Liquid Ring Compressors and Vacuum Pumps in Petroleum, Chemical, and Gas Industry Services

API STANDARD 681

SECOND EDITION, JULY 2021

Special Notes

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Copyright © 2021 American Petroleum Institute

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Foreword

Nothing contained in any API publication is to be construed as granting any right, by implication or otherwise, for the manufacture, sale, or use of any method, apparatus, or product covered by letters patent. Neither should anything contained in the publication be construed as insuring anyone against liability for infringement of letters patent.

The verbal forms used to express the provisions in this document are as follows.

Shall: As used in a standard, “shall” denotes a minimum requirement to conform to the standard.

Should: As used in a standard, “should” denotes a recommendation or that which is advised but not required to conform to the standard.

May: As used in a standard, “may” denotes a course of action permissible within the limits of a standard. Can: As used in a standard, “can” denotes a statement of possibility or capability.

ThisdocumentwasproducedunderAPIstandardizationproceduresthatensureappropriatenotificationandparticipation in the developmental process and is designated as an API standard. Questions concerning the interpretation of the content of this publication or comments and questions concerning the procedures under which this publication was developed should be directed in writing to the Director of Standards, American Petroleum Institute, 200 Massachusetts Avenue, NW, Suite 1100, Washington, DC 20001. Requests for permission to reproduce or translate all or any part of the material published herein should also be addressed to the director.

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Suggested revisions are invited and should be submitted to the Standards Department, API, 200 Massachusetts Avenue, NW, Suite 1100, Washington, DC 20001, standards@api.org.

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Contents

Page

1. Scope 1

2. Normative References 1

3. Terms, Definitions, Acronyms, and Abbreviations 4

   1. Terms and Definitions 4

   2. Acronyms and Abbreviations 11

4. General 11

   1. Unit Responsibility 11

   2. Nomenclature 11

5. Requirements 11

   1. Units of Measure 11

   2. Statutory Requirements 11

   3. Conflicting Requirements 11

6. Basic Design 12

   1. General 12

   2. Pressure Casings 15

   3. Casing Connections 17

   4. Flanges 19

   5. External Forces and Moments 20

   6. Rotating Elements 22

   7. Mechanical Shaft Seals 23

   8. Dynamics 25

   9. Bearings, Bearing Housings, and Lubrication 27

   10. Materials 31

   11. Sealless Design 36

   12. Nameplates and Rotation Arrows 37

7. Accessories 38

   1. Drivers 38

   2. Couplings 40

   3. Guards 41

   4. Belt Drives 42

   5. Baseplates 43

   6. Controls and Instrumentation 46

   7. Ring Liquid System and Auxiliaries 55

   8. Piping 58

8. Inspection, Testing, and Preparation for Shipment 60

   1. General 60

   2. Inspection 61

   3. Testing 63

   4. Preparation for Shipment 68

9. Vendor’s Data 71

Annex A (informative) Data Sheets 72

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Contents

Page

Annex B (informative) Contract Documents and Engineering Design Data 98

Annex C (informative) Liquid Ring Compressor and Vacuum Pump Nomenclature 114

Annex D (normative) Materials and Material Specifications 119

Annex E (informative) Ring Liquid System Schematics 128

Annex F (informative) System Considerations, Operating Variables, and Test Performance Conversion 133

