Specifications for the Approval, Verification, Reverification, Installation and Use of Cone-shaped Differential Pressure Meters

Specifications for the Approval, Verification, Reverification, Installation and Use of Cone-shaped Differential Pressure Meters (PDF, 60 KB, 10 pages)

1.0 Scope
2.0 Authority
3.0 Definitions
4.0 Metrological Requirements
5.0 Technical Requirements
- 5.1 Design, Composition, and Construction
- 5.2 Installation and Use
6.0 Administrative Requirements
- 6.1 Nameplate Markings
- 6.2 Verification Marking
Appendix A - Algorithms Used to Evaluate DPC Meter Performance

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 flowrate measured by the meter under test, and the volume or flowrate 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 rate or volumetric flow rate at metering conditions.
Mean Deviation: The flow rate weighted mean error of a meter, calculated according to clause 4.1.3.
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 Permissible Error: The largest allowable deviation within the specified operational range of the meter.
Minimum Flow Rate (Qmin): The lowest actual volumetric 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.
Maximum Flow Rate (Qmax): The maximum rated actual volumetric flow rate of a meter, 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 equaling 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.

4.1.1 Meters shall be tested using a suitable test medium ¹at 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 meter². 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 tested flow rates shall be approximately equally spaced between 0.1Qmax and Qmax, and shall include as a minimum; 0.1Qmax³, 0.25Qmax, 0.5Qmax, 0.75Qmax and Qmax.

NOTE 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 intended gas.

NOTE 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 can be predicted using a dimensionless number such as the Reynolds number.

NOTE 3: If Qmin is less than 0.1Qmax, use Qmin. If Qmin is not less than 0.1Qmax, use 0.1Qmax.

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 for the 95.45% confidence interval
u_ci = the standard combined measurement uncertainly of the calibration ⁴

NOTE 4: The standard combined measurement uncertainly u_ci includes contributions from the DPC 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.

4.1.3 Calculation of Deviation and Linearity

eqn. (1)

eqn. (2)

eqn. (3)

NOTE 5: Q(i) is defined as the volume or volumetric flowrate, as determined by the meter under test using its flow algorithm, and Q(i) ref. is the volume or volumetric flowrate determined by the reference meter. As an alternative, for type approval, the linearity of meter can be assessed by replacing Q in eqn. (1) with the discharge coefficient Cd.

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, tests shall be conducted on a suitable test medium 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.

NOTE 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, and use of that test media authorized in the Notice of Approval.

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:

eqn. (4)

Maximum permissible deviation = ±1.0% × Q_attained ÷ Q_max

4.3 Reverification Interval

Meters shall be reverified in accordance with the reverification periods stipulated in Bulletin G-18.

4.4 Conditions for Metrological Characteristics

4.4.1 Disturbance Factors

For pattern approval purposes, the meter accuracy tests 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 inlet to the installation configuration recommended by the manufacturer shall be preceded by a swirl generator, constructed of two ninety-degree elbows connected together orthogonally.

5.0 Technical Requirements

5.1 Design, Composition, and Construction

5.1.1 The case of a 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.

5.2 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 reverse flow 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. Where necessary, shelter and heaters or other arrangements shall be provided 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, 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 length of pipe equal to three times the meter's nominal diameter, shall be installed downstream of the meter. In cases where the thermowell in permanently installed in the downstream meter tube, the entire meter run (which includes the upstream and downstream straight pipes) shall 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 state and 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.

6.0 Administrative Requirements

6.1 Nameplate Markings

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

manufacturer's name
model number
serial number
direction of positive flow (e.g. arrow)
minimum and maximum flow rate (at line conditions)
minimum and maximum rated operating pressure
ambient temperature range, where less than -30 °C to +40 °C
Notice of Approval number
Beta ratio

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 Marking

Upon verification or reverification, the DPC 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 marking may either be placed in the vicinity of 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
Measurement Canada

Appendix A - Algorithms Used to Evaluate DPC Meter Performance

Unless the applicant of the DPC meter specifies otherwise, the meter's performance will be evaluated using a similar equation for differential pressure measuring elements presented in AGA Report No. 3 (1992) for orifice meters. 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 DPC 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 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.

eqn. (A.1)

eqn. (A.2)

where,

eqn. (A.3)

eqn. (A.4)

Fext = √(2ρ_fΔP)

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

eqn. (A.5)

Y1 = 1 - (0.649 + 0.696β^4)(ΔP) / (k′P)

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

eqn. (A.6)

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:

eqn. (A.7)

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 meter factor (M_f,_(i)) at each test point. The M_f,_(i)values will then be programmed in the flow computer.

eqn. (A.8)

eqn. (A.9)

List of Symbols Used in this Appendix

Symbol	Description
β	beta ratio
ΔP	pressure differential across meter
C_d	discharge coefficient
C_d,mean	mean C_d (value programmed as a constant C_d in flow computer)
C_d,(i)	discharge coefficient at the specific Reynolds number (Re)
d	outside diameter of the cone
D	inside diameter of the meter pipe
E_v	approach velocity
F_ext	expansion factor
k'	isentropic exponent
M_f,(i)	meter factor at the specific Reynolds number (Re)
P	static pressure absolute
Q_m	mass flowrate
Q	non-converted volumetric flowrate (not converted to reference conditions of standard pressure and standard temperature)
Q_ref,(i)	mass flowrate through the reference standard at the specific Reynolds number (Re) or (i) test point, 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