1. Relief System Design Guide
2015 Edition 1
EZ Relief Systems Consulting, Inc.
September 2015
2. Property of EZ Relief Systems Consulting, Inc.
Proprietary -- To Be Maintained in Confidence
1 BACKGROUND....................................................................................................1-1
1.1 Scope............................................................................................................................................1-1
1.2 Terms, Definitions, Acronyms, and Abbreviations .................................................................1-1
1.2.1 Terms and Definitions...............................................................................................................1-1
1.2.2 Acronyms and Abbreviations..................................................................................................1-10
1.2.3 Should versus Shall versus Must ...........................................................................................1-11
1.3 Overview of Pressure Relief System Design .........................................................................1-11
1.3.1 Overpressure Protection Philosophy......................................................................................1-11
1.3.2 Pressure Relief System Design Philosophy ...........................................................................1-12
1.3.3 Pressure Relief System Design Flow Chart ...........................................................................1-13
1.3.4 Minimum Qualifications for Relief System Design..................................................................1-16
1.3.5 Accountability..........................................................................................................................1-17
1.4 Documentation Requirements.................................................................................................1-17
1.4.1 General ...................................................................................................................................1-17
1.4.2 Facility and Process Documentation ......................................................................................1-18
1.4.3 Overpressure Scenario Documentation .................................................................................1-18
1.4.4 Relief Requirements Documentation......................................................................................1-19
1.4.5 Pressure Relief Device (PRD) Documentation.......................................................................1-19
1.4.6 PRD Installation Documentation.............................................................................................1-20
1.4.7 Flare System Documentation .................................................................................................1-20
1.4.8 Vendor Supplied Packaged Equipment..................................................................................1-21
1.5 Management of Change (MOC)................................................................................................1-21
1.6 Industry Loss Experience ........................................................................................................1-22
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CODES AND STANDARDS ........................................................................................................ 2‐1
2.1 General .................................................................................................................................................. 2‐1
2.2 Occupational Safety & Health Administration (OSHA) ............................................................................. 2‐1
2.2.1 OSHA 29 CFR1910.119 ............................................................................................................................ 2‐1
2.2.2 OSHA 29 CFR1910.106 ............................................................................................................................ 2‐2
2.2.3 OSHA CPL 03‐00‐010 ............................................................................................................................... 2‐2
2.3 ASME Boiler and Pressure Vessel Codes .................................................................................................. 2‐3
2.3.1 General ................................................................................................................................................... 2‐3
2.3.2 ASME Section I Rules for Construction of Power Boilers ........................................................................ 2‐3
2.3.2.1 General .......................................................................................................................................... 2‐3
2‐2.3.2 Number of PRVs ............................................................................................................................ 2‐5
2.3.2.2 Overpressure Scenarios/Relief Load Basis .................................................................................... 2‐5
2.3.2.3 PRV Opening (Set) Pressure/Maximum Allowable Pressure ......................................................... 2‐7
2.3.3 ASME Section II ‐ Materials‐Part D‐Properties ....................................................................................... 2‐8
2.3.4 ASME Section IV Rules for Construction of Heating Boilers ................................................................... 2‐8
2.3.5 ASME Section VIII Rules for Construction of Pressure Vessels ............................................................. 2‐10
2.3.5.1 General ........................................................................................................................................ 2‐10
2.3.5.2 Scope of ASME Section VIII ......................................................................................................... 2‐11
2.3.5.3 Need for a Pressure Relief Device (PRD) ..................................................................................... 2‐13
2.3.5.3.1 General ................................................................................................................................... 2‐13
2.3.5.3.2 Overpressure Protection by System Design (UG‐140) ........................................................... 2‐13
2.3.5.4 Pressure Relief Devices Protecting Multiple Vessels .................................................................. 2‐15
2.3.5.4.1 General ................................................................................................................................... 2‐15
2.3.5.4.2 Intervening Block Valves ‐ Historical Requirements ............................................................... 2‐16
2.3.5.4.3 Intervening Block Valves ‐ Current Requirements ................................................................. 2‐17
2.3.5.4.4 M‐5.5: Block Valves in Systems Where the Pressure Originates Exclusively from an Outside
Source ................................................................................................................................................ 2‐19
