How does an orifice plate measure flow?

An orifice plate is a thin plate with a precisely machined bore installed between flanges. As fluid accelerates through the bore its pressure falls, and the differential pressure measured across the plate is related to the flow rate. Flow is proportional to the square root of the differential pressure, so doubling the flow gives four times the differential pressure.

What is the beta ratio of an orifice plate?

The beta ratio is the orifice bore diameter divided by the internal pipe diameter. ISO 5167-2 covers beta ratios from 0.10 to 0.75. A typical design target is 0.4 to 0.6, which balances a strong measurable differential pressure against the permanent pressure loss and keeps the plate within the best-characterised part of the standard.

What is the discharge coefficient of an orifice plate?

The discharge coefficient C is the ratio of actual flow to the theoretical flow predicted from the pressure drop. For orifice plates it is calculated from the Reader-Harris/Gallagher equation in ISO 5167-2 and depends on the beta ratio, the pipe Reynolds number, the tapping arrangement and, for small pipes, the pipe diameter. It is typically close to 0.60 to 0.61, and tends to 0.5961 as the beta ratio approaches zero at very high Reynolds number.

What is the expansibility factor?

The expansibility factor, sometimes called the expansion factor, corrects for the density change of a compressible fluid as it expands through the orifice. It equals 1 for liquids and is less than 1 for gases and steam, falling further as the pressure ratio and beta ratio increase. ISO 5167-2 restricts its use to a downstream to upstream pressure ratio of 0.75 or greater.

What is the permanent pressure loss of an orifice plate?

Only part of the differential pressure is recovered downstream; the rest is lost permanently and represents pumping or compression energy. For a beta ratio of 0.6 roughly 63 percent of the differential pressure is lost permanently, rising to over 90 percent at a beta ratio of 0.2 and falling to about 45 percent at 0.75. This is why a very small bore gives a strong signal at a high running cost.

What is the difference between corner, flange and D and D/2 tappings?

They are the standard locations for the pressure connections. Corner tappings are taken at the plate faces, flange tappings are 25.4 mm each side of the plate, and D and D/2 tappings are one pipe diameter upstream and half a diameter downstream. Each has its own terms in the discharge coefficient equation, so the tapping type must be entered correctly; using the wrong one introduces a small but systematic error.

What Reynolds number does ISO 5167 require?

For orifice plates ISO 5167-2 requires a pipe Reynolds number of at least 5000 for beta ratios from 0.10 to 0.56, and at least 16000 multiplied by the square of the beta ratio for beta above 0.56. Below these values the discharge coefficient is not reliable, which typically happens with viscous liquids or very low flows, and a different meter should be considered.

What pipe sizes does ISO 5167 cover for orifice plates?

ISO 5167-2 applies to pipe internal diameters from 50 mm to 1000 mm, with an orifice bore of at least 12.5 mm. Outside that range the equation is being extrapolated. Below 50 mm the small-pipe correction term applies within the standard, but pipes under 50 mm fall outside the stated scope.

How much straight pipe does an orifice plate need?

ISO 5167-2 tabulates the required straight lengths, which depend on the beta ratio and the upstream fitting. As a guide, allow roughly 20 to 44 pipe diameters upstream and about 4 to 7 downstream, with the longest runs needed after two bends in different planes or after a reducer at a high beta ratio. Where the straight run is not available, a flow conditioner can be used and the length reduced.

How do I choose the differential pressure for an orifice plate?

The differential pressure is a design choice traded against permanent pressure loss. Common practice is to size for a full-scale differential of 25 to 250 mbar, often 100 mbar or 250 mbar to suit standard transmitter ranges. A higher differential improves resolution and turndown but increases the unrecovered pressure loss and the pumping cost.

Can an orifice plate be used for custody transfer?

Yes, orifice metering is widely used for fiscal and custody transfer measurement of gas and liquids, but it requires full compliance with ISO 5167 or the applicable API MPMS and AGA procedures, including verified straight lengths, plate condition and edge sharpness, surface roughness, documented tolerances and calibrated transmitters. A design calculation alone is not sufficient for custody transfer.

How is orifice plate flow measurement uncertainty calculated?

Following ISO 5167-1 and GUM principles, the relative uncertainties of the discharge coefficient, the expansibility, the orifice bore, the pipe diameter, the differential pressure and the density are each multiplied by a sensitivity coefficient and combined by root sum square. The sensitivity coefficients come from the flow equation itself: 1 for the discharge coefficient and expansibility, 2 divided by 1 minus beta to the fourth for the bore, 2 beta to the fourth divided by 1 minus beta to the fourth for the pipe, and one half for differential pressure and density. The expanded uncertainty uses a coverage factor of 2 for about 95 percent confidence.

Why does orifice accuracy get worse at low flow?

Flow is proportional to the square root of differential pressure, so at 30 percent of design flow the differential is only 9 percent of full scale. A transmitter error expressed as a percentage of span therefore becomes a much larger percentage of the measured flow, growing as one over the square of the flow fraction. This is why a single orifice run rarely gives better than about 3 to 1 or 4 to 1 usable turndown.

What is sour service under NACE MR0175?

A system is classified as sour when the partial pressure of hydrogen sulphide reaches 0.3 kPa, which is 0.05 psia absolute. The partial pressure is the total absolute pressure multiplied by the mole fraction of hydrogen sulphide. In sour service, carbon and low alloy steels must meet controlled hardness limits, typically a maximum of 22 HRC, along with chemistry, heat treatment and welding requirements, to resist sulphide stress cracking.

Is this orifice plate calculator free and accurate?

The calculator is free and works offline in the browser. It implements the ISO 5167-2 Reader-Harris/Gallagher discharge coefficient, the expansibility factor and the permanent pressure loss equations, and iterates the discharge coefficient against the Reynolds number to a converged solution. It is engineering grade for design and checking. Fluid properties are reference values, so enter the density and viscosity at your operating conditions, and confirm any custody transfer application against the full standard.

AIENGINEAR · Orifice Plate Flow Calculator

Name: Orifice Plate Flow Calculator Professional Suite
Author: AIEnginear

Professional Suite · ISO 5167 Ultimate Edition · Engineering Intelligence Powered by AI