Chapter 13: Safety relief systems

13.0 Introduction 8
13.1 Requirement of Pressure Relief Valves 8
13.1.1 Vessels 8
13.1.2 Heat Exchangers 9
13.1.3 Fired Heaters 9
13.1.4 Steam Boiler Systems 9
13.1.5 Piping 9
13.1.6 Rotating and Mechanical Equipment 9
13.1.7 Temperature Relief Valves 10
13.2 Requirement of Vacuum Relief Protection 10
13.2.1 Vacuum Relief Requirement 10
13.2.2 Vacuum Breaker/Preventer Systems 11
13.2.3 Vacuum Relief Valves 11
13.3 Technical Requirements of Pressure Relief Valves 11
13.3.1 Set Pressure 11
13.3.1.1 Margin above Maximum Operating Pressure 11
13.3.1.2 Pressure Relief Valve Set Pressure 12
13.3.1.3 Temperature Relief Valve Set Pressure 12
13.3.1.4 Vacuum Relief Valve Set Pressure 12
13.3.1.5 Spring Setting (Cold Differential Test Pressure) 12
13.3.2 Accumulation 12
13.3.3 Effect of Back Pressure 12
13.3.3.1 Effect on Opening Pressure 12
13.3.3.2 Effect on Relieving Capacity 15
13.4 PSV Types 17
13.4.1 Non-ASME Devices 18
13.4.2 Codes and Standards 18
13.4.3 Testing and Certification 19
13.4.4 Conventional Pressure Relief Valves 20
13.4.4.1 Operating Characteristics 22
13.4.4.2 Applications 26
13.4.4.3 Design Considerations 26
13.4.4.4 Operating Pressure 27
13.4.4.5 Superimposed Back Pressure 27
13.4.4.6 Inlet Loss 27
13.4.4.7 Back Pressure 28
13.4.5 Balanced Bellows Pressure Relief Valves 29
13.4.5.1 Operating Characteristics 29
13.4.5.2 Applications 30
13.4.5.3 Design Considerations 30
13.4.6 Pilot Operated Pressure Relief Valves 31
13.4.6.1 Pilot Operating Description 33
13.4.6.2 Pop and Modulating Action Pilots 33
13.4.6.3 Flowing and Non-flowing Pilots 34
13.4.6.4 Restricted Lift 34
13.4.6.5 Applications 34
13.5 Rupture Disks 37
13.5.1 Operating Characteristics 37
13.5.1.1 Bursting Pressure 38
13.5.1.2 Operating ratio 39
13.5.2 Rupture Disc disadvantages 41
13.5.3 Applications 42
13.5.3.1 Rupture Disk on Inlet to Pressure Relief Valves 42
13.5.3.2 Rupture Disk on Discharge from Pressure Relief Valve 42
13.5.3.3 Rupture Disk in Parallel with Pressure Relief Valve 43
13.5.4 Types of Rupture Disks 43
13.5.4.1 Conventional Tension Loaded Disks 43
13.5.4.2 Prescored Tension Loaded Disks 44
13.5.4.3 Composite Rupture Disks 46
13.5.4.4 Reverse Buckling Disks with Knives 46
13.5.4.5 Prescored Reverse Buckling Disks 48
13.5.4.6 Reverse Buckling Disk for Liquid Service 50
13.5.4.7 Graphite Disks 51
13.5.5 Design Considerations 51
13.5.5.1 Rupture Disc Special Features 53
13.6 Other Types of Pressure Relief Devices 55
13.6.1 Surface Condenser Pressure Relief Valves 55
13.6.2 Sentinel Valves 56
13.7 Design Philosophy for Determining Relief Load 56
13.7.1 Process Evaluation Basis 56
13.7.1.1 Material Balance Rates and Duties 57
13.7.1.2 Material Balance Rates and Duties plus Specified Margin(s) 57
13.7.1.3 Loads Based on Equipment or Process Limitations 58
13.7.2 Double Jeopardy 58
13.7.3 Utility Losses 58
13.7.3.1 Loss of Cooling Water 59
13.7.3.2 Loss of Electric Power 59
13.7.3.3 Loss of Steam 61
13.7.3.4 Loss of Fuel 61
13.7.3.5 Loss of Instrument Air 61
