The most common way to measure flow in a pressurised line is to restrict the flow and measure the pressure difference it creates. An orifice flange set is the connection that makes that measurement possible.
How it works
A drilled plate is installed between two flanges. Flow accelerates through the bore and a pressure difference appears across the plate. Tappings in the flanges carry those two pressures to a transmitter, and flow is calculated from the differential.
A set typically includes two orifice flanges, the orifice plate, gaskets, studs and nuts, jack bolts to help remove the plate, and the tapping valves.
Tapping location
Where the pressure is taken directly affects the result. In flange tappings the holes sit in the flange body, close to the plate. Different standards define different distances, so the flange must be ordered to suit the arrangement the project specifies.
What spoils the measurement
- Insufficient straight run. A defined length of straight pipe is needed before and after the plate; elbows and valves too close distort the reading.
- Plate installed backwards. The sharp edge must face the flow; reversed installation gives a systematic error.
- Gasket bore too small. A gasket protruding into the flow disturbs the pressure profile.
- Air in the impulse lines. They must stay full in liquid service.
- Worn plate. A dulled edge shifts the reading, so periodic checks are needed.
When ordering
State size, pressure class, face type, material grade and pipe schedule, plus the type (NPT or welded) and position of the tappings. Plate material is normally stainless and is chosen for the fluid, while the bore diameter comes from the process calculation.
Let us quote the whole measurement set on one list: enquiry form. See our instrument valves and manifolds page for the instrument connections.
FAQ
How does an orifice flange differ from a normal flange?
It has pressure tappings and jack bolt provisions in the body; otherwise dimensions and class are the same as a standard flange.
What material is the orifice plate?
Usually 316 stainless; more resistant grades are chosen for abrasive or corrosive fluids.
What happens if the plate is fitted backwards?
With the sharp edge away from the flow, the measurement carries a systematic error and flow is consistently misread.
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