PS-G-16—Provisional Specifications for the Approval, Verification, Reverification, Installation and Use of Conditioning Orifice Plates

PS-G-16—Provisional Specifications for the Approval, Verification, Reverification, Installation and Use of Conditioning Orifice Plates (PDF, 64 KB, 11 pages)

Category: GAS
Bulletin: PS-G-16
Issue Date: 2011-06-23
Effective Date: 2011-06-23

Table of Contents

1.0 Scope

These specifications apply to the approval, verification, reverification, installation and use of Conditioning Orifice Plates (COPs) and related meter run assembly components, in addition to the applicable requirements in Measurement Canada (MC) specifications LMB-EG-08 and S-G-03.

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

Conditioning Orifice Plate (COP)

A multi-hole orifice plate designed to simultaneously function as a differential pressure element and as a flow conditioner. COPs are designed for use with meter tubes that are shorter in length than those used with conventional orifice plates.

Deviation (relative error)

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. Corrections must be made for the differences in gas pressure, temperature and compressibility between the two meters. The deviation shall be calculated according to equation (1) in clause 4.1.3.

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 equation (3) in clause 4.1.3.

Maximum Flow rate (Qmax)

The maximum rated flow rate of a meter, as specified 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 a meter, calculated according to equation (2) in 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 values prescribed in clause 4.1.2., as specified 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.

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 using a suitable test medium1 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 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 test flow rates shall be approximately equally spaced between 0.1Qmax and Qmax, and shall include as a minimum the following test points: 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 type of gas intended to be measured in service.

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 may be predicted using a dimensionless number such as the Reynolds number.

Note 3: If the Qmin declared by the applicant for an approval is less than 0.1Qmax, use Qmin. If this Qmin value is not less than 0.1Qmax, use 0.1Qmax.

Note 4: Baseline tests are performed to establish a COP’s performance under fully developed ideal flowing gas conditions.

4.1.2 Deviations between Qmin and Qmax determined using the approved flow equation shall not exceed the following values:

Test Limits:
  • The lesser of TLu = 1.50% - kuci or TLu = +1.0%
  • The greater of TLL = -1.50% + kuci or TLL = -1.0%
Repeatability:
±0.2%
Linearity:
±0.5%

where

k =
appropriate coverage factor (1.645) for the 95.45% confidence interval
uci =
the standard combined measurement uncertainty of the calibration5

Note 5: The standard combined measurement uncertainty uci includes contributions from the COP 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

Deviation(i)(%) = ((Q(i)m-Q(i)ref) × 100 / Q(i)ref^(note:6) eqn. (1)

Mean Deviation(%) = 1 / n × D1 + 1 / n × D2 + … + 1 / n × Dn eqn. (2)

Linearity(i)(%) = Deviation(i)(%) × 100% / Mean Deviation(i)(%) eqn. (3)

Note 6: Q(i)m is defined as the volume or volumetric flow rate as determined by the flow algorithm by the meter under test. Q(i) ref. is the volume or volumetric flow rate as determined by the reference meter. This may also be expressed in terms of mass flow rate. As an alternative, for type approval, the linearity of the meter can be assessed by replacing Q in eqn. (1) with the discharge coefficient Cd.

4.1.4 Unless otherwise indicated by the 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, the algorithm shall be authorized for use in the Notice of Approval.

4.1.5 Dimensional Tolerances for COPs

Where a manufacturer intends to have a COP reverified by means of dimensional inspection, the following conditions shall apply:

  1. Critical physical dimensions and their permitted deviations (tolerances) shall be provided by the manufacturer/applicant, for placement in the Notice of Approval, examples of which may include, but are not necessarily limited to, the following:
    • pipe inside diameter
    • plate thickness
    • average of bore diameters
    • diameter of bore hole centres
    • surface roughness
    • bevel, if applicable
    • overall plate diameter
  2. The manufacturer/applicant shall provide test data demonstrating that the dimensional tolerances do not introduce a deviation in performance that is greater than that specified in clause 4.1.2.

4.2 Verification and Reverification

4.2.1 Subject to clause 4.1.3, initial verification tests shall be conducted on a suitable test medium7 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 7: 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.

4.2.2 When tested at a test facility, the COP shall be flow calibrated together with a metering section (i.e. upstream and downstream meter tubes, flow conditioner and any required plate carrier) supplied either by the test facility (laboratory) or the meter owner (customer). If the metering section is supplied by the test facility, the test facility shall provide an attestation that the components comply with the specification requirements set out in Report No. 3 of the American Gas Association (AGA): Orifice Metering of Natural Gas, Part 2: Specification and Installation Requirements, 4th Edition (2000).

4.2.3 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:

Maximum permissible deviation = TL × Qattained ÷ Qmax eqn. (4)

4.2.4 Subject to the requirements of section 5.2, for purposes of verification and reverification, the meter tubes, flow conditioner and plate carrier (used with conventional/universal type plates) or flange union (used with paddle type plates) shall be inspected to verify conformance with the specification and installation requirements of the AGA’s Report No. 3, Orifice Metering of Natural Gas, Part 2: Specification and Installation Requirements, 4th Edition (2000).

