Process Measurement Instrumentation

API RECOMMENDED PRACTICE 551 FIRST EDITION, MAY 1993

REAFFIRMED, FEBRUARY 2007

Process Measurement Instrumentation

Manufacturing, Distribution and Marketing Department

API RECOMMENDED PRACTICE 551 FIRST EDITION, MAY 1993

REAFFIRMED, FEBRUARY 2007

SPECIAL NOTES

1. API PUBLICATIONS NECESSARILY ADDRESS PROBLEMS OF A GENERAL NATURE. WITH RESPECT TO PARTICULAR CIRCUMSTANCES, LOCAL, STATE, AND FEDERAL LAWS AND REGULATIONS SHOULD BE REVIEWED.

2. API IS NOT UNDERTAKING TO MEET THE DUTIES OF EMPLOYERS, MANU- FACTURERS, OR SUPPLIERS TO WARN AND PROPERLY TRAIN AND EQUIP THEIR EMPLOYEES, AND OTHERS EXPOSED, CONCERNING HEALTH AND SAFETY RISKS AND PRECAUTIONS, NOR UNDERTAKING THEIR OBLIGATIONS UNDER LOCAL, STATE, OR FEDERAL LAWS.

3. INFORMATION CONCERNING SAFETY AND HEALTH RISKS AND PROPER PRECAUTIONS WITH RESPECT TO PARTICULAR MATERIALS AND CONDI- TIONS SHOULD BE OBTAINED FROM THE EMPLOYER, THE MANUFACTURER OR SUPPLIER OF THAT MATERIAL, OR THE MATERIAL SAFETY DATA SHEET.

4. NOTHING CONTAINED IN ANY API PUBLICATION IS TO BE CONSTRUED AS GRANTING ANY RIGHT, BY IMPLICATION OR OTHERWISE, FOR THE MANU- FACTURE, SALE, OR USE OF ANY METHOD, APPARATUS, OR PRODUCT COV- ERED BY LETTERS PATENT. NEITHER SHOULD ANYTHING CONTAINED IN THE PUBLICATION BE CONSTRUED AS INSURING ANYONE AGAINST LIABIL- ITY FOR INFRINGEMENT OF LETTERS PATENT.

5. GENERALLY, API STANDARDS ARE REVIEWED AND REVISED, REAF- FIRMED, OR WITHDRAWN AT LEAST EVERY FIVE YEARS. SOMETIMES A ONE- TIME EXTENSION OF UP TO TWO YEARS WILL BE ADDED TO THIS REVIEW CYCLE. THIS PUBLICATION WILL NO LONGER BE IN EFFECT FIVE YEARS AF- TER ITS PUBLICATION DATE AS AN OPERATIVE API STANDARD OR, WHERE AN EXTENSION HAS BEEN GRANTED, UPON REPUBLICATION. STATUS OF THE PUBLICATION CAN BE ASCERTAINED FROM THE API AUTHORING DEPART- MENT [TELEPHONE (202) 682-8000]. A CATALOG OF API PUBLICATIONS AND MATERIALS IS PUBLISHED ANNUALLY AND UPDATED QUARTERLY BY API, 1220 L STREET, N.W., WASHINGTON, D.C. 20005.

Copyright ? 1993 American Petroleum Institute

FOREWORD

The procedures for installation of the instruments covered in this recommended practice are based on experience with and evaluation of many installations. They represent the in- stallation practices that yield the most consistently accurate results and have proved to be practical and safe.

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 pub- lication and hereby expressly disclaims any liability or responsibility for loss or damage re- sulting 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 Manufac- turing, 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 associ- ated 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 se- rious 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 product 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 As- bestos, Tremolite, Anthophyllite, and Actinolite, 29 Code of Federal Regulations Section 1910.1001; the U.S. Environmental Protection Agency, National Emission Standard for As- bestos, 40 Code of Federal Regulations Sections 61.140 through 61.156; and the U.S. En- vironmental Protection Agency (EPA) rule on labeling requirements and phased banning of asbestos products, published at 54 Federal Register 29460 (July 12, 1989).

There are currently in use and under development a number of substitute materials to re- place 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.

