Chapter 13: Safety relief systems

13.0Introduction8
13.1Requirement of Pressure Relief Valves8
13.1.1Vessels8
13.1.2Heat Exchangers9
13.1.3Fired Heaters9
13.1.4Steam Boiler Systems9
13.1.5Piping9
13.1.6Rotating and Mechanical Equipment9
13.1.7Temperature Relief Valves10
13.2Requirement of Vacuum Relief Protection10
13.2.1Vacuum Relief Requirement10
13.2.2Vacuum Breaker/Preventer Systems11
13.2.3Vacuum Relief Valves11
13.3Technical Requirements of Pressure Relief Valves11
13.3.1Set Pressure11
13.3.1.1Margin above Maximum Operating Pressure11
13.3.1.2Pressure Relief Valve Set Pressure12
13.3.1.3Temperature Relief Valve Set Pressure12
13.3.1.4Vacuum Relief Valve Set Pressure12
13.3.1.5Spring Setting (Cold Differential Test Pressure)12
13.3.2Accumulation12
13.3.3Effect of Back Pressure12
13.3.3.1Effect on Opening Pressure12
13.3.3.2Effect on Relieving Capacity15
13.4PSV Types17
13.4.1Non-ASME Devices18
13.4.2Codes and Standards18
13.4.3Testing and Certification19
13.4.4Conventional Pressure Relief Valves20
13.4.4.1Operating Characteristics22
13.4.4.2Applications26
13.4.4.3Design Considerations26
13.4.4.4Operating Pressure27
13.4.4.5Superimposed Back Pressure27
13.4.4.6Inlet Loss27
13.4.4.7Back Pressure28
13.4.5Balanced Bellows Pressure Relief Valves29
13.4.5.1Operating Characteristics29
13.4.5.2Applications30
13.4.5.3Design Considerations30
13.4.6Pilot Operated Pressure Relief Valves31
13.4.6.1Pilot Operating Description33
13.4.6.2Pop and Modulating Action Pilots33
13.4.6.3Flowing and Non-flowing Pilots34
13.4.6.4Restricted Lift34
13.4.6.5Applications34
13.5Rupture Disks37
13.5.1Operating Characteristics37
13.5.1.1Bursting Pressure38
13.5.1.2Operating ratio39
13.5.2Rupture Disc disadvantages41
13.5.3Applications42
13.5.3.1Rupture Disk on Inlet to Pressure Relief Valves42
13.5.3.2Rupture Disk on Discharge from Pressure Relief Valve42
13.5.3.3Rupture Disk in Parallel with Pressure Relief Valve43
13.5.4Types of Rupture Disks43
13.5.4.1Conventional Tension Loaded Disks43
13.5.4.2Prescored Tension Loaded Disks44
13.5.4.3Composite Rupture Disks46
13.5.4.4Reverse Buckling Disks with Knives46
13.5.4.5Prescored Reverse Buckling Disks48
13.5.4.6Reverse Buckling Disk for Liquid Service50
13.5.4.7Graphite Disks51
13.5.5Design Considerations51
13.5.5.1Rupture Disc Special Features53
13.6Other Types of Pressure Relief Devices55
13.6.1Surface Condenser Pressure Relief Valves55
13.6.2Sentinel Valves56
13.7Design Philosophy for Determining Relief Load56
13.7.1Process Evaluation Basis56
13.7.1.1Material Balance Rates and Duties57
13.7.1.2Material Balance Rates and Duties plus Specified Margin(s)57
13.7.1.3Loads Based on Equipment or Process Limitations58
13.7.2Double Jeopardy58
13.7.3Utility Losses58
13.7.3.1Loss of Cooling Water59
13.7.3.2Loss of Electric Power59
13.7.3.3Loss of Steam61
13.7.3.4Loss of Fuel61
13.7.3.5Loss of Instrument Air61
13.7.3.6Loss of Instrument Power61
13.7.3.7Loss of Refrigeration62
13.7.3.8Loss of Inert Gas62
13.7.4Blocked Exits62
13.7.5Fire62
13.7.5.1Vertical Height Limit for Fire Heat Input63
13.7.5.2Wetted Surface Area Exposed to Fire Heat Input64
13.7.5.3Fire Circle64
13.7.5.4Fire Heat Input Causing Vaporization of Liquid64
13.7.5.5Fire Relief Rate from Vessels Containing Liquid65
13.7.5.6Fire Relief Rate from Vessels Containing Only Gas65
13.7.5.7Additional Fire Protection Considerations65
13.7.5.8Basic Assumptions for Fire Case Relief Analysis66
13.7.5.9Heat Flux Equations66
13.7.5.10Determination of Wetted Area67
13.7.5.11Insulation credit69
13.7.5.12Liquid Filled Systems70
13.7.5.13Protection of System with Individual Relief Valve70
13.7.5.14Maintenance Isolation71
13.7.5.15Determination of Latent Heat for Boiling Applications71
13.7.5.16High Boiling Point Fluids72
13.7.5.17Latent Heat of Hydrocarbon/Water Mixtures72
13.7.5.18Critical or Super-Critical Fluids72
