demonstrate hydrogen safety ! 3/11/2021 Preliminary Data
The Role of Risk Assessment • Various levels of risk assessment can be utilized to assess the risk associated with a hydrogen facility: – Qualitative methods such as Failure Modes Effects Analysis are typically used to identify hazards and specify accident prevention and mitigation features – Semi-Quantitative methods such as risk matrices can identify risk-significant accidents Quantitative Risk Assessment (QRA) evaluates the facility risk and provides much more! 3/11/2021 Preliminary Data
Quantitative Risk Assessment • QRA is a systematic process for evaluating the risk associated with a facility for: – Verification that it meets an accepted risk criteria – Identification of important accidents, components, operations contributing to risk – Identification and evaluation of risk reduction and control measures – Identification of risk management requirements (e. g. , maintenance intervals) 3/11/2021 Preliminary Data
Applications of QRA • Primary application is to determine that a facility is safe! – Can be performed for evaluating early prototype facilities and for evaluating standard designs – Can be used as part of facility permitting process • Can be used to generate risk-informed code and standard requirements – National Fire Protection Association has generated guidance for this application 3/11/2021 Preliminary Data
Risk-Informed Codes and Standards • Use of a risk-informed process is one way to establish the requirements necessary to ensure public safety – Comprehensive QRA used to identify and quantify scenarios leading to hydrogen release and ignition – Accident prevention and mitigation requirements identified based on QRA – Results combined with other considerations to establish minimum code and standard requirements needed for an established risk level • Required prevention and mitigation features can be specified as a function of important facility parameters: – Hydrogen generation method – Volume and pressure of hydrogen – Location of components (e. g. , inside versus outside) 3/11/2021 Preliminary Data
Separation Distances • Specified distances in codes separating H 2 components from the public, structures, flammable material, and ignition sources – Distance vary with possible consequences from hydrogen releases (e. g. , radiation heat fluxes or overpressures) – Distances influenced by facility design parameters (e. g. , hydrogen pressure and volume), available safety features (e. g. , isolation valves), and release parameters (e. g. , leak size and location) • Options for evaluating: – Consequence-based (deterministic) • Worst case leak size (e. g. , equivalent to flow area) • More probable break size (e. g. , 20% flow area) – Risk-informed (based on acceptable risk level) Separation distances based solely on the consequences of hydrogen leaks can be long for high pressure systems! 3/11/2021 Preliminary Data
Sandia Hydrogen Leak Model • Used to evaluate separation distances for jet releases • Model predicts (as function of system volume, pressure, and leak size): – Radiant heat flux from hydrogen jet flames • Radial and axial positions downstream of the jet as a function of time – Visible flame length for ignited jets – Hydrogen concentrations in jets – Separation distances based on free jets • Model validated against Sandia/SRI experiments • Reference: Houf and Schefer, “Predicting Radiative Heat Fluxes and Flammability Envelopes from Unintended Releases of Hydrogen, ” IJHE Paper GI-353 3/11/2021 Preliminary Data
Example of Consequence-Based Separation Distances for a Jet Fire Leak Diameter (mm) 3/11/2021 Preliminary Data
Separation Distances for Different Consequence Measures – Jet Fires Consequence Parameter 3/11/2021 Preliminary Data
Separation Distances for Different Consequence Measures – Jet Fires 3/11/2021 Preliminary Data
Risk-Informed Approach • Uses risk insights from QRA plus other considerations to help define code requirements Risk = Frequency X Consequence from all accidents • Requires definition of important consequences • Requires definition of acceptable risk levels • Requires comprehensive evaluation of all possible accidents • Accounts for parameter and modeling uncertainty present in analysis 3/11/2021 Preliminary Data
Risk Approach for Establishing Separation Distances Cumulative frequency of accidents resulting in consequences that requires this separation distance 3/11/2021 Preliminary Data
Application to Example Facility • Evaluated risk-based separation distances for a representative fueling facility – To demonstrate risk methodology – To evaluate important facility features (e. g. , gas volume and leak isolation features) – To determine importance of modeling parameters (e. g. , data, geometry, temporal effects) – To identify key risk scenarios and identify possible ways to reduce the risk to acceptable levels • Work presented is focused on hydrogen jet releases from gas pipes and gas storage cylinders 3/11/2021 Preliminary Data
Example Facility Description • Facility can refuel 100 cars/day • All components located outside • Gas storage was sized for 500 kg of hydrogen(12. 63 m 3 for 70 MPa facility) • Analyzed gas storage area and distribution piping 3/11/2021 Preliminary Data
Analysis Assumptions • No protective barriers are installed around the outside gas storage area • No isolation of a gas storage leak from upstream of isolation valve is possible • Pipe leaks downstream of isolation valve can be isolated • State-of-the art hydrogen leak or flame detector sends signal to isolation valve resulting in closure within 10 s • Immediate ignition results in injury or damage at separation distance if not automatically detected and isolated (no credit for manual detection and isolation) • Delayed ignition of un-isolated gas jet results in flash fire – Injury or damage assumed out to a distance corresponding to ignitable concentration levels (used to determine separation distance) • Pipe leak orientation was assumed directed at lot line or structures/equipment resulting in maximum separation distances • Impact of surfaces on jet flames not included 3/11/2021 Preliminary Data
Failure Data Used in Analysis Hydrogen component leakage frequencies. Component Mean Component Leakage Frequency Small Leak Large Leak Rupture 1 E-4/yr 1 E-5/yr 1 E-6/yr 3 E-6/m-yr 3 E-7/m-yr 3 E-8/m-yr 0. 1/yr 1 E-2/yr 1 E-3/yr Pump 3 E-3/yr 3 E-4/yr 3 E-5/yr Compressor 3 E-2/yr 3 E-3/yr 3 E-4/yr Electrolyser 1 E-4/yr 1 E-5/yr 1 E-6/yr Vaporizer 1 E-3/yr 3 E-4/yr 5 E-5/yr Valve 1 E-3/yr 1 E-4/yr 1 E-5/yr Pipe Joints and Unions 3 E-2/yr 4 E-3/yr 5 E-4/yr Flange 3 E-4/yr 3 E-5/yr NA Filter 3 E-3/yr 3 E-4/yr 3 E-5/yr Instrument Line 1 E-3/yr 3 E-4/yr 5 E-5/yr Vessel Pipe Refueling Hose 3/11/2021 Preliminary Data
Leakage Frequency Distributions Large leak assumed = 1 mm diameter; leak diameter distributed as inverse function of diameter 3/11/2021 Preliminary Data
Gas Pipe Leak Event Tree 3/11/2021 Pipe Leak or Rupture Downstream of Immediate Ignition of Hydrogen Jet Detection of Hydrogen or Flame Automatic Isolation of Pipe within 10 s Delayed Ignition of Hydrogen PIPE_LEAK I-IGNITION DETECTION ISOLATION D-IGNITION pipe leak - (New Event Tree) Preliminary Data # END-STATE-NAMES 1 JET-FIRE-(10 -S) 2 JET-FIRE 3 JET-FIRE 4 GAS-RELEASE-(10 -S) 5 FLASH-FIRE 6 GAS-RELEASE 7 FLASH-FIRE 8 GAS-RELEASE 2007/01/27 Page 0
Gas Pipe Results: Un-isolated Jet Fires Mean frequency of any size un-isolated pipe leak < 1 E-6/yr Decreasing leak frequency is countered by increasing ignition probability 3/11/2021 Preliminary Data
Gas Pipe Results: Isolated Jet fire Frequency of isolated jet fires are higher but the exposure time is short (10 s) which reduces potential for structural or equipment damage and personnel injury. 3/11/2021 Preliminary Data
Gas Pipe Results: Isolated Jet fire 3/11/2021 Preliminary Data
Gas Pipe Results: Flash Fires Delayed ignition assumed to result in flash fire and injury/damage out to various hydrogen concentration levels. 3/11/2021 Preliminary Data
Gas Storage Leak Event Tree 3/11/2021 Preliminary Data
Gas Storage Results: Un-isolated Jet Fires Accident frequencies are affected by leakage contribution from different components. Heat Flux 3/11/2021 Preliminary Data
Gas Storage Results: Flash Fires Flash fires require longer separation distances than jet fires. Hydrogen Concentration 3/11/2021 Preliminary Data
Gas Storage Sensitivity Study – Volume of Stored Gas Limiting gas storage volume can lead to reduced separation distances. Mass of Gaseous Hydrogen 3/11/2021 Preliminary Data
Gas Storage Sensitivity Study – Storage Pressure 3/11/2021 Preliminary Data
Leak Orientation Sensitivity Leak orientation is important in determining separation distances for jet fires. Leak Orientation to Target 3/11/2021 Preliminary Data
Transient Effects Integrated dose for 60 s = 1070 (kw/m 2)1. 333 s. Probability of second degree burns = 0. 7, fatality = 0. 5. 3/11/2021 Preliminary Data
Example Parameter Uncertainties 3/11/2021 Preliminary Data
Use of Low Consequence Measures Can Lead To Wrong Separation Distances 3/11/2021 Preliminary Data