SECTION 1?GENERAL

CONTENTS

Page

1. Scope 1

2. Referenced Publications 1

SECTION 2?FLOW

1. Scope 2

2. General 2

   1. Categories 2

   2. Transmission Practice 2

   3. Accessibility 2

   4. Local Indication 2

   5. Vibration 2

   6. Pulsation 2

   7. Purging and Sealing 2

   8. Piping 3

3. Measurement Devices 3

   1. Differential-Pressure Meters 3

   2. Variable-Area Meters 5

   3. Magnetic Flowmeters 7

   4. Turbine Meters 10

   5. Positive-Displacement Meters 11

   6. Vortex Meters 12

   7. Mass Flowmeters 13

SECTION 3?LEVEL

1. Scope 14

2. General 14

   1. Introduction 14

   2. Accessibility 14

   3. Readability 14

   4. Connections to Vessels 14

   5. Multiple-Instrument Mounting 14

   6. Block Valves 14

   7. Strain Relief 15

   8. Vibration 15

   9. Drains and Vents 15

3. Locally Mounted Indicating Gauges 15

   1. General 15

   2. Tubular Gauge Glasses 15

   3. Armored Gauge Glasses 15

   4. Magnetic Gauges 17

4. Level Transmitters 17

   1. General 17

   2. Displacement Transmitters 18

   3. Differential-Pressure Transmitters 19

   4. Hydrostatic-Head Transmitters 20

   5. Nuclear Level Transmitters 20

   6. Ultrasonic Level Transmitters 22

   7. Capacitance/Radio-Frequency Level Transmitters 22

5. Locally Mounted Controllers 24

   1. General 24

   2. Displacement Controllers 24

   3. Internal Ball-Float Controllers 25

   4. Differential-Pressure Controllers 26

6. Level Switches 26

   1. General 26

   2. Installation of Float Switches 26

   3. Installation of Other Switches 27

   4. Testing 27

   5. Over?ll Protection 27

7. Tank Gauging 27

8. Accessories 27

   1. Seals and Purges 27

   2. Weather Protection 27

SECTION 4?PRESSURE

1. Scope 27

2. General 27

   1. Introduction 27

   2. Application Practice 27

   3. Accessibility 27

   4. Local Indication 28

   5. Vibration 28

   6. Pulsation 28

   7. Purging and Sealing 28

   8. Piping 29

   9. Enclosures 30

   10. Element and Socket (Wetted) Materials 30

3. Pressure Gauges and Switches 30

   1. Connections 30

   2. Supports 30

   3. Safety Devices 30

   4. Siphons 30

   5. Case Material and Size 31

4. Pressure Transmitters 31

   1. Connections 31

   2. Installation Considerations 31

   3. Differential-Pressure Transmitters 31

5. Locally Mounted Controllers and Recorders 32

   1. Connections 32

   2. Supports 32

   3. Installation Considerations 32

SECTION 5?TEMPERATURE

1. Scope 34

2. Thermowells 34

   1. General 34

   2. Insertion Length 34

   3. Immersion Length 34

   4. Materials 34

   5. Construction 34

3. Thermocouple Temperature Instruments 37

   1. General 37

   2. Applications 37

   3. Tube-Surface Temperature Measurement 38

      Page

   4. Firebox Temperature Measurement 39

   5. Extension Wires 39

   6. Signal Conditioning 39

   7. Input Circuits 41

4. Resistance Temperature Measurement 41

   1. Application 41

   2. Resistance Temperature Devices 41

   3. Extension Wires 42

   4. Resistance Transmitters 42

5. Dial Thermometers for Local Temperature Measurement 42

6. Filled-System Temperature Instruments 42

   1. General 42

   2. Applications 42

   3. Self-Acting Temperature Regulators 42

   4. Temperature Transmitters 42

   5. Installation Guidelines 42

7. Radiation Pyrometers 42

SECTION 6?PROCESS AND ENVIRONMENTAL PROTECTION

1. Scope 43

2. General 44

3. Seals 44

   1. Diaphragm Seals 44

   2. Liquid Seals 45

4. Purges 45

   1. General 45

   2. Purge Fluids 46

   3. Rate of Flow 46

5. Heating 48

   1. General 48

   2. Steam Heating 49

   3. Electrical Heating 52

Figures

1?Flow Nozzle, Venturi Tube, and Flow Tubes 3

2?Pitot Tube Installation 4

3?Close-Coupled Differential-Pressure Flowmeters 6

4?Remotely Mounted Differential-Pressure Flowmeters for Liquid and

Gas Service 8

5?Remotely Mounted Differential-Pressure Flowmeters for Steam or

Condensable-Vapor Service 9

6?Variable-Area Meter Piping Con?gurations 10

7?Gauge-Glass Assemblies 16

8?Gauge-Glass Mounting Arrangements for Horizontal Vessels and for

Interface Measurement 17

9?Typical Magnetic Gauge 18

10?External-Cage Displacement Instrument 19

11?External-Cage Displacement Controller With Standpipe 20

12?Standpipe With External-Cage Displacement Instrument and Multiple

Sight Gauges 21

13?Typical Stilling Well 22

14?Typical Installations of Differential-Pressure-Level Instruments 23

15?Flange-Type Differential-Pressure-Level Transmitter 24

16?Typical Arrangement for Nuclear Level Transmitter 25

17?Capacitance/Radio-Frequency-Type Level Transmitter 25

18?Arrangement of High- and Low-Level Alarm Switches With Parallel

Gauge Glass 26

19?Piping for Pressure Instruments That Share a Common Process

Connection 28

20?Piping for Pressure Gauges in Pulsating, Corrosive, Slurry, or Freezing

Fluid Service 28

21?Pressure Gauges Supported by Piping 29

22?Field-Mounted Gauge Supports 30

23?Gauges With Siphon Required in Hot Condensable-Vapor Service 31

