Manual of Petroleum Measurement Standards Chapter 14.10

Natural Gas Fluids Measurement—Measurement of Flow to Flares

SECOND EDITION, DECEMBER 2021

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Contents

Page

1. Scope 1

   1. General 1

   2. Field of Application 1

2. Normative References 1

3. Terms and Definitions 2

4. Application Considerations for Meters in Flare Systems 3

   1. Overview 3

   2. Flare Metering Technologies 4

   3. General Considerations for Design and Selection of an FFMS 6

   4. Location of Flare Meters 6

   5. Application-specific Factors Affecting Flow Meter Performance 7

   6. Meter Sizing 8

   7. Measurement Uncertainty 9

   8. Flow Meter Selection 9

   9. Specific Meter Considerations 12

   10. Secondary Instrumentation 17

   11. Design Considerations 17

   12. Record-keeping 18

5. Factory Calibrations/Verifications 18

   1. Flow Meter 18

   2. Pressure And Temperature Instruments 18

6. Commissioning and Startup 19

   1. General 19

   2. Flare Meter Commissioning 19

7. Periodic Verification 21

   1. General 21

   2. Periodic Verification Method—Flow Meter 21

   3. Periodic Verification Method—Secondary Devices 22

8. Reevaluation of Existing FFMS 22

   1. General 22

   2. Reevaluation Procedure 22

9. Performance Test Protocol Scope 23

10. Uncertainty and Propagation of Error 23

    1. Objective 23

    2. Uncertainty Analysis Procedure 23

    3. Simplified Uncertainty Analysis Procedure 24

    4. Uncertainty Estimate for Flare Composition 26

    5. Meter-specific Examples 29

11. Documentation 30

    1. Procedural Documentation 30

    2. Scaling Documentation 30

       Contents

       Page

    3. Other Documentation 30

    4. Audit Trail Documentation and Retention 30

Annex A (informative) Process Stream Data Sheet 32

Annex B (informative) Flare Meter Calculations 33

Annex C (informative) Compressibility Effects on Flare Gas Measurement Uncertainty 38

Annex D (informative) General Flare Design Considerations 39

Annex E (informative) Guidance on Management of Change Process FFMS Systems 48

Annex F (informative) Velocity Profile and Velocity Integration for Flare Gas Measurement 50