13.7.5.19Relief Loads for Vessels Containing Vapor73
13.7.5.20Depressurizing74
13.7.5.21Sizing of Depressurizing system lines76
13.7.6Aerial Cooler Failure78
13.7.7Condensing Duty Failure78
13.7.8Reflux Failure78
13.7.9Accumulation of Non-Condensables78
13.7.10Abnormal Heat Input78
13.7.11Abnormal Vapour Input78
13.7.12Abnormal Chemical Reaction79
13.7.12.1Process Flow79
13.7.12.2Start-of-Run and End-of-Run Conditions79
13.7.12.3Reaction Process Characteristics79
13.7.12.4Alternate Operation Modes79
13.7.12.5Causes of Overpressure80
13.7.12.6Heat and Material Balance Considerations80
13.7.12.7Reactor Yields80
13.7.12.8Condensation Curves80
13.7.12.9Pressure Profiles81
13.7.12.10Pressure Relief and Depressurizing Facilities82
13.7.12.11Location of Pressure Relief Valves82
13.7.12.12Presence of Block Valves in the Loop83
13.7.12.13Equipment Shutdown83
13.7.12.14Hydraulic Expansion83
13.7.12.15Entrance of Volatile Liquid83
13.7.12.16Combination of Causes83
13.7.12.17Thermal Relief84
13.7.12Mechanical Equipment84
13.7.13.1Pumps84
13.7.13.2Compressors85
13.7.13.3Mechanical Driver Considerations85
13.7.14Unsteady state Conditions86
13.7.14.1Heat Exchange Tube Rupture87
13.7.14.2Double Pipe Exchangers90
13.7.14.3Fractionation/Distillation Tower Upsets90
13.7.14.4Depressurizing Impact90
13.7.14.5Block Valves, Check Valves and Control Valves90
13.7.14.6Heat Transfer Equipment Performance93
13.7.15HIPPS System94
13.7.16Effect of Instrumentation95
13.8Relief Load Calculation for Fractionating Tower PSV Sizing96
13.8.1System Description96
13.8.2Causes of Overpressure96
13.8.3H&MB Considerations for Upset Conditions97
13.8.3.1Basic Assumptions for Relief Case H&MB97
13.8.3.2Heat Balance for Upset Conditions97
13.8.4Maximum Capacity100
13.8.5Example of Relief load calculation for Fractionation column101
13.9Safety Relief Valve Sizing107
13.9.1Sizing for Gas or Vapour Relief- Critical Flow Pressure Ratio108
13.9.2PSV sizing for Gas or Vapor – Critical Flow109
13.9.3PSV sizing for Subcritical Gas or Vapor Flow112
13.9.4PSV sizing for Steam Flow114
13.9.5PSV sizing for Liquid Flow – Liquid Trim Relief Valves requiring capacity certification115
13.9.6PSV sizing for Liquid Flow – Conventional Pressure Relief Valves (Capacity certification not required)118
13.9.7PSV sizing as per Manufacturer’s Equations119
13.9.7.1Pilot Operated Pressure Relief Valves120
13.9.8PSV sizing for Two Phase Flow120
13.9.9Rupture Disc Sizing121
13.10Special Engineering Tips for PSV sizing121
13.11Specification of Safety/Relief Valves and Rupture Devices124
13.11.1Documentation124
13.11.2Specification124
13.11.3Depressurizing or Blowdown Valve Sizing125
13.11.4Specification of Depressurizing or Blowdown Valves125
13.11.5Dynamic Simulation126
13.12FLARE SYSTEM SIZING127
13.12.1OVERALL FLARE SYSTEM LOAD EVALUATION127
13.12.2Flare Load Determination for Each Significant Case128
13.12.3Effect of Instrumentation on Flare System Flare Loads128
13.12.4Effect of Auto-lockouts on Heat Input Duties130
13.12.5Fire Circle Flare Loads130
13.12.6Instrument Air Failure Flare Loads130
13.12.7Instrument Power Failure Flare Loads131
13.12.8Electric Power Failure Flare Loads131
13.12.9Steam Failure Flare Loads131
13.12.10Cooling Water Failure Flare Loads132
13.12.11Combination Cause Flare Loads132
13.12.12Flare Header Sizing and Design132
13.12.13Loads from Depressurizing Systems133
13.12.13.1Documentation of Flare Load Cases133
13.12.14Flare Load Minimization136
13.12.14.1System Design and Modifications137
13.12.14.2Percentage Reduction137
13.12.14.3Time Frame Analysis138
13.12.14.4Response of Control Instruments138
13.12.14.5Pump Driver Selection Philosophy139
13.12.14.6Auto Start Spares139
13.12.14.7Instrumentated Shutdown System140
13.12.14.8High Integrity Pressure Protective Instrumentation
(HIPPS) Systems
140
