Theory Gas Orifice Meter

The most common method of measuring gas is through an orifice meter. Gas flows through a piece of straight pipe with an orifice plate inserted in the middle.

The orifice plate is a steel circular plate with a hole in the middle; the hole is smaller than the internal diameter of the pipe. The plate is placed perpendicular to the gas flow, and is sealed so that all the gas flows through the hole. The hole is "tapered", meaning that the edge of the orifice hole is sharp. The plate must be inserted in such a way as to have the direction of flow of gas from the smaller to larger diameter, i.e. if the gas is flowing from left to right, the sharp edge of the orifice (the smaller diameter) must be at the left. Reversing the orifice plate will give an incorrect measurement.

TheoryGasOrifice1.jpg

The orifice plate is actually a highly machined component. Its dimensions must adhere to certain specifications. If the plate is damaged in any way in or around the "hole", it can no longer measure gas rates with any accuracy, and it must be replaced.

The plates come in different sizes for measuring different rates. Each time an orifice plate is changed, the size of the new plate must be recorded because the orifice size is needed for calculating flow rates.

In addition to knowing the size of the orifice plate, there are other parameters, which also must be known and reported. The relationship for calculating flow rate, using an orifice meter, is described by the orifice equation.

Reference

Orifice Metering of Natural Gas, Gas Measurement Committee Report #3,  American Gas Association, 1992,2003

Orifice Meter Equations

The Orifice meter calculation changes slightly  depending on its physical configuration. FieldNotes™ supports the following configurations

Flange Tapped Orifice Calculation:

Upstream

OrificeMeterFormula1.jpg

Downstream

OrificeMeterFormula3.jpg

The above equations are referred to as the "Factors Approach" as defined by appendix 3-B in AGA 3 Part 3

Symbol

Description

Reference

 

 

 

D

Meter tube internal diameter, calculated at Tf (FieldNotes™ assumes that the meter tube is constructed from carbon steel)

Input

d

Orifice plate bore diameter, calculated at Tf  (FieldNotes™ assumes that the orifice plate is constructed from stainless steel)

Input

Tf

Absolute flowing temperature

Input

Pfl

Absolute flowing pressure

Input

hw

Orifice differential pressure

Input

Ts

Standard Temperature

Input

Ps

Standard Pressure

Input

Qv

Volume flow rate at standard conditions

3-B-2

C

Composite orifice flow factor

3-B-4

Fn

Numeric conversion  factor

3-B-5

Fc

Orifice calculation factor (Displayed as the Fb column in FieldNotes™)

3-B-7

Fst

Orifice slope factor  (Displayed as the Fr column in FieldNotes™)

3-B-9

Y1

Expansion Factor (upstream tap)

3-32

Y2

Expansion Factor (downstream tap)

3-37

Fpb

Base pressure factor

3-B-10

Ftb

Base temperature factor

3-B-11

Ftf

Flowing temperature factor 3-B-12

Fgr

Specific gravity factor

3-B-13

Ftpv

Supercompressibility factor

3-B-14

Flow Calculation Example

This example is taken from AGA 3 Part 3 Appendix 3-C

Symbol Input Published Ver 4.1 Ver 3.xx
         

D

8.071 inches      

d

4.000 inches      

Tf

65 F      

Pfl

370 psi(a)      

hw

50 inches      

Ts

50 F      

Ps

14.65 psi(a)      

Gr

0.570      

N2

1.10 %      

CO2

0 %      

Qv

  608,394 SCF/min 608,405 SCF/min 608,623 SCF/min

C

    4561.453 n/a

Fn

  5581.82 5581.82 n/a

Fc

  0.601767 0.601767 n/a

Fst

  0.001189 0.001177 n/a

Y1

  0.998383 0.998383 n/a

Y2

  n/a n/a n/a

Fpb

  1.000000 0.980757 n/a

Ftb

  1.000000 1.000000 n/a

Ftf

  0.995224 0.995226 n/a

Fgr

  1.32453 1.324532 n/a

Ftpv

  1.02423 1.024249 n/a

 

 

Before the days of computers, engineers and operators used a complex system of tables to create an orifice coefficient for each size of orifice and various flowing conditions. FieldNotes™ does these calculations automatically, and the correct value of C is always applied to current calculation of gas flow rates. Some field operators use a "Sony Circular Slide Rule" or handed down spreadsheets to calculate gas flow rates. The answers obtained from this means are only approximate, and may differ from those calculated in the FieldNotes™ program.