2.3.5.4.5 M‐5.6: PRD Maintenance Block Valves .................................................................................. 2‐20
2.3.5.4.6 M‐5.7: Block Valves in Relief Path Where There is Normally Process Flow ........................... 2‐20
2.3.5.4.7 M‐5.8: Block Valves in Relief Path Where There is Normally Process Flow with Fire the Only
Source of Pressure ..................................................................................................................................... 2‐21
2.3.5.4.8 Example Equipment Configurations ....................................................................................... 2‐21
2.3.5.5 PRD Installation Requirements ................................................................................................... 2‐24
2.3.5.6 ASME Section VIII Code Interpretations and Code Cases ........................................................... 2‐25
2.3.5.7 ASME Section VIII Design Margin/Safety Factor ......................................................................... 2‐26
2.4 ASME Piping Codes ............................................................................................................................... 2‐26
2.4.1 General ................................................................................................................................................. 2‐26
2.4.2 ASME B31.1 Power Piping .................................................................................................................... 2‐26
2.4.3 ASME B31.3 Process Piping .................................................................................................................. 2‐28
2.4.3.1 Scope ........................................................................................................................................... 2‐28
2.4.3.2 Need for a PRD ............................................................................................................................ 2‐28
2.4.3.3 PRD Requirements ...................................................................................................................... 2‐29
2.4.3.4 PRD Installation and Other Considerations ................................................................................. 2‐30
2.4.4 ASME B31.4 ‐ Pipeline Transportation Systems for Liquid Hydrocarbons and Other Liquids .............. 2‐31
2.4.5 ASME B31.8 ‐ Gas Transmission and Distribution Piping Systems ....................................................... 2‐31
2.4.5.1 Scope ........................................................................................................................................... 2‐31
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2.4.5.2 Pressure‐Limiting Requirements in Compressor Stations ........................................................... 2‐32
2.4.5.3 Pressure Relief and Pressure‐Limiting Installations Design Requirements ................................. 2‐32
2.5 National Fire Protection Agency (NFPA) ................................................................................................. 2‐34
2.5.1 General ................................................................................................................................................. 2‐34
2.5.2 NFPA 30, Flammable and Combustible Liquids Code ........................................................................... 2‐35
2.5.2.1 General ........................................................................................................................................ 2‐35
2.5.2.2 Atmospheric Storage Tanks ........................................................................................................ 2‐35
2.5.2.3 Low Pressure Storage Tanks ....................................................................................................... 2‐36
2.5.3 NFPA 58, Liquefied Petroleum Gas Code .............................................................................................. 2‐36
2.5.4 NFPA 68, Standard on Explosion Protection by Deflagration Venting .................................................. 2‐37
2.5.5 NFPA 69, Standard on Explosion Prevention Systems .......................................................................... 2‐37
2.6 American Petroleum Institute (API) ....................................................................................................... 2‐38
2.6.1 General ................................................................................................................................................. 2‐38
2.6.2 API Standard 520 Part I ......................................................................................................................... 2‐39
2.6.3 API Standard 520 Part II ........................................................................................................................ 2‐40
2.6.4 API Standard 521 .................................................................................................................................. 2‐40
2.6.5 API Standard 526 .................................................................................................................................. 2‐41
2.6.6 API Standard 527 .................................................................................................................................. 2‐41
2.6.7 API Standard 537 .................................................................................................................................. 2‐41
2.6.8 API Standard 576 .................................................................................................................................. 2‐42
2.6.9 API Standard 2000 ................................................................................................................................ 2‐42
2.7 ISO (International Organization for Standardization) Standards ............................................................. 2‐42
2.8 State/Local Codes ................................................................................................................................. 2‐44
2.9 Owner/Operator Company Standards ................................................................................................... 2‐44
2.10 Design Institute for Emergency Relief Systems (DIERS) ...................................................................... 2‐44
2.11 Other Resources ............................................................................................................................... 2‐45
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PRESSURE SETTINGS AND TEMPERATURE EFFECTS OF RELIEF DEVICES............ 3‐1
3.1 General .................................................................................................................................................. 3‐1
3.2 Pressure Definitions ............................................................................................................................... 3‐2