Annex G (informative) Uncertainty Calculation Examples 56

Bibliography 63

Figures

1. Flare Flow Measurement System (FFMS) Graphical Representation of an FFMS and its

   Relation to Other Devices 4

2. Vortex Shedding Principle 6

3. Thermal Flow Meter Detail 15

4. In-line and Insertion Vortex Shedding Meters 16

5. FFMS Commissioning and Verification 20

6. Measurement Error Caused by Gas Composition Analysis Delay 28

A.1 Example Process Stream Data Sheet 32

1. Single Point Flare Burner 39

2. Purge Reduction Seals 40

3. Plant or Operating Unit Knock-out Drum 40

4. Local Knock-out Drum 40

5. Local Water Seal 40

6. Local Water Seal with Local Knock-out Drum 41

7. Air-assisted Flare 41

8. Enclosed Ground Flare 41

9. Flare Gas Recovery Unit 41

10. Waste Gas Riser 42

11. Staged Multi-burners 42

12. Purge Gas Injection Point 42

13. Assist Gas Injection Point 42

14. FFMS in a Horizontal Section with No Additional Equipment 43

15. FFMS with an Optional Connection and an LWS 44

16. FFMS with an LKO 45

17. FFMS Following a PKO 46

18. FFMS in a Staged Multi-burner Flare System 46

19. Typical Staging Curve 46

1. Annulus Area vs Distance from the Center of the Pipe 51

   Contents

   Page

2. Point Velocity vs Area Weighted Velocity 52

3. Predicted Velocity Contours through the Downstream of the Single Bend: a) Flow through the

   Bend (Insert—Predicted Vortices), b) Development after the Bend 53

4. Comparison of Axial Velocity on the Horizontal Axis with NIST Data at Various Axial Distances

from the Bend 54

Tables

1. Guidance on Sensitivity to Entrained Mist, Liquid, Particulates, and Fouling 11

2. Guidance on Installation effects and Secondary Instrument Requirements for Pressure,

   Temperature, and Composition 11

3. Upstream or Downstream Location of Pressure Devices Relative to the Flare Flow Meter 17

4. Example Table of Combined Uncertainties 26

5. Errors Related to Use of Fixed Composition for Different Meter and Calculations Types (Absolute

   Value of Error) 29

6. Procedures and Responsibilities for Documentation 30

B.1 Meter Equation Matrix 33

1. Velocity/Pipe Bulk Average Velocity 53

2. Table of Meter Errors Meter Using the Fully Developed Profile vs 11.2D Profile 54

3. Table of Meter Errors Meter Using the Fully Developed Profile vs 2.7D Profile 54

1. Linear Volume Meter Measuring Standard Volumetric Flowrate 57

2. Random Uncertainty—Linear Volume Meter Measuring Standard Volumetric Flowrate 57

3. Systematic Uncertainty—Linear Volume Meter Measuring Standard Volumetric Flowrate 58

4. Averaging Pitot Tube Measuring Standard Volumetric Flowrate 60

5. Random Uncertainty—Averaging Pitot Tube Measuring Standard Volumetric Flowrate 60

6. Systematic Uncertainty—Averaging Pitot Tube Measuring Standard Volumetric Flowrate 61

7. Thermal Flow Meter Measuring Mass Flowrate 61

8. Random Uncertainty—Thermal Flow Meter Measuring Mass Flowrate 62

9. Systematic Uncertainty—Thermal Flow Meter Measuring Mass Flowrate 62

Introduction

Measurement of flow to flares is important from accounting, mass balance, energy conservation, emissions reduction, and regulatory perspectives. However, measurement of flow to flares remains distinctly different from traditional fiscal measurement practices. Flares are safety-relief systems, which typically receive highly unpredictable rates of flow and varying compositions; for safety reasons, they do not often lend themselves to being taken out of service to accommodate measurement concerns, even for short periods. Therefore, some of the traditional paradigms applicable to fiscal measurement systems, such as reasonably predictable flow rates and composition, the use of in-line proving, capability to readily remove meters from the piping system, the use of by-pass connections, and the use of master meters, for example, have to be abandoned altogether or highly modified in flare measurement applications. Use of measurement systems with diagnostic and verification capability might be one solution to ensure the performance.

Natural Gas Fluids Measurement—Measurement of Flow to Flares

1. Scope

   
   

   1. General

      
      

      The standard addresses measurement of flow to flares, and includes:

      
      

      • application considerations;

        
        

      • selection criteria and other considerations for flare meters and related instrumentation;

        
        

      • installation considerations;

        
        

      • limitations of flare measurement technologies;

        
        

      • calibration;

        
        

      • operation;

        
        

      • uncertainty and propagation of error;

        
        

      • calculations.

      
      

      The scope of this standard does not include analytical instrumentation.

      
      

   2. Field of Application

      
      

      For safety and other considerations, it is highly undesirable to directly flare multiphase mixtures of liquids and gases. Therefore, this standard is primarily concerned with flare flow measurement in the gas or vapor phase. However, considering that fouling substances such as liquid droplets and/or mist or other contaminants may be present even in well-designed flare systems, this standard provides appropriate cautionary detail as to the effects of such contaminants that may impact flare flow measurements.

      
      

      Most flare header applications are designed to operate during non-upset conditions at near-atmospheric pressure and ambient temperature, where compressibility of the mixture is near unity. Extreme conditions have been noted to be between 97.925 kPa-a (14.196 psia) and 414 kPa-a (60 psia), and between –150 °C and 300 °C (–238 °F and 572 °F). Flare gas compositions are highly variable and can range from average molecular weights approaching that of hydrogen to that of iso-pentanes and heavier. The uncertainty in flare gas density associated with varying pressure, temperature, and composition is discussed in more detail in 10.4.

      
      

      Most flare headers are designed to operate at maximum velocities of 91 m/s (300 ft/s), with extremes up to 183 m/s (600 ft/s). This standard does not exclude pressures, temperatures, and velocity ranges different than those suggested above, as long as all applicable requirements are met.

      
      

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 Manual of Petroleum Measurement Standards (MPMS) Chapter 21.1, Flow Measurement Using Electronic Metering Systems—Electronic Gas Measurement