S-G-03–Specifications for the approval of type of gas meters, ancillary devices and associated measuring instruments

Category: Gas
Issue date:
Effective date:
Revision number: 1
Supersedes: LMB-EG-08 and S-G-03


Appendix D: Algorithms used to evaluate the performance of conditioning orifice plate meters

Unless the applicant of the conditioning orifice plate meter specifies otherwise, the meter's performance shall be evaluated using a modified version of AGA Report No.3: Orifice Metering of Natural Gas and Other Related Hydrocarbon Fluids – Concentric, Square-edged Orifice Meters, Part 3: Natural Gas Applications (American Petroleum Institute Manual of Petroleum Measurement Standards, chapter 14.3.3, fourth edition, 2013). The equation is modified to account for the physical differences between a differential pressure measuring plate meter and a conventional orifice meter. The differential pressure measuring plate meter can be considered as an orifice plate with a slight bias shift from AGA calculations. The discharge coefficient shall be calculated according to AGA Report No. 3, Part 3 and adjusted for each conditioning orifice plate according to an empirically derived calibration factor (Fc). Mass flow shall be calculated using standard orifice plate flow equations:

Eqn. (A.1)

(Q m) = K × (C c) × beta^2 × (uppercase D^2) × (E v) × (F ext) × (Y1)

Eqn. (A.2)

(C c) = (C (lowercase d)) × (F c)

Eqn. (A.3)

(E v) = 1 ÷ (√(1-beta^4))

Eqn. (A.4)

(F ext) = √((rho f) × (Delta P))

The gas expansion factor Y1 shall be calculated as per AGA Report No. 3, Part 3. The beta ratio (ß), as described in AGA Report No. 3, is not directly applicable. A geometrically equivalent beta ratio is defined by the following relationship:

Eqn. (A.5)

Beta = (2 × (lowercase d)) ÷ (uppercase D)

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 following equation:

Eqn. (A.6)

(Cd i) = (Q i, ref) ÷ ((K) × beta^2 × ((uppercase D)^2) × (E v) × (F ext) × (Y1))

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 (Mf) at each test point. The Mf values shall then be programmed into the flow computer.

Eqn. (A.7)

(M f i) = (C d i) ÷ (C d, mean)

Eqn. (A.8)

R e = (rho × V × (uppercase D)) ÷ mu

List of symbols used in appendix D
Symbol Description
ß beta ratio
ΔP pressure differential across meter
Cc discharge coefficient corrected by calibration factor
Cd discharge coefficient
Cd,mean mean Cd (value programmed as a constant Cd in flow computer)
d inside diameter of the conditioning orifice plate meter orifices
D 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

top of page

Date modified: