Flare System Process Design Manual - Flour Daniel

Flare System
Flare System
Flare System
Flare System
Process Design Manual

Cover Page.doc
PROCESS DESIGN MANUAL
FLARE SYSTEM
670-225-9048
PLEASE READ MESSAGE BELOW BEFORE YOU
PROCEED
It is recognized that this manual will require further improvement and updating. However, this manual
may be used in actual projects with suitable caution. A listing of recommended action items is
provided on the next page to identify issues, which have been targeted as areas for improvement. A
brief review of those items is warranted before using the manual. Any suggestions for improvement
are welcomed and should be forwarded to David Kang.
The risk based assessment and recommendations of relief load mitigation instrumentation in Sections
5.3.4 and 5.3.5 shall be used only as a reference. The values of reliability and/or probability used in
these sections require further verification and approval based on the latest industry practices. The
recommendations for use of load mitigation instruments may be changed depending on the finalized
instrument reliability values.
Please E-Mail David Kang of Process Engineering in the Irvine office for questions or comments
regarding this Design Manual.
ISSUE DATE MADE CHECKED APPROVED DESCRIPTION
0 11-Aug-98 CAS DKK DDC Review & Comment
1 14-Apr-00 YVB DKK DDC Issuance

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SECTION 0.0
PROCESS MANUAL
AREAS FOR FURTHER IMPROVEMENTS
PAGE 1
DATE 07-00
Chap0-r1.doc
SECTION 3.11.5.4 TWO PHASE OR FLASHING FLOW SERVICE
Provide the methodology for using Diers HEM to calculate relief valve size for two phase or
flashing flow relief. This method has been recently adopted by API.
SECTION 5.3.4 THRU 5.3.5 RISK BASED ASSESSMENT AND RECOMMENDATIONS FOR
RELIEF LOAD MITIGATION INSTRUMENTATION
Update reliability and probability values for relief load mitigation instruments, and establish
standards for use of the load mitigation instruments and mitigated flare load calculation
methodology. The Fluor Daniel Houston office is currently (1998) reviewing Fluor Daniel’s
position on this subject.
SECTION 7.5.1.3 WATER SEALS
Review number of contraints on seal drum criteria depicted in Figure 7.3.
APPENDIX B-1 TOWER RELIEF LOAD
Develop an improved methodology to calculate energy balances, using process simulation, for
steam stripped crude towers or absorbers which used high boiling point materials to recover
light ends from gases. Currently, the spreadsheet is best suited for simple fractionation towers
which separate relatively close boiling point materials.
APPENDIX B-2 REACTOR LOOP RELIEF LOAD
Develop a spreadsheet which can calculate change of energy balances in a reactor loop. This
spreadsheet will include simplified exchanger rating methods for different kinds of exchangers.

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REV 1 LIST
PAGE 1
DATE 07-00
Rev1 List.doc
The following summarizes the changes made in the Revision 1 of the Flare System Manual for
all items other than cosmetic items:
Cover Page: Notes, contact name, date.
Chapter 0: Added items to Sections 3.11.5.4 & 7.5.1.3.
Delete item 1 under Appendix B-1.
Chapter 1: Edited Item 5 in Table 1.2.
The following items were modified in Table 1.1:
• Edited Item 1 Under Task, Secondary/Consultant Responsibility
and Notes
• Edited Item 2. Under Primary Responsibility,
Secondary/Consultant Responsibility and Notes
• Edited Item 2.D Under Primary Responsibility,
• Edited Item 3.B Under Primary Responsibility,
• Edited Item 3.C Under Primary Responsibility,
Secondary/Consultant Responsibility
• Added Item 3.D
• Edited Item 4.B Under Primary Responsibility,
• Added Item 4.D
• Edited Item 5 Under Primary Responsibility,
Edited Figure 1.2 to reflect Table 1.1 editions
Chapter 2: Added English units.
Edited Section 2.1.5.1, item 2 for liquid services.
Added equations for English and Metric (Bar) units to Equations
3.1,3.4,3.6,3.7,3.8).
Equation 3.8 was also corrected for a typo.

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Edited Section 3.2.4 to clarify when to specify Liquid Trim Valves.
Edited Section 3.11.5.4 to include discussion on HEM method for two
phase flow.
Figure 3.1 was replaced with the correct picture.
Edited the constant of Equation 3.6 (metric) from 1.342 to 1.000 in
Section 3.11.1.5.
Added equations for English and Metric (Bar) units to Equations
4.1,4.3a,4.3b,4.5,4.6a,4.6b,4.11,4.12,4.13,4.21).
Section 4.3.1.1 was edited.
Heater Duty sub-section under section 4.3.3.2 was edited
Chapter 7: Section 7.5.1.3.e was edited.
Table 7.1, item 8 was edited. Item 9 was added.
Figure 7.3, Note 2 was corrected.
Appendix B-1: Rewritten with new examples and backup calculations.
Appendix B-3: Rewritten with new example.
Appendix B-4: Rewritten with new example.
Appendix B-5: More explanation on examples.
Appendix D-1: New copies of Excel spreadsheets, new examples, and reference of
location of the electronic spreadsheets.

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TABLE OF CONTENTS DATE 06-00
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1.0 INTRODUCTION
1.1 DESIGN CONSIDERATIONS
1.1.1 Precedence of Laws, Local Regulations, Client Standards,
Design Codes, Client Guides and this Manual
1.1.2 Design Objectives
1.1.3 Design Impact Factors
1.1.4 Administrative Procedures
1.2 DESIGN RESPONSIBILITIES
1.2.1 Establish Design Philosophy and Standards
1.2.2 System Assessment
1.2.3 Relief Source Identification
1.2.4 Preliminary PSV and Vessel Nozzle Sizing
1.2.5 Final Data Sheet Preparation
1.2.6 Final Relief Load Computation
1.2.7 “As Purchased” Equipment Performance Review
1.2.8 Relief Device Installation Review
1.2.9 Monitor Design Changes
1.2.10 Engineering Documentation
1.3 CODES, STANDARDS AND PRACTICES
1.3.1 ASME Boiler and Pressure Vessel Code
1.3.2 API Publications
1.3.3 NFPA Standards
1.3.4 ANSI Standards
1.3.5 International Conference of Building Officials (ICBO)
1.3.6 American Institute of Steel constructors (AISC)

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1.3.7 American Society for Testing Materials (ASTM)
1.3.8 American Welding Society (AWS)
1.4 DESIGN GUIDE SUMMARY
1.4.1 Establish Design Pressure of Vessels and Piping (Chapter 2)
1.4.2 Establish Design Temperature of Vessels and Piping (Chapter 2)
1.4.3 Select Type of Relieving Device (Chapter 3)
1.4.4 Establish Individual Relief Loads (Chapter 4)
1.4.5 Calculate Required Relief Device Orifice Area (Chapter 3)
1.4.6 Review Disposal Options (Chapter 6)
1.4.7 Establish Equipment Depressuring Requirements (Chapter 4)
1.4.8 Size Thermal Relief Valves (Chapter 4)
1.4.9 Evaluate Process Flow Loops (Chapter 4)
1.4.10 Evaluate Total Relief Loads to the Flare, by Contingency,
to Include Depressuring (Chapter 5)
1.4.11 Consider Mitigation for Relief Load Reduction (Chapter 5)
1.4.12 Review Depressuring Loads for Time Smoothing (Chapter 5)
1.4.13 Review and Perhaps Modify Control Valves
for Favorable Control Actions (Chapter 5)
1.4.14 Size Relief Valve Piping Inlet/Outlet (Chapter 7)
1.4.15 Establish Required Purging Rates by Converting
Velocities Given Below into Rates [lb/hr (kg/hr) or SCFH (nm3/hr)]
for the Relief Piping (Chapter 7)
1.4.16 Select and Specify the Following Equipment
where Appropriate (Chapter 8)
1.4.17 Develop Flare Stack and Tip Details (Chapter 9)
Table 1.1 Pressure Relief System Design Responsibilities

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Table 1.2 Essential Criteria for Flare and Relief System
Figure 1.1 Typical Relief System Engineering Schedule
Figure 1.2 Typical Relief System Activity Flow Chart
2.0 DESIGN PRESSURE AND TEMPERATURE SELECTION
2.1 DESIGN PRESSURE SELECTION
2.1.1 Operating Pressure
2.1.2 Maximum Operating Pressure
2.1.3 Settling Out Pressure
2.1.4 Design Pressure
2.1.5 Design Pressure Selection
2.1.6 Design Vacuum
2.1.7 Maximum Allowable Working Pressure (MAWP)
2.2 DESIGN TEMPERATURE SELECTION
2.2.1 Definition of Operating Temperature
2.2.2 Maximum Operating Temperature
2.2.3 Definition of Design Temperature
2.2.4 Design Temperature Selection
2.3 PSV RELATED PRESSURES
2.3.1 PSV Set Pressure for Vessels
2.3.2 Spring Setting (Cold Differential Test Pressure)
2.3.3 Permissible Overpressure or Accumulation
2.3.4 Superimposed Back Pressure
2.3.5 Built-Up Back Pressure
2.3.6 Back Pressure

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2.3.7 Pressure Tolerances
2.3.8 Blowdown Pressure
2.4 EQUIPMENT RERATING
Figure 2.1 Typical Pressure Levels per API RP 521
Figure 2.2 Allowable Design Stress vs. Temperature
3.0 RELIEVING DEVICES
3.1 INTRODUCTION
3.2 TYPES OF PRESSURE RELIEF DEVICES
3.2.1 Safety Valves
3.2.2 Relief Valves
3.2.3 Safety-Relief Valves
3.2.4 Liquid Trim Relief Valves
3.2.5 Pilot Operated Pressure Relief Valves
3.2.6 Rupture Disks
3.2.7 Non-ASME Devices
3.3 CODES AND STANDARDS
3.3.1 ASME Section I
3.3.2 ASME Section VIII
3.3.3 ANSI/API Standard 526
3.3.4 API RP-520, Part I
3.3.5 Testing and Certification
3.3.6 Code Stamps
3.4 CONVENTIONAL PRESSURE RELIEF VALVES

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3.4.1 Operating Characteristics
3.4.2 Applications
3.4.3 Design Considerations
3.5 BALANCED BELLOWS PRESSURE RELIEF VALVES
3.5.2 Applications
3.6 LIQUID TRIM RELIEF VALVES
3.6.2 Applications
3.7 SPECIAL FEATURES
3.8 PILOT OPERATED PRESSURE RELIEF VALVES
3.8.2 Applications
3.8.4 Special Features
3.9 RUPTURE DISKS
3.9.2 Applications
3.9.4 Rupture Disk Burst Pressure Example
3.9.5 Special Features
3.10 OTHER TYPES OF PRESSURE RELIEF DEVICES

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3.10.1 Surface Condenser Pressure Relief Valves
3.10.2 Sentinel Valves
3.10.3 Pressure/Vacuum Breather Valves
3.10.4 Explosion Hatches
3.10.5 Non-ASME Pressure Relief Valves
3.10.6 Liquid Seals
3.10.7 Vacuum Relief Valves
3.11 PRESSURE RELIEF DEVICE SIZING
3.11.1 API Sizing Equations
3.11.2 Manufacturer’s Equations
3.11.3 Pilot Operated Pressure Relief Valves
3.11.4 Safety Valves
3.11.5 Sizing Procedures
3.11.6 Rupture Disk Sizing
3.12 REFERENCES
Table 3.1 API Nozzle Sizes and Areas
Table 3.2 Effective and Actual Areas/Coefficients of Discharge
Figure 3.1 Cross Section of Conventional Pressure Relief Valve
Figure 3.2-A Operating Characteristics of Conventional
Safety Relief Valves in Vapor Service
Figure 3.2-B Operating Characteristics of Conventional Spring
Opposed Pressure Relief Valve in Liquid Service
Figure 3.2-C Operating Characteristics of Liquid Trim Pressure
Relief Valve in Liquid Service
Figure 3.3 Cross Section of Balanced Bellows Pressure Relief Valve
Figure 3.4 Cross Section of Piston Type Pilot Operated Relief Valve

