Pressure-relieving and Depressuring Systems

API STANDARD 521

SEVENTH EDITION, JUNE 2020

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Foreword

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Contents

Page

1. Scope 1

2. Normative References 1

3. Terms, Definitions, Acronyms, and Abbreviations 1

   1. Terms and Definitions 1

   2. Acronyms and Abbreviations 10

4. Causes of Overpressure and Their Relieving Rates 11

   1. General 11

   2. Overpressure Protection Philosophy 12

   3. Determination of Individual Relieving Rates 14

   4. Individual Overpressure Causes and Their Relieving Rates 16

   5. Guidance on Vacuum Relief 64

   6. Vapor Depressuring 66

   7. Relief System Design Documentation. 74

   8. Flare Header Design Documentation 78

   9. Special Considerations for Individual PRDs 78

5. Disposal Systems 80

   1. General 80

   2. Fluid Properties That Influence Selection and Design of Disposal Systems 80

   3. System Design Load 82

   4. System Arrangement 85

   5. Piping 88

   6. Disposal to a Lower-pressure System 104

   7. Disposal to Flare 104

   8. Disposal to Atmosphere 125

   9. Design Details for Seal and Knockout Drums 142

Annex A (informative) An Analytical Methodology for Fire Evaluations 144

Annex B (informative) Special System Design Considerations 162

Annex C (informative) Sample Calculations 165

Annex D (informative) Typical Details and Sketches 204

Annex E (informative) High-integrity Protection Systems 207

Annex F (informative) Flare Background Information 213

Annex G (informative) Vapor Breakthrough into Liquid-containing Systems 219

Annex H (informative) Flow-induced Vibration 222

Bibliography 223

Figures

1. Average Rate of Heating Steel Plates Exposed to Open Gasoline Fire on One Side 40

2. Effect of Overheating Carbon Steel (ASTM A515, Grade 70) 41

3. Isothermal Flow Chart 91

4 Adiabatic Flow of k = 1.00 (i.e. Isothermal Flow) Compressible Fluids Through Pipes

at High Pressure Drops 93

1. Determination of Drag Coefficient 117

   Contents

   Page

2. Typical Flare Gas Recovery System 122

3. Flare Gas Recovery Inlet Pressure 124

4. Maximum Downwind Vertical Distance from Jet Exit to Lean-flammability

   Concentration Limit for Petroleum Gases 128

5. Maximum Downwind Horizontal Distance from Jet Exit to Lean-flammability

   Concentration Limit for Petroleum Gases 129

6. Axial Distance to Lean- and Rich-flammability Concentration Limits for Petroleum Gases 130

7. Sound Pressure Level at 30 m (100 ft) from the Stack Tip 141

1. Effect of Fuel Air Stoichiometry on Pool and Jet Fire Heat Fluxes 144

2. Typical Effect of Wall Temperature on Absorbed Heat Flux for Pool Fires 147

3. Illustration of Actual Stress Compared with the UTS for a Fire Exposed Pipe

   That Is Depressurized 154

4. Fire Depressurization Work Flow Diagram 155

5. Minimum Depressuring Rates to Avoid Failure of a Gas-filled Vessel Fabricated

   from SA-516 Carbon Steel and Exposed to a Pool Fire 159

6. Vapor Pressure and Heat of Vaporization of Pure, Single-component Paraffin

Hydrocarbon Liquids 161

B.1 Typical Flow Scheme of a System Involving a Single PRD Serving Components

in a Process System with Typical Pressure Profiles 163

1. Equilibrium Phase Diagram for a Given Liquid 166

2. Dimensional References for Sizing a Flare Stack 171

3. Flame Center for Flares and Ignited Vents—Horizontal Distance, xc (SI Units) 176

4. Flame Center for Flares and Ignited Vents—Horizontal Distance, xc (USC Units) 177

5. Flame Center for Flares and Ignited Vents—Vertical Distance, yc (SI Units) 178

6. Flame Center for Flares and Ignited Vents—Vertical Distance, yc (USC Units) 179

7. Flare Knockout Drum 182

8. Use of the Analytical Method to Reproduce API Fire Test Data (See Plate 2 of Figure 1) 190

9. Use of the Analytical Method to Reproduce API Figure 1, Plates 1, 3, and 4 191

10. BRL Test Data Illustrating Fire Temperature vs Time at the Top of the Front

    and Rear Walls of a Rail Tank Car Containing LPG and Exposed to a JP-4 Pool Fire 192

11. BRL Test Data Illustrating Wall Temperature vs Time at the Top of the Front

    and Rear Walls of a Rail Tank Car Containing LPG and Exposed to a JP-4 Pool Fire 192