Annex G (informative) Packaging 136

Bibliography 139

Figures

1. Typical Gusset Design 18

2. Coordinate System for the Forces and Moments in Table 4, Top Suction/Top Discharge LRC/VP Design 21

3. Coordinate System for the Forces and Moments in Table 4—Top Suction/Side Discharge LRC/VP Design 22

4. Coordinate System for the Forces and Moments in Table 4—Side Suction/Side Discharge LRC/VP Design 22

5. Indicative Locations for Taking Vibration on Overhung and Between Bearing LRC/VP 27

1. LRC/VP—Two Stage, Between Bearing, Plate Design 114

2. LRC/VP—Single Stage, Between Bearing, Conical Design, Single Suction 115

3. LRC/VP—Single Stage, Between Bearing, Conical Design, Single Suction 115

4. LRC/VP—Single Stage, Overhung, Plate Design 115

5. Magnetic Drive LRC/VP—Single Stage, Between Bearing, Plate Design, Double Suction 116

6. LRC/VP Typical Flow Paths 117

1. LRC/VP Once-through System 128

2. LRC/VP Partial Recirculation System (NOTE 5) 129

3. LRC/VP Total Recirculation System (Vertical Separator) 130

4. LRC/VP Total Recirculation System (Horizontal Separator) 131

5. Typical Three-phase Separator 132

Tables

1. SI and U.S. Standard Conditions 9

2. Conditions for Cooling Water System Design Parameters 13

3. Material Casting Factors 16

4. Nozzle Loadings 20

5. Power Ratings for Motor Drives 39

6. Minimum Thickness for Control Panels 48

7. Minimum Alarm, Shutdown, and Trip Recommendations 51

8. Minimum Tubing Wall Thickness 59

9. Materials Inspection Standards 62

   
   

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   Contents

   
   

   Page

10. Maximum Allowable Free Air Gauss Levels 63

11. Performance Tolerances 67

1. Material Classes for LRC/VP Parts 119

2. Material Specifications for LRC/VP Parts 120

3. Non-metallic Wear Part Materials 125

4. Piping Materials 126

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Introduction

Users of this standard should be aware that further or differing requirements may be needed for individual applications. This standard is not intended to inhibit a vendor from offering, or the purchaser from accepting, alternative equipment or engineering solutions for the individual application. This may be particularly appropriate where there is innovative or developing technology. Where an alternative is offered, the vendor should identify any variations from this standard and provide details.

• Annex A contains data sheets which purchasers are encouraged to use.

  
  

• Annex B contains guidelines for submittal of contract documents and engineering design data including typical forms which may be used to indicate vendor drawing and data requirements.

  
  

• Annex C contains nomenclature for the various equipment components.

— Annex D specifies requirements and gives guidance on materials selection.

• Annex E contains schematic drawings of ring liquid systems.

  
  

• Annex F contains system considerations, operating variables, and test performance guidance.

  
  

• Annex G contains guidance on packaging.

This standard requires the purchaser to specify certain details and features. A bullet [•] in the margin indicates that either a decision by, or further information from, the purchaser is required. Further information should be shown on the data sheets (see example in Annex A) or stated in the quotation request and purchase order.

In this standard, U.S. customary units are included in brackets for information.

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Liquid Ring Compressors and Vacuum Pumps in Petroleum, Chemical, and Gas Industry Services

1. Scope

   
   

   1. This standard covers the minimum requirements for liquid ring compressor and vacuum pump (LRC/VP) systems for service in the petroleum, chemical, and gas industries. The requirements include basic equipment design, materials, fabrication, inspection, testing, and preparation for shipment.

      
      

   2. This standard requires the purchaser to specify certain details and features. A bullet [•] in the margin indicates that a decision by, or further information from, the purchaser is required. Further information should be stated in the quotation request and purchase order.

      
      

   3. The purchaser and the vendor shall mutually determine the measure that shall be taken to comply with governmental codes, regulations, ordinances, or rules that are applicable to the equipment.

      
      

2. Normative References

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any addenda) applies.

API Recommended Practice 500, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division 1 and Division 2

API Recommended Practice 686, Machinery Installation, and Installation Design

API Recommended Practice 691, Risk-based Machinery Management

API Standard 520, Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries; Part I— Sizing

and Selection

API Standard 520, Sizing, Selection, and Installation of Pressure- relieving Devices in Refineries; Part II— Installation

API Standard 526, Flanged Steel Pressure Relief Valves

API Standard 541, Form-wound Squirrel- cage Induction Motors—500 Horsepower and Larger

API Standard 547, General-purpose Form- wound Squirrel-cage Induction Motors—250 Horsepower and Larger

API Standard 614, Lubrication, Shaft-sealing, and Control-oil Systems and Auxiliaries

API Standard 670: Machinery Protection Systems

API Standard 677, General-Purpose Gear Units for Petroleum, Chemical and Gas Industry Services

API Standard 682, Pumps—Shaft Sealing Systems for Centrifugal and Rotary Pumps

ABMA Standard 7, 1 Shaft, Housing Fits for Metric Radial Ball and Roller Bearings (Except Tapered Roller Bearings) Conforming to Basic Boundary Plans

ABMA 19.1, Tapered Roller Bearings—Radial Metric Design

1 American Bearing Manufacturers Association, 2025 M Street, NW, Suite 800, Washington, DC 20036, www.abma-dc.org.

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