3.3 PRD Set Pressure (Opening Pressure) ..................................................................................................... 3‐3
3.4 Relieving Pressure .................................................................................................................................. 3‐4
3.5 Operating Margins for PRVs .................................................................................................................... 3‐5
3.6 Rupture Disk Pressures........................................................................................................................... 3‐7
3.7 Buckling Pin Devices .............................................................................................................................. 3‐11
3.8 Temperature Effects on PRD Opening Pressure ...................................................................................... 3‐11
3.8.1 General ................................................................................................................................................. 3‐11
3.8.2 PRV Set Pressure Temperature Effects ................................................................................................. 3‐11
3.8.3 Rupture Disk Burst Pressure Temperature Effects ............................................................................... 3‐15
3.9 Design Pressure versus MAWP .............................................................................................................. 3‐18
3.10 Corrected Hydrotest Pressure Examples ............................................................................................ 3‐18
3.10.1 Application to Overpressure Scenarios ............................................................................................ 3‐18
3.10.2 Overpressure Scenario Temperature Equals Design Temperature ................................................. 3‐18
3.10.3 Correction for Other Temperatures ................................................................................................. 3‐20
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SCENARIO SELECTION ............................................................................................................. 4‐1
4.1 Background ............................................................................................................................................ 4‐1
4.2 Scenario Selection Guidelines ................................................................................................................. 4‐1
4.2.1 General ................................................................................................................................................... 4‐1
4.2.2 Scenario Review Team/Individual .......................................................................................................... 4‐3
4.2.3 Integration with HAZOP .......................................................................................................................... 4‐4
4.2.4 Information Needs .................................................................................................................................. 4‐5
4.2.5 Scenario Development Process .............................................................................................................. 4‐5
4.2.5.1 Step 1) Define the equipment to be protected ............................................................................. 4‐5
4.2.5.2 Step 2) Identify sources of energy ................................................................................................ 4‐6
4.2.5.3 Step 3) Determine how the energy translates into a pressure deviation ..................................... 4‐6
4.2.5.4 Step 4) Document the team's conclusions .................................................................................... 4‐7
4.2.6 Scenario Characterization Guidance....................................................................................................... 4‐9
4.3 Allowable/Non‐Allowable Credits ......................................................................................................... 4‐10
4.3.1 General ................................................................................................................................................. 4‐10
4.3.2 Credit for Instrumentation ................................................................................................................... 4‐10
4.3.3 Credit for Procedures (Administrative Controls) .................................................................................. 4‐11
4.3.4 Credit for Alternate Relief Paths ........................................................................................................... 4‐12
4.3.5 Credit for Paths Containing Car‐Sealed /Chain‐Locked Open Valves ................................................... 4‐13
4.3.6 Non‐Allowable Credits .......................................................................................................................... 4‐15
4.3.7 Scenarios Where Relief Devices Do Not Generally Protect .................................................................. 4‐17
4.3.8 Is a PRD Required? ................................................................................................................................ 4‐18
4.4 Common Mode Failures / Multiple Contingencies .................................................................................. 4‐19
4.4.1 Common Mode (Common Cause) Failures ........................................................................................... 4‐19
4.4.2 Multiple Contingencies/Likelihood Considerations .............................................................................. 4‐20
4.5 Analysis of 20 Standard Scenarios ......................................................................................................... 4‐21
4.5.1 General ................................................................................................................................................. 4‐21
4.5.2 Scenario 1 ‐ Closed Outlet (Blocked Outlet) ......................................................................................... 4‐22
4.5.3 Scenario 2/3/4 – Loss of Cooling and/or Loss of Refluxes .................................................................... 4‐24
4.5.4 Scenario 5 – Accidental Mixing ............................................................................................................. 4‐26
4.5.4.1 General ........................................................................................................................................ 4‐26
4.5.4.2 Hot Oil/Volatile Fluid (e.g., Water) Accidental Mixing ................................................................ 4‐26
4.5.4.3 Accidental Mixing of Fluids with Volatility Differences ............................................................... 4‐27
4.5.4.4 Truck Delivery Accidental Mixing ................................................................................................ 4‐29
4.5.5 Scenario 6 ‐ Overfilling .......................................................................................................................... 4‐29