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Figure 3.5 Cross Section of Diaphragm Type Relief Valve
Figure 3.6 Conventional Tension Loaded Rupture Disk
Figure 3.7 Prescored Tension Loaded Rupture Disk
Figure 3.8 Composite Disk
Figure 3.9 Reverse Buckling Disk with Knifes
Figure 3.10 Prescored Reverse Buckling Disk
Figure 3.11 Graphite Disk
Figure 3.12 Rupture Disk Telltale Installation
Figure 3.13 KB Versus Back Pressure for Conventional Pressure
Relief Valves
Figure 3.14 Back Pressure Sizing Factor KB for Balanced Bellows
Pressure Relief Valve (Vapors and Gases)
Figure 3.15 Typical Back Pressure Correction Factor, KW , for
Liquid Service Balanced Bellows Valve (Vapor or
Liquid Trim)
Figure 3.16 Typical Overpressure Correction Factor, KP, for
Conventional Pressure Relief Valve in Liquid Service
Figure 3.17 Rupture Disk Burst Pressure and Manufacturing
Range Tolerances
4.0 DETERMINATION OF INDIVIDUAL RELIEF LOADS
4.1 BASIC PHILOSOPHY
4.1.1 Process Evaluation Basis
4.1.2 Double Jeopardy
4.1.3 Utility Losses
4.1.4 Unsteady State Conditions
4.1.5 Block Valves, Check Valves and Control Valves

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4.1.6 Control System Response
4.1.7 Operator Intervention
4.1.8 Heat Transfer Equipment Performance
4.1.8.1 Air Cooled Exchangers
4.1.8.2 Shell and Tube Exchangers
4.1.8.3 Fired Heaters
4.1.9 Use of DIERS Methodology
4.2 CAUSES OF OVERPRESSURE
4.2.1 General
4.2.2 Operator Error
4.2.3 Utility Failure
4.2.4 Local Equipment/Operation Failure
4.2.5 External Fire
4.2.6 Depressurization
4.2.7 Thermal Expansion
Table 4.1 Cubical Expansion Coefficient
4.2.8 Chemical Reaction
4.2.9 Miscellaneous
Table 4.2 Bases for Relief Capacities under Selected Conditions
4.3 FRACTIONATION AND DISTILLATION
4.3.1 System Description
4.3.2 Causes of Overpressure
4.3.3 Heat and Material Balance Considerations
4.3.3.1 Basic Assumptions for Relief Case
Heat and Material Balance

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4.3.3.2 Heat Balance for Upset Conditions
4.3.4 Maximum Capacity
4.3.5 Determination of Relief Loads
4.4 REACTOR LOOPS
4.4.1 System Description
4.4.1.1 Process Flow
4.4.1.2. Start-of-Run and End-of-Run Conditions
4.4.1.3 Reaction Process Characteristics
4.4.1.4 Alternate Operation Modes
4.4.2 Causes of Overpressure
4.4.3 Heat and Material Balance Considerations
4.4.3.1 Basic Assumptions for Operational Upsets
4.4.3.2 Reactor Yields
4.4.3.3 Condensation Curves
4.4.4 Pressure Profiles
4.4.4.1 Operating Pressure Profiles
4.4.4.2 Design Pressure Profile
4.4.4.3 Relieving Pressure Profile
4.4.4.4 Settle-Out Pressure
4.4.5 Pressure Relief and Depressuring Facilities
4.4.5.1 Code Criteria
4.4.5.2 Location of Pressure Relief Valve
4.4.5.3 Presence of Block Valves in the Loop
4.4.5.4 Pilot Operated Pressure Relief Valve Applications

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4.4.5.5 Depressuring
4.4.6 Maximum Capacity
4.4.7 Determination of Relief Loads
4.4.7.1 Loss of Feed
4.4.7.2 Loss of Effluent Cooling
4.4.7.3 Loss of Quench
4.4.7.4 Recycle Compressor Failure
4.4.7.5 Utility Failure
4.4.7.6 Control Failure
4.4.7.7 Blocked Exits
4.4.7.8 Abnormal Heat Input
4.4.7.9 Change in Feed Composition
4.4.7.10 Chemical Reaction
4.4.7.11 Fire
4.5 LIQUID FILLED SYSTEMS
4.5.1 Blocked Discharge
4.5.2 Thermal Relief
4.6 MECHANICAL EQUIPMENT
4.6.1 Pumps
4.6.2 Compressors
4.6.3 Mechanical Driver Considerations
Table 4.3 Condensing Turbines. Atmospheric Safety Valves Sizes
4.7 HEAT EXCHANGER TUBE RUPTURE
4.7.1 Determining Required Relief Flow Rate

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4.7.2 Steady State Relief Analysis
4.7.3 Dynamic Relief Analysis
4.7.4 Relief Devices and Locations
4.7.5 Double Pipe Exchangers
4.8 FIRE
4.8.1 Basis Assumptions for Fire Case Relief Analysis
4.8.2 Heat Flux Equations
4.8.3 Determination of Relief Loads for Equipment Containing Liquid
4.8.4 Relief Loads for Vessels Containing Vapor
4.8.5 Depressuring
4.9 CHEMICAL REACTIONS
4.10 ATMOSPHERIC STORAGE TANK PROTECTION
4.10.1 Relief Device Accumulation
4.10.2 Non-refrigerated Aboveground Tanks
4.10.3 Refrigerated Aboveground and Belowground
4.10.4 Means of Venting
4.11 REFERENCES
Figure 4.1 Isothermal Flow of Compressible Fluids Through
Pipes at High Pressure Drops
5.0 OVERALL RELIEF SYSTEM LOAD EVALUATIONS
5.1 INTRODUCTION
5.2 FLARE LOAD CALCULATIONS
5.2.1 General Methodology
5.2.2 Determination of Relief System Loads

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5.2.2.1 Determination of Area Fire Loads
5.2.2.2 Utility Failure
5.2.2.3 Other Contingencies
5.2.2.4 Loads from Depressuring Systems
5.3 FLARE LOAD MINIMIZATION
5.3.1 Background
5.3.2 System Design and Modifications
5.3.3 General Approaches
5.3.4 Risk Based Assessment
5.3.5 Recommendations for Relief Load Mitigation Instrumentation
Table 5.1 Pump Autostart Load Reduction Credits
Table 5.2 Dual Loop Shutdown System Load Reduction Credits
5.3.6 Dynamic Simulation
5.3.7 Probability Analysis
5.4 REFERENCES
Figure 5.1 Triple Loop Shutdown System
6.0 RELIEF MATERIAL RECOVERY AND DISPOSAL
6.1 GENERAL
6.2 DISPOSAL OPTIONS
6.2.1 Discharge to Atmosphere
6.2.2 Discharge to Grade or Sewer
6.2.3 Discharge to a Process Vessel
6.2.4 Discharge to a Closed Collection System

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6.3 HAZARD AND RISK ASSESSMENT
6.4 ENVIRONMENTAL FACTORS
6.5 VAPOR RELEASE CRITERIA
6.5.1 General
6.5.2 Atmospheric Release Criteria
6.5.3 Safety Review
6.6 LIQUID RELEASE CRITERIA
6.6.1 Non-Hazardous Streams
6.6.2 Non-Hazardous Hydrocarbons
6.6.3 Hazardous Streams
6.6.4 Two Phase Releases
6.6.5 Prevention of Liquid Releases
6.6.6 Pressure Relief Device Failure
6.7 DISPOSAL INTO A PROCESS
6.7.1 Capacity
6.7.2 Destination Pressure
6.7.3 Process Upsets
6.7.4 In-Service Requirements
6.8 CLOSED DISPOSAL SYSTEMS
6.8.1 Intermediate Collection Systems
6.8.2 Flare Systems
6.8.3 Vapor Recovery
6.8.4 Incinerators & Burn Pits
6.8.5 Liquid Handling Systems

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6.8.6 Treating Systems
6.9.1 Atmospheric Releases
6.9.2 Intermediate Collection Systems
6.9.3 Flare Systems
6.9.4 Vapor Recovery
6.9.5 Incinerators
6.9.6 Liquid Handling Systems
6.9.7 Treating Systems
6.10 REFERENCES
Table 6.1 Typical Threshold Limit Values for Toxic or
Hazardous Chemicals Found in Refineries
Figure 6.1 Typical Flare Gas Recovery System
Figure 6.2 Flare Gas Recovery Inlet Pressure Control System
Figure 6.3 Typical Quench Drum
Figure 6.4 Typical Scrubber System
7.0 RELIEF SYSTEM PIPING & SEALING/PURGING
7.1.1 Piping Layout Guidelines
7.1.2 Design Temperature
7.1.4 Stress
7.1.5 Isolation Valves
7.1.6 Design Criteria for Relief Valve Inlet Piping

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7.1.7 Design Criteria for Relief Headers
7.1.8 Piping Metallurgy
7.1.9 Winterization, Safety Insulation and Steam Tracing
7.2 LINE SIZING
7.2.1 Relief Valve Inlet/Outlet Piping Sizing
7.2.2 Line Sizing of the Main Relief Header
7.3 COMPUTER MODELING OF FLARE HEADERS
7.3.1 Network Method
7.3.2 Flare Method
7.3.3 Contingency Allowance
7.3.4 Pipe Roughness ( ε )
7.3.5 Hydraulic Evaluation
7.4 FLOW METERING
7.4.1 Design Discussion
7.4.2 Methods
7.5 SEALING AND PURGING
7.5.1 Sealing
7.5.2 Purge Gas
7.6 REFERENCES
Table 7.1 PSV Inlet/Outlet Calculations, Design Criteria
Table 7.2 Orifice/Inlet Area Ratio for Standard Relief Valves
Table 7.3 Maximum Allowable Equivalent Lengths of Inlet
Piping to Comply with 3% Inlet Loss Criteria for Relief Valve
Table 7.4 Orifice/Outlet Area for Standard Relief Valves
Table 7.5 Typical Outlet Nozzle Lengths

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Table 7.6 Typical Reducer Angles
Table 7.7 Typical Weld Neck Flange Lengths
Table 7.8 Typical Friction Factors for Clean Carbon Steel Pipe
Figure 7.1 Baffle Type Seal
Figure 7.2 Labyrinth Type Seal
Figure 7.3 Vertical Water Seal Drum
Figure 7.4 Flare Purge Gas Supply
Figure 7.5 Typical Pressure Relief Valve Installation:
Atmospheric (Open) Discharge
Figure 7.6 Typical Pressure Relief Valve Installation:
Closed System Discharge
Figure 7.7 Typical Pressure Relief Valve Mounted on
Process Line
Figure 7.8 Typical Pressure Relief Valve Mounted on
Long Inlet Pipe
Figure 7.9 Typical Pilot-Operated Pressure Relief
Valve Installation
Figure 7.10 Typical Rupture Disk Assembly Installed in
Combination with a Pressure Relief Valve
Figure 7.11 Typical Pressure Relief Valve Installation
with an Isolation Valve
8.0 KNOCKOUT, BLOWDOWN, SEAL, QUENCH DRUMS AND PUMPS
8.1 KNOCKOUT DRUM
8.1.1 Purpose
8.1.2 Design Parameters
8.1.3 Design Details
8.2 BLOWDOWN DRUM

