Throughout the oil and gas industry, there stems the need for accurate, economical measurement of process fluids. Orifice metering satisfies most flow measurement applications and is the most common flow meter in use  today. The orifice meter, sometimes called the head loss flow meter, is chosen most frequently because of its long history of use in many applications, versatility, and low cost, as compared to other flow meter available.

The orifice meter consists of a primary element and secondary element(s). The primary element includes a section of straight run pipe with a constrictive device, most commonly and orifice plate, which causes change in energy. The energy changes in the form of a loss in static pressure and increased velocity through the orifice.The secondary  element senses the change in pressure, or differential pressure.

This differential pressure  combined with correction factors for the primary device and physical characteristics of  the fluid being measured allows computation of rate of flow. Proven flow factors and established procedures convert the differential pressure into flow rate. These factors and / or coefficients are based on measurable dimensions of the primary device, such as the pipe inside diameter and the orifice bore diameter, along with the physical properties of the fluid being measured, such as specific gravity, density, and viscosity.

Most flow meters require flow calibration to verify accuracy. However, the properly designed orifice meter will not require flow calibration if designed  and manufactured in accordance with acceptable design practices.

Guidelines for design and manufacture of orifice meter tubes are detailed in the AGA Report No. 3-Third Edition, Part 2.

Smith Metering (2000) Fundamentals of Orifice

Smith Metering (2000) Fundamentals of Orifice
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