4.5.6 Scenario 7 ‐ Accumulation of Non‐Condensibles ................................................................................. 4‐37
4.5.7 Scenario 8 ‐ Failure of Automatic Controls ........................................................................................... 4‐37
4.5.7.1 General ........................................................................................................................................ 4‐37
4.5.7.2 Incidents ...................................................................................................................................... 4‐39
4.5.8 Scenario 9 – Abnormal Heat or Vapor Input ........................................................................................ 4‐42
4.5.8.1 Scenario 9a ‐ Abnormal Heat Input ............................................................................................. 4‐42
4.5.8.2 Scenario 9b ‐ Abnormal Vapor Input........................................................................................... 4‐42
4.5.8.2.1 General ................................................................................................................................... 4‐42
4.5.8.2.2 Bypass Valves ......................................................................................................................... 4‐42
4.5.8.2.3 Check Valves ........................................................................................................................... 4‐45
4.5.8.2.4 Choke Valve Failure ................................................................................................................ 4‐47
4.5.9 Scenario 10 ‐ Internal Explosions or Transient Pressure Surges ........................................................... 4‐47
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4.5.9.1 Internal Explosions ...................................................................................................................... 4‐47
4.5.9.2 Transient Pressure Surges ........................................................................................................... 4‐48
4.5.10 Scenario 11 ‐ Chemical Reactions .................................................................................................... 4‐49
4.5.10.1 General ........................................................................................................................................ 4‐49
4.5.10.2 Incidents ...................................................................................................................................... 4‐49
4.5.10.3 Scenario Guidance ...................................................................................................................... 4‐51
4.5.10.4 Emergency Vent Sizing for Runaway Reactions .......................................................................... 4‐54
4.5.11 Scenario 12/13 – Thermal Expansion / Blocked in Cold‐Side of Exchanger ..................................... 4‐54
4.5.11.1 Background ................................................................................................................................. 4‐54
4.5.11.2 Liquid Thermal Expansion in Vessels ........................................................................................... 4‐56
4.5.11.3 Liquid Thermal Expansion in Heat Exchangers ............................................................................ 4‐56
4.5.11.4 Liquid Thermal Expansion in Piping and Other Equipment ......................................................... 4‐57
4.5.11.5 Other Criteria/Considerations for Determining Thermal Relief Needs ....................................... 4‐57
4.5.12 Scenario 14 ‐ Fire ............................................................................................................................. 4‐71
4.5.13 Scenario 15 – Heat Transfer Equipment Failure .............................................................................. 4‐77
4.5.13.1 General ........................................................................................................................................ 4‐77
4.5.13.2 Shell‐and‐tube Heat Exchangers ................................................................................................. 4‐77
4.5.13.3 Double‐pipe Exchangers ............................................................................................................. 4‐79
4.5.13.4 Plate‐and‐frame Exchangers ....................................................................................................... 4‐80
4.5.13.5 Vessels Equipped with External Heating or Cooling Jackets ....................................................... 4‐80
4.5.14 Scenario 16 ‐ Utility Failure .............................................................................................................. 4‐80
4.5.14.1 General ........................................................................................................................................ 4‐80
4.5.14.2 Allowable and Non‐Allowable Credits ........................................................................................ 4‐82
4.5.15 Scenario 17 ‐ Closed Outlet on Fired Heater ................................................................................... 4‐83
4.5.16 Scenario 18 ‐ Loss of Heat ................................................................................................................ 4‐85
4.5.17 Scenario 19 – Loss of Absorbent ...................................................................................................... 4‐86
4.5.18 Scenario 20 ‐ Abnormal Flow through a Vent Valve ........................................................................ 4‐86
4.6 Maintenance Scenarios ......................................................................................................................... 4‐87
4.7 Overpressure Scenarios for Piping Systems ............................................................................................ 4‐90
4.8 Vacuum Scenarios ................................................................................................................................. 4‐90
4.8.1 General ................................................................................................................................................. 4‐90
4.8.2 Vacuum Causes ..................................................................................................................................... 4‐92
4.8.3 Vacuum Prevention/Protection ............................................................................................................ 4‐92
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RELIEF REQUIREMENTS ......................................................................................................... 5‐1
5.1 Introduction ........................................................................................................................................... 5‐1