13.7.3.6 Loss of Instrument Power 61
13.7.3.7 Loss of Refrigeration 62
13.7.3.8 Loss of Inert Gas 62
13.7.4 Blocked Exits 62
13.7.5 Fire 62
13.7.5.1 Vertical Height Limit for Fire Heat Input 63
13.7.5.2 Wetted Surface Area Exposed to Fire Heat Input 64
13.7.5.3 Fire Circle 64
13.7.5.4 Fire Heat Input Causing Vaporization of Liquid 64
13.7.5.5 Fire Relief Rate from Vessels Containing Liquid 65
13.7.5.6 Fire Relief Rate from Vessels Containing Only Gas 65
13.7.5.7 Additional Fire Protection Considerations 65
13.7.5.8 Basic Assumptions for Fire Case Relief Analysis 66
13.7.5.9 Heat Flux Equations 66
13.7.5.10 Determination of Wetted Area 67
13.7.5.11 Insulation credit 69
13.7.5.12 Liquid Filled Systems 70
13.7.5.13 Protection of System with Individual Relief Valve 70
13.7.5.14 Maintenance Isolation 71
13.7.5.15 Determination of Latent Heat for Boiling Applications 71
13.7.5.16 High Boiling Point Fluids 72
13.7.5.17 Latent Heat of Hydrocarbon/Water Mixtures 72
13.7.5.18 Critical or Super-Critical Fluids 72
13.7.5.19 Relief Loads for Vessels Containing Vapor 73
13.7.5.20 Depressurizing 74
13.7.5.21 Sizing of Depressurizing system lines 76
13.7.6 Aerial Cooler Failure 78
13.7.7 Condensing Duty Failure 78
13.7.8 Reflux Failure 78
13.7.9 Accumulation of Non-Condensables 78
13.7.10 Abnormal Heat Input 78
13.7.11 Abnormal Vapour Input 78
13.7.12 Abnormal Chemical Reaction 79
13.7.12.1 Process Flow 79
13.7.12.2 Start-of-Run and End-of-Run Conditions 79
13.7.12.3 Reaction Process Characteristics 79
13.7.12.4 Alternate Operation Modes 79
13.7.12.5 Causes of Overpressure 80
13.7.12.6 Heat and Material Balance Considerations 80
13.7.12.7 Reactor Yields 80
13.7.12.8 Condensation Curves 80
13.7.12.9 Pressure Profiles 81
13.7.12.10 Pressure Relief and Depressurizing Facilities 82
13.7.12.11 Location of Pressure Relief Valves 82
13.7.12.12 Presence of Block Valves in the Loop 83
13.7.12.13 Equipment Shutdown 83
13.7.12.14 Hydraulic Expansion 83
13.7.12.15 Entrance of Volatile Liquid 83
13.7.12.16 Combination of Causes 83
13.7.12.17 Thermal Relief 84
13.7.12 Mechanical Equipment 84
13.7.13.1 Pumps 84
13.7.13.2 Compressors 85
13.7.13.3 Mechanical Driver Considerations 85
13.7.14 Unsteady state Conditions 86
13.7.14.1 Heat Exchange Tube Rupture 87
13.7.14.2 Double Pipe Exchangers 90
13.7.14.3 Fractionation/Distillation Tower Upsets 90
13.7.14.4 Depressurizing Impact 90
13.7.14.5 Block Valves, Check Valves and Control Valves 90
13.7.14.6 Heat Transfer Equipment Performance 93
13.7.15 HIPPS System 94
13.7.16 Effect of Instrumentation 95
13.8 Relief Load Calculation for Fractionating Tower PSV Sizing 96
13.8.1 System Description 96
13.8.2 Causes of Overpressure 96
13.8.3 H&MB Considerations for Upset Conditions 97
13.8.3.1 Basic Assumptions for Relief Case H&MB 97
13.8.3.2 Heat Balance for Upset Conditions 97
13.8.4 Maximum Capacity 100
13.8.5 Example of Relief load calculation for Fractionation column 101
13.9 Safety Relief Valve Sizing 107
13.9.1 Sizing for Gas or Vapour Relief- Critical Flow Pressure Ratio 108
