Venting Atmospheric and Low-Pressure Storage Tanks

Nonrefrigerated and Refrigerated

API STANDARD 2000

FIFTH EDITION, APRIL 1998

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Venting Atmospheric and Low-Pressure Storage Tanks

Nonrefrigerated and Refrigerated

Manufacturing, Distribution and Marketing Department

API STANDARD 2000

FIFTH EDITION, APRIL 1998

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Copyright ? 1998 American Petroleum Institute

FOREWORD

This standard covers the normal and emergency vapor venting requirements for above- ground liquid petroleum storage tanks and aboveground and underground refrigerated stor- age tanks designed for operation at pressures from vacuum through 15 pounds per square inch gauge (1.034 bar gauge). Discussed in this standard are the causes of overpressure or vacuum; determination of venting requirements; means of venting; selection, installation, and maintenance of venting devices; and testing and marking of relief devices.

This standard has been developed from the accumulated knowledge and experience of quali?ed engineers in the petroleum-processing industry and its related industries. The vapor venting requirements in this standard are based on studies using hexane. Intended for petro- leum products, this standard may be applied to other materials; however, sound engineering analysis and judgment should be used whenever this standard is applied to other materials.

Engineering studies of a particular tank may indicate that the appropriate venting capacity for the tank is not the venting capacity estimated in accordance with this standard. The many variables associated with tank venting requirements make it impractical to set forth de?nite, simple rules that are applicable to all locations and conditions. Larger venting capacities may be required on tanks in which liquid is heated, on tanks that receive liquid from wells or traps, and on tanks that are subjected to pipeline surges. Larger venting capacities may also be required on tanks that use ?ame arresters or have other restrictions that may build up pres- sure under certain conditions.

This standard does not apply to external ?oating roof tanks or free vented internal ?oating roof tanks.

API publications may be used by anyone desiring to do so. Every effort has been made by the Institute to assure the accuracy and reliability of the data contained in them; however, the Institute makes no representation, warranty, or guarantee in connection with this publication and hereby expressly disclaims any liability or responsibility for loss or damage resulting from its use or for the violation of any federal, state, or municipal regulation with which this publication may con?ict.

Suggested revisions are invited and should be submitted to the director of the Manufactur- ing, Distribution and Marketing Department, American Petroleum Institute, 1220 L Street, N.W., Washington, D.C. 20005.

IMPORTANT INFORMATION CONCERNING USE OF ASBESTOS OR ALTERNATIVE MATERIALS

Asbestos is speci?ed or referenced for certain components of the equipment described in some API standards. It has been of extreme usefulness in minimizing ?re hazards associated with petroleum processing. It has also been a universal sealing material, compatible with most re?ning ?uid services.

Certain serious adverse health effects are associated with asbestos, among them the serious and often fatal diseases of lung cancer, asbestosis, and mesothelioma (a cancer of the chest and abdominal linings). The degree of exposure to asbestos varies with the prod- uct and the work practices involved.

Consult the most recent edition of the Occupational Safety and Health Administration (OSHA), U.S. Department of Labor, Occupational Safety and Health Standard for Asbestos, Tremolite, Anthophyllite, and Actinolite, 29 Code of Federal Regulations Section 1910.1001; the U.S. Environmental Protection Agency, National Emission Standard for Asbestos, 40 Code of Federal Regulations Sections 61.140 through 61.156; and the U.S. Environmental Protection Agency (EPA) rule on labeling requirements and phased banning of asbestos products (Sections 763.160-179).

There are currently in use and under development a number of substitute materials to replace asbestos in certain applications. Manufacturers and users are encouraged to develop and use effective substitute materials that can meet the speci?cations for, and operating requirements of, the equipment to which they would apply.

SAFETY AND HEALTH INFORMATION WITH RESPECT TO PARTICULAR PRODUCTS OR MATERIALS CAN BE OBTAINED FROM THE EMPLOYER, THE MANUFACTURER OR SUPPLIER OF THAT PRODUCT OR MATERIAL, OR THE MATERIAL SAFETY DATA SHEET.

CONTENTS

Page

0 INTRODUCTION 1

1. SCOPE 1

2. REFERENCES 1

   1. Standards 1

   2. Other References 1

3. DEFINITION OF TERMS 1

4. NONREFRIGERATED ABOVEGROUND TANKS 2

   1. General 2

   2. Causes of Overpressure or Vacuum 2

   3. Determination of Venting Requirements 4

   4. Means of Venting 12

   5. Selection, Installation, and Maintenance of Venting Devices 13

   6. Testing and Marking of Venting Devices 15

5. REFRIGERATED ABOVEGROUND AND BELOWGROUND TANKS 17

   1. General 17

   2. Causes of Overpressure or Vacuum 17

   3. Determination of Venting Requirements 19

   4. Means of Venting 21

   5. Selection, Installation, and Maintenance of Venting Devices 25

   6. Testing and Marking of Venting Devices 26

APPENDIX A BASIS OF THE NORMAL VENTING FOR TABLES 1 AND 2 27

APPENDIX B BASIS OF EMERGENCY VENTING FOR TABLES 3 AND 5 29

APPENDIX C TYPES AND OPERATING CHARACTERISTICS OF

VENTING DEVICES 33

Figures

1 Typical Ratio Limits for Capacity Testing of Venting Devices Using the

Coef?cient of Discharge Method 16

B-1 Curve for Determining Requirements for Emergency Venting During

Fire Exposure 31

1. Open Vent 33

2. Side-by-Side Pressure/Vacuum Vent 35

3. Large Weight-Loaded Emergency Vent 35

4. Direct-Acting Vents 36

5. Capacity/Overpressure Characteristic of Vent 37

6. Pilot-Operated Pressure Vent (Single Diaphragm) 38

7. Pilot-Operated Pressure/Vacuum (Double Diaphragm) 39

v

Page

Tables

1A Normal Venting Requirements (SCFH of Air per Barrel per Hour of Liquid Flow) A. English Units 4

