S-G-06—Specifications for the Approval, Verification, Reverification, Installation and Use of Cone-Shaped Differential Pressure Meters

S-G-06—Specifications for the Approval, Verification, Reverification, Installation and Use of Cone-Shaped Differential Pressure Meters (PDF, 103 KB, 11 pages)

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Category: Gas
Specification: S-G-06
Document(s):
Distribution Date: 2011-07-26
Effective Date: 2011-07-26
Supersedes:

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Table of Contents

• 1.0 Scope
• 2.0 Authority
• 3.0 Definitions
• 4.0 Metrological Requirements
  • 4.1 Pattern Approval Baseline Accuracy Tests
  • 4.2 Verification and Reverification Accuracy Tests
  • 4.3 Reverification Interval
  • 4.4 Conditions for Metrological Characteristics
    • 4.4.1 Disturbance Factors
      • 4.4.1.1 Swirl Susceptibility Testing
• 5.0 Technical Requirements
  • 5.1 Design, Composition and Construction
  • 5.2 Installation and Use
    • 5.2.1 General
    • 5.2.2 Meter Tubes
    • 5.2.3 Upstream Meter Tube Length
    • 5.2.4 Downstream Meter Tube Length
    • 5.2.5 Meter Alignment
    • 5.2.6 Thermometer Well
    • 5.2.7 Flow Computer
• 6.0 Administrative Requirements
  • 6.1 Nameplate Marking
  • 6.2 Verification Marks
• Appendix A–Algorithms Used to Evaluate CSDP Meter Performance

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1.0 Scope

These specifications apply to the approval, verification, reverification, installation and use of cone-shaped differential pressure (DPC) meters.

2.0 Authority

These specifications are issued under the authority of sections 12 and 18 of the Electricity and Gas Inspection Regulations.

3.0 Definitions

Deviation

The difference between the volume or flow rate measured by the meter under test and the volume or flow rate measured by a reference meter, calculated according to clause 4.1.3. Note: Corrections must be made in the calculations to account for the differences in gas pressure, temperature and compressibility between the two meters.

Flow algorithm

The mathematical relationship used to transform the measured pressure differential in the meter to a mass flow or volumetric flow rate at metering conditions.

Linearity

The maximum difference between the meter's mean deviations and any of the error deviations for test points between the maximum and minimum rated flow rates, calculated according to clause 4.1.3.

Maximum flow rate (Qmax)

The maximum flow rate of a meter, as listed in the meter's Notice of Approval.

Maximum permissible error

The largest allowable deviation within the specified operational range of the meter.

Mean deviation

The mean error of the meter, calculated according to clause 4.1.3.

Minimum flow rate (Qmin)

The lowest flow rate at which the meter's deviation is less than the maximum permissible error and the linearity is less than the maximum prescribed values in clause 4.1.2., as listed in the meter's Notice of Approval..

Repeatability

The largest spread of errors of a given meter when several successive measurements are performed at the same flow rate under the same operating conditions.

Reynolds number (Re)

The dimensionless number equalling the ratio of inertial forces to viscous forces in closed pipe flow (see equation A.9 in Appendix A).

Specification Limit

The maximum permissible error permitted for a meter's performance characteristic.

Test Limit (TL)

The limit established when the specification limit is adjusted for the associated measurement uncertainty.

4.0 Metrological Requirements

4.1 Pattern Approval Baseline Accuracy Tests

4.1.1 Meters shall be baseline tested to establish performance under fully developed ideal flowing gas conditions. This testing shall be performed using a suitable test medium ^{Footnote 1} at flow rates over the range of 10% to 100% of Qmax, and at Reynolds numbers which are representative of the meter’s intended use over the specified operating range of the meter ^{Footnote 2}. Each test point shall consist of at least three measurements, each measurement being of sufficient duration to provide an error resolution of 0.1% or better. The test flow rates shall be approximately equally spaced between 0.1Qmax and Qmax, and shall include as a minimum the following test points: 0.1Qmax ^{Footnote 3}, 0.25Qmax, 0.5Qmax, 0.75Qmax and Qmax.

4.1.2 The deviation(s) determined using the approved flow equation between Qmin and Qmax shall not exceed the following values:

Test Limits:

• The lesser of TL_u = 1.50% - ku_ci or TL_u = +1.0%
• The greater of TL_L = -1.50% + ku_ci or TL_L = -1.0%
• Repeatability: ±0.2%
• Linearity: ±0.5%

where,

• k = appropriate coverage factor (= 1.645, unless otherwise specified) for the 95.45% confidence interval
• u_ci = the standard combined measurement uncertainty of the calibration ^{Footnote 4}

4.1.3 Deviation and linearity shall be calculated in accordance with the equations given below.

where,

• Q_(i) = volumetric or mass flowrate or accumulated volume determined by meter under test using its flow algorithm ^{Footnote 5}
• Q_ref(i) = volumetric or mass flowrate or accumulated volume determined by reference meter ^{Footnote 5}

where,

• D_(i) = percent deviation at test point (i) determined using eqn.(1)

4.1.4 Unless otherwise indicated by the approval applicant, the algorithm used to translate the primary measurements of pressure and temperature into volumetric flow shall be the algorithm derived in Appendix A. Where the applicant can show sufficient evidence that an alternative algorithm is suitable, that algorithm shall be authorized for use in the Notice of Approval.