5.1.1 Background ............................................................................................................................................. 5‐1
5.1.2 Required Relief Rate ............................................................................................................................... 5‐1
5.1.3 Relieving Conditions ............................................................................................................................... 5‐2
5.1.4 Relief Capacity Credits ............................................................................................................................ 5‐2
5.1.5 Multiphase Vent Flow Determination .................................................................................................... 5‐6
5.1.5.1 Background to Multiphase Flow ................................................................................................... 5‐6
5.1.5.2 Multiphase Flow Potential ............................................................................................................ 5‐6
5.1.5.3 Multiphase Flow Models ............................................................................................................... 5‐7
5.1.5.4 Homogeneous Equilibrium Model (HEM) ..................................................................................... 5‐9
5.1.5.5 DIERS Vessel Disengagement ...................................................................................................... 5‐10
5.2 Information Needs ................................................................................................................................ 5‐12
5.3 Determining Relief Requirements .......................................................................................................... 5‐13
5.4 Relief Requirements Guidance for the 20 Scenarios ............................................................................... 5‐17
5.4.1 Scenario 1 ‐ CLOSED OUTLETS .............................................................................................................. 5‐17
5.4.1.1 General ........................................................................................................................................ 5‐17
5.4.1.2 High Pressure Upstream Inventory ............................................................................................. 5‐17
5.4.1.3 Pump or Compressor as the Source of Pressure ......................................................................... 5‐18
5.4.1.4 Additional Energy Sources .......................................................................................................... 5‐22
5.4.1.5 Multiple Sources ......................................................................................................................... 5‐22
5.4.2 Scenarios 2/3/4 ‐ LOSS OF COOLING/REFLUX FAILURES ...................................................................... 5‐22
5.4.3 Scenario 5 ‐ ACCIDENTAL MIXING ........................................................................................................ 5‐24
5.4.3.1 Mixing a Hot Liquid with a Cold Volatile Liquid .......................................................................... 5‐24
5.4.3.2 Mixing Reactive Fluids ................................................................................................................. 5‐24
5.4.3.3 Volatility Differences ................................................................................................................... 5‐24
5.4.4 Scenario 6 ‐ OVERFILLING STORAGE OR SURGE VESSELS ..................................................................... 5‐24
5.4.5 Scenario 7 ‐ ACCUMULATION OF NON‐CONDENSIBLES ....................................................................... 5‐25
5.4.6 Scenario 8 ‐ FAILURE OF AUTOMATIC CONTROLS ................................................................................ 5‐25
5.4.6.1 General Guidance........................................................................................................................ 5‐25
5.4.6.2 Example ....................................................................................................................................... 5‐26
5.4.6.2.1 Scenario Definition ................................................................................................................. 5‐26
5.4.6.2.2 Example Part 1 ....................................................................................................................... 5‐27
5.4.6.2.3 Example Part 2 ....................................................................................................................... 5‐28
5.4.7 Scenario 9. ABNORMAL HEAT OR VAPOR INPUT .................................................................................. 5‐29
5.4.7.1 Scenario 9A. ABNORMAL HEAT INPUT ........................................................................................ 5‐29
5.4.7.1.1 General ................................................................................................................................... 5‐29
5.4.7.1.2 Venting of Storage Tanks ....................................................................................................... 5‐30
5.4.7.1.2.1 Normal Venting of Storage Tanks Background ............................................................... 5‐30
5.4.7.1.2.2 Historical Method to Determine Storage Tanks Normal Venting Rates ......................... 5‐31
5.4.7.1.2.3 New Method to Determine Storage Tanks Inbreathing and Out‐breathing Rates ......... 5‐37
5.4.7.2 Scenario 9B ‐ ABNORMAL VAPOR INPUT .................................................................................... 5‐40
5.4.7.2.1 Block Valves or Bypass Valves ................................................................................................ 5‐40
5.4.7.2.2 Check Valves ........................................................................................................................... 5‐42
5.4.8 Scenario 10 ‐ INTERNAL EXPLOSIONS OR OTHER PRESSURE SURGES .................................................. 5‐43
5.4.8.1 Scenario 10A ‐ Internal Explosions .............................................................................................. 5‐43
5.4.8.2 Scenario 10B ‐ Pressure Surges ................................................................................................... 5‐43
5.4.9 Scenario 11 ‐ CHEMICAL REACTIONS .................................................................................................... 5‐45
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5.4.10 Scenario 12/13 ‐ THERMAL EXPANSION / COLD SIDE OF EXCHANGER BLOCKED IN WHILE HOT FLUID
CONTINUES ......................................................................................................................................................... 5‐45
5.4.11 Scenario 14 ‐ EXTERNAL FIRE ........................................................................................................... 5‐47
5.4.11.1 Background ................................................................................................................................. 5‐47