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8.2.1 Purpose
8.3 SEAL DRUM
8.3.1 Purpose
8.4 QUENCH DRUMS
8.4.1 Purpose
8.5 PUMPS
8.6 REFERENCES
Table 8.1 Table of Geometry for Circles and Arcs
Figure 8.1 Typical Horizontal Knockout Drum
Figure 8.2 Drag Coefficient, C
Figure 8.3 Typical Horizontal Blowdown Drum
Figure 8.4 Typical Horizontal Seal Drum
Figure 8.5 Schematic for Combined Ground Flare and
Elevated Flare
Figure 8.6 Typical Operating and Emergency Flares
Figure 8.7 Typical Quench Drum (Condensable)
Figure 8.8 Typical Quench Drum (Emergencies)

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9.0 FLARE
9.1 DESIGN DISCUSSION
9.1.1 Selection of Flare Stack Location
9.1.2 Flare System Data Sheet
9.2 TYPES OF FLARES
9.2.1 Discussion
9.2.2 Elevated Flare
9.2.3 Ground Flares
9.2.4 Offshore Platform Flares
9.3 FLARE SYSTEM METALLURGY
9.3.1 Hydrocarbon Flaring
9.3.2 H2S Flaring
9.4 ELEVATED FLARE SIZING
9.4.1 Discussion of Sizing Methods
9.4.2 Nomenclature
9.4.3 Stack Diameter
9.4.4 Stack Height
9.4.5 Radiation Considerations
9.4.6 Equipment Surface Temperature
9.5 GROUND FLARE SIZING
9.5.1 Enclosed Ground Flares
9.5.2 Open Pit Ground Flares
9.5.3 Burn Pit
9.6 SMOKELESS FLARING

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9.6.1 Smokeless Flaring Requirements
9.6.2 Steam Injection
9.6.3 Air Assisted Flaring
9.6.4 Miscellaneous
9.6.5 Smokeless Flaring Control
9.7 FLARE TIP DESIGN OPTIONS
9.7.1 Flare Tip Characteristics
9.7.2 Open Pipe Flare Tip
9.7.3 Forced Draft (Air Assisted) Flare Tips
9.7.4 Multi Tip Flares
9.7.5 Coanda Flare Tip
9.7.6 High Velocity Tips
9.8 NOISE & ENVIRONMENTAL
9.8.1 Noise Standards
9.8.2 Noise Discussion
9.8.3 Environmental
9.9 FLARE IGNITION
9.9.1 Discussion
9.9.2 Pressure Ignitor
9.9.3 Electronic Ignitor
9.9.4 Atmospheric Ignitor
9.9.5 Pilot Monitoring
9.10 REFERENCES
Table 9.1 Comparison of Flare Types

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Table 9.2A Lower Limits of Flammability of Gases and Vapors in Air
Table 9.2B Lower Limits of Flammability of Gases and Vapors in Air
Table 9.3 Recommended Surface Emissivity Values (εs)
Table 9.4 Air Required for Stoichiometric Combustion of Gases
Figure 9.1 Stack & Flame Geometry
Figure 9.2 XL versus SL
Figure 9.3 Temperature of Steel vs Time of Exposure
Figure 9.4 Typical Enclosed Ground Flare Configuration
Figure 9.5 Typical Burn Pit
Figure 9.6 Typical Air-Assisted Flare System
Figure 9.7 Steam/Hydrocarbon Ration vs Flare Gas Molecular
Weight for Smokeless Flaring
Figure 9.8 Conventional Pipe Flare
Figure 9.9 Conventional Flare with Steam Water Spray
Figure 9.10 High Velocity Tip
Figure 9.11 Air Assisted Flare Tip (Top View)
Figure 9.12 Coanda Nozzle (Internal)
Figure 9.13 Coanda Flare (External)
Figure 9.14 Offshore Flare Support Types
APPENDIX A - NOMENCLATURE
APPENDIX B - SAMPLE CALCULATIONS
B-1 Tower Relief Load
B-2 Reactor Loop Relief Load
B-3 Rupture Tube Relief Load

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B-4 Fire Relief Load
B-5 Rating of the Horizontal Flare K.O. Drum
B-6 Flare Radiation
B-7 Dynamic Simulation
B-8 Flare System Hydraulics Calculations
APPENDIX C – PSV SIZING SOFTWARE
APPENDIX D - Flare System Calculation Spreadsheets
D-1 Flare System Spreadsheet Calculations
D-2 Typical Calculation Index

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1.0 INTRODUCTION
1.1.1 Precedence of Laws, Local Regulations, client standards, Design Codes, client
guides and this Manual
It is intended that this manual supplement rather than replace or supersede any of
the laws, regulations, standards, design codes or guides listed in Section 1.3.0. A
thorough knowledge of these design criteria is essential for a safe design. Any
apparent conflicts with this manual are to be resolved in such a manner as to satisfy
the order of the following precedence:
1. National Laws
2. Local Regulations
3. Client Standards
4. Design Codes
5. Client Design Guides
6. Fluor Daniel Flare System Manual
7. Industry Standards and Guidelines
1.1.2 Design Objectives
The purpose of a flare system is to safely limit the pressure on operating equipment
and interconnecting piping to the maximum allowable pressure. The relieving
system size is dictated by the volume to be relieved and the pressure available to
transfer this volume to the flare.
This manual achieves this purpose and more when the design objectives listed
below are met:
• The system provides adequate safety for personnel and equipment, thus
concurring with all safety laws, design codes and standards.
• Atmospheric emissions are lower, enhancing the environmental acceptability
of the plant.
• Energy is conserved by the recovery and reuse of valuable hydrocarbons as
fuel, providing added profits to the client.
• Plant siting problems are minimized by the reduction of flare emissions,
luminescence, noise and smoke.
1.1.3 Design Impact Factors
Relief and flare system designs begin with a collection of preliminary engineering
information which may impact on the proposed design. A partial list of these would
include:

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• Heat and material balances of process units.
• Power distribution oneline diagram (conceptual).
• The anticipated relieving quantities for various emergency conditions such as
cooling water failure, total power failure, etc., where the plant has been
designed in a traditional conservative manner.
• The anticipated relieving quantities for the same emergency conditions as in
the previous item for a plant using improvements which might incorporate
some, or all, of the following functions to minimize flare quantities and
contain, retain, and recirculate hydrocarbons which would otherwise be
vented to the atmosphere:
° Design of power distribution system to minimize relieving quantities.
° Increased design pressures of key equipment.
° Highly reliable double-lead electrical systems from dual power grids
to ensure high on-stream reliability.
° Instrumentation to lock out reboiler heat sources for fractionation
towers.
° Reliable driver selection for reflux pumps in key fractionating
systems.
° Cascading hot vapor relief streams through compatible cooling
systems to maximize liquid condensation before relieving.
• Operating and investment costs for the flare and relief system components.
• The expected frequency of normal operational upsets and of major
emergency situations, which will activate the flare system.
• The methods used for venting gases generated during start-up, shutdown
and depressuring operations.
• Recovery of gases from sources previously vented to the atmosphere such
as atmospheric storage tanks, sour water storage tanks, and compressor
distance piece vents.
• Liquid recovery from the pumpout and/or blowdown header.
• Quench and scrubbing systems in the flare header to recover valuable
substances.

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• Segregation of sweet and sour flare gases into two headers to provide an
economical means for recovery, treating, and reuse of valuable vapor and
liquid components.
• Segregation of high and low pressure headers to minimize required piping
sizes.
• Equipment required for a Vapor Recovery System to contain, retain and
recirculate the gases and liquids from sources such as relief valve leaks,
minor upsets, and header purge gases.
• Consideration of the benefits of multiple combustion systems to handle small
and large flare quantities in separate systems such as open pit combustion,
ground flares, and enclosed thermal oxidizers.
• The environmental and safety standards which must be met in the area
surrounding the process unit protected by the flare system.
• The concerns of local communities for the impact the plant will have on
them.
1.1.4 Administrative Procedures
Administrative procedures have an important economic role in the safe design and
operation of pressure relief systems. Application of administrative procedures,
however, places a burden on management of the refinery for maintenance of the
required procedures. For this reason, these procedures are to be applied only when
the benefits exceed the burdens.
Administrative procedures which are related to pressure relief systems must be
clearly defined, clearly communicated to unit operators and strictly enforced. Plant
management has direct responsibility for accepting the risks that can be associated
with administrative procedures and for assuring that administrative procedure
policies are established and enforced.
A partial listing of possible administrative procedures follows:
• Lock (or car seal) procedures for block valves associated with pressure relief
valves. The procedures should include a list of all block valves which are
required to be locked in position, definition of who is authorized to unlock and
move block valve positions, procedures for maintaining logs of locked block
valve movements and definition of how the procedures will be enforced.
• Requirements that equipment be continuously attended during certain
operations, such as when a pressure relief valve is blocked in or when

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equipment is operated in a mode, such as steam out or pump out, that it is
known the pressure relief system is not designed to protect against.
• Limitations on modification of equipment without the proper engineering
review of the effect on the pressure relief system. Examples of these types
of limitations are restrictions on changes of pump impeller sizes or turbine
driver speed settings, operating control valves with their bypass valves
partially or fully open, adjustment or removal of control valve minimum or
limit stops or revisions to control valve internal trim.
• Operating procedures for shutting down a unit under pre-identified failure
conditions.
• Vent and drain procedures for equipment maintenance.
1.2 DESIGN RESPONSIBILITIES
1.2.1 Establish Design Philosophy and Standards
The Unit Process Engineer determines causes of overpressure and flows to the
relief and flare system for the process units. The Upset Condition Checklist and
Individual Relief Summary Sheet are utilized for these tasks. Process data for relief
devices are summarized and transmitted to the Control systems engineer on the
Relief Valve Calculations Sheet.
The Unit Process Engineer for the relief and flare system reviews the relief loads for
the various units and determines the disposal methods for the streams from each
unit and maximum flow to each disposal system and calculates the sizes for all lines
including relief device inlet piping. Other duties include developing process and
system flow diagrams, providing process operating and design data for equipment
and piping, and reviewing piping isometric drawings.
The Flare System Process Engineer provides assistance in development of the
design philosophy and assures that the client standards and guides and applicable
Laws and Codes are properly applied to the design. He will review plot plans for
location of equipment containing volatile liquids and participate in the system design
development and reviews as the project proceeds.
Design and maintenance of pressure relief systems is a multi-discipline activity
which begins with the conceptual design of a new process unit, utility or offsite
facility and continues throughout its use. Once a pressure relieving system has
been designed and installed, legal obligations and responsible practice require that
records of the engineering design basis, inspection and maintenance history for the
system be kept.
The sizing basis for pressure relief systems is often directly related to process
variables such as flow rates, equipment capacities or operating characteristics,

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process fluid properties, etc. As a consequence, the design requirements for
pressure relief systems may change if modifications are made to system equipment
or the piping configuration or if the process equipment is operated differently than
planned for in the original design. The sizing basis for a pressure relief system must
be reviewed every time a significant change in minimum or maximum process rate,
stream composition, operating conditions, equipment capacity or equipment line-up
is made.
The various tasks and responsibilities associated with design and ownership of
pressure relief systems are discussed in this Section. The groups typically
responsible for each task are identified for the purposes of discussion only and
actual assignments should be designated by management.
Figure 1.1 and 1.2 diagram the typical activities in designing a pressure relief
system. Typical tasks and responsibilities are listed in Table 1.1. Finally, a Flare
and Relief System Checklist, Table 1.2, is included at the end of Section 1. This
table is not intended to be all inclusive with respect to flare system design but rather
to assure that the more important design parameters have been addressed in the
design.
1.2.2 System Assessment
1.2.2.1 Set Equipment Design Conditions
As part of process design development, the Unit Process Engineer
determines the maximum pressure and temperature for which each piece
of equipment must be designed. Section 2 of this manual discusses the
selection of design pressures and temperatures. This activity should
recognize the interaction between equipment design and overpressure
protection.
In addition to consideration of normal and maximum operating pressure
and temperature, the design condition selected should include
consideration of:
• Pressure and temperature excursions due to process upsets
• Maximum pressure of external sources
• Maximum pressures that rotating equipment can develop under
both normal and abnormal conditions
• Performance limitations of pressure relief devices
• Potential relief stream disposal problems.

