Pressure-relieving and Depressuring Systems

ANSI/API STANDARD 521 FIFTH EDITION, JANUARY 2007

(INCLUDES ERRATA JUNE 2007) ADDENDUM, MAY 2008

ISO 23251 (Identical), Petroleum and natural gas industries—Pressure-relieving and depressuring systems

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20005.

Copyright © 2006, 2008 American Petroleum Institute

API Foreword

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Contents Page

API Foreword ii

Foreword v

Introduction vi

1. Scope 1

2. Normative references 1

3. Terms and definitions 1

4. Causes of overpressure 10

   1. General 10

   2. Overpressure protection philosophy 10

   3. Potentials for overpressure 11

   4. Recommended minimum relief system design content 17

   5. List of items required in flare-header calculation documentation 20

   6. Guidance on vacuum relief 20

5. Determination of individual relieving rates 22

   1. Principal sources of overpressure 22

   2. Sources of overpressure 24

   3. Effects of pressure, temperature, and composition 24

   4. Effect of operator response 24

   5. Closed outlets 24

   6. Cooling or reflux failure 25

   7. Absorbent flow failure 26

   8. Accumulation of non-condensables 26

   9. Entrance of volatile material into the system 26

   10. Failure of process stream automatic controls 27

   11. Abnormal process heat input 29

   12. Internal explosion (excluding detonation) 30

   13. Chemical reaction 30

   14. Hydraulic expansion 31

   15. External pool fires 36

   16. Jet fires 51

   17. Opening manual valves 52

   18. Electric power failure 52

   19. Heat-transfer equipment failure 52

   20. Vapour depressuring 55

   21. Special considerations for individual pressure-relief devices 63

   22. Dynamic simulation 64

   23. Overfilling process or surge vessel 65

6. Selection of disposal systems 66

   1. General 66

   2. Fluid properties that influence design 66

   3. Atmospheric discharge 67

   4. Disposal by flaring 77

   5. Disposal to a lower-pressure system 96

   6. Disposal of liquids and condensable vapours 97

   7. Disposal through common vent stack 98

7. Disposal systems 99

   1. Definition of system design load 99

   2. System arrangement 102

   3. Design of disposal system components 104

   4. Flare gas recovery systems 141

Annex A (informative) Determination of fire relief requirements 146

Annex B (informative) Special system design considerations 150

Annex C (informative) Sample calculations for sizing a subsonic flare stack 153

Annex D (informative) Typical details and sketches 170

Annex E (informative) High integrity protection systems (HIPS) 173

Bibliography 180

Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights.

ISO 23251 was prepared by Technical Committee ISO/TC 67, Materials, equipment and offshore structures for petroleum, petrochemical and natural gas industries, Subcommittee SC 6, Processing equipment and systems.

This corrected version of ISO 23251:2006 incorporates corrections to Table 4, column 2, second row under the header, and the five rows of data in column 3.

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Introduction

This International Standard is based on the draft 5th edition of API RP 521, with the intent that the 6th edition of API RP 521 will be identical to this International Standard.

The portions of this International Standard dealing with flares and flare systems are an adjunct to API Std 537 [10], 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 data sheets, which can be found in of API Std 537, Appendix A. The use of these data sheets is both recommended and encouraged as a concise, uniform means of recording and communicating design information.

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API Standard 521 / ISO 23251

Petroleum, petrochemical and natural gas industries — Pressure- relieving and depressuring systems

1. Scope

   
   

   This International Standard is applicable to pressure-relieving and vapour-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 International Standard is intended to supplement the practices set forth in ISO 4126 or API RP 520-I for establishing a basis of design.

   
   

   This International Standard specifies requirements and gives guidelines for examining the principal causes of overpressure; and determining individual relieving rates; and selecting and designing disposal systems, including such component parts as piping, vessels, flares, and vent stacks. This International Standard does not apply to direct-fired steam boilers.

   
   

   Piping information pertinent to pressure-relieving systems is presented in 7.3.1.

   
   

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.

   
   

   ISO 4126 (all parts), Safety devices for protection against excessive pressure

   
   

   API RP 520-I:2000, Sizing, Selection and Installation of Pressure-Relieving Devices in Refineries — Part I: Sizing and Selection 1)

   
   

3. Terms and definitions

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

3.1

accumulation

pressure increase over the maximum allowable working pressure of the vessel allowed 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.

1) American Petroleum Institute, 1220 L Street, N.W., Washington, D.C., 20005-4070, USA.

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