4.2 Verification and Reverification Accuracy Tests

4.2.1 Subject to clause 4.1.3, initial verification tests shall be conducted on a suitable test medium ^{Footnote 6} at Reynolds numbers which are representative of the meter's intended use. Where Qmax cannot be achieved because of limitations of the test facility, the upper test flow rate shall be at least 0.4Qmax and the meter shall be tested at least five flow rates approximately equally spaced between 0.1Qmax and the maximum flow rate attainable by the test facility.

4.2.2 The maximum permissible error of the meter shall not exceed the tolerances set out in clause 4.1.2. Where the meter's maximum test flow rate cannot be achieved, the errors of the meter shall not exceed the following tolerance limits:

4.3 Reverification Interval

Meters shall be reverified in accordance with the applicable initial reverification period or subsequent reverification period prescribed in MC bulletin G-18.

4.4 Conditions for Metrological Characteristics

4.4.1 Disturbance Factors

For pattern approval purposes, the difference between the meter accuracy established during baseline testing and the meter accuracy established during meter flow disturbance testing shall not exceed the errors set out in clause 4.1.2 for the following disturbance factor.

4.4.1.1 Swirl Susceptibility Testing

The configuration recommended by the manufacturer at the inlet to the installation shall be preceded by a swirl generator constructed of two ninety-degree elbows connected together orthogonally. The outlet of the upstream disturbance shall be placed a distance of two times the nominal pipe diameters from the upstream face of the CSDP meter. Downstream piping shall be straight and equal in diameter to the upstream piping and a minimum of two pipe diameters in length. The meter shall be tested using the flow rates established in clause 4.1.1.

5.0 Technical Requirements

5.1 Design, Composition and Construction

5.1.1 The case of a CSDP meter shall be designed and constructed to operate without leakage or deformation over the expected range of operating pressures, flowing gas temperatures and environmental conditions.

5.1.2 The case of a meter intended for outdoor use shall be waterproof, dustproof and dimensionally stable over the range of environmental conditions expected during the service life of the meter.

5.1.3 The construction of the meter shall be mechanically sound and the materials, finish, etc., shall be such as to provide assurance of long life and sustained accuracy.

5.1.4 The body end connections shall be designed in accordance with appropriate flange or threaded connection standards.

5.1.5 A static pressure tap shall be located on the meter body.

5.1.6 Where the piping configuration requirements differ from the requirements of section 5.2, the Notice of Approval shall specify the required installation piping configuration.

5.2 Installation and Use

5.2.1 General

5.2.1.1 Unless otherwise stated in the Notice of Approval, the meter shall be installed either vertically or horizontally. In a vertically-installed meter, the gas shall flow downward.

5.2.1.2 Where conditions of reverse flow may occur during meter usage, the installation shall incorporate features to prevent the reverse flow of gas through the meter.

5.2.1.3 The meter shall be installed in a manner designed to prevent the accumulation of contaminants.

5.2.1.4 Where a meter is subject to flow pulsations and it has been shown that the meter's accuracy is affected by such disturbances, appropriate provisions shall be made to reduce the intensity of the disturbance to a level that will not induce a measurement error greater than the maximum permissible error specified in clause 4.1.2.

5.2.1.5 The meter shall not be used outside the range of ambient temperature for which it is approved. At the time of approval, the manufacturer shall provide an attestation concerning the meter's intended operating range for ambient temperature. Where necessary, shelter and heaters or other arrangements shall be provided by the meter owner to ensure that this requirement is met.

5.2.2 Meter Tubes

The meter shall be installed in a manner consistent with the manufacturer's installation requirements and any other special installation requirements set out in the Notice of Approval for the type of service that the meter is intended to be used.

5.2.3 Upstream Meter Tube Length

Unless otherwise authorized by the Notice of Approval, a straight pipe equal in length to five times the nominal meter diameter shall be installed upstream of the meter.

5.2.4 Downstream Meter Tube Length

Unless otherwise authorized by the Notice of Approval, a straight pipe equal in length to three times the nominal meter diameter shall be installed downstream of the meter. In cases where the thermowell is permanently installed in the downstream meter tube, the entire meter run (which includes the upstream and downstream straight pipes) must be calibrated as an integral system, unless otherwise authorized by the Notice of Approval.

5.2.5 Meter Alignment

For inclusion in the meter's Notice of Approval, the manufacturer shall provide supporting test data relating to the maximum permissible meter / meter tube step change and maximum permissible meter non-axial alignment.

5.2.6 Thermometer Well

Unless otherwise authorized by the Notice of Approval, the thermometer well used for measuring the flowing gas temperature shall be installed between two and five diameters downstream of the metering element.

5.2.7 Flow Computer

The flow computer used to calculate the volumetric flow rate or mass flow through a CSDP meter shall be a model which has been type approved for use of the algorithms listed in Appendix A.