13.13Relief Material Recovery and Disposal141
13.13.1Disposal Options141
13.13.2Hazard and Risk Assessment142
13.13.3Environmental Factors142
13.13.4Vapor Release Criteria142
13.13.4.1Atmospheric Release Criteria143
13.13.5Liquid Release Criteria144
13.13.5.1Non-Hazardous Streams144
13.13.5.2Non-Hazardous Hydrocarbons144
13.13.5.3Hazardous Streams144
13.13.5.4Two Phase Releases144
13.13.6Prevention of Liquid Releases144
13.13.6.1Disposal into a Process145
13.13.6.2Closed Disposal Systems146
13.13.6.2.1Intermediate Collection Systems146
13.13.6.2.2Flare Systems146
13.13.6.2.3Vapor Recovery147
13.13.6.2.4Incinerators and Burn Pits147
13.13.6.2.5Liquid Handling Systems147
13.13.6.2.6Treating Systems148
13.13.7Design Considerations148
13.13.7.1Atmospheric Releases148
13.13.7.2Intermediate Collection Systems149
13.13.7.3Flare Systems149
13.13.7.4Vapor Recovery- Flare Gas Recovery Systems150
13.13.7.5Incinerators153
13.13.7.6Liquid Handling Systems153
13.13.7.7Treating Systems153
13.14Relief System Piping158
13.14.1Pressure Relief Valve Installation158
13.14.2Spare Pressure Relief Valves158
13.14.3Location158
13.14.4PSV Installation Position159
13.14.5Block Valves161
13.14.6Inlet Piping162
13.14.7Discharge Piping164
13.14.8Pressure Relief Valve Bonnet Vents164
13.14.9Pilot Operated Pressure Relief Valve Installation164
13.14.10Temperature Relief Valve Installation165
13.14.11Rupture Disk Installation165
13.15Relief Discharge System166
13.15.1Discharge to Atmosphere166
13.15.2Discharge to Closed System166
13.15.3Permissible Back Pressure on Pressure Relief Valves167
13.15.4Relief System Piping Design Considerations168
13.15.4.1Piping Layout Guidelines168
13.15.4.2Design Temperature168
13.15.4.3Design Pressure169
13.15.4.4Stress169
13.15.4.5Isolation Valves169
13.15.4.6Design Criteria for Relief Valve Inlet Piping169
13.15.4.7Design Criteria for Relief Headers170
13.15.4.8Piping Metallurgy171
13.15.4.9Winterization, Safety Insulation and Steam Tracing171
13.15.5Line Sizing171
13.15.5.1Relief Valve Inlet/Outlet Piping Sizing171
13.15.5.2Line Sizing of the Main Relief Header175
13.15.6Flow Metering177
13.15.6.1Design177
13.15.6.2Methods177
13.15.7Sealing and Purging178
13.15.7.1Sealing178
13.15.7.2Gas Seals179
13.15.7.3Water Seals181
13.15.7.4Purge Gas183
13.16Knockout, Blowdown, Seal, Quench Drums and Pumps185
13.16.1Knockout Drum185
13.16.1.1Inlets to Knock out drums187
13.16.1.2Flare Knock-Out Drum Elevation188
13.16.2Blowdown Drum190
13.16.3Seal Drum191
13.16.3.1Vertical Water Seal Drum194
13.16.3.2Horizontal Water Seal Drum194
13.16.4Quench Drums194
13.16.5Pumps196
13.17Flare System198
13.17.1Flare Location198
13.17.1.1Radiation198
13.17.1.2Liquid Carryover199
13.17.1.3Ground Level Concentrations of Toxic Compounds (GLC’s)199
13.7.1.4Selection of Flare Stack Location199
13.17.2Types of Flares201
13.17.2.1Elevated Flares202
13.17.2.2Ground Flares206
13.17.2.3Offshore Platform Flares212
13.17.3Flare System Metallurgy213
13.17.3.1Hydrocarbon Flaring213
13.17.3.2H2S Flaring214
13.17.4Elevated Flare Sizing214
13.17.4.1Stack Diameter215
13.17.4.2Stack Height215
13.17.4.3Radiation Considerations222
13.17.5Ground Flare Sizing224
13.17.5.1Enclosed Ground Flares224
13.17.5.2Open Pit Ground Flares226
13.17.5.3Burn Pit226
13.17.6Smokeless Flaring227
13.17.6.1Smokeless Flaring Requirements228
13.17.6.2Steam Injection228
13.17.6.3Air Assisted Flaring228
13.17.6.4Miscellaneous methods for smokeless flaring233
13.17.6.5Smokeless Flaring Control234
13.17.6.6Flare Tip Design Options234
13.17.7Noise and Environmental242
13.17.7.1Environmental242
13.17.8Flare Ignition243
13.17.8.1Pilot Ignition243
13.17.8.2Flame Front Generator (FFG)245
13.17.8.3Other Accessories246
13.17.8.4Pilot Monitoring247
13.17.9Flare Header Sizing Methodology247
13.17.10Flare System Data Sheet252
13.18Glossary253
13.19References255