Pressure-relieving and Depressuring Systems

API STANDARD 521

SIXTH EDITION, JANUARY 2014

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Copyright © 2014 American Petroleum Institute

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

      1. Hierarchy of Protective Measures 12

      2. Use of Administrative Controls if Corrected Hydrotest Pressure Not Exceeded 12

      3. Double Jeopardy 13

      4. Latent Failures 13

      5. Operator Error/Effect of Operator Response 13

      6. Role of Instrumentation in Overpressure Protection 14

   3. Determination of Individual Relieving Rates 14

      1. General Philosophy 14

      2. Effects of Pressure, Temperature, and Composition 15

      3. Dynamic Simulation 15

   4. Individual Overpressure Causes and Their Relieving Rates 16

      1. General 16

      2. Closed Outlets 16

      3. Cooling or Reflux Failure 18

      4. Absorbent Flow Failure 20

      5. Accumulation of Noncondensables 20

      6. Entrance of Volatile Material into the System 20

      7. Overfilling 21

      8. Failure of Automatic Controls 22

      9. Abnormal Process Heat or Vapor Input 26

      10. Internal Explosions or Transient Pressure Surges 29

      11. Chemical Reaction 30

      12. Hydraulic Expansion 31

      13. Fires 36

      14. Heat Transfer Equipment Failure 54

      15. Utility Failure 58

      16. Overpressure Prevention During Maintenance 60

   5. Guidance on Vacuum Relief 61

      1. General 61

      2. Causes for Vacuum 61

      3. Protection Against Vacuum. 62

   6. Vapor Depressuring 62

      1. General 62

      2. Initiation of Depressuring 63

      3. Low Temperatures During Depressuring 63

      4. Application Criteria 64

      5. Acceptance and Design Criteria 64

      6. Depressuring Rate 66

      7. Vapor Flows 66

         Page

   7. Relief System Design Documentation 71

      1. General 71

      2. Purpose of Documentation 71

      3. Potential Elements of Relief System Design Documentation 71

   8. Flare Header Design Documentation 75

   9. Special Considerations for Individual PRDs 76

      1. General 76

      2. Liquid-Vapor Mixture and Solids Formation 76

      3. Location of a PRD in a Normally Liquid System 76

      4. Multiple PRDs 76

5. Disposal Systems 77

   1. General 77

   2. Fluid Properties That Influence Selection and Design of Disposal Systems 77

      1. Physical, Chemical, and Reactive Properties 77

      2. Temperature 78

      3. Hazardous and Nuisance Properties 79

      4. Viscosity and Solidification 79

      5. Miscibility 79

      6. Recovery Value 80

   3. System Design Load 80

      1. General 80

      2. Loads from Pressure Systems 80

      3. Establishing Design Load for the Disposal System 81

      4. Refinement of the Disposal System Design Load 82

   4. System Arrangement 82

      1. General 82

      2. Single-device Disposal Systems 83

      3. Multiple-device Disposal System 84

      4. Header Segregation 84

   5. Piping 85

      1. General 85

      2. Backpressure 86

      3. Line Sizing 86

      4. Multiple-relief Scenarios 87

      5. Isothermal Pressure Drop Calculation Method 87

      6. Lapple Pressure Drop Calculation Method 90

      7. Fanno Lines Pressure Drop Calculation Method 93

      8. Nonideal Gas Behavior 93

      9. Frictional Resistance of Fittings (K-factors) 94

      10. Mixed Phase Fluids 94

      11. Mechanical Design of the Disposal System 97

      12. Acoustic Fatigue 97

      13. Setting the Mechanical Design Temperature for Flare Headers 100

      14. Reaction Forces 100

      15. Shock Loading 101

      16. Pipe Anchors, Guides, and Supports 101

      17. Self-draining/Heat Tracing 101

      18. Routing of Discharge Piping/Sloping 101

   6. Disposal to a Lower-pressure System 102

   7. Disposal to Flare 102

      1. General 102

      2. Combustion Properties 103

         Page

      3. Combustion Methods 112

      4. Flare Systems Designs 120

      5. Sizing 122

      6. Purging 124

      7. Ignition of Flare Gases 127

      8. Liquid Seal Drum 128

      9. Flare Knockout Drum 133

      10. Siting Considerations for the Flare 140

      11. Flare Gas Recovery Systems 140

   8. Disposal to Atmosphere 144

      1. General 144

      2. Formation of Flammable Mixtures 144

      3. Exposure to Toxic Vapors or Corrosive Chemicals 151

      4. Ignition of a Relief Stream at the Point of Emission 152

      5. Excessive Noise Levels 154

      6. Air Pollution 154

      7. Knockout Drums Venting to Atmosphere 154

      8. Disposal Through Common Vent Stack 156

      9. Sewer 157

      10. Vent Stacks 157

   9. Design Details for Seal and Knockout Drums 161

Bibliography 233

Figures

1. Average Rate of Heating Steel Plates Exposed to Open Gasoline Fire on One Side 39

2. Effect of Overheating Carbon Steel (ASTM A515, Grade 70) 40

3. Isothermal Flow Chart 89

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

   Pressure Drops 91

5. Flame Length vs Heat Release—Industrial Sizes and Releases (SI Units) 109

6. Flame Length vs Heat Release—Industrial Sizes and Releases (USC Units) 110

7. Approximate Flame Distortion Due to Lateral Wind on Jet Velocity from Flare Stack 111

8. Steam-injected Smokeless Flare Tips 116

9. Typical Flare Systems 118

10. Flare Structures 121