6.0 Administrative Requirements

6.1 Nameplate Marking

The following information shall be indelibly marked on the meter or on a nameplate securely fastened to the meter:

1. manufacturer's name
2. model number
3. serial number
4. direction of positive flow (e.g., arrow)
5. minimum and maximum flow rate (at line conditions)
6. minimum and maximum rated operating pressure
7. ambient temperature range, where less than -30 °C to +40 °C
8. Notice of Approval number
9. Beta ratio
10. inside diameter of meter tube

In addition to the above markings, space shall be provided on the meter or on a securely affixed nameplate for marking the meter's inspection number (i.e., the unique identification number assigned by the meter owner).

6.2 Verification Marks

Upon verification or reverification, the CSDP meter body shall be marked with a verification mark. This may be done using a special steel punch designed to produce an imprint that uniquely identifies the meter verifier. Such marks may either be placed near the meter nameplate or placed on the outer edge of the meter's inlet connection flange. Alternatively, if the meter body has suitable provisions (e.g., drilled hole) for securely attaching a sealing wire, a conventional verification seal may be affixed.

Alan Johnston
President

Appendix A–Algorithms Used to Evaluate CSDP Meter Performance

Unless the applicant of the CSDP meter specifies otherwise, the meter's performance shall be evaluated using an equation for differential pressure measuring elements similar to those presented in Report No. 3 of the American Gas Association (AGA): Orifice Metering of Natural Gas, Part 3–Natural Gas Applications (1992). The equation differs in order to account for the physical differences between the two types of meters. The equation for the mass flow rate for CSDP meters can be written as shown below. An important difference is that the discharge coefficient (C_d) is not determined by the modeling equation used in AGA Report No. 3, but rather by using an empirical equation developed through experimentation specific to the meter type. It has been shown that C_d can be assumed as reasonably constant in the calibrated flow range. It will be corrected by using a Re correlated meter factor in the flow computer.

where,

The following empirical equation has been developed to describe the upstream gas expansion factor:

The Beta ratio as described in AGA Report No. 3 is not directly applicable. A similar ratio has been successful developed for a CSDP meter and is defined by the following relationship:

Use of Data in Flow Computers

Knowing, from a reference standard, the true mass flow rate at each of the prescribed test points (Q_ref(i)), C_d,(i) can be calculated using the following equation:

In the first method, once the values for C_d have been calculated, the values will be used to determine the relationship between Re and the meter factor (M_f(i)) at each test point. The M_f(i) values will then be programmed in the flow computer.

List of Symbols Used in this Appendix

Symbol	Description
β	beta ratio
ΔP	pressure differential across meter
Cd	discharge coefficient
Cd,mean	mean Cd (value programmed as a constant Cd in flow computer)
Cd(i)	discharge coefficient at the specific Reynolds number ( Re)
d	outside diameter of the cone
D	inside diameter of the meter pipe
Ev	approach velocity
Fext	expansion factor
k′	isentropic exponent
Mf(i)	meter factor at the specific Reynolds number (Re)
P	static pressure absolute
Qm	mass flow rate
Q	non-converted volumetric flow rate (not converted to reference conditions of standard pressure and standard temperature)
Qref(i)	mass flow rate through the reference standard at the specific Reynolds number (Re) or (i) testpoint, where i = 1, 2, 3, 4, …
Re	Reynolds number
Y1	upstream gas expansion factor
V	bulk velocity of flowing gas
ρ	gas density at actual flowing gas conditions
μ	dynamic viscosity of the flowing gas

Footnotes

Footnote 1

The test medium will normally be natural gas. However, if an applicant provides data demonstrating that other media are suitable for performing tests, tests on those media will be accepted. Unless otherwise specified in the Notice of Approval, a meter shall be calibrated at or near its intended operating conditions using the type of gas intended to be measured in service.

Return to footnote 1 referrer

Footnote 2

Where test facilities are not available to perform tests over the entire operating pressure range of the meter, the applicant shall provide test data demonstrating that the pattern to which the meter belongs is either insensitive to operating pressure or may be predicted using a dimensionless number such as the Reynolds number.

Return to footnote 2 referrer

Footnote 3

If the Qmin declared by an approval applicant is less than 0.1Qmax, use Qmin. If this Qmin value is not less than 0.1Qmax, use 0.1Qmax.

Return to footnote 3 referrer

Footnote 4

The standard combined measurement uncertainty u_ci includes contributions from the CSDP element and its associated pressure and temperature instrumentation as well as the measuring apparatus. Long term stability of the flow element need not be included.

Return to footnote 4 referrer

Footnote 5

As an alternative, for type approval, the linearity of the meter can be assessed by replacing Q in eqn. (1) with the discharge coefficient C_d.

Return to footnote 5 referrer

Footnote 6

The test medium will normally be natural gas. However, if an applicant provides test data demonstrating that other media are suitable for performing tests, tests on those media will be accepted. Use of those test media shall be authorized in the Notice of Approval.

Return to footnote 6 referrer