13.9.2 PSV sizing for Gas or Vapor – Critical Flow 109
13.9.3 PSV sizing for Subcritical Gas or Vapor Flow 112
13.9.4 PSV sizing for Steam Flow 114
13.9.5 PSV sizing for Liquid Flow – Liquid Trim Relief Valves requiring capacity certification 115
13.9.6 PSV sizing for Liquid Flow – Conventional Pressure Relief Valves (Capacity certification not required) 118
13.9.7 PSV sizing as per Manufacturer’s Equations 119
13.9.7.1 Pilot Operated Pressure Relief Valves 120
13.9.8 PSV sizing for Two Phase Flow 120
13.9.9 Rupture Disc Sizing 121
13.10 Special Engineering Tips for PSV sizing 121
13.11 Specification of Safety/Relief Valves and Rupture Devices 124
13.11.1 Documentation 124
13.11.2 Specification 124
13.11.3 Depressurizing or Blowdown Valve Sizing 125
13.11.4 Specification of Depressurizing or Blowdown Valves 125
13.11.5 Dynamic Simulation 126
13.12 FLARE SYSTEM SIZING 127
13.12.1 OVERALL FLARE SYSTEM LOAD EVALUATION 127
13.12.2 Flare Load Determination for Each Significant Case 128
13.12.3 Effect of Instrumentation on Flare System Flare Loads 128
13.12.4 Effect of Auto-lockouts on Heat Input Duties 130
13.12.5 Fire Circle Flare Loads 130
13.12.6 Instrument Air Failure Flare Loads 130
13.12.7 Instrument Power Failure Flare Loads 131
13.12.8 Electric Power Failure Flare Loads 131
13.12.9 Steam Failure Flare Loads 131
13.12.10 Cooling Water Failure Flare Loads 132
13.12.11 Combination Cause Flare Loads 132
13.12.12 Flare Header Sizing and Design 132
13.12.13 Loads from Depressurizing Systems 133
13.12.13.1 Documentation of Flare Load Cases 133
13.12.14 Flare Load Minimization 136
13.12.14.1 System Design and Modifications 137
13.12.14.2 Percentage Reduction 137
13.12.14.3 Time Frame Analysis 138
13.12.14.4 Response of Control Instruments 138
13.12.14.5 Pump Driver Selection Philosophy 139
13.12.14.6 Auto Start Spares 139
13.12.14.7 Instrumentated Shutdown System 140
13.12.14.8 High Integrity Pressure Protective Instrumentation
(HIPPS) Systems		 140
13.13 Relief Material Recovery and Disposal 141
13.13.1 Disposal Options 141
13.13.2 Hazard and Risk Assessment 142
13.13.3 Environmental Factors 142
13.13.4 Vapor Release Criteria 142
13.13.4.1 Atmospheric Release Criteria 143
13.13.5 Liquid Release Criteria 144
13.13.5.1 Non-Hazardous Streams 144
13.13.5.2 Non-Hazardous Hydrocarbons 144
13.13.5.3 Hazardous Streams 144
13.13.5.4 Two Phase Releases 144
13.13.6 Prevention of Liquid Releases 144
13.13.6.1 Disposal into a Process 145
13.13.6.2 Closed Disposal Systems 146
13.13.6.2.1 Intermediate Collection Systems 146
13.13.6.2.2 Flare Systems 146
13.13.6.2.3 Vapor Recovery 147
13.13.6.2.4 Incinerators and Burn Pits 147
13.13.6.2.5 Liquid Handling Systems 147
13.13.6.2.6 Treating Systems 148
13.13.7 Design Considerations 148
13.13.7.1 Atmospheric Releases 148
13.13.7.2 Intermediate Collection Systems 149
13.13.7.3 Flare Systems 149
13.13.7.4 Vapor Recovery- Flare Gas Recovery Systems 150
13.13.7.5 Incinerators 153
13.13.7.6 Liquid Handling Systems 153
13.13.7.7 Treating Systems 153
13.14 Relief System Piping 158