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The need for pressure relief systems cannot be eliminated by
specification of higher design conditions. Often however, pressure relief
system designs can be simplified or minimized by careful selection of
mechanical design conditions.
1.2.2.2 Review Plot Plan for Fire Exposure
The plot distribution of vessels and exchangers containing volatile liquids
must be reviewed. Too close a grouping of these equipment types could
lead to a very large relief during a fire, resulting in an excessively large
relief header. This activity must be performed by the Flare System
Process Engineer early in the design to prevent expensive and untimely
changes to the plot plan later in the design phases.
1.2.2.3 Relief Stream Disposal
Relief streams must be disposed of in a safe, economical and
environmentally acceptable manner.
As part of the process design development, the methods of disposal of
relief streams must be determined by the Unit Process Engineer. The
evaluation for this decision must consider the following:
• Local environmental requirements: Unit operating permits may
be contingent upon disposal of all or some relief streams to a
closed disposal system. Releases of some types of streams to
atmosphere may have to be reported to pollution control
authorities.
• Potentially dangerous or toxic relief streams: In refineries,
relatively few relief streams are considered to be toxic. However,
release of flammable liquid or two-phase mixtures, or of high
molecular weight condensable vapors, may pose unacceptable
hazards and must be avoided or contained.
Guidelines for disposal of relief streams are discussed in Section 10.
1.2.2.4 System Review
Before a process design is released for detailed engineering, the Unit
Process Engineer, Flare System Process Engineer, Control Systems
Engineer, Mechanical Engineer, Piping Engineer and Refinery
Operations should conduct a joint review under the direction of the
Project Engineer of the preliminary pressure relief system design. The
purpose of this review is to determine where unit economics or safety

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might be improved by upgrading equipment design conditions. Typically
this review is performed as part of the P&ID review.
1.2.3 Relief Source Identification
The services which require pressure relief devices and their physical location are
provisionally established during process design and confirmed during the detailed
design phase. The basic requirements for determining where pressure relief
devices need to be located are discussed in Section 7.
Final determination of pressure relief device services and locations is dependent
upon completion of the P&ID and the piping isometrics. However, the need for and
location of the major pressure relief devices must be identified during process
design by the Unit Process Engineer. Be aware that during detailed design, “as-
purchased” equipment performance characteristics or addition of equipment, piping,
block valves, or control valves may change the pressure relief system requirements.
Each project should conduct a pressure relief system safety review at the
appropriate stage of design development, as discussed in Section 1.2.8.
1.2.4 Preliminary PSV and Vessel Nozzle Sizing
1.2.4.1 Preliminary Relief Load Calculations
When the process design is complete and sufficient mechanical
information is available, the Unit Process Engineer is responsible for
determination of relieving rates and completion of the Relief Valve
Calculation Sheet for each service and a unit summary on the Individual
Relief Load Summary.
The basis and techniques for developing relief flow rates are discussed
in Section 4.
Evaluated relief cases and calculated relief loads for all major services,
such as fractionation towers or reactor loops, should be reviewed at this
time with control systems engineering. Other pressure relief services
which are deemed critical either due to their complexity or the magnitude
or quantity of their relief loads should also be included in this preliminary
review.
1.2.4.2 Preliminary Valve Sizing
The initial sizing of pressure relief devices by the Control Systems
Engineer should begin as soon as there is enough information for an
effective analysis. This is usually before final mechanical design data
such as rotating equipment performance curves are available. In order
to generate a sizing basis, the unit Process Engineer will have to rely on
reasonable assumptions where firm design information is lacking. It is
the Unit Process Engineer’s further responsibility to monitor purchased

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equipment performance data as well as flow sheet developments
throughout the job to ensure that previously specified relief quantities
remain valid or are modified accordingly.
1.2.4.3 Preliminary Nozzle Sizing
The Unit Process Engineer is responsible for determining the size and
location of nozzles for mounting pressure relief devices on process
vessels. Although final pressure relief device sizing information may not
be available when the process vessel specifications are first issued, all
known pressure relief connections should be indicated on the vessel
sketch, with sizes placed on “hold”, if necessary. When the Control
Systems Engineer receives the Relief Valve Calculation Sheet and
carries out preliminary pressure relief device sizing and selection, the
process engineer can then confirm nozzle sizing following appropriate
hydraulic calculations.
Nozzles for pressure relief valves should be generously sized to minimize
the chance that a change will be required after vessel fabrication has
started. Any requirements such as rounded entrance nozzles should be
specified at this time.
1.2.5 Final Data Sheet Preparation
After the Unit Process Engineer issues the Relief Valve Calculation Form (Section
1.2.4), the Control Systems Engineer performs preliminary sizing calculations and
makes a preliminary selection of pressure relief device types and sizes. Once the
P&ID has been fully developed and all equipment specified, the Unit Process
Engineer finalizes relief loads. At this time, the Control Systems Engineer performs
final sizing calculations and prepares detailed purchase specifications which include
specification of materials of construction, accessories and required code stamps in
addition to the pressure relief device size and type.
Guidelines for pressure relief device sizing and selection are discussed in Section 3.
1.2.6 Final Relief Load Computation
The Unit Process Engineer is responsible for summarizing the case loads for all the
events causing relief to the flare headers and establishing the controlling cases for
design. Some recycle of work occurs at this time to optimize the flare system
design. For this reason, an early start to this activity is important.
1.2.7 “As Purchased” Equipment Performance Review
When the equipment is committed for purchase, the assumptions taken during early
development of the flare and relief system must be confirmed. The Unit Process
Engineer is responsible for performing detailed relief case evaluations based on the

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actual purchased capacities of the equipment such as pumps, compressors, control
valves, etc..
1.2.8 Relief Device Installation Review
Pressure relief devices may operate marginally or not at all if improperly installed.
Additionally, codes contain specific installation requirements and limitations. During
P&ID development, the Control Systems Engineer shall be responsible for reviewing
the designs and advising the Project Engineer where the client standards and local
codes are not being met.
Installation requirements are discussed in Section 7.
As soon as piping sketches are available, the Piping Designer and the Unit Process
Engineer must verify that the pressure relief valve inlet and outlet piping pressure
losses are within acceptable tolerances for the installation. This evaluation will serve
as a check of the inlet loss determination which was done (possibly using an
assumed inlet piping configuration) to establish the vessel relief valve nozzle sizes.
If any pipe or nozzle sizes appear marginal, they should be increased at this time to
avoid costly rework. Evaluation of any services which discharge to a common
blowdown header may have to be deferred until all pressure relief valves connected
to that header have been specified.
Maximum allowable losses and methods for computing them are covered in
Section 7.
During detailed piping design, the Piping Designer is responsible for ensuring that
the piping design meets all criteria for pressure relief device installation. This
includes verification that all block valves are properly specified, the piping is
correctly supported, and that it is adequately designed for anticipated thermal
stresses and reactive forces. If necessary, the piping design should be reviewed
with the Control Systems Engineer. In addition, the Piping Designer has a
continuing responsibility for advising the Unit Process Engineer of any significant
changes in piping arrangement, so that hydraulic calculations can be reviewed as
necessary. Copies of all calculations must be retained and included with the
pressure relief device documentation package.
1.2.9 Monitor Design Changes
As the design development for the flare and relief system proceeds, changes
usually occur in the design basis for both the flare and relief system and the process
being protected. This would include establishment of those items for which
assumptions were originally taken to allow development to proceed. The impact of
these changes on the flare and relief system must be constantly monitored by the
Project Engineer to provide the best system design with respect to operation and
cost. The Unit Process Engineer and the Control Systems Engineer are responsible
to keep the Project Engineer informed of these changes and their impacts. An
example of these types of impacts would be the cooling water pump drivers. If an

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early decision had been made that the drivers would all be electrical then total loss
of power would also mean total loss of cooling water. This impact could be
minimized if turbine drivers were mixed with the motor drivers allowing only partial or
no loss of cooling water during total power failure.
1.2.10 Engineering Documentation
As mentioned earlier, design of pressure relief systems is sensitive to processing
conditions, processing rates, equipment size and equipment configuration.
Whenever a change in a unit’s operation or equipment is planned, the effect upon
pressure relief system design must be evaluated. A key factor in being able to
perform these evaluations is availability of engineering records.
Pressure relief system design is complex, and unless detailed engineering records
of the existing systems are kept, it is often prohibitive, both in terms of cost and
time, to reassemble all the process and mechanical data required and to recreate
process relief calculations every time the design basis for a pressure relief system
needs to be reviewed. Therefore, a complete engineering record for each pressure
relief device should be provided to the client, up to the level of detail consistent with
Fluor Daniel’s scope of work on the project. Responsibility for completion of these
files, updated to include as-built conditions, should be clearly defined with the client
when the files are transferred.
All cases in which special operating limits or procedures form part of the relief
system design basis must be clearly identified. Among the items which must be
defined jointly with the client and provided in the design basis are:
• A block valve lock and seal administrative policy and a list of all block valves
which are covered by it.
• Specific operating procedures, such as vent and drain procedures which
apply to the pressure relief system.
• Identification of limitations on equipment operating ranges or modifications
which may be performed.
• Allowable line-ups in systems with installed spare pressure relief devices.
The client has the responsibility for ensuring that operators have clearly defined
procedures and restrictions relative to pressure relief systems, that all operators are
properly trained in these procedures and restrictions, and that the procedures and
restrictions are carefully observed.

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1.3 CODES, STANDARDS AND PRACTICES
1.3.1 ASME Boiler and Pressure Vessel Code
• Section I Power Boiler
• Section IV Heating Boilers
• Section VIII Pressure Vessels
These codes are for design of pressure vessels and boilers in excess of 15 psig
(1.03 barg, 1.05 kg/cm
2
G) internal pressure. This represents the majority of the
vessels in refinery operations. The primary sections of interest in Section I are PG-
67 to PG-73. The primary sections of interest in Section VIII are UG-15 to UG-136
and Appendices M and 11.
1.3.2 API Publications
Standards:
API Std 526 Flanged Steel Safety-Relief Valves
This standard specifies dimensions of carbon and alloy
steel safety-relief valves.
API Std 620 Recommended Rules for Design and Construction of
Large, Welded, Low-Pressure Storage Tanks
This standard is for design of low pressure storage
tanks less than 15 psig (1.03 barg, 1.05 kg/cm2
G) but
greater than 0.28 psig (0.02 barg, 0.02 kg/cm
2
G).
API Std 650 Welded Steel Tanks for Oil Storage
This standard is for design of atmospheric pressure
storage tanks with internal pressures up to 0.28 psig
(0.02 barg, 0.02 kg/cm2
G).
API Std 2000 Venting Atmospheric and Low-Pressure Storage
Tanks (Non-refrigerated and Refrigerated)
This standard covers the specification of relief valves
for vessels and tanks with design pressures less than
15 psig (1.03 barg, 1.05 kg/cm2
G).