Separation Distance Results • Separation distances are significantly affected by facility operating parameters (H 2 pressure and volume) • Consequence-based separation distances can be prohibitively long for large leak diameters • If small leak diameters can be justified, short separation distances even for high pressures can be justified • Risk methods can be used as a basis to help justify selection of leak diameter and separation distances • Risk-informed separation distances are significantly affected by component leakage frequency data and selected consequence parameters and risk criteria • Installation of mitigation features can reduce the frequency and consequences from leakage events • Selection of low consequence parameters to set separation distance can result in unacceptable risk 3/11/2021 Preliminary Data
Backup Slides 3/11/2021 Preliminary Data
Current Separation Distances in ICC International Fire Code for H 2 Gas Outdoor Equipment or Feature Distance (ft) Lot line 10 Outdoor public assembly 25 Offsite sidewalks and onsite/offsite parked vehicles 15 Ignition sources 10 Noncombustible building walls (2 hour fire barrier) Combustible building walls 5 25 Above ground flammable liquid storage 50 Below ground flammable liquid storage 20 Above ground flammable gas storage 3/11/2021 Preliminary Data 25 or 50
Failure Data Used in Analysis 3/11/2021 Preliminary Data
Failure Data Used in Analysis 3/11/2021 Preliminary Data
Potential of Injury from Jet Fires Reduced time of exposure to heat flux reduces the radiation dose and the magnitude of injury. Average Thresholds: Third Degree Burn Second Degree Burn 25 k. W/m 2 First Degree Burn 4. 7 k. W/m 2 1. 6 k. W/m 2 3/11/2021 Preliminary Data
How Do You Select Leak Diameter? • Ideally, examine appropriate leak data to determine leak distribution – Select leak size that encompasses a designated percentage of leaks (e. g. , 50% or 95%) • Use precedents (e. g. , 20% flow area cited in several documents) • Base selection on available analyses (e. g. , offshore process leakage data) 3/11/2021 Preliminary Data
Leak Distribution Sensitivity 3/11/2021 Preliminary Data
Offshore Leakage Data 3/11/2021 Preliminary Data
Use of Risk Can Eliminate Large Leaks from Consideration Risk Criteria Increasing Leak Diameter 3/11/2021 Preliminary Data
Example Facility 3/11/2021 Preliminary Data
Consequence Parameters and Risk Criteria Used in Current Analysis Consequence Parameters – Radiant Heat Flux from Jet Fires: • 1. 6 k. W/m 2 – no harm to individuals for long exposures • 4. 7 k. W/m 2 – injury (second degree burns) within 20 seconds • 25 k. W/m 2 – 100% lethality within 1 minute; equipment and structural damage – Hydrogen Concentration from Un-ignited Releases: • 4%, 6%, and 8% concentrations – lower flammability limit • Risk Criteria – Frequency of Fatality to Individual at Separation Distance Used as Upper Bound Accident Frequency Criteria • <2 E-4/yr – fatality risk from all other high-risk hazards in society 3/11/2021 Preliminary Data
Example Separation Distances Due to Flash Fires Gas Storage Leaks- Flash Fires 1 Consequence-Based Separation Distance (m)2 Risk Criteria 35 MPa 70 MPa 105 MPa 2 E-4/yr 22. 7, 72. 3 30. 0, 96. 6 35. 0, 112. 7 26 32 36 5 E-5/yr - - - 30 44 49 1 E-5/yr - - - 59 76 87 5 E-6/yr - - - 72 82 92 1. 2. 3/11/2021 Maximum Risk-Informed Separation Distances (m) Separation distances for - 4% H 2 concentrations. Values are for 4. 2 mm and 13. 5 mm leaks, respectively. Preliminary Data
Summary • QRA can help ensure hydrogen facility safety directly – Identifies important accidents – Evaluates effectiveness of preventive and mitigation features – Used to establish risk management strategies • QRA can help establish hydrogen code and standard requirements – Compliance with minimum requirements ensures an accepted level of risk is achieved 3/11/2021 Preliminary Data
Future Efforts • Continue evaluating safety distances for example facility to further demonstrate methodology – Examine other gas-related accidents (e. g. , vapor cloud explosions) – Examine liquid hydrogen storage leaks/ruptures • Evaluate facilities using different methods for onsite hydrogen production (gas reforming and electrolysis) • Improve risk methodology including consideration of time-dependent impacts, geometry factors, and incorporation of uncertainty • Get consensus on failure data for use in QRA (e. g. , leak frequencies and component failure rates); Bayesian approaches to data analysis • Identify methods for determining accident phenomenology probabilities (e. g. , ignition and detection probabilities) • Identify key risk drivers for hydrogen facilities and identify what can be done to reduce the risk and separation distances to acceptable levels • Extend evaluation to other types of hydrogen facilities 3/11/2021 Preliminary Data
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