5.4.11.2 API Empirical Method for Pressure Vessels and Equipment (Wetted Surfaces with Liquid Boiling) ..
.................................................................................................................................................... 5‐48
5.4.11.2.1 General ................................................................................................................................. 5‐48
5.4.11.2.2 Wetted Area .......................................................................................................................... 5‐49
5.4.11.2.3 Open Pool Fire Heat Input into Wetted Surfaces ................................................................. 5‐50
5.4.11.2.4 Fire Relief Loads .................................................................................................................... 5‐53
5.4.11.3 API Method for Pressure Vessels and Equipment (Unwetted Surfaces with Vapor Expansion) . 5‐57
5.4.11.4 Fire Heat Input Correlations for Storage Tanks ........................................................................... 5‐57
5.4.11.5 Equipment Specific Considerations ............................................................................................. 5‐61
5.4.11.5.1 Piping .................................................................................................................................... 5‐61
5.4.11.5.2 Double‐Pipe Heat Exchangers ............................................................................................... 5‐61
5.4.11.5.3 Air Cooled Heat Exchangers .................................................................................................. 5‐61
5.4.11.5.4 Shell and Tube Heat Exchangers: .......................................................................................... 5‐63
5.4.11.5.5 Small Equipment ................................................................................................................... 5‐63
5.4.11.6 Effect of Fire on the Unwetted Surface of a Vessel .................................................................... 5‐63
5.4.11.7 Other Fire Relief Requirement Considerations ........................................................................... 5‐69
5.4.11.7.1 Heat‐Up Time ........................................................................................................................ 5‐69
5.4.11.7.2 Continued Flow during a Fire ................................................................................................ 5‐69
5.4.11.7.3 Light‐End Pool Fires .............................................................................................................. 5‐70
5.4.11.7.4 Liquid Swelling ...................................................................................................................... 5‐70
5.4.11.7.5 Fire Zone ............................................................................................................................... 5‐70
5.4.11.7.6 Jet Fires ................................................................................................................................. 5‐70
5.4.12 Scenario 15 ‐ SPLIT EXCHANGER TUBE ............................................................................................ 5‐71
5.4.12.1 Scenario Definition ...................................................................................................................... 5‐71
5.4.12.2 Tube Break Location/Characteristics .......................................................................................... 5‐72
5.4.12.3 Modeling Approach ..................................................................................................................... 5‐73
5.4.12.4 Mixing effects .............................................................................................................................. 5‐74
5.4.12.5 Calculating Flow through the Tube Break ................................................................................... 5‐76
5.4.12.5.1 General ................................................................................................................................. 5‐76
5.4.12.5.2 Frictional Velocity Head Loss for the Tubesheet................................................................... 5‐77
5.4.12.5.3 Friction Velocity Head Loss for the Tube .............................................................................. 5‐79
5.4.12.5.4 Equivalent Length of Tubesheets .......................................................................................... 5‐79
5.4.12.5.5 Equivalent Length of Tubes ................................................................................................... 5‐81
5.4.12.5.6 Phase of Flow through the Tube Break ................................................................................. 5‐82
5.4.12.5.7 Mass Flux per Unit Area (Non‐Flashing Liquids) ................................................................... 5‐82
5.4.12.5.8 Mass Flux per Unit Area (All‐Vapor or All‐Gas) ..................................................................... 5‐83
5.4.12.5.9 Mass Flux Per Unit Area (Multiphase) .................................................................................. 5‐84
5.4.12.5.10 Total Mass Flux Through the Tube Break............................................................................ 5‐86
5.4.12.5.11 Calculating the Flow and Properties of the Relief Stream .................................................. 5‐87
5.4.12.6 Alternate Relief Paths ................................................................................................................. 5‐88
5.4.12.7 Shock Effects ............................................................................................................................... 5‐89
5.4.12.8 Risk Reduction Methods ............................................................................................................. 5‐89
5.4.13 Scenario 16 ‐ UTILITY FAILURE ......................................................................................................... 5‐90
5.4.14 Scenario 17 ‐ CLOSED OUTLET ON FIRED HEATER ........................................................................... 5‐90
5.4.15 Scenario 18 ‐ LOSS OF HEAT............................................................................................................. 5‐91
5.4.16 Scenario 19 ‐ LOSS OF ABSORBENT.................................................................................................. 5‐91