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API Std 2510 Design and Construction of LP-Gas (LPG) Installations
at Marine and Pipeline Terminals, Natural Gas
Processing Plants, Refineries, and Tank Farms
Bulletins:
API Bulletin 2521 Use of Pressure Vacuum Vent Valves for Atmospheric
Pressure Tanks to Reduce Evaporation Losses.
Recommended Practices:
API RP 520 Design and Installation of Pressure Relieving Systems
in Refineries - Part I (Design) and Part II (Installation)
This recommended practice has been accepted as the
most authoritative set of rules for sizing and
specification of individual relief devices. As indicated
above, the first part is for design and the second part
is for installation. It is anticipated that this standard will
be used in conjunction with API RP 521 to provide a
consistent design basis for flare and relief system
design.
This practice is intended for use for relief valves to be
installed on vessels and tanks with design pressures
of 15 psig (1.03 barg, 1.05 kg/cm
2
G) or greater.
API RP 521 Guide for Pressure Relief and Depressuring Systems
This recommended practice has also been accepted
as an authoritative set of rules. Its application is for
design of a relief system to safely dispose of the
individual relief loads established by designs
conforming to API RP 520.
API RP 576 Inspection of Pressure-Relieving Devices
1.3.3 NFPA Standards
NFPA 30 Flammable and Combustible Liquids Code
Use this standard for non-refinery low pressure
storage, less than 15 psig (1.03 barg, 1.05 kg/cm2
G).
NFPA 58 Standard for Storage and Handling of Liquefied
Petroleum Gases
Use for non-refinery gas plant LPG storage

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1.3.4 ANSI Standards
B9.1 Safety Code for Mechanical Refrigeration
B19.3 Safety Standard for Compressors for Process
Industries
B31.1 Power Piping
B31.2 Industrial Gas and Air Piping
B31.3 Petroleum Refinery Piping
B31.4 Liquid Petroleum Transportation Piping
B31.5 Refrigeration Piping
B31.6 Chemical Process Piping
B31.8 Gas Transmission and Distribution Piping
B95.1 Terminology for Pressure Relief Devices
1.4.9 NEMA Standards
SM21 Multistage Steam Turbines for Mechanical Drive
Service
SM22 Single Stage Steam Turbines for Mechanical Drive
Service
1.3.5 International Conference of Building Officials (ICBO)
Uniform Building Code (UBC)
1.3.6 American Institute of Steel Constructors (AISC)
Manual of Steel Construction
1.3.7 American Society For Testing Materials (ASTM)
A320 Specification for Alloy-Steel Bolting Materials for Low
Temperature Service.
1.3.8 American Welding Society (AWS)
D.1.1 Structural Welding Code

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D.14.4 Classification and Application of Welded Joints for
Machinery and Equipment.
1.4 DESIGN GUIDE SUMMARY
This Flare System Manual was developed to provide guidance for the engineering effort
involved in the development of safe and efficient flare and relief systems. The tasks
involved in this development include proactively obtaining the inputs of the project
engineering task force at appropriate times as well as performing the design engineering
functions. For this reason a matrix of approaches appear in this Section. Figure 1.1,
Typical Relief System Engineering Schedule, presents the developmental tasks from Table
1.1 on a timeline to clarify the timeliness of tasks number 1 through 10. The same tasks
are presented again in Figure 1.2, Typical Relief System Activity Flow Chart, to give a
perspective to the engineer developing the relief system. Note the recycle of changes in
Figure 1.2. These recycle activities are frustrating but necessary to the most efficient
overall development of a project. Prior knowledge that these recycles will occur and
contingency planning to minimize the impacts will add significantly to the smooth flow of the
project and a timely completion. The temptation to wait until all the necessary inputs are
available and final must be avoided if project schedules are to be met.
The Essential Criteria for Flare and Relief System, Table 1.2, provides a checklist that
should be referred to by the Lead Process Engineer as the tasks in Table 1.1 are
performed. Because of the uniqueness of each project, additional essential criteria should
be identified and developed by the Lead Process Engineer on a project by project basis. If
any of the criteria in Table 1.2 are interpreted to be in conflict with the later Sections of this
design guide, the detailed guidelines in the individual Sections should be given precedence.
The same is true for the summary of technical tasks below.
The following technical tasks must be performed in the development of a flare and relief
system design. Each task is a summary or condensation of the items which are discussed
in further detail in the referenced location in the following Sections. The use of this
summary presupposes that the engineer is knowledgeable about the background
information contained in these Sections and API RP 520 and 521. Where this is not the
case, the appropriate material should be reviewed.
1.4.1 Establish Design Pressure of Vessels and Piping (Section 2)
• Vessels - Select the highest of:
1. Operating pressure plus 25 psi (1.8 bar, 1.8 kg/cm
2
)
2. Operating pressure times 1.1 for vapor and 1.2 for liquid
3. 50 psig (3.4 barg, 3.5 kg/cm
2
G) if a PSV on the vessel PSV relieves
to the flare header
4. 30 psig (2.1 barg, 2.1 kg/cm
2
G) if the vessel PSV relieves to
atmosphere
5. 15 psig (1.03 barg, 1.05 kg/cm
2
G) if the vessel is vented to
atmosphere

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• Piping - Select the higher of:
1. Vessel design pressure for vapor
2. Vessel design pressure plus static head for liquids
3. Centrifugal pump shutoff pressure
4. Positive displacement pump, based on relief valve set pressure,
which is typically set at the lower of:
a. Casing mechanical design pressure
b. Higher of rated pressure plus 25 psi (1.8 bar, 1.8 kg/cm
2
) or
110% of rated pressure.
1.4.2 Establish Design Temperature of Vessels and Piping (Section 2)
Set the design temperature at the maximum operating temperature coincident with
the design pressure selected from section 1.5.1 above plus a design margin as
shown:
• Add 50 °F (28 °C) for operating temperatures up to 650 °F (343 °C)
• Add 25 °F (14 °C) for operating temperatures over 650 °F (343 °C)
1.4.3 Select Type of Relieving Device (Section 3)
• Conventional Pressure Relief Valve
Spring loaded, built-up back pressure tends to reseat the valve beyond about
10% of set pressure.
•
•
•
• Balanced Pressure Relief Valve (Piston & Bellows types)
Spring loaded, the effects of built-up back pressure are reduced greatly by
use of piston or bellows to allow up to 30 or 40% of set pressure without
capacity reduction.
•
•
•
• Pilot Operated Pressure Relief Valve (Piston & Diaphragm types)
Consists of a main valve and an external pilot valve that can be modulating
or “pop” action.
•
•
•
• Safety Valve
Primarily for ASME Section I relief service. These spring loaded valves
provide full opening with minimum overpressure.
•
•
•
• Relief Valve
Spring loaded pressure relief device for liquid relief service.

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•
•
•
• Safety Relief Valve
Liquid and vapor service. The majority of the refinery relief devices are of
this type. This type valve performs like a safety valve in vapor service (pop
action) and a relief valve in liquid service.
• Rupture Disk
Rupture disks are used only infrequently for special applications
1.4.4 Establish Individual Relief Loads (Section 4)
• Establish Sizing Basis for the PSV
• Add the relief load to the total relief summary on a case by case basis
1.4.5 Calculate Required Relief Device Orifice Area (Section 3)
API Sizing Equations
• Vapor in Critical Flow
A = [ W / (C Kd P1 Kb) ] [ (T Z) / MW ]
0.5
3.1 (English)
A = 1.316 [ W / (C Kd P1 Kb) ] [ (T Z) / MW ]0.5
3.1 (Metric)
A = 1.342 [ W / (C Kd P1 Kb) ] [ (T Z) / MW ]0.5
3.1 (Metric)
a. Check if Back Pressure Corrections are required by valve
manufacturer (Kb)
b. Check if Sub-Critical Flow Equation applies (Equation 3.4)
• Sub-Critical Vapor Flow
A = [ W /(735 F2 Kd )] [(Z T) / (MW P1 (P1 - P2))]0.5
3.4 (English)
A = 1.316 [ W /(735 F2 Kd )] [(Z T) / (MW P1 (P1 - P2))]0.5
3.4 (Metric)
A = 1.342 [ W /(735 F2 Kd )] [(Z T) / (MW P1 (P1 - P2))]0.5
3.4 (Metric)
• Steam Flow
A = Ws / (51.5 P1 Kd Kn Ksh ) 3.6 (English)
A = 1.316 (Ws) / (51.5 P1 Kd Kn Ksh ) 3.6 (Metric)
A = 1.342 (Ws) / (51.5 P1 Kd Kn Ksh ) 3.6 (Metric)

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• Liquid Trim Valves
A = [Q / (38 Kd Kw Kv )][G / (P1 - P2 )]
0.5
3.7 (English)
A = 7.456 [Q / (38 Kd Kw Kv )][G / (P1 - P2 )]
0.5
3.7 (Metric)
A = 7.528 [Q / (38 Kd Kw Kv )][G / (P1 - P2 )]0.5
3.7 (Metric)
• Conventional PSV in Liquid Service
A = [Q / (38 Kd Kw Kv Kp )][G /(1.25Ps - Pb )]0.5
3.8 (English)
A = 7.456 [Q / (38 Kd Kw Kv Kp )][G /(1.25Ps - Pb )]0.5
3.8 (Metric)
A = 7.528 [Q / (38 Kd Kw Kv Kp )][G /(1.25Ps - Pb )]0.5
3.8 (Metric)
Note: See Appendix C for a listing of vendor computer programs for valve
sizing
1.4.6 Review Disposal Options (Section 6)
Perform a HAZOP and risk assessment to establish the best location for disposal of
refinery waste streams. See Table 6.1 for typical toxic or hazardous chemicals
encountered around the refinery. The following locations should be considered:
• Atmosphere
• Grade or sewer
• Process Vessel
• Closed System (Flare Header)
1.4.7 Establish Equipment Depressuring Requirements (Section 4)
1.4.8 Size Thermal Relief Valves (Section 4)
1.4.9 Evaluate Process Flow Loops (Section 4)
• Settling Out Pressure
• PSV Relieving rate, if a PSV is required
1.4.10 Evaluate Total Relief Loads to the Flare, by Contingency, to Include Depressuring
(Section 5)
1.4.11 Consider Mitigation for Relief Load Reduction (Section 5)
1.4.12 Review Depressuring Loads for Time Smoothing (Section 5)

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1.4.13 Size Relief Valve Piping Inlet/Outlet (Section 7)
The Relief system piping is to be designed according to the following guidelines:
• Relief device inlet/outlet piping must meet the pressure drop limitations in
Table 7.1
• Use the SIMSCI INPLANT program for hydraulics
1.4.14 Establish Required Purging Rates by Converting Velocities Given Below into Rates
[lb/h (kg/h) or SCFH (Nm
3
/h)] for the Relief Piping (Section 7)
Note: The quantity of purge gas for an elevated flare depends upon the type of seal
selected.
• Normal Purge Gas Rate:
a. Elevated Flare - 0.10 ft/s (0.03 m/s) in stack/tip [design for 0.23 ft/s
(0.07 m/s) maximum]
b. Enclosed Ground Flare - 0.10 ft/s (0.03 m/s) in each first stage tip
[design for 0.23 ft/s (0.07 m/s) maximum]
• Special Purge Gas Rate (intermittent):
a. Elevated Flare - 3.3 ft/s (1.0 m/s) in stack/tip
b. Ground Flare - 3.3 ft/s (1.0 m/s) in main header
1.4.15 Select and Specify the Following Equipment where Appropriate (Section 8)
o Blowdown Drum
Required for sizable liquid relief loads only
o Knock Out (KO) Drum
o Seal Drum
This drum is mandatory and will be included in every design.
o Quench Drum
This type of drum is seldom required and is a specialty design item.

