Orifice flanges
An orifice meter measures flow from the pressure drop across a plate with a hole in it, and the orifice flanges hold that plate and carry the pressure taps. They are machined in pairs and have their own standard, ASME B16.36.
Orifice flanges hold an orifice plate between them for flow measurement, with pressure tap holes drilled through the flange rim on each side of the plate and provision for jackscrews that spread the flanges when the plate is changed. ASME B16.36 covers weld neck orifice flanges from Class 300 to Class 2500, and slip-on and threaded orifice flanges in Class 300. We make all three types in A105, A350 LF2, 304L and 316L, in raised face or ring type joint facing, with an EN 10204 3.1 mill test report for every heat.
Types and classes in ASME B16.36
| Type | Classes | Notes |
|---|---|---|
| Weld neck | 300, 400, 600, 900, 1500, 2500 | The usual choice; butt welded to the meter run |
| Slip-on | 300 | Lower cost; two fillet welds |
| Threaded | 300 | Small lines where welding is not wanted |
The standard starts at Class 300 because a lighter flange is too thin to take the pressure tap holes, so Class 150 lines use Class 300 orifice flanges.
Pressure taps and jackscrews
Each flange has tap holes through its rim into the bore, usually threaded 1/2 NPT, so that the differential pressure across the plate can be piped to the transmitter. With flange taps, the tap centers sit 1 in (25 mm) from the faces of the orifice plate. ASME B16.36 also shows corner taps, and an angled tap for ring type joint flanges, where the groove is in the way of a straight tap. Jackscrew holes let the flanges be pushed apart to change or inspect the plate without cutting the pipe.
Tell us the number of taps per flange, the tap size and their position relative to the bolt holes, and whether you need taps on both sides. Taps are usually placed at the same angle on both flanges so the impulse lines run straight.
Bore and the meter run
The accuracy of an orifice meter depends on a smooth, round bore with no step at the flange. Order weld neck orifice flanges bored to the inside diameter of the meter run pipe, giving the schedule or the actual inside diameter. The meter run itself needs straight pipe upstream and downstream of the plate, as AGA Report No. 3 and ISO 5167 set out; that is a matter for your instrument engineer, but it decides where the flanges go.
Grades
| Grade | Specification | Typical service |
|---|---|---|
| A105 | ASTM A105 | Gas, oil, steam and water meter runs |
| A350 LF2 | ASTM A350, Charpy tested at -50 °F | Cold gas and outdoor meter stations |
| F304L, F316L | ASTM A182 | Corrosive and clean process streams |
How to order
A typical line reads: "Orifice flange pair, weld neck RF, NPS 4, Class 600, bored for Sch 80, two 1/2 NPT taps per flange, ASME B16.36, ASTM A105, 2 pairs". Send the type, size, class, facing, bore or schedule, material and quantity for each line, and we quote every line as specified. Every order ships from our works in Gujarat, India, with an EN 10204 3.1 mill test report for every heat; ordering from India explains the documents, marking, packing and customs.
Frequently asked questions
Why are there no Class 150 orifice flanges?
ASME B16.36 starts at Class 300 because Class 150 flanges are too thin for the pressure tap holes. Class 300 orifice flanges are used on Class 150 lines.
Do orifice flanges come as a pair?
Yes. They are ordered and machined as pairs, so that the taps line up and the bores match. Quote the quantity in pairs.
Can you make RTJ orifice flanges?
Yes. ASME B16.36 provides for ring type joint facing, with an angled tap that clears the ring groove. Give the ring number on the line list.
Sources
- ASME B16.36, Orifice Flanges
- ASME B16.5, Pipe Flanges and Flanged Fittings, NPS 1/2 through NPS 24 Metric/Inch Standard
- AGA Report No. 3, Orifice Metering of Natural Gas
- ISO 5167, Measurement of fluid flow by means of pressure differential devices