5.4.17 Scenario 20 ‐ ABNORMAL FLOW THROUGH VENT VALVE ............................................................... 5‐91
5.4.18 Vacuum Relief .................................................................................................................................. 5‐91
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5.5 Depressuring Systems ........................................................................................................................... 5‐91
5.5.1 Background ........................................................................................................................................... 5‐91
5.5.2 Acceptance and Design Criteria ............................................................................................................ 5‐92
5.5.3 Depressuring Rate for Vessel Fire Exposure ......................................................................................... 5‐93
5.5.4 Low Temperatures during Depressuring .............................................................................................. 5‐96
5.5.5 Other Depressuring System Design Considerations ............................................................................. 5‐96
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SIZING PRESSURE RELIEF VALVES ...................................................................................... 7‐1
7.1 Introduction ........................................................................................................................................... 7‐1
7.2 Origin of PRV (Orifice) Sizing Formulas ................................................................................................... 7‐1
7.3 API Versus ASME Orifice Areas and Discharge Coefficients ...................................................................... 7‐2
7.4 Use of PRV Sizing Equations for Sizing Stand‐Alone Rupture Disks .......................................................... 7‐5
7.5 Single Phase PRV Sizing Equations .......................................................................................................... 7‐6
7.5.1 Gas/Vapor PRV Sizing Equations ............................................................................................................ 7‐6
7.5.1.1 Compressible Flow ........................................................................................................................ 7‐6
7.5.1.2 Origin of the Sizing Equations ....................................................................................................... 7‐7
7.5.1.3 Determination Whether Flow is Critical or Sub‐Critical ................................................................ 7‐8
7.5.1.4 Gas/Vapor Sizing Equations for Critical Flow ................................................................................ 7‐9
7.5.1.5 Gas/Vapor Sizing Equations for Sub‐Critical Flow ....................................................................... 7‐13
7.5.1.6 Modification for Real Gases (i.e., Z < 0.8) ................................................................................... 7‐14
7.5.2 Non‐Flashing Incompressible Liquids Sizing Equations ........................................................................ 7‐15
7.5.3 Vapor versus Liquid Trim ...................................................................................................................... 7‐19
7.6 Multiphase / Two‐Phase Flow PRV Sizing Equations .............................................................................. 7‐19
7.6.1 Background ........................................................................................................................................... 7‐19
7.6.2 Omega Correlating Parameter .............................................................................................................. 7‐19
7.6.3 Two‐Point Method to Calculate ω ........................................................................................................ 7‐21
7.6.4 Single Point Method to Calculate ω ..................................................................................................... 7‐23
7.6.5 Multiphase Critical Flow Equation ........................................................................................................ 7‐26
7.6.6 Multiphase Sub‐Critical Flow Equation ................................................................................................. 7‐28
7.6.7 Subcooled Liquids ................................................................................................................................. 7‐28
7.7 Relieving Fluids above Thermodynamic Critical Conditions .................................................................... 7‐31
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PIPING DESIGN AND CONSIDERATIONS ............................................................................ 8‐1
8.1 Background ............................................................................................................................................ 8‐1
8.2 Pressure Drop Equations / Design Requirements .................................................................................... 8‐1
8.2.1 Basis ........................................................................................................................................................ 8‐1
8.2.2 Rated Capacity versus Required Flow Rate ............................................................................................ 8‐2
8.2.3 Problematic PRVs and PRV Sizes ............................................................................................................ 8‐3
8.2.4 Pressure Relief Valve Inlet Line Sizing .................................................................................................... 8‐3
8.2.4.1 Recoverable versus Non‐recoverable Pressure Loss ..................................................................... 8‐3
8.2.4.2 Current Inlet Pressure Loss ASME Code/API Requirements ......................................................... 8‐5
8.2.4.3 Historical Source/Basis of the 3% Criteria ..................................................................................... 8‐5
8.2.4.4 Consequences of Exceeding Inlet Pressure Loss Criteria .............................................................. 8‐7