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1.4.16 Develop Flare Stack and Tip Details (Section 9):
o Select flare location
o Select flare type (reference Table 9.1)
• Elevated Flare
• Ground Flare
o Check if assist fuel is required
o Select gas seal type
o Select metallurgy for piping, flare tip and seal
o Select the flare diameter based on required design flow rate and velocity
limitations and verify the tip diameter proposed by vendor.
o Calculate the flare stack height based on allowable ground level radiant heat
intensity, using Brzustowski’s and Summer’s method per API RP 521 (see
Section 9.4.4):
• 500 Btu/ft2
/h (5,677 kJ/m2
/h, 1,356 kcal/m2
/hr) for operating areas
• 1,500 Btu/ft
2
/h (17,040 kJ/m2
/h, 4,069 kcal/m2
/h) for short term
exposure
• 3,000 Btu/ft
2
/h (34,070 kJ/m
2
/h, 8,137 kcal/m
2
/h) for limited access
areas with shelter
o Confirm that acceptable emission limits of Section 9.8.3.3 are met by
selected stack height or adjust height.
o Check adjacent equipment surface temperatures during design release
(reference Section 9.4.6).
o Set smokeless flaring rate (reference Section 9.6 and Figure 9.6)
o Review noise specifications SP-45820 (Equipment Noise Level Limits) and
SP-45230 (Noise Abatement)
o Review Environmental Emissions to meet local environmental regulations.
o Establish type of Flare Ignitor:
a. Pressure Ignitor

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This type ignitor is traditional for flare systems
b. Electronic Ignitor
This type ignitor is cheaper and has been providing higher reliability
than in the past. Review vendor designs with special concern for
experience with reliability. This will probably become the standard in
the future.

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TABLE 1.1
PRESSURE RELIEF SYSTEM DESIGN RESPONSIBILITIES
Task Primary
Responsibility
Secondary/Consultant
Responsibility
Notes
1. Establish Scope,
Design
Philosophy and
Standards
Lead Process
Engineer
Unit Process Engineer
Flare System Process
Engineer
Review Scope, Standards and
Guides, and applicable Laws
and Codes for application to the
design. Add client involvement.
2. System
Assessment
System assessment consists of
a review of the process design
to identify where relief system
requirements may affect design
pressures, plot plans or other
basic design specifications. Key
information required: Process
H & M Balances and PFD.
A.) Design
Pressures and
Temperature
Unit Process
Engineer
Control Systems
Engineer
Mechanical Engineer
Piping Engineer
Consider pump/compressor
shutoff heads, additional
throughput, and potential relief
cases.
B.) Relief Stream
Disposal
Unit Process
Engineer
Environmental
Engineer
Define needs for recovery
systems, flares, etc. Identify
any special permit
requirements.
C.) Fire Exposure Flare System
Process Engineer
Mechanical Engineer
Review plot plan for location of
vessels and exchangers
containing volatile liquids.
D.) Utility Failure
Basis
Flare System
Process Engineer
Lead Process
Engineer
Establish relief system design
philosophy and utility failure
basis.
E.) System Review Lead Process
Engineer
Control Systems
Engineer
Engineer
Refinery Operations
Mechanical Engineer
Piping Engineer
Review design pressures, utility
failure basis, plot plans and
preliminary relief system
requirements. Identify any
design pressure or layout
changes required or any relief
cases, which need special
attention. Identify any changes
in the utility system plans, which
are beneficial to relief system
design.

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TABLE 1.1 (Continued)
Task Primary
Responsibility
Secondary/Consultant
Responsibility
Notes
3. Relief Source
Identification
A.) Basic Design Unit Process
Engineer
Mechanical Engineer
Engineer
Control Systems
Engineer
Identify sources of process
stream relief loads during
P&ID development, identify
services which require relief
devices.
B.) Design Review Unit Process
Engineer
Lead Process
Engineer Control
Systems
Engineer
Engineer
Mechanical Engineer
Perform at time of review of
Issue for Approval P&ID’s.
Review P&ID’s to verify that all
services requiring
overpressure protection have
relief devices and that they are
properly located. Review relief
cases, which should be
evaluated.
C.) Design
Monitoring
D.) Flare System
Design Activity
Plan
Unit Process
Engineer
Flare System
Engineer
Lead Process
Engineer Control
Systems
Engineer
Mechanical Engineer
Lead Process
Engineer
Piping Engineer
Continue to assure that earlier
design premises remain either
unchanged or within
acceptable limits.
Establish flare design activity
plan and identify limitations for
flare. Develop load summary
format and have Unit Process
Engineer fill flare load
summary. Estimate flare load
in reference to past similar
plant for early start of flare
system design.

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Task Primary
Responsibility
Secondary/consultant
Responsibility
Notes
4. Preliminary PSV
and Vessel
Nozzle Sizing
A.) Preliminary
Relief Load
Calculation
Unit Process
Engineer
Control Systems
Engineer
This step assists in identifying
early needs for adequately
sized PSVs and vessel nozzles
B.) Preliminary
Valve Sizing
Unit Process
Engineer
Control Systems
Engineer
Process engineer performs
sizing calculations.
C.) Preliminary
Nozzle Size
D.) Issue
Preliminary PSV
Data Sheets
Unit Process
Engineer
Unit Process
Engineer
Mechanical Engineer
Piping Engineer
Flare System Engineer
Control System
Engineer
coordinates with vessel group.
Final nozzle size after relief
device selection.
Issue PSV data sheets to
control system engineer
5. Relief Device
Installation
Review
Unit Process
Engineer
Control Systems
Engineer
Piping Engineer
Mechanical Engineer
performs a review of inlet &
outlet piping losses. Markup
process P&ID's to reflect
installation.
Piping Engineer performs force
and moment calculations.
Unit Process Engineer and
Piping Engineer to verify that
calculations have been done
and that the installation meets
code and regulatory
requirements.

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Task Primary
Responsibility
Responsibility
Notes
6. Flare System
Design
A.) Flare Load
Summary
B.) Preliminary Flare
System Design
C.) Optimize Flare
System
D.) Finalize Flare
Load and Relief
Valve Sizes
E.) Issue P&ID's for
Construction
Flare System
Engineer
Flare System
Engineer
Flare System
Engineer
Flare System
Engineer
Flare System
Engineer
Unit Process
Engineers
Mechanical Engineer
Piping Engineer
Piping Engineer
Piping Engineer
Mechanical Engineer
Piping Engineer
Mechanical Engineer
Collect unit flare load summary
with relief device calculation
sheets. Check consistency.
Perform preliminary sizing
based on collected relief load,
produce P&ID's, size
preliminary flare equipment
consulting with mechanical
engineer, and discuss flare
sterile area and header routing
with Piping Engineer.
Obtain flare piping Iso sketches
from piping and perform
detailed flare header hydraulic
analysis. Optimize flare size by
analyzing relief valve
backpressure. Check any
changes of utility failure basis
per project development.
Finalize flare load and flare
piping to meet the project
objective (cost and schedule).
Inform Unit Process Engineer
and Piping Engineer about the
final size requirement and
buildup backpressure.
Issue P&ID's for construction
after client review.

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Task Primary
Responsibility
Responsibility
Notes
7. Final Relief Load
Computation and
sizing
Unit Process
Engineer
Control Systems
Engineer
Engineer
performs final check of relief
load case evaluations and relief
valve sizing based on finalized
flare system design.
8. Final PSV Data
Sheet Revision
Unit Process
Engineer
Control Systems
Engineer
Issue final revision for Control
Systems Engineer to prepare
instrument data sheets and
purchase specifications.
9. “As Purchased”
Equipment
Performance
Review
Unit Process
Engineer
Control Systems
Engineer
Mechanical Engineer
The Unit System Process
Engineers to review relief load
calculations to assess any
effect that “As Purchased”
pump, compressor or control
valve capacities may have
upon relief loads.
10. Monitor design
changes
Lead Process
Engineer
Control Systems
Engineer
Review any design changes
occurring during project
development and evaluate for
impacts on the design of the
flare and relief system.
Examples would include "As
Purchased " equipment
checks, electrical system
development, cooling water
system design basis and steam
system design basis.

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Task Primary
Responsibility
Responsibility
Notes
11. Engineering
Documentation
Unit Process
Engineer
Project Engineer
Lead Process
Engineer
Prepare relief device
engineering files. Client
Control Systems Engineer and
Unit Process Engineer review
for completeness. Client
project engineer responsible
for requiring that as-built
updates are properly
completed and placed into the
files.

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TABLE 1.2
ESSENTIAL CRITERIA FOR FLARE AND RELIEF SYSTEM
ITEM RECOMMENDED PRACTICE
1. GENERAL
1. Client relief standards Provide Applicable Client Engineering
Standards, Standard Specifications and Design
Guides to design team or contractor.
2. Identify flare type required See Section 9.2
3. Identify criteria for process relief to
atmosphere
All hydrocarbon relief loads vent to relief header.
See ES1100 for reference. See Section 6.5.2.
4. Consideration of flare gas vapor recovery For significant flare quantities, vapor recovery
economics should be reviewed. See Section
6.8.3.
2. EQUIPMENT/PIPING CRITERIA
Select between elevated or ground flare. See
Section 9.2.
2A. ELEVATED FLARE See Section 9.
1. Design radiation levels Per API RP 521. See Section 9.4.5.
Personnel protection
Long Term Exposure 500 Btu/ft
2
/h (5,677 kJ/m
2
/h, 1,356 kcal/m
2
/h)
Short Term Exposure 1,500 Btu/ft
2
/h (17,040 kJ/m
2
/h, 4,069 kcal/m
2
/h).
Limited access is required around areas where
this limit can be exceeded, for personnel
protection.
Restricted Access 3,000 Btu/ft
2
/h (34,070 kJ/m
2
/h, 8,137 kcal/m
2
/h).
Shelter to be available
2. Spacing for flare stack See Section 9.4.4. Note: US insurance industry
standards suggest minimum spacing of at least
330 feet (100 meters) from process units,
hazardous storage, tanks and loading or
unloading facilities. This minimum distance
reduces to 213 feet (65 meters) if flare stack
height is greater than 82 feet (25 meters).

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3. Allowable noise See Section 9.8
50 ft (15 m) from base of flare Per client standards. Generally, 90 dBA at
maximum smokeless rate and 125 dBA limit
for design flare relief.
At plant boundary Per client standards. Generally, 75 dBA at
maximum smokeless relief rates
4. Air Pollutants Must be below the allowable limits specified
in Client Environmental Emission & Effluent
Limits. See Section 9.8.3.3.
5. Smokeless flaring load Maximum continuous load which can
exceed 5 minutes during any 2 consecutive
hours. If continuous load is not identifiable,
use the smaller of 10% of peak load or flare
tip maximum smokeless capacity given by
the tip supplier. See Section 9.6.
6. Radiation calculation method
7. Fraction of heat radiated
Use Brzustowski’s and Sommer’s method
(Sect. 9.4.4).
0.15 for maximum flare load. 0.25 for
continuous flare load. See Section 9.4.5.
8. Design wind velocity Maximum wind velocity for maximum flare
load [if none available use 30 ft/s (8.9 m/s)].
See Section 9.4.4.
Normal wind velocity for continuous flare
load [if none available use 10 ft/s (3 m/s)].
9. Maximum flare tip mach number Max. 0.5 for initial design. Max. 0.7 for
revamp (establish with flare vendor). See
Section 9.4.3.

