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Orifice gas meters
The orifice flowmeter consists of a thin, flat plate sandwiched between flanges or installed in a dedicated fitting. The plate has a precise, sharp-edged orifice, bored concentric with the pipe axis. The flow pattern contracts as it approaches the orifice—the contraction continuing to a distance of approximately one orifice diameter downstream. This point of minimum cross section is called the vena contracta. Thereafter, the jet diverges to the full-pipe section.
As a result of its longevity and widespread usage in the industry, the orifice plate is an extremely well documented and regulated measurement device. There are two main standards for orifice metering: ISO Standard 5167 and AGA Standard 3.  This chapter is based around the requirements and guidance of ISO Standard 5167. 
A mathematical model, generated from experimental data, of the conditions in the meter stream must be applied to calculate the flow. Refining this mathematical model is a continual process. The uncertainty in the flow-rate measurement can be predicted in accordance with ISO Standard 5167. 
There are many ways of locating an orifice plate within a pipeline. These range from a simple orifice flange to a more specialized fitting, such as the long standing Daniel Senior Fitting, which permits removal of the plate under pressure (Fig. 1). It should be noted that other manufacturers offer orifice fittings with the similar design objectives.
There are also guidelines as to how the orifice flowmeter should be mounted in the pipeline. Because the orifice flowmeter is particularly sensitive to flow profile distortions, care should be taken to ensure fully developed flow. ISO Standard 5167 provides details on meter tube design. Fig. 2 provides a representation of the "catch all" meter tube.
This tube incorporates a 2-diameter straightening vane within the 44-diameter upstream meter tube. Shorter meter-tube configurations may be achieved by using flow conditioners other than simple vanes. These devices may include shorter tube bundles in combination with a perforated "flow conditioning plate" or a thicker perforated plate as a standalone device.
Theory of operation
The installation of the orifice plate causes a static pressure difference between the upstream side and the throat or downstream side of the plate (Fig. 3). The rate of flow can be determined from the measured value of this pressure difference and from knowledge of:
- The flowing gas properties
- Upstream or downstream pressure
- Gas temperature
- The circumstances under which the device is being used
There are modifications to generally accepted equations for flow rate calculations, because of:
- Frictional pressure losses
- Expansibility factors
- Other empirically derived coefficients
Various internationally recognized equations may be applied and normally take the form of a discharge coefficient and an expansibility factor. A full analysis may be found in ISO TR 5168, Annex E. 
Advantages and disadvantages
All meter types have advantages and disadvantages. Table 1 summarizes them for orifice flowmeters.
Orifice meter size is determined largely by the range of differential pressures that are deemed acceptable to measure. For example, a user who is willing to operate at differential pressures of 200 in. of water column would be able to flow more than 40% more gas through an identical device than a user who limits the differential pressure to 100 in. of water column. Similarly, the choice of beta ratio (the ratio of the outer diameter of the orifice plate and the diameter of the plate opening) will also impact the range of measurement. Typical sizing is accomplished by limiting beta ratios to values no larger than 0.65 and differential pressures between 10 and 100 in. of water column.
- ISO Standard 5167, Measurement of Fluid Flow by Means of Pressure Differential Devices—Part 1: Orifice Plates, Nozzles and Venturi Tubes Inserted in Circular Cross-Section Conduits Running Full. 1991. Geneva, Switzerland: ISO.
- Orifice Metering of Natural Gas and Other Related Hydrocarbon Fluids, Report No. 3. 2000. Washington, DC: AGA.
- ISO Standard 5168, Measurement of Fluid Flow—Evaluation of Uncertainty of a Flow Rate Measurement. 1978. Geneva, Switzerland: ISO.
Noteworthy papers in OnePetro
- Morrow, T.B., McKee, R.J. 1991. Effects of Orifice Meter Installation Condition on Orifice Coefficient Accuracy. Presented at the SPE Gas Technology Symposium, Houston, Texas, 22-24 January. SPE-21509-MS. http://dx.doi.org/10.2118/21509-MS.
- Shen, J.J.S. 1991. Velocity Profile Survey in a 16-in. Custody-Transfer Orifice Meter for Natural Gas. SPE Production Engineering. SPE-19075-PA. http://dx.doi.org/10.2118/19075-PA
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