8.2.4.5 Inlet Pressure Loss Criteria for Thermal Relief Valves................................................................... 8‐8
8.2.4.6 Inlet Pressure Loss Equation for All Fluid Phases .......................................................................... 8‐8
8.2.4.7 Adjusting PRV Capacity Based on Inlet Pressure Loss ................................................................. 8‐10
8.2.4.8 Options for High Inlet Line Pressure Loss.................................................................................... 8‐11
8.2.4.9 Engineering Analysis ................................................................................................................... 8‐13
8.2.4.9.1 Background/Caveats .............................................................................................................. 8‐14
8.2.4.9.2 Engineering Analysis Topics ................................................................................................... 8‐14
8.2.4.9.3 Force Balance Equations ........................................................................................................ 8‐15
8.2.4.9.4 Force Balance Examples ......................................................................................................... 8‐18
8.2.4.9.5 Acoustic Interaction ............................................................................................................... 8‐19
8.2.5 Outlet Vent Pipe Sizing ......................................................................................................................... 8‐21
8.2.5.1 Current Outlet Pressure Loss ASME Code/API Requirements .................................................... 8‐21
8.2.5.2 Backpressure ............................................................................................................................... 8‐21
8.2.5.2.1 Background ............................................................................................................................ 8‐21
8.2.5.2.2 Backpressure Effects on Set Pressure .................................................................................... 8‐22
8.2.5.2.3 Backpressure Effects on PRV Capacity ................................................................................... 8‐23
8.2.5.2.4 Other Backpressure Considerations ....................................................................................... 8‐25
8.2.5.2.5 Conventional versus Balanced Bellows PRVs ......................................................................... 8‐25
8.2.5.3 Piping Pressure Drop ‐ Incompressible Liquid Flow .................................................................... 8‐26
8.2.5.4 Piping Pressure Drop ‐ Isothermal Gas/Vapor Flow .................................................................... 8‐27
8.2.5.5 Multiphase Flow (General Compressible Flow Solution) ............................................................ 8‐30
8.3 Stand‐Alone Rupture Disk Relief Systems .............................................................................................. 8‐32
8.4 Piping Design and Layout Considerations ............................................................................................... 8‐33
8.4.1 Code and Standard Requirements ........................................................................................................ 8‐33
8.4.2 Isolation Valves in Relief Systems ......................................................................................................... 8‐35
8.4.2.1 Intent of Block Valves in Relief Systems...................................................................................... 8‐35
8.4.2.2 Code/Standard Requirements .................................................................................................... 8‐35
8.4.2.3 Applications of Block Valves in Relief Systems............................................................................ 8‐36
8.4.2.4 Design Criteria for Block Valves in Relief Systems ...................................................................... 8‐37
8.4.3 PRD Inlet/Outlet Piping Drainage ......................................................................................................... 8‐38
8.4.4 Bellows PRV Bonnet Venting ................................................................................................................ 8‐44
8.4.5 Reaction Forces / PRV Support ............................................................................................................. 8‐48
8.4.6 PRV Location in Proximity to Devices Causing Unstable Flow Patterns ............................................... 8‐51
8.4.7 Brittle Fracture Potential ...................................................................................................................... 8‐52
8.4.8 Acoustic‐Induced Fatigue ..................................................................................................................... 8‐54
8.4.8.1 Background ................................................................................................................................. 8‐54
8.4.8.2 Screening Method Using the Sound Level .................................................................................. 8‐55
14. Property of EZ Relief Systems Consulting, Inc.
Proprietary -- To Be Maintained in ConfidencePage 8-2
8.4.8.3 Mitigation Options ...................................................................................................................... 8‐57
8.4.8.4 Effect of Piping Thickness ............................................................................................................ 8‐58
8.4.8.5 Turbulence‐Induced Fatigue ....................................................................................................... 8‐58
8.5 Discharge to Atmosphere ...................................................................................................................... 8‐60
8.5.1 General Requirements .......................................................................................................................... 8‐60
8.5.2 Atmospheric Discharge of Non‐Hazardous Materials .......................................................................... 8‐61
8.5.3 Flammable and Toxic Discharge from Atmospheric Vents ................................................................... 8‐61
8.5.4 Dispersion and Consequence Modeling ............................................................................................... 8‐62
