Fuel Gas Measurement

API TECHNICAL REPORT 2571 FIRST EDITION, MARCH 2011

Fuel Gas Measurement

Measurement Coordination

API TECHNICAL REPORT 2571 FIRST EDITION, MARCH 2011

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Copyright © 2011 American Petroleum Institute

Foreword

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iii

1. Scope 1

2. Terms and Definitions 1

3. Performance Characteristics and Measurement by Meter Type 3

   1. General 3

   2. Differential Pressure Type Flow Meters 4

   3. Linear Flow Meters 10

   4. Selection Criteria and Documentation of Fuel Gas Meters 14

4. Secondary and Tertiary Instrumentation 15

   1. Introduction 15

   2. Equipment Selection Considerations 15

   3. Equipment Location and Installation 17

5. Uncertainty Calculations 18

   1. Objective 18

   2. Uncertainty Analysis Procedure 18

   3. Combining the Uncertainty of Multiple Meters 20

   4. Differential Producers 21

   5. Linear Meters (Vortex, Ultrasonic, Turbine, PD) 25

   6. Linear Meter (Coriolis) 28

6. Inspection, Verification, and Calibration 28

   1. General 28

   2. System Overview for Selection of Maintenance Activity, Frequency, and Tolerance 29

   3. Relative Stream Significance 29

   4. Identify Devices to be Inspected, Verified, or Calibrated 30

   5. Influence of Meter Type on Frequency Selection 31

   6. Calibration Results Influence on Frequency Selection 33

Annex A (informative) Uncertainty Examples 34

Bibliography 65

Figures

1. Schematic of a Fuel Gas Measurement System 15

2. Example of Fuel Gas Measurement System 16

A.1 Example of Fuel Gas Measurement System (for Illustration Purposes Only) 35

Tables

1. Installation Effect Sensitivity and Secondary Instrument Requirements 3

2. Significant Common DP Meter Influence Parameters 5

3. Additional Orifice Meter Influence Parameters 6

4. Additional Venturi and Nozzle Meter Influence Parameters 7

5. Additional Multiport Averaging Pitot, and Wedge Meter Influence Parameters. 8

6. Variable Area Meter Influence Parameters 9

7. Additional Pitot, Pitot-Static Tube Meter Influence Parameters 9

8. Common Volumetric Linear Meter Influence Parameters 10

9. Additional Displacement Meter Influence Parameters 11

10. Additional Turbine Meter Influence Parameters 11

11. Additional Ultrasonic Meter Influence Parameters 12

    
    

12. Additional Vortex Meter Influence Parameters 12

13. Additional Thermal Meter Influence Parameters 13

14. Additional Coriolis Meter Influence Parameters 14

15. Static Pressure Tap Location 17

16. DP Meter Static Pressure Variance Effect 23

17. DP Meter Temperature Variance Effect 24

18. DP Meter Compressibility Effect Typical Natural Gas vs Typical Natural Gas + 20 % H2 25

19. Linear Meter Static Pressure Variance Effect 26

20. Linear Meter Temperature Variance Effect 26

21. Compressibility–Typical Natural Gas 27

22. Linear Meter Compressibility Effect Typical Natural Gas vs Typical Natural Gas + 20 % H2 27

23. Plant Determination of Relative Stream Significance 30

24. Inspection, Verification, and Calibration Summary 31

25. Meter Station F101: Initial Calibration Program 32

26. Meter Station F101: Revised Calibration Program 33

A.1 Instrumentation on Meter Types 34

Introduction

This document provides a performance-based methodology for the measurement and reporting of fuel gas consumption. Specifically, considerations are provided for measurement device selection, installation, maintenance, calibration and documentation to achieve the targeted performance in terms of availability and uncertainty. If the performance of any installed measurement device is determined not to be in compliance with an acceptable level of uncertainty, the measurement device, its installation, or maintenance practices, etc. can be upgraded. Techniques are described to assess the uncertainty contribution of individual components of fuel gas measurement systems and the overall facility fuel gas measurement uncertainty. By following the guidance and calculation procedures of this document, cost effective fuel gas measurements of appropriate quality can be achieved. In most cases the rigorous requirements of industry standards intended for custody transfer quality measurements can be reduced and still achieve the desired measurement uncertainty. For this document, a fuel gas system in a facility could be comprised of multiple fuel gas meters or a single meter.

This document addresses the most common fuel gas measurement devices in use at the time of its development. This does not advocate the use of these devices or preclude the utilization of other types of devices, provided the targeted performance is achieved.

This Technical Report (TR) includes a brief description of the working principles of different types of fuel gas meters and their influence parameters, installation recommendations, a uniform method to ascertain the measurement uncertainty, a recommended method to determine the frequency of maintenance, performance verification or calibration of the meter and secondary instruments, and other relevant and necessary information.

Fuel gas can be measured by different types of flow meters. The selection of a meter typically depends on several factors such as:

• desired accuracy for the application;

  
  

• desired accuracy verification capability (i.e. calibration, inspection, replacement);

  
  

• life expectancy;

  
  

• operating conditions and their variability—flow rate, pressure, temperature, gas composition/density, etc.;

  
  

• cost of initial installation;

  
  

• operational requirements;

  
  

• regulatory requirements.

  
  

  Listed below are different flow meters that are typically installed to measure the fuel gas flows in the industry. The selection of the fuel gas meter by the user may include other types of meters not included in this list:

  
  

• differential-pressure or head-type flow meters;

  
  

• displacement flow meters;

  
  

• turbine flow meters;

  
  

• thermal dispersion flow meters;

  
  

• Coriolis force flow meters;

  
  

• ultrasonic flow meters;

  
  

• vortex flow meter.

vii

Fuel Gas Measurement

1. Scope

   
   

   This Technical Report (TR) provides guidance in the following areas to allow the user to achieve a targeted uncertainty of measurement:

   
   

   • selection of flow meter type; differential pressure (DP), displacement, ultrasonic, Coriolis, vortex, turbine, thermal, and others;

     
     

   • associated instrumentation for measuring fluid properties and flowing conditions, such as pressure and temperature transmitters, densitometers, gas chromatographs;

     
     

   • obtaining and use of gas composition or other analytical data;

     
     

   • design and installation requirements of the measurement system;

     
     

   • inspection, verification and calibration practices of flow meters and their associated accessory instrumentation; and

     
     

   • simplified uncertainty calculations with examples to illustrate the methodology.

   
   

2. Terms and Definitions

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

2.1

accuracy

The ability of a measurement instrument to indicate values closely approximating the true value of the quantity measured.

2.2

bias

Any influence on a result that produces an incorrect approximation of the true value of the variable being measured. Bias is the result of a predictable systematic error.

2.3

calibration

The process or procedure of adjusting an instrument so that its indication or registration is in satisfactorily close agreement with a reference standard.

2.4

carbon content

The fraction of carbon in the fluid expressed as percent by weight.

2.5

compensation

The adjustment of the measured value to reference conditions (e.g. pressure compensation).

2.6

fuel gas

Typically a mixture of light hydrocarbon and other molecules (e.g. H2, N2) in a gaseous state that are consumed in fired heaters. Fuel gas is often a mixture of recovered gaseous molecules from plant operations and purchased natural gas.

1