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10. Flare tip type See Section 9.7
11. Identify any stack height limitations See Section 9.4.4
Ground level heat flux See Item 2A.1 and Client Standard SP-
46410
Pollutant Dispersion Must maintain ground level concentration of
air pollution below allowable limits specified
in client standards or government codes .
Aircraft Clearance Per local aviation regulations
12. Availability of Utilities
Fuel Gas To be developed on a project basis
Propane To be developed on a project basis
Compressed Air To be developed on a project basis
Steam To be developed on a project basis
Power To be developed on a project basis
13. Acceptable Seal Types: Use at least two out of the listed items, one
of which must be gas purge. Water seal
drums, if used, must be protected against
freezing. See Section 7.5.
Liquid Seal
Flame Arrestor
Gas Seal
Gas Purge (Nitrogen or fuel gas) See Section 7.5.2 for normal purge rate
Other

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14. Pilot requirements Continuous pilot required. See Section 9.9
15. Pilot monitoring Continuous pilot monitoring required. Direct
sensing should be used; thermocouples for
pressure igniters and self-contained system
for electronic igniters. Backup system can
include remote sensor (IR, visual, other).
16. Identify available plot area locations
(per plot plans).
Per project requirements
2B. GROUND FLARE See Section 9.5
1. Enclosed Ground Flare Used primarily if normally visible flame is
not acceptable.
Establish enclosure dimensions A reputable vendor must size the enclosure
Confirm clearance from top of flare
enclosure to adjacent equipment, buildings
or platforms.
Enclosure is sufficiently deep to contain
flames.
2. Other Types See Section 9.5
2C. FLARE KNOCK OUT DRUM See Section 8.1
1. Select horizontal or vertical vessel Normally a horizontal vessel is selected.
However, spacing and capacity requirements
may dictate a vertical drum.

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2. Sizing Method See Section 8.1
3. Steam Heater Required to prevent freezing
4. Pumpout rate criteria Largest of:
• 425 ft
3
/h (12 m3
/h)
• 1.5 x maximum continuous relief rate
• 0.25 x maximum instantaneous relief rate
See Section 8.5
5. Pump Sparing philosophy/driver Preferred configurations are either one
motor drive with spare turbine drive or two
motor drives with emergency power backup.
Two motor drives supplied with power from
independent low voltage feeders can be an
acceptable alternative, provided that
analysis confirms simultaneous failure of
the two feeders does not also result in
significant liquid relief load. See Section
8.5.
6. Design droplet size For initial design: 400 micron
For revamp: 600 micron
7. Design liquid gravity Default value = 0.7 for drum sizing, 0.7-1.0
for pump selection.
8. Design L/D ratio 2:5 to 3:1, unless at maximum drum
diameter for shipping
9. Indicate if any liquid relief services
should discharge to a separate
knockout drum before entering a
common header with vapor relief
loads.
Normally all relief streams are combined in
a common unit header with liquid separation
in a unit knockout drum.

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10. Liquid Surge Capacity
w/auto transfer pump start 15 minutes of max. instantaneous liquid
drop out
w/manual transfer pump start 30 minutes of max. instantaneous liquid
drop out
Where no clear drop out rate has
been established
Horizontal Drum 25 % of the KO Drum diameter
Vertical Drum 20% of the KO Drum Tangent-to-Tangent
length
2D. UNIT BLOWDOWN DRUMS
1. Unit Blowdown Drum to be
provided
Job Specific
2. Sizing Methods Maximum volume relieved over 30 minute
period. See Section 8.2.
2E. SEAL DRUM See Section 8.3 and Figure 7.3
1. Seal leg on the inlet line Minimum 10 ft (3 m) seal leg. See Section
8.3.2.1.
2. Seal Depth 1 ft (0.3 m) minimum or as required to
maintain specified backpressure. See
Section 8.3.2.1.
3. Water above the minimum seal Sufficient to allow a 10 ft (3 m) vacuum to
be pulled on the flare header without losing
the water seal. See Section 8.3.2.1.
4. Overflow seal height Greater of 10 ft (3 m) or 150 % of maximum
back pressure. See Section 8.3.2.1.

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5. Winterization Steam heated coil or nonreactive antifreeze
sealant in the seal drum.
6. Seal water supply Confirm level is automatically maintained
7. Transfer pump start Confirm autostart
8. Spare transfer pump Confirm a spare is available
9. Anti-slosh baffling
Horizontal Drums Add transverse baffles as required. See
Figure 8.4
Vertical Drums See Figure 7.3
2F. FLARE HEADER See Section 7
1. Minimum slope of header 8 in/330 feet (21 cm/100 meters) (0.21%),
per API RP 521. See Section 7.1.1.
2. Design Pressure Maximum back pressure plus 10 percent or
50 psig (3.4 barg, 3.5 kg/cm
2
G), whichever
is greater. See Section 7.1.3.
3. Design Temperature Stress is based on maximum temperature
possible including fire relief. Heat losses
from the header may be considered. Wall
thickness is based on maximum
temperature excluding fire relief. Minimum
temperature is based on auto-refrigeration
of relief fluids. See Section 7.1.2.

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4. Purge gas source and preferred
method of purging.
Continuous sweep of gas through the
headers using fuel gas or inert gas. See
Section 7.5.
Normal purge gas rate:
• Elevated flare
- Normal operation: 0.10 ft/s (0.03 m/s)
in stack/tip
- Upset condition: 3.3 ft/s (1.0 m/s) in
stack/tip
• Enclosed ground flare
- Normal operation: 0.10 ft/s (0.03 m/s)
in first stage tips
- Upset condition: 3.3 ft/s (1.0 m/s) in
main header
5. Pressure test method X-ray welds; can also add air/nitrogen test.
See Section 7.1.3.
6. Sonic velocity at branch outlet
acceptable?
Yes, if not in prolonged service. See
Section 7.2.1.6.
7. Define relief header normal
operating pressure w/o emergency
relief loads.
Typical:
• 0.43 psig (0.03 barg, 0.03 kg/cm
2
G)
normal [1.0 ft (0.3 m) seal depth]
• 1.4 psig (0.1 bar, 0.1 kg/cm
2
G) with
vapor recovery
• Can be higher with enclosed ground
flare
See Section 7.1.7.
8. Define relief header operating
pressure for major relief load
cases.
Per Project Basis

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2G. RELIEF VALVE CRITERIA See Section 3
1. Indicate any specific limitations on
the use of bellows or conventional
valves.
Normally bellows will be used for relief to
header unless conventional valves are
clearly acceptable based on back pressure.
2. Are pilot valves acceptable?
Indicate any special restrictions or
instructions relating to the use of
pilot valves
Pilot valves will be considered for: light
gases at high pressure where operating
pressures are close to the set pressure, for
system conditions that may cause
chattering in spring loaded valves, and
where remote sensing is required.
3. Indicate any preference or
requirements for locating PSV
directly on vessel nozzle versus on
the overhead piping.
Either option is acceptable. Choice is
based on inlet loss criteria, access and
cost.
4. Indicate any relief services which
are required to be spared and
related inlet and outlet block valve
arrangements.
No spares are normally provided.
Exceptions are equipment governed by
ASME Boiler Code and equipment in critical
services.
5. PSV Installation Criteria See project design criteria and API RP 520
Part-2
2H. PSV ISOLATION VALVES Block valves are not normally installed, with
the following possible exceptions:
o Dual type PSV installed on one vessel
o Spare pressure vessel w/PSV used for
standby

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o PSV in parallel w/auto depressuring valve
o Thermal relief valves on piping
Block valves must be car sealed or locked
in open position as required to protect
operating system.
3. ENGINEERING CALCULATIONS
3A. ADMINISTRATIVE EXEMPTIONS
See Section 4
1. All exchangers isolated for
maintenance only are to be
immediately drained.
Eliminates the fire relief case for
maintenance situations. See Section
4.8.3.5.
2. Lock/car seal procedures are to be
rigidly observed and monitored.
This allows maintenance isolation without
adding blocked discharge case.
3B. INDIVIDUAL RELIEF CASES
1. General Fire See Sections 4.8 and 4.10
a. Applicable Codes:
General Refinery, Section VIII
vessels. (Greater than 15
psig (1.03 barg, 1.05 kg/cm
2
G)
API RP 520 and 521 (To be confirmed)
Refinery LPG Storage API RP 520 and 521 with wetted area per
API 2000 for conservative design.
Refinery Low Pressure Storage,
[Less than 15 psig (1.03 barg,
1.05 kg/cm
2
G)]
API 2000

FLUOR DANIEL
FLARE SYSTEM
SECTION 1.0
PROCESS MANUAL
INTRODUCTION
PAGE 37
DATE 06-00
Chap1-r1.doc
Sheet 11 of 16
Non-Refinery Low Pressure
Storage, [Less than 15 psig
(1.03 barg, 1.05 kg/cm
2
G)]
NFPA 30
Non-Refinery/Gas Plant LPG
Storage
NFPA 58
b. Any special insurance
requirements?
By Client Insurance Underwriter
c. Insulation/banding criteria to
allow credit for insulation.
Stainless steel wire or banding and
aluminum jacketing. See Section 4.8.3.2
d. Insulation credit (F Factor) 0.075 to 1.0, based on calculations per API
guidelines. A value lower than 0.075
should only be used under special
circumstances, based on detailed
calculations and review. See Equation
4.12
e. Include vertical vessel bottom
head in fire heat flux area
calculation
Unless contradicted by specific guidelines
based on skirt design.
2. Air Cooler Fire Mitigate by free draining and locating away
from fire zone, rather than by relief valve.
3. Exchanger Tube Rupture Analysis is not required if the low pressure
side test pressure (adjusted for
temperature) is equal to or greater than the
high design pressure. For a typical
hydrotest pressure of 1.5 times design
pressure, this equates to a low pressure
side design pressure greater than or equal
to 2/3 the design pressure of the high
pressure side.

FLUOR DANIEL
FLARE SYSTEM
SECTION 1.0
PROCESS MANUAL
INTRODUCTION
PAGE 38
DATE 06-00
Chap1-r1.doc
Sheet 12 of 16
Where relief protection is provided on the
high pressure side, relief valve set
pressure may be substituted for design
pressure, if lower.
See Section 4.7 for calculation format
4. Air cooler cooling credit during
power failure
25% of normal duty (20 to 30% per API RP
521), provided louvers do not fail closed
due to power failure.
5. Use of instruments to eliminate
individual relief valve load
See Section 5.3 for flare load minimization.
6. Operator response time/criteria to
prevent relief case.
10 minutes allowed. Time begins from alert
from an alarm independent from any
instrument that could cause upset
conditions. Upset must be readily resolved
by one clear operator action. See Section
4.1.7.
3C. UNIT DESIGN BASIS
1. Heat and material balance
See Section 4.1.1
All loads are based on process design
material balance rates and duties, unless
client requests any specific margin(s) for
future debottlenecking.

FLUOR DANIEL
FLARE SYSTEM
SECTION 1.0
PROCESS MANUAL
INTRODUCTION
PAGE 39
DATE 06-00
Chap1-r1.doc
Sheet 13 of 16
2. Relief Valve Sizing
a. Size relief valve inlet/outlet for
maximum flow based on these
limits, or maximum valve capacity
per requirements in Table 7.1.
b. Turbine drivers rated at 105% of
normal speed.
c. Fired heaters are rated at 125% of
rated firing.
d. Towers operated up to flood or by
reboiler/condenser limits
e. Exchangers in clean condition for
heat addition and fouled condition
for heat withdrawal.
4. RELIEF SYSTEM LOADS
4A. CALCULATE MAJOR FLARE
SYSTEM LOADS
See Section 5
See Section 5.2
1. Fire Area 60 feet (18 meter) diameter fire circle [2500
to 5000 ft
2
(232 to 465 m2
) per API RP 520
criteria] minimum, unless special
containment provided (dikes, walls, etc.)
Entire area of diked storage system
considered to be a single fire circle.
2. Identify Major Utility Power Failure
Modes
Identify any recommended exclusions such
as total power failure. Prepare utility failure
system basis.