11. Purge-reduction Seal—Buoyancy Seal 125

12. Determination of Drag Coefficient 137

13. Typical Flare Gas Recovery System 141

14. Flare Gas Recovery Inlet Pressure 143

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

    Limit for Petroleum Gases 148

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

    Page

    Limit for Petroleum Gases 149

17. Axial Distance to Lean- and Rich-flammability Concentration Limits for Petroleum Gases 150

18. Sound Pressure Level at 30 m (100 ft) from the Stack Tip 160

1. Effect of Fuel Air Stoichiometry on Pool and Jet Fire Heat Fluxes 163

2. Typical Effect of Wall Temperature on Absorbed Heat Flux for Pool Fires 166

3. Illustration of Actual Stress Compared with the UTS for a Fire Exposed Pipe That is Depressurized 173 A.4 Fire Depressurization Work Flow Diagram 174

1. Minimum Depressuring Rates to Avoid Failure of a Gas-filled Vessel Fabricated from SA-516

   Carbon Steel and Exposed to a Pool Fire 178

2. Vapor Pressure and Heat of Vaporization of Pure, Single-component Paraffin Hydrocarbon Liquids 180

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

System with Typical Pressure Profiles 182

1. Equilibrium Phase Diagram for a Given Liquid 185

2. Dimensional References for Sizing a Flare Stack 190

3. Flame Center for Flares and Ignited Vents—Horizontal Distance, xc (SI Units) 196

4. Flame Center for Flares and Ignited Vents—Horizontal Distance, xc (USC Units) 197

5. Flame Center for Flares and Ignited Vents—Vertical distance, yc (SI Units) 198

6. Flame Center for Flares and Ignited Vents—Vertical distance, yc (USC Units) 199

7. Flare Knockout Drum 203

8. Use of the Analytical Method to Reproduce API Fire Test Data (See Plate 2 of Figure 1) 211

9. Use of the Analytical Method to Reproduce API Figure 1, Plates 1, 4, and 5 212

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 [30] 212

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 [30] 213

12. Use of the Analytical Method to Reproduce BRL Fire Test Data (Constant Fire Temperature) 213

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 [109] 215

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 [109] 215

15. Use of the Analytical Method to Reproduce BAM Fire Test Data (Variable Fire Temperature) 217

16. Use of the Analytical Method to Reproduce BAM Fire Test Data (Constant Fire Temperature) 217

17. Liquid Temperature vs Time Profile from the BAM Fire Test [109] 219

18. Comparison of the API Empirical Method and the Analytical Method with BRL Fire Test Data 221

19. Calculated Wetted Area Exponent of the API Empirical Method Equation (7) Based on BRL

Fire Test Data 221

1. Typical Horizontal Flare Seal Drum 224

2. Quench Drum 225

3. Typical Flare Installation 226

Tables

1. Guidance for Required Relieving Rates Under Selected Conditions 17

2. Typical Values of Cubic Expansion Coefficient for Hydrocarbon Liquids and Water 32

3. Values of Linear Expansion Coefficient, l, and Modulus of Elasticity, E 35

4. Effects of Fire on the Wetted Surfaces of a Vessel 38

5. Environment Factor 41

6. Thermal Conductivity Values for Typical Thermal Insulations 49

7. Possible Utility Failures and Equipment Affected 59

8. Design Basis for PRD Laterals and Disposal System Headers 83

9. Typical K-factors for Pipe Fittings 92

   Page

10. Typical K-factors for Reducers (Contraction or Enlargement) 93

11. Exposure Times Necessary to Reach the Pain Threshold 105

12. Recommended Design Thermal Radiation for Personnel 106

13. Radiation from Gaseous Diffusion Flames 108

14. Suggested Injection Steam Rates 114

1. Comparison of Heat Absorption Rates in Fire Tests 165

2. Typical Range in Equation (A.1) Parameters for an Open Pool Fire 168

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

   Data or Information are Unavailable 169

4. Typical Range in Equation (A.1) Parameters for a Jet Fire 170

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

   Information are Unavailable 171

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

   Unwetted Walls Exposed to Jet Fires 175

7. High-temperature Tensile Strength of Carbon Steel and 18-8 Stainless Steel [154] 178

1. Optimizing the Size of a Horizontal Knockout Drum (SI Units) 206

2. Optimizing the Size of a Horizontal Knockout Drum (USC Units) 206

3. Equation (A.1) Parameters Used to Reproduce Curves in Figure 1 210

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 214

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

   the Rail Tank Car Temperatures vs Time 216

6. Comparison of Absorbed Heat Duties Calculated with the API Empirical and Analytical Method Applying the BAM Fire Test Data 219

E.1 SIL vs Availability 230

Introduction

The portions of this standard dealing with flares and flare systems are an adjunct to API Standard 537 [8], which addresses mechanical 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 537, Appendix 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 U.S. customary (USC) units.

x

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 520-1:2008 [5], Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries—Part 1, Sizing and Selection

   
   

   API Recommended Practice 520-2:2003 [6], Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries—Part 2, Installation

   
   

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.

1