Orifice Coefficient for Hot Tap Sizing

Owned by Joseph Pikas
Last updated: Dec 12, 2022
2 min read

Orifice coefficient for Hot Tap Sizing

 

When a compressible fluid, such as natural gas or air, is passed through an orifice, the rate of flow is determined by the area of the orifice opening; the absolute upstream pressure is 𝑃1; and the absolute downstream pressure is 𝑃2: unless the ratio 𝑃2/𝑃1. equals or is less than the critical ratio. When 𝑃2/𝑃1 equals or is less than the critical ratio downstream pressure no longer effects rate of flow through the orifice, and flow velocity at the vene contracta is equal to the speed of sound in that fluid under that set of condition. This is commonly referred to as critical or sonic flow. Orifice equations are therefore classified as “sonic” or “subsonic” equations.

Critical Flow Ratio – The equations for the critical flow of a compressible gas based on 𝑃1 and the ratio, k of the specific heats of the gas for constant pressure, 𝐶𝑝 and constant volume 𝐶𝑣.

 

For natural gas this ratio is estimated to be at 0.55.  This value will need to be overwritten in the program.

It should be noted that is difficult to accurately calculate the flow of a gas or liquid through a tiny orifice because of the leading edge of the orifice. In the case of a sharp-edged orifice, the effective diameter is 0.65 times the actual diameter. However, in the case of an orifice whose leading edge is rounded with a radius roughly equal to that of the orifice itself, the factor is approximately 1.0. This factor, which can vary between 0.6 and 1.0 is known as the orifice coefficient. As the flow of any fluid through an orifice is proportional to diameter squared, the flow is reduced by up to (0.65) = 44% depending on the shape of the leading edge. Figure A below shows flow through a sharp-edged orifice.

 

See AGA Report No. 3 on the following website for additional information.

 https://docslide.net/documents/aga-report-no-3-orifice-metering-of-natural-gas.html

There are many ways to determine these values based on installation and the type of hot tap history of how these taps behaved.