12. Use of the Analytical Method to Reproduce BRL Fire Test Data (Constant Fire Temperature) 193

13. BAM Test Data Illustrating Fire Temperature vs Time at Various Locations

    Around a Rail Tank Car Containing LPG and Exposed to a Fuel Oil Pool Fire 194

14. BAM Test Data Illustrating Wall Temperature vs Time at Various Locations

    Around a Rail Tank Car Containing LPG and Exposed to a Fuel Oil Pool Fire 195

15. Use of the Analytical Method to Reproduce BAM Fire Test Data (Variable Fire Temperature) 196

16. Use of the Analytical Method to Reproduce BAM Fire Test Data (Constant Fire Temperature) 197

17. Liquid Temperature vs Time Profile from the BAM Fire Test 199

18. Comparison of the API Empirical Method and the Analytical Method with BRL Fire Test Data 201

19. Calculated Wetted Area Exponent of the API Empirical Method

Equation (8) Based on BRL Fire Test Data 201

1. Typical Horizontal Flare Seal Drum 204

2. Quench Drum 205

3. Typical Flare Installation 206

1. Flame Length vs Heat Release—Industrial Sizes and Releases (SI Units) 216

2. Flame Length vs Heat Release—Industrial Sizes and Releases (USC Units) 217

3. Approximate Flame Distortion Due to Lateral Wind on Jet Velocity from Flare Stack 218

Contents

Tables

Page

1. Guidance for Required Relieving Rates Under Selected Conditions 17

2. Typical Values of Cubic Expansion Coefficient for Hydrocarbon Liquids and Water 33

3. Values of Linear Expansion Coefficient, al, and Modulus of Elasticity, E 36

4. Effects of Fire on the Wetted Surfaces of a Vessel 39

5. Environmental Factor 42

6. Thermal Conductivity Values for Typical Thermal Insulations 50

7. Possible Utility Failures and Equipment Affected 62

8. Design Basis for PRD Laterals and Disposal System Headers 85

9. Typical K-factors for Pipe Fittings 94

10. Typical K-factors for Reducers (Contraction or Enlargement) 95

11. Exposure Times Necessary to Reach the Pain Threshold 106

12. Recommended Design Thermal Radiation for Personnel 106

1. Comparison of Heat Absorption Rates in Fire Tests 146

2. Typical Range in Equation (A.1) Parameters for an Open Pool Fire 149

3. Recommended Values for Equation (A.1) Parameters for an Open Pool Fire

   Where Other Data or Information is Unavailable 150

4. Typical Range in Equation (A.1) Parameters for a Jet Fire 151

5. Recommended Values for Equation (A.1) Parameters for a Jet Fire Where Other

   Data or Information Is Unavailable 152

6. Typical Starting Points for Step 1 in Figure A.4 When Designing Depressurization

   System for Unwetted Walls Exposed to Jet Fires 156

7. High-temperature Tensile Strength of Carbon Steel and 18-8 Stainless Steel 159

1. Optimizing the Size of a Horizontal Knockout Drum (SI Units) 185

2. Optimizing the Size of a Horizontal Knockout Drum (USC Units) 186

3. Equation (A.1) Parameters Used to Reproduce Curves in Figure 1 190

4. Equation (A.1) Parameters Used to Reproduce BRL Fire Test Data

   for the Rail Tank Car Top Front and Rear Wall Temperatures vs Time 193

5. Equation (A.1) Parameters Used to Reproduce BAM Fire Test Data for the

   Rear Locations of the Rail Tank Car Temperatures vs Time 195

6. Comparison of Absorbed Heat Duties Calculated with the API Empirical

and Analytical Method Applying the BAM Fire Test Data 198

E.1 Safety Integrity Level vs Availability 210

F.1 Radiation from Gaseous Diffusion Flames 215

Introduction

The portions of this standard dealing with flares and flare systems are an adjunct to API Standard 537, which addresses design, operation, and maintenance of flare equipment. It is important for all parties involved in the design and use of a flare system to have an effective means of communicating and preserving design information about the flare system. To this end, API has developed a set of flare datasheets, which can be found in API Standard 537, Annex E. The use of these datasheets is both recommended and encouraged as a concise, uniform means of recording and communicating design information.

The Bibliography lists the documents that are referenced informatively in this standard, as well as other documents not cited in this standard but which contain additional useful information. Some of the content of the documents listed might not be suitable for all applications and therefore needs to be assessed for each application before use.

In this standard, quantities are expressed in the International System (SI) of units and the US customary (USC) units.

Pressure-relieving and Depressuring Systems

1. Scope

   
   

   This standard is applicable to pressure-relieving and vapor depressuring systems. Although intended for use primarily in oil refineries, it is also applicable to petrochemical facilities, gas plants, liquefied natural gas (LNG) facilities, and oil and gas production facilities. The information provided is designed to aid in the selection of the system that is most appropriate for the risks and circumstances involved in various installations.

   
   

   This standard specifies requirements and gives guidelines for the following:

   
   

   • examining the principal causes of overpressure;

     
     

   • determining individual relieving rates;

     
     

   • selecting and designing disposal systems, including such component parts as piping, vessels, flares, and vent stacks.

   
   

   This standard does not apply to direct-fired steam boilers.

   
   

2. Normative References

   
   

   The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

   
   

   API Standard 520, Sizing, Selection, and Installation of Pressure-relieving Devices, Part 1—Sizing and Selection, Ninth Edition, 2014

   
   

   API Standard 520, Sizing, Selection, and Installation of Pressure-relieving Devices, Part 2—Installation, Sixth Edition, 2015

   
   

   API Standard 537, Flare Details for Petroleum, Petrochemical, and Natural Gas Industries, Third Edition, Addendum 1, 2020

   
   

3. Terms, Definitions, Acronyms, and Abbreviations

3.1 Terms and Definitions

For the purposes of this document, the following definitions apply.

3.1.1

accumulation

Pressure increase over the maximum allowable working pressure (MAWP) of the vessel during discharge through the pressure-relief device.

NOTE Accumulation is expressed in units of pressure or as a percentage of MAWP or design pressure. Maximum allowable accumulations are established by pressure design codes for emergency operating and fire contingencies.

3.1.2

administrative controls

Procedures intended to ensure that personnel actions do not compromise the overpressure protection of the equipment.