Chapter 13: Safety relief systems

13.1Requirement of Pressure Relief Valves8
13.1.2Heat Exchangers9
13.1.3Fired Heaters9
13.1.4Steam Boiler Systems9
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 above Maximum Operating Pressure11 Relief Valve Set Pressure12 Relief Valve Set Pressure12 Relief Valve Set Pressure12 Setting (Cold Differential Test Pressure)12
13.3.3Effect of Back Pressure12 on Opening Pressure12 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 Characteristics22 Considerations26 Pressure27 Back Pressure27 Loss27 Pressure28
13.4.5Balanced Bellows Pressure Relief Valves29 Characteristics29 Considerations30
13.4.6Pilot Operated Pressure Relief Valves31 Operating Description33 and Modulating Action Pilots33 and Non-flowing Pilots34 Lift34
13.5Rupture Disks37
13.5.1Operating Characteristics37 Pressure38 ratio39
13.5.2Rupture Disc disadvantages41
13.5.3Applications42 Disk on Inlet to Pressure Relief Valves42 Disk on Discharge from Pressure Relief Valve42 Disk in Parallel with Pressure Relief Valve43
13.5.4Types of Rupture Disks43 Tension Loaded Disks43 Tension Loaded Disks44 Rupture Disks46 Buckling Disks with Knives46 Reverse Buckling Disks48 Buckling Disk for Liquid Service50 Disks51
13.5.5Design Considerations51 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 Balance Rates and Duties57 Balance Rates and Duties plus Specified Margin(s)57 Based on Equipment or Process Limitations58
13.7.2Double Jeopardy58
13.7.3Utility Losses58 of Cooling Water59 of Electric Power59 of Steam61 of Fuel61 of Instrument Air61 of Instrument Power61 of Refrigeration62 of Inert Gas62
13.7.4Blocked Exits62
13.7.5Fire62 Height Limit for Fire Heat Input63 Surface Area Exposed to Fire Heat Input64 Circle64 Heat Input Causing Vaporization of Liquid64 Relief Rate from Vessels Containing Liquid65 Relief Rate from Vessels Containing Only Gas65 Fire Protection Considerations65 Assumptions for Fire Case Relief Analysis66 Flux Equations66 of Wetted Area67 credit69 Filled Systems70 of System with Individual Relief Valve70 Isolation71 of Latent Heat for Boiling Applications71 Boiling Point Fluids72 Heat of Hydrocarbon/Water Mixtures72 or Super-Critical Fluids72 Loads for Vessels Containing Vapor73 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 Flow79 and End-of-Run Conditions79 Process Characteristics79 Operation Modes79 of Overpressure80 and Material Balance Considerations80 Yields80 Curves80 Profiles81 Relief and Depressurizing Facilities82 of Pressure Relief Valves82 of Block Valves in the Loop83 Shutdown83 Expansion83 of Volatile Liquid83 of Causes83 Relief84
13.7.12Mechanical Equipment84 Driver Considerations85
13.7.14Unsteady state Conditions86 Exchange Tube Rupture87 Pipe Exchangers90 Tower Upsets90 Impact90 Valves, Check Valves and Control Valves90 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 Assumptions for Relief Case H&MB97 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 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
13.11.3Depressurizing or Blowdown Valve Sizing125
13.11.4Specification of Depressurizing or Blowdown Valves125
13.11.5Dynamic Simulation126
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 of Flare Load Cases133
13.12.14Flare Load Minimization136 Design and Modifications137 Reduction137 Frame Analysis138 of Control Instruments138 Driver Selection Philosophy139 Start Spares139 Shutdown System140 Integrity Pressure Protective Instrumentation
(HIPPS) Systems
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 Release Criteria143
13.13.5Liquid Release Criteria144 Streams144 Hydrocarbons144 Streams144 Phase Releases144
13.13.6Prevention of Liquid Releases144 into a Process145 Disposal Systems146 Collection Systems146 Systems146 Recovery147 and Burn Pits147 Handling Systems147 Systems148
13.13.7Design Considerations148 Releases148 Collection Systems149 Systems149 Recovery- Flare Gas Recovery Systems150 Handling Systems153 Systems153
13.14Relief System Piping158
13.14.1Pressure Relief Valve Installation158
13.14.2Spare Pressure Relief Valves158
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 Layout Guidelines168 Temperature168 Pressure169 Valves169 Criteria for Relief Valve Inlet Piping169 Criteria for Relief Headers170 Metallurgy171, Safety Insulation and Steam Tracing171
13.15.5Line Sizing171 Valve Inlet/Outlet Piping Sizing171 Sizing of the Main Relief Header175
13.15.6Flow Metering177
13.15.7Sealing and Purging178 Seals179 Seals181 Gas183
13.16Knockout, Blowdown, Seal, Quench Drums and Pumps185
13.16.1Knockout Drum185 to Knock out drums187 Knock-Out Drum Elevation188
13.16.2Blowdown Drum190
13.16.3Seal Drum191 Water Seal Drum194 Water Seal Drum194
13.16.4Quench Drums194
13.17Flare System198
13.17.1Flare Location198 Carryover199 Level Concentrations of Toxic Compounds (GLC’s)199 of Flare Stack Location199
13.17.2Types of Flares201 Flares202 Flares206 Platform Flares212
13.17.3Flare System Metallurgy213 Flaring213 Flaring214
13.17.4Elevated Flare Sizing214 Diameter215 Height215 Considerations222
13.17.5Ground Flare Sizing224 Ground Flares224 Pit Ground Flares226 Pit226
13.17.6Smokeless Flaring227 Flaring Requirements228 Injection228 Assisted Flaring228 methods for smokeless flaring233 Flaring Control234 Tip Design Options234
13.17.7Noise and Environmental242
13.17.8Flare Ignition243 Ignition243 Front Generator (FFG)245 Accessories246 Monitoring247
13.17.9Flare Header Sizing Methodology247
13.17.10Flare System Data Sheet252