13.14.1 Pressure Relief Valve Installation 158
13.14.2 Spare Pressure Relief Valves 158
13.14.3 Location 158
13.14.4 PSV Installation Position 159
13.14.5 Block Valves 161
13.14.6 Inlet Piping 162
13.14.7 Discharge Piping 164
13.14.8 Pressure Relief Valve Bonnet Vents 164
13.14.9 Pilot Operated Pressure Relief Valve Installation 164
13.14.10 Temperature Relief Valve Installation 165
13.14.11 Rupture Disk Installation 165
13.15 Relief Discharge System 166
13.15.1 Discharge to Atmosphere 166
13.15.2 Discharge to Closed System 166
13.15.3 Permissible Back Pressure on Pressure Relief Valves 167
13.15.4 Relief System Piping Design Considerations 168
13.15.4.1 Piping Layout Guidelines 168
13.15.4.2 Design Temperature 168
13.15.4.3 Design Pressure 169
13.15.4.4 Stress 169
13.15.4.5 Isolation Valves 169
13.15.4.6 Design Criteria for Relief Valve Inlet Piping 169
13.15.4.7 Design Criteria for Relief Headers 170
13.15.4.8 Piping Metallurgy 171
13.15.4.9 Winterization, Safety Insulation and Steam Tracing 171
13.15.5 Line Sizing 171
13.15.5.1 Relief Valve Inlet/Outlet Piping Sizing 171
13.15.5.2 Line Sizing of the Main Relief Header 175
13.15.6 Flow Metering 177
13.15.6.1 Design 177
13.15.6.2 Methods 177
13.15.7 Sealing and Purging 178
13.15.7.1 Sealing 178
13.15.7.2 Gas Seals 179
13.15.7.3 Water Seals 181
13.15.7.4 Purge Gas 183
13.16 Knockout, Blowdown, Seal, Quench Drums and Pumps 185
13.16.1 Knockout Drum 185
13.16.1.1 Inlets to Knock out drums 187
13.16.1.2 Flare Knock-Out Drum Elevation 188
13.16.2 Blowdown Drum 190
13.16.3 Seal Drum 191
13.16.3.1 Vertical Water Seal Drum 194
13.16.3.2 Horizontal Water Seal Drum 194
13.16.4 Quench Drums 194
13.16.5 Pumps 196
13.17 Flare System 198
13.17.1 Flare Location 198
13.17.1.1 Radiation 198
13.17.1.2 Liquid Carryover 199
13.17.1.3 Ground Level Concentrations of Toxic Compounds (GLC’s) 199
13.7.1.4 Selection of Flare Stack Location 199
13.17.2 Types of Flares 201
13.17.2.1 Elevated Flares 202
13.17.2.2 Ground Flares 206
13.17.2.3 Offshore Platform Flares 212
13.17.3 Flare System Metallurgy 213
13.17.3.1 Hydrocarbon Flaring 213
13.17.3.2 H2S Flaring 214
13.17.4 Elevated Flare Sizing 214
13.17.4.1 Stack Diameter 215
13.17.4.2 Stack Height 215
13.17.4.3 Radiation Considerations 222
13.17.5 Ground Flare Sizing 224
13.17.5.1 Enclosed Ground Flares 224
13.17.5.2 Open Pit Ground Flares 226
13.17.5.3 Burn Pit 226
13.17.6 Smokeless Flaring 227
13.17.6.1 Smokeless Flaring Requirements 228
13.17.6.2 Steam Injection 228
13.17.6.3 Air Assisted Flaring 228
13.17.6.4 Miscellaneous methods for smokeless flaring 233
13.17.6.5 Smokeless Flaring Control 234
13.17.6.6 Flare Tip Design Options 234
13.17.7 Noise and Environmental 242
13.17.7.1 Environmental 242
13.17.8 Flare Ignition 243
13.17.8.1 Pilot Ignition 243
13.17.8.2 Flame Front Generator (FFG) 245
13.17.8.3 Other Accessories 246
13.17.8.4 Pilot Monitoring 247
13.17.9 Flare Header Sizing Methodology 247
13.17.10 Flare System Data Sheet 252
13.18 Glossary 253
13.19 References 255

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