GB/T 21446-2025Measurement of natural gas flow by means of standardized orifice meter (English PDF)

1 Scope

GB/T 21446-2025 is the Chinese national standard covering custody transfer by orifice plate - the plate and the pipe geometry and the tolerances on both, the straight lengths and the flow conditioner, the differential pressure measurement, the gas composition and compressibility, and the uncertainty of the final volume. At 48,500 words. This is the standard behind the billing of most of the natural gas sold in China. Issued on 5 October 2025, it has been in force since 1 May 2026, replacing GB/T 21446-2008.

This document specifies the technical requirements for the processing and installation of standard orifice plates, and provides the mass flow rate of natural gas and the volumetric flow rate under standard reference conditions. Methods for calculating energy flow and the assessment of measurement uncertainty. This document applies to throttling devices for single-phase natural gas, with pressure tapping methods including flange and corner connections, and orifice plate opening diameter equal to or greater than 12.5mm, measuring tube inner diameter equal to or greater than 50mm and equal to or less than 1000mm, diameter ratio equal to or greater than

0.1 and equal to or less than

0.1 For cases where the pipeline Reynolds number is

0.75 or greater than 5000.

2 Normative references

The contents of the following documents, through normative references within the text, constitute essential provisions of this document. Dated citations are not included. For references to documents, only the version corresponding to that date applies to this document; for undated references, the latest version (including all amendments) applies. This document.

GB/T 8170 Rules for rounding off numerical values and the representation and determination of limiting values

GB/T 11062 Calculation Methods for Calorific Value, Density, Relative Density and Wobbe Index of Natural Gas

GB/T 13610 Compositional Analysis of Natural Gas - Gas Chromatography

GB/T 15464 General Technical Requirements for Packaging of Instruments and Meters

GB/T 17747 (all parts) Calculation of natural gas compressibility factor

GB/T 18603 Technical Requirements for Natural Gas Metering Systems

GB/T 30491 (all parts) Calculation of thermodynamic properties of natural gas

GB/T 35186 Performance Evaluation of Natural Gas Metering Systems JJG640 Differential Pressure Flow Meter Verification Procedure

3 Terms, Definitions and Symbols

3.1 Terms and Definitions The following terms and definitions apply to this document.

3.1.1 A thin plate with circular perforations, obtained through machining. Its throttling orifice has a cylindrical surface perpendicular to the upstream end face of the orifice plate, and its edges are... The orifice plate is sharp, and its thickness is relatively small compared to its diameter. It should be designed within the scope and requirements specified in this document. Manufacturing, installation and use.

Note. Hereinafter referred to as "perforated plate".

3.1.2 orificeplateholder A pressurized piping assembly used to output the static pressure difference generated by the orifice plate and to house and position the orifice plate. [Source:

GB/T 8423.4-2022, 4.1.65]

7 Tolerances and Tolerance Factors for Platinum Resistance Therapy (PTT)

32 Table A.1 Linear Expansion Coefficient Values of Metallic Materials (20°C~100°C) (Table 38) Table A.2 Dynamic viscosity µ of methane at different pressures and temperatures. (Table 38) Table A.3 Physical property parameters of commonly used components of natural gas (Table 39) Table A.4 Methane cp and cv values at different pressures and temperatures. Table A.5 Equivalent Absolute Roughness K Values of Steel Pipe Inner Wall (Table 40) Table B.1 Pressure-conducting tube length and inner diameter (Table 43) Table D.1 Relationship between bK and rK/d

50 Table D.2 Standard orifice plate gammao values (Table 51) Table E.1 Calculation of Uncertainty Components of Measurement Results for Standard Gas Flow Standard Device Verification or Calibration Standard Throttling Device (Table 54) Table E.2 Calibration Results of Differential Pressure Device Discharge Coefficient Method

54 Table F.1 Composition of Natural Gas

56 Table F.2 Calculation of Natural Gas Compressibility Factor and Overcompressibility Factor

58 Table F.3 Calculation of Natural Gas Density and Relative Density

60 Table F.4 Calorific Value Calculation Table

62 Table F.5 Calculation of Natural Gas Isotropic Index (Table 63) Table F.6 Calculation of Natural Gas Dynamic Viscosity

65 Table F.7 Natural Gas Flow Calculation Order and Significant Digit Values Table

65 Table G.1 Format for Flow Iteration Calculation and Energy Calculation

76 Table G.2 Iterative Calculation Format for Orifice Plate Opening Diameter

77 Table G.3 Differential Pressure Range Iterative Calculation Format

77 Table H.1 Maximum Permissible Error of Flow Metering System and Accuracy Class of Flow Computer

81 Table H.2 Accuracy Classes and Maximum Permissible Errors of Flow Calculators

82 Table H.3 Maximum Permissible Error of Flow Calculator Functional Units and Measurement Channels 82

9 Table

2.Permissible straight pipe section length between the orifice plate and the 19-tube bundle flow conditioner (the distance between the flow throttling element and the orifice plate is LZ, and the value is expressed as follows). (Multiples of pipe inner diameter D)

11 Angle-connected pressure-taking plate holder

22 Figure B.1 Schematic diagram of pressure tap location

43 Figure B.2 Differential pressure gauge installed above the throttling device (no corrosive media)

44 Figure B.3 Differential pressure gauge installed below the throttling device (no corrosive media)

45 Figure B.4 Differential pressure gauge installed above the throttling device (including corrosive media)

45 Figure B.5 Differential pressure gauge installed below the throttling device (including corrosive media)

46 Figure C.1 Chevron hydrocyclone

48 Figure G.1 Flowchart of Flow Calculation Program

78 Figure G.2 Flowchart of the orifice plate opening diameter calculation procedure

79 Figure G.3 Flowchart of Differential Pressure Range Calculation Program

1.Permissible straight pipe section length between orifice plate and flow obstruction (without flow conditioner, values are expressed as multiples of pipe inner diameter D)

12 Figure

5 Example 14 of installing a 19-tube bundle flow adjuster downstream of a single elbow. Figure

13 Table

3.Limits for orifice diameter, measuring tube inner diameter, diameter ratio, and pipe Reynolds number (Table 24) Table

4.Maximum value of 104Ra/D.

5.Minimum value of 104Ra/D.

15 Figure

7 Schematic diagram of the deviation between the orifice plate and the measuring tube axis