1B Normal Venting Requirements (Nm3/hr of Air per Cubic Meter

per Hour of Liquid Flow)

B. Metric Units 5

2A Requirements for Thermal Venting Capacity

1. English Units 6

   2B Requirements for Thermal Venting Capacity

2. Metric Units 7

3A Emergency Venting Required for Fire Exposure Versus Wetted Surface Area

1. English Units 9

   3B Emergency Venting Required for Fire Exposure Versus Wetted Surface Area

2. Metric Units 9

4A Environmental Factors for Nonrefrigerated Aboveground Tanks

1. English Units 10

   4B Environmental Factors for Nonrefrigerated Aboveground Tanks

2. Metric Units 11

5A Emergency Venting Required for Fire Exposure Versus Wetted Surface Area

1. English Units 22

   5B Emergency Venting Required for Fire Exposure Versus Wetted Surface Area

2. Metric Units 22

6A Environment Factors for Refrigerated Aboveground and Partially Belowground Tanks

1. English Units 23

   6B Environment Factors for Refrigerated Aboveground and Partially Belowground Tanks

2. Metric Units 24

C-1 Operating Characteristics of Venting Devices 34

Venting Atmospheric and Low-Pressure Storage Tanks

0 Introduction

The venting requirements provided in this standard are based on studies of hexane stored in steel tanks. Sound engi- neering judgment should be applied when extrapolating these results to other liquids and nonmetallic tanks.

Detailed engineering studies of a particular tank and its operating conditions may indicate that the appropriate vent- ing capacity for the tank is not the venting capacity estimated in accordance with this standard. If a tank?s operating condi- tions could deviate from those used in developing this stan- dard, detailed engineering studies should be performed.

1. Scope

   This standard covers the normal and emergency vapor venting requirements for aboveground liquid petroleum or petroleum products storage tanks and aboveground and underground refrigerated storage tanks designed for operation at pressures from vacuum through 15 pounds per square inch gauge (1.034 bar gauge). Discussed in this standard are the causes of overpressure or vacuum; determination of venting requirements; means of venting; selection, installation, and maintenance of venting devices; and testing and marking of relief devices.

   
   

2. References

   Unless otherwise speci?ed, the referenced sections of the most recent editions or revisions of the following standards, codes, and speci?cations shall, to the extent speci?ed herein, form a part of this standard.

   
   

   1. STANDARDS

      API

      Std 620 Design and Construction of Large, Welded, Low-Pressure Storage Tanks

      Std 650 Welded Steel Tanks for Oil Storage

      Std 2510 Design and Construction of Lique?ed Petroleum Gas (LPG) Installations

      ASME1

      PTC 19.5 Fluid Meters: Interim Supplement on Instruments and Apparatus, Part II?

      ?Application?

      PTC 25 Pressure Relief Devices

      Boiler & Pressure Vessel Code, Section VIII, Division 1, Rules for Construction of Pressure Vessels

      
      

      1American Society of Mechanical Engineers, 345 East 47th Street, New York, New York 10017

      1

      
      

   2. OTHER REFERENCES

      API

      RP 520 Sizing, Selection, and Installation of Pres- sure-Relieving Devices in Re?neries, Part I

      ??Sizing and Selection?

      RP 521 Guide for Pressure-Relieving and Depres- suring Systems

      RP 576 Inspection of Pressure-Relieving Devices

      Publ 2210 Flame Arresters for Vents of Tanks Storing Petroleum Products

      RP 2350 Over?ll Protection for Petroleum Storage Tanks

      Bull 2521 Use of Pressure-Vacuum Vent Valves for Atmospheric Pressure Tanks to Reduce Evaporation Loss

      
      

3. Definition of Terms

   For the purposes of this standard, the following de?nitions apply:

   1. accumulation: The pressure increase in a tank over its maximum allowable working pressure when the vent valve is relieving (expressed in pressure units or percentage of the maximum allowable working pressure). Maximum allowable accumulations are typically established by applicable codes for operating and ?re contingencies.

   2. barrel: A liquid unit of measure equal to 42 US gallons (0.159 cubic meters).

   3. BTU: British Thermal Unit, a unit of heat that will increase the temperature of one pound of water one degree Fahrenheit.

   4. emergency venting: The venting required when an abnormal condition, such as ruptured internal heating coils or an external ?re, exists either inside or outside of a tank.

   5. nonrefrigerated tank: A container that stores mate- rial in a liquid state without the aid of refrigeration either by evaporation of the tank contents or by a circulating refrigera- tion system. Generally, the storage temperature will be close to or higher than ambient temperature.

   6. normal venting: The venting required because of operational requirements or atmospheric changes.

   7. overpressure: The pressure increase at the valve inlet above the set pressure, when the valve is relieving, expressed in pressure units or as a percentage of the set pres- sure. It is the same as accumulation when the valve is set at the maximum allowable working pressure and the inlet pip- ing losses are zero.