24?Typical Installation of Pressure Transmitters for Gas, Liquid, and Steam

Service 32

25?Schematic for Measurement of Pressure Differential Across a Reactor or

Section of Tower 33

26?Thermowell Installation 35

27?Elbow Installation of Thermowells 36

28?Vessel and Line Installation of Thermowells 36

29?Metal-Sheathed, Mineral-Insulated Thermocouple Assemblies 37

30?Sheathed-Type Thermocouple and Head Assembly 38

31?Thermocouple-to-Conduit Connections 38

32?Knife-Edge Tube-Surface Thermocouple for Heater Tube 39

33?Typical Firebox Thermocouple Installations 40

34?Typical Radiation Pyrometer Installation 43

35?Seals for Pressure Gauges 44

36?Diaphragm and Capillary System 45

37?Liquid Seal Installations 46

38?Freezing Points of Ethylene-Glycol?Water Mixtures 47

39?Purge Installations 47

40?Ori?ce Tap Purges for Flowmeters 48

41?Steam Tracing and Insulation for Instrument Lines and Pressure Gauges .. 50 42?Steam Tracing and Heating 51

43?Instrument Housings 52

44?Steam Tracing and Insulation for External Displacement Level

Instrument 53

45?Typical Steam Tracing for Flow Transmitter 54

46?Typical Steam Tracing for Pressure Transmitter 55

47?Typical Electrical Tracing and Instrument Enclosures 55

48?Electrical Tracing and Insulation for Pressure Gauges 55

49?Electrical Tracing and Insulation for Level Instruments 56

50?Electrical Tracing and Insulation for Instruments 57

Tables

1?Thermocouple Materials and Temperature Range 37

2?Thermocouple Extension-Wire Materials 41

viii

Process Measurement Instrumentation

SECTION 1—GENERAL

1. Scope

   The material in Sections 2?6 was previously presented in several sections of Part 1 of API Recommended Practice 550, which is now out of print:

   1. Section 2?API Recommended Practice 550, Part I, Sec- tion 1, ?Flow.?

   2. Section 3?API Recommended Practice 550, Part I, Sec- tion 2, ?Level.?

   3. Section 4?API Recommended Practice 550, Part I, Sec- tion 4, ?Pressure.?

   4. Section 5?API Recommended Practice 550, Part I, Sec- tion 3, ?Temperature.?

   5. Section 6?API Recommended Practice 550, Part I, Sec- tion 8, ?Seals, Purges, and Winterizing.?

   The procedures for installation of the instruments covered in this recommended practice are based on experience with and evaluation of many installations. They represent the in- stallation practices that yield the most consistently accurate results and have proved to be practical and safe.

   Process and environmental protection is covered in a gen- eral fashion in Section 6. Where required, speci?c instances of process and environmental protection are covered in Sec- tions 2?5.

   Tank gauging is outside the scope of Section 3. The ap- plicable publication is referenced in that section.

   Where appropriate, installation drawings, cautionary notes, and explanations are included. Valves and piping that are typically covered in piping standards have been omitted from most installation drawings.

   
   

2. Referenced Publications

The latest edition or revision of the following standards, codes, and speci?cations shall, to the extent speci?ed, form a part of this recommended practice:

ANSI1

MC 96.1 Temperature Measurement: Thermocouples

API

Heaters and Inert Gas Generators? (out of print)

RP 552 Signal Transmitters and Air Supplies (in press)

RP 554 Control Centers, Instruments, and Control Systems (in press)

RP 555 Process Analyzers (in press)

Std 606 Compact Carbon Steel Gate Valves?Ex- tended Body

Publ 2218 Fireproofing Practices in Petroleum and Petrochemical Processing Plants

RP 2350 Overfill Protection for Petroleum Storage Tanks

Std 2545 Method of Gaging Petroleum and Petroleum Products

Manual of Petroleum Measurement Standards, Chapter 4,

?Proving Systems?; Chapter 5, ?Metering?; Chapter 6, ?Metering Assemblies?; Chapter 7, ?Temperature Determination?; Chapter 14, ?Natural Gas Fluids Measurement,? Sec- tion 3, ?Concentric, Square-Edged Orifice Meters,? Part 1, ?General Equations and Un- certainty Guidelines?; Chapter 18, ?Custody Transfer?

ASME2

B16.5 Pipe Flanges and Flanged Fittings

PTC 19.3 Performance Test Code?Temperature Mea- surement

ASTM3

STP 470B Manual on the Use of Thermocouples in Temperature Measurement

NFPA4

70 National Electrical Code

1American National Standards Institute, 11 West 42nd Street, New York,

RP 550 Manual on Installation of Refinery Instru-

ments and Control Systems, Part I, ?Process Instrumentation and Control,? Section 7,

?Transmission Systems? (out of print)

RP 550 Manual on Installation of Refinery Instru- ments and Control Systems, Part III, ?Fired

New York 10036.

2American Society of Mechanical Engineers, 22 Law Drive, Box 2300, Fair?eld, New Jersey 07007-2300.

3American Society for Testing and Materials, 1916 Race Street, Philadel- phia, Pennsylvania 19103.

4National Fire Protection Association, 1 Batterymarch Park, P.O. Box 9101, Quincy, Massachusetts 02269-9101.

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