Article
An orifice flange assembly is one of the oldest and most reliable ways to measure flow in a pipeline. A thin plate with a precise hole is clamped between two special flanges, and the pressure difference across the plate tells the instrument how much fluid is passing.
What makes an orifice flange different
- Pressure tap holes – each flange has radial holes drilled through the ring to the bore, usually tapped 1/2" NPT, for connecting the differential pressure transmitter.
- Extra thickness – the ring is thicker than a standard flange so the taps can be drilled. ASME B16.36 orifice flanges start at Class 300 for this reason.
- Jack screws – to separate the flanges and replace the plate without dismantling the line.
Standard
ASME B16.36 covers orifice flanges in Classes 300 to 2500. Where a Class 150 line needs metering, Class 300 orifice flanges are normally used. The usual types are weld neck, with slip-on and threaded types for smaller, lower-pressure duties.
Typical assembly
| Component | Qty per set |
|---|---|
| Orifice flanges (weld neck) | 2 |
| Orifice plate | 1 |
| Gaskets | 2 |
| Stud bolts with nuts | As per size and class |
| Jack screws | 2 |
| Plugs for unused taps | As needed |
Bore and straight run
For accurate measurement, the flange bore must match the pipe bore exactly, and there must be enough straight pipe before and after the assembly. The exact straight-run requirement depends on the upstream fittings and the beta ratio; the instrument designer will specify it.
A complete description
Orifice Flange WN 4" Class 300 RF, bore to SCH 40S, ASTM A182 F316L, ASME B16.36, with 1/2" NPT taps and jack screws – 2 sets
Frequently asked questions
Why do orifice flanges start at Class 300?
The ring must be thick enough to drill the pressure tap holes, so ASME B16.36 starts at Class 300 even for lower-pressure lines.
How many pressure taps does an orifice flange have?
Usually one or two per flange, drilled radially and tapped 1/2" NPT.
What are jack screws for?
They push the flanges apart so the orifice plate can be inspected or replaced without dismantling the line.
Key takeaways
- Orifice flanges hold an orifice plate and provide pressure taps.
- ASME B16.36, Classes 300 to 2500.
- Match the bore to the pipe and order full sets with plate, gaskets and bolts.
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