FLUOR DANIEL
FLARE SYSTEM
SECTION 1.0
PROCESS MANUAL
INTRODUCTION
PAGE 40
DATE 06-00
Chap1-r1.doc
Sheet 14 of 16
3. Identify impact of power failure on
other utility systems, particularly
cooling water, instrument air, steam
and fuel gas.
Per Project Basis
4. Identify impact of calculated flare
load.
Prepare load summary, calculate radiation
and hydraulic impacts for major loads. If
loads are excessive, identify design
changes or mitigation steps to reduce
loads.
4B. DESIGN MODIFICATIONS TO
REDUCE LOADS
See Section 5.3.2
1. Electrical system configuration Modify to reduce simultaneous relief loads.
2. Equipment design pressure Increase design pressure to reduce tower
loads by inducing reboiler pinch.
3. Column accumulator capacity Increase accumulator volume to avoid
flooding during upset conditions.
4C. RELIEF LOAD MITIGATION
OPTIONS
See Section 5.3.3 through 5.3.6
1. Non-normal automatic
instrumentation (single loop)
Auto-start spare pumps and single loop
shutdown systems. Only 3 out of first 6
assumed to work. See Table 5.1.

FLUOR DANIEL
FLARE SYSTEM
SECTION 1.0
PROCESS MANUAL
INTRODUCTION
PAGE 41
DATE 06-00
Chap1-r1.doc
Sheet 15 of 16
2. Redundant dual loop trips High reliability. Four out of 5 assumed to
function on demand. See Table 5.2. Used
normally to cut off heat supply to columns.
3. Redundant loop trips with triple
modular redundant (TMR)
architecture.
Two out of three voting logic. Can be very
expensive and therefore are used only for
most critical services, typically for fired
heaters. Nine out of ten assumed to
function on demand. See Sections 5.3.4.4
and 5.3.5.3.
4. High integrity protective instrument
systems.
Reliability calculated to be in excess of relief
valve reliability. Similar to item 3 above, but
with independent trip sources. Only to be
used where relief valves can not practically
provide protection. Requires calculations to
verify reliability and Client management’s
approval. See Sections 5.3.4.5 and 5.3.5.3.
5. Documentation, maintenance and
testing.
For load reduction credit to be taken for any
of these systems, reliability must be
maintained and proven through testing.
Documentation and quality controls must be
consistent with normal relief system
practices
6. Dynamic simulation Detailed dynamic simulation can sometimes
provide a basis for significantly reducing
key column loads.

FLUOR DANIEL
FLARE SYSTEM
SECTION 1.0
PROCESS MANUAL
INTRODUCTION
PAGE 42
DATE 06-00
Chap1-r1.doc
Sheet 16 of 16
ITEM RECOMMENDED PRACTICE R
5. HYDRAULICS
1. Flare header/subheader
2. Individual laterals
3. Relief valve inlet
4. Relief valve outlet
See Section 7
Based on worst case (for hydraulics) total
calculated simultaneous releases. See
Section 7.3.
Based on relief valve flow used for relief
outlet.
See Table 7.1

FLUOR DANIEL
FLARE SYSTEM
SECTION 1.0
PROCESS MANUAL
INTRODUCTION
PAGE 43
DATE 06-00
Chap1-r1.doc
FIGURE 1.1
TYPICAL RELIEF SYSTEM
ENGINEERING SCHEDULE
TASK
INITIAL SCOPE (PROCESS TEAM)
A. FRONT END WORK
Basic Process Design
Basic Flare System Design Basis
Rough Equipment Specifications
Relief System Assessment Preliminary
B. PROJECT DEVELOPMENT
P&ID Development (Process)
P&ID Development (Flare)
Relief System Assessment Definitive
C. REVIEW AND DETAILED ENGINEERING
P&ID Review & Approval for Engineering (Process)
P&ID Review/Approval for Engineering (Flare)
Review Relief System Assessment
PSV Location
PSV Sizing (Preliminary)
D. COMMIT DESIGN & EQUIPMENT
P&ID’s Issued Approved for Construction
PSV Final Data Sheet Issue
Relief Load Summaries (Final)
Equipment Purchased
PSV Installation Reivew
Design Changes
Engineering Documentation

FLUOR DANIEL
FLARE SYSTEM
SECTION 1.0
PROCESS MANUAL
INTRODUCTION
PAGE 44
DATE 06-00
Chap1-r1.doc
FIGURE 1.2
TYPICAL RELIEF SYSTEM
ACTIVITY FLOW CHART
Note: Numbers in parentheses at the top of the boxes are the Pressure Relief System Design
Responsibilities from Table 1.1
(1)
Establish Scope
Design Philosophy
and Standards
(2a to 2e)
Relief System
Assessment
(3a to 3d)
Identify Relief
Sources
(4a)
Preliminary Relief
Load Calculation
(6d to 6e)
Final Flare Load
and System Design.
Issue P&ID's for
Construction
(5)
Review Relief
Device Installation
(7)
Final Relief Load
Calculations and
PSV Sizing
(4b to 4d)
Preliminary PSV
and Vessesl Nozzle
Sizing
(8)
PSV Data Sheet
Revision
(6c)
Optimize Flare
System
(6a to 6b)
Preliminary Flare
System Design
Prelimary Estimate
(9)
Review "As
Purchased"
Equipment
Performance
(10)
Monitor Design
Changes
(11)
Engineering
Documents

FLUOR DANIEL
FLARE SYSTEM
SECTION 2.0
PROCESS MANUAL
DESIGN TEMPERATURE AND
PAGE 1
TEMPERATURE SELECTION DATE 06-00
Chap2-r1.doc
2.0 DESIGN PRESSURE AND TEMPERATURE SELECTION
2.1 DESIGN PRESSURE SELECTION
In order to make an appropriate selection of design pressure it is necessary to understand
the relationship between design pressure and other parameters, such as set pressure, back
pressure, accumulation, MAWP, blowdown, etc. Simple definitions of these parameters
are presented in Section 2.3 and will be discussed in greater detail in later Sections. If the
design pressure is selected without consideration of the relief device requirements then the
design pressure may need adjustment at a later date with negative impacts on project
schedule, cost, and performance of the unit.
2.1.1 Operating Pressure
Operating pressure (psig, barg, kg/cm
2
G) is the expected fluid pressure in the
equipment during normal operation; used as a basis for determining design
pressure.
2.1.2 Maximum Operating Pressure
Maximum operating pressure (psig, barg, kg/cm2
G) is the worst case pressure
expected to occur due to process upsets, start-up and shutdown operating cases,
and shut-in operating pressures of compressors and pumps. This is always less
than the design pressure defined below.
The pressure increases are usually caused by equipment characteristics such as
the rise of the pump discharge head caused by higher than normal upstream
pressures and increased pressure rise across the pump due to low flow
(approaching shutoff head), fouling of catalyst beds and filter media area reductions
associated with end of run conditions prior to regeneration or replacement.
2.1.3 Settling Out Pressure
In a reaction loop, the flow of process fluids through a system is achieved by
creating a pressure differential with a pump and a compressor. Where the system
can be shut in and the process flow stopped, the pressure will decrease in the
upstream volumes and increase in the downstream volumes, if the fluid is
compressible. The final pressure is defined to be the settling out pressure and will
be constant throughout the loop after equalization. It is important to establish the
relief device set pressure sufficiently above settling out pressure to prevent flare
relief due to reductions or stoppage of process flow. (See API RP 520 Part I,
Appendix B).

FLUOR DANIEL
FLARE SYSTEM
SECTION 2.0
PROCESS MANUAL
PAGE 2
Chap2-r1.doc
Design pressure (psig, barg, kg/cm
2
G) is the most severe condition of coincident
internal or external pressure and temperature (minimum or maximum) to be
contained by the equipment. These values are used as the basis for mechanical
design of the equipment. Since allowable design stresses vary with temperature,
the design pressure is always specified with a coincident design temperature.
Some types of mechanical equipment have a design pressure set by conditions
other than those required to contain the maximum anticipated pressure. Where this
is the case, the equipment manufacturer is to be required to provide the maximum
working pressure, the relief device set pressure for this equipment (if one is needed)
and the basis for this required set pressure. For example, compressor casings are
usually based on standard designs which frequently can withstand pressure in
excess of the maximum level anticipated.
Fired heaters are a special case where two design pressures are specified:
• Elastic Design Pressure
This is the maximum pressure the furnace coil will experience for short
periods of time. This pressure is usually related to pressure relief valve
settings, pump shut-in conditions, etc.
• Rupture Design Pressure
This is the maximum long-term pressure in the coil during normal operation.
The rupture design pressure is usually the lower of the two design pressures.
When establishing heater design conditions, the process engineer must
therefore identify both the short-term design pressure as well as the
maximum operating pressure, as both figures may be needed to determine
tube wall thickness. For pressure relief system considerations, the short-
term (elastic) design pressure should be considered as the equivalent of
MAWP, and should be selected based on the discussion in Section 2.17.
2.1.5 Design Pressure Selection
2.1.5.1 Pressure Vessels
Select the highest of the following:
1) Operating pressure plus 25 psi (1.7 bar, 1.8 kg/cm
2
).
2) Operating pressure times 1.1 [the margin may be reduced to 1.05 or
a minimum of 100 psi (6.9 bar, 7.0 kg/cm2
)], whichever is greater, in

FLUOR DANIEL
FLARE SYSTEM
SECTION 2.0
PROCESS MANUAL
PAGE 3
Chap2-r1.doc
vessels or reactors with operating pressures over 1000 psig (69 barg,
70 kg/cm
2
G). For liquid-full vessels, to avoid frequent liquid relief due
to system hydraulic variations, use 1.2 times operating pressure. For
LPG services, use shutoff pressure times 1.1 to be conservative.
3) 50 psig (3.4 barg, 3.5 kg/cm
2
G) if the vessel relieves to the flare.
4) 30 psig (2.0 barg, 2.1 kg/cm2
G) if the vessel relieves to atmosphere.
5) 15 psig (1.03 barg, 1.05 kg/cm2
G) if the vessel is vented to
atmosphere.
The determination of design pressure should be based on the operating
pressure at the top of the vessel. Towers or vessels with significant
pressure drop (top to bottom) should also be specified with a bottom
design pressure.
Vessels which can be subject to a vacuum condition under normal or upset
conditions will be designed for full vacuum. Full vacuum condition resulting
from steamout should also be considered.
All calculated design pressures should be rounded up to the nearest 1.4
psi (0.1 bar, 0.1 kg/cm
2
).
The reactor loop profile and the settling out pressure shall be considered
when setting the design pressure of a vessel in a recycle loop, per API RP
520 Part I, Appendix B.
The process engineer should be aware that selection of the design
pressure actually defines service requirements for the relief system. Often,
a small increase in the design pressure can reduce the cost and complexity
of the relief system or even eliminate the need for pressure relief for
particular contingencies. If the equipment design pressure is low or the
anticipated relieving rates are high, additional care should be taken in this
selection. It should be kept in mind that higher design pressure selection
may reduce or eliminate frequent venting to the flare system.
The flare seal drum, K.O. drum(s) and the flare piping are usually designed
for 50 psig (3.4 barg, 3.5 kg/cm
2
G). However, higher pressures shall be
specified if required by hydraulic evaluation.
2.1.5.2 Heat Exchangers
Exchanger design pressure can be set to minimize the need for relief by
selecting the highest of the following, when appropriate:

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