4.2.5 Dimensional Reverification

Where permitted by a Notice of approval, the COP may be dimensionally reverified, provided that:

  1. The COP has been initially flow calibrated;
  2. The dimensions of the COP were measured and recorded at the time of initial verification; and
  3. The measured dimensions are within the tolerances specified in the Notice of Approval.

4.2.6 Reverification Interval

COPs shall be reverified in accordance with the applicable initial reverification period and subsequent reverification period prescribed in MC Bulletin G-18.

4.3 Conditions for Metrological Characteristics

4.3.1 Disturbance Factors

For pattern approval purposes, the difference between the meter accuracy established during baseline testing and the meter accuracy established during flow disturbance testing shall not exceed the errors set out in clause 4.1.2 for the disturbance factor set out in 4.3.1.1 below.

4.3.1.1 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 COP. Downstream piping shall be straight and equal in diameter to the upstream piping and be 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 COP shall be used in conjunction with an industry-approved standard orifice fitting.

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

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 include features to prevent reverse flow through the meter.

5.2.1.3

The COP shall be installed such that the high and low pressure taps are located directly between two of the bore holes, perpendicular to the plane of the pipe.

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 ofdisturbances 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. At the time of type approval assessment, the COP manufacturer shall provide MC with a letter of attestation regarding the COP’s intended operating range for temperature or mathematical modelling calculations which support its rated temperature range.

5.2.1.6

In the case of orifice fittings that include a second set of telemetry taps for redundant pressure measurements, these taps are not to be used.

5.2.1.7

Plate carriers shall include a method to prevent the COP from rotating within the plate carrier during insertion. Methods may include tabs in the seal ring or a setscrew installed through the plate carrier.

5.2.2 Meter Installation

The meter shall be installed in a manner consistent with the manufacturer’s installation requirements and any other special installation requirements shall be 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 two 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 two 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.

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 mass flow through the COP must 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 COP, without distorting the plate:

  1. manufacturer’s name
  2. model number
  3. serial number
  4. Notice of Approval number
  5. Beta ratio
  6. calibration factor (Fc)

6.2 Verification Marks

6.2.1 Upon verification and reverification, the COP, tubes and plate carrier shall each be marked with a verification mark, without distorting the plate. This may be done using a special steel punch designed to produce an imprint that uniquely identifies the meter verifier and the year of verification.

6.2.2 For paddle type plates, the verification mark shall be applied to the downstream side of the paddle handle. For conventional/universal type plates, the verification mark shall be applied on the downstream face, near the outer edge of the plate.

6.2.3 For orifice plate carriers, the verification mark shall be applied near the nameplate.

6.2.4 For meter tubes, the verification mark shall be applied on the outer edge of either flange or near the nameplate.

Alan Johnston
President

Appendix A - Algorithms Used to Evaluate the Performance of Conditioning Orifice Plate Meters

Unless the applicant of the COP meter specifies otherwise, the meter’s performance shall be evaluated using a modified version of AGA Report No. 3, Orifice Metering of Natural Gas, Part 3: Natural Gas Applications (1992). The equation is modified to account for the physical differences between a COP meter and a conventional orifice meter. The COP meter can be considered as an orifice plate with a slight bias shift from AGA calculations.

The discharge coefficient of the COP shall be calculated according to AGA Report No. 3, Part 3 and adjusted for each individual COP according to an empirically derived Calibration Factor (Fc). Mass flow shall be calculated using standard orifice plate flow equations:

Qm = KCc β2 D2 Ev Fext Y1 eqn. (A1)

where,

Cc = Cd Fc eqn. (A2)

Ev = 1 / √(1 - β4) eqn. (A3)

Fext = √ρf ΔP eqn. (A4)

The gas expansion factor, Y1, shall be calculated as per AGA Report No. 3, Part 3.

The Beta ratio (β), as described in AGA Report No. 3, is not directly applicable. A geometrically equivalent Beta ratio is defined by the following relationship:

β = 2d / D eqn. (A5)

Use of Data in Flow Computers

Knowing, from a reference standard, the true mass flow rate at each of the prescribed test points (Q(i), ref), Cd shall be calculated using the equation below:

Cd,Re = Q(i),ref / (K β2 D2 Ev Fext Y1) eqn. (A6)

In the first method, once the values for Cd have been calculated, the values shall be used to determine the relationship between Re and meter factor (Mƒ) at each test point. The Mƒ values shall then be programmed into the flow computer.

Mƒ = (Cd,Re) / (Cd,mean) eqn. (A7)

Re = ρVD / μ eqn. (A8)

List of Symbols Used in this Appendix

β
beta ratio
ΔP
pressure differential across meter
Cc
discharge coefficient corrected by calibration factor
Cd
discharge coefficient
Cd,fwm
mean Cd (value programmed as a constant Cd in flow computer)
d
the inside diameter of the COP orifices
D
the inside diameter of the meter pipe
Ev
approach velocity
Fc
calibration factor
K
dimensionless constant
Mf,Re
meter factor at the specific Reynolds number (Re)
P
pressure absolute
Qm
mass flow rate
Re
Reynolds number
Y1
upstream gas expansion factor
V
bulk velocity of flowing gas
ρ
flowing gas density
μ
dynamic viscosity of the flowing gas