Unternehmensnachrichten über Orifice Flanges: ASME B16.36 Tap Arrangements, Plate Centering and Flow Measurement Accuracy

Most flanges are chosen once, at the piping level, and then forgotten. Orifice flanges are not that kind of part. They are bought as an instrument accessory, installed as a matched pair, and judged by a question unrelated to pressure rating: can the flow computer trust the differential pressure they deliver?

That changes the procurement conversation, because an orifice flange is not a weld neck flange with two holes drilled in it. Tap position, jack screws, face finish, plate centring and material grade all interact with the measurement, not only with the pressure boundary.

Why Orifice Flanges Are Bought and Installed as a Pair

A differential pressure flow element is an assembly, not a component. Two flanges, one plate, two gaskets and a set of bolts form the primary element, and the axial distance from each tap to the nearest plate face is fixed by the flange geometry. A plain B16.5 flange still holds pressure, but that tap position is lost — and with it the calibration basis of the measurement.

Order the pair together and keep it together through receiving, storage and installation. Mixing one flange from a paired set with a spare from another order introduces an offset that no amount of transmitter zeroing will remove. The flow element tag number should travel with the flanges from purchase order to pipe spool.

What ASME B16.36 Covers, and What It Leaves to You

ASME B16.36, Orifice Flanges, is the product standard for this family; the current edition is B16.36-2025. Its scope is narrow: pressure-temperature ratings, materials, dimensions, tolerances, testing and marking of flanges similar to those in ASME B16.5 that carry orifice pressure differential connections.

Coverage is limited in two ways. Welding neck orifice flanges are addressed in Classes 300, 400, 600, 900, 1500 and 2500. Slip-on and threaded orifice flanges are addressed only in Class 300, so a threaded Class 600 request falls outside the stated coverage and should reach the project engineer before anyone quotes it.

Equally important is what the standard does not do: it does not design the plate, specify straight lengths, define impulse line layout or compute flow. Those belong to the flow measurement standard referenced by the project and to the instrument engineer. B16.36 tells you the flange is dimensionally and materially correct, not that the installation will measure accurately — a division of responsibility that causes most confusion in this category.

Tap Arrangements: Corner, Flange and D–D/2

Three arrangements dominate, each implying different hardware. Getting this wrong is the most expensive mistake in the category, because it surfaces after the flanges are welded in.

Corner taps

Corner taps sit at the corner formed by the upstream plate face and the pipe wall. They are compact, suit small line sizes, and are common in laboratory and skid-mounted metering. With the tap effectively at the plate, the geometry is tightly defined and the discharge coefficient stable — but the drilling is a specialised feature a general-purpose flange does not carry.

Flange taps

Flange taps are drilled radially through the hub so each tap centre sits roughly one inch (25 mm) from the adjacent plate face, one upstream and one downstream. This is the arrangement B16.36 orifice flanges are built around and the default in chemical, refining and utility service: the tap is accessible and impulse tubing runs clear of the pipe. Unqualified, "orifice flange" means flange taps.

D and D/2 taps

D and D/2 taps place the upstream tap one diameter upstream of the plate and the downstream tap half a diameter downstream — used where corner or flange taps will not fit, as on large lines or thin plates. In practice this means pipe taps on the spool, so confirm a D–D/2 request with the instrument engineer.

The Hardware Buyers Forget: Jack Screws and Tap Plugs

Jack screws matter because a plate clamped under bolt load and possibly scaled does not slide out politely. Pairs are normally drilled and tapped for screws that push it free once the main bolts are slackened. Without them the plate is often damaged during removal, and a damaged plate is a replacement, not a recalibration.

Tap plugs matter because taps stay open to atmosphere until impulse lines are connected, so they ship plugged. Material, thread and sealant all count: stainless plugs in stainless flanges without anti-galling compound can seize permanently. A third detail is invisible on a drawing — tap clocking. Specify the angle against the isometric, plus whether bolt holes straddle or fall between centrelines.

Face Finish, Gasket Choice and the Pressure Boundary

Orifice joints are broken far more often than ordinary pipe joints — for plate inspection, cleaning or a change of bore size — and that pattern should drive facing and gasket selection. Raised face is the common choice; ring joint facing is used where class, fluid or specification demands a metal-to-metal seal, in which case the groove must match the ring.

Finish must suit the gasket: a spiral wound gasket needs a defined finish range to seat without damaging its sealing element, while a soft gasket tolerates a rougher face but creeps under cyclic load. Because the joint is repeatedly disturbed, gaskets and bolts are consumables — order spares with the flanges. When a joint weeps, inspect for radial scratches and refinish the face rather than adding bolt load, which distorts the flanges.

Plate Centering and Installation Tolerances

The plate must sit concentric with the bore and square to the flow axis. An off-centre plate changes the effective open area and biases the indicated differential pressure — a systematic error that looks like instrument drift and is often chased for weeks in the wrong place.

Two checks catch most of it: confirm the plate sits flat with no gasket material protruding into the bore, then confirm that plate plus gasket thickness fits the face-to-face gap without pre-loading. Concentric plates with the bore low suit liquids carrying solids; eccentric plates with the bore high suit gases carrying condensate, so verify orientation — an inverted eccentric plate changes the reading.

Material Grades and When They Matter

Flange material follows the line material, with one addition: an orifice flange is also an instrument connection, so it sees stagnant fluid in the tap drillings that the pipe may never see. Stagnant conditions are where crevice and pitting attack start.

Austenitic stainless steels

Grades such as 304, 304L, 316 and 316L cover most general chemical, water and food-grade service, with low-carbon variants preferred where the flange is welded without a subsequent solution anneal. They are supplied as forged flanges under ASTM A182/A182M, which covers forged or rolled alloy and stainless steel flanges and limits individual products to 10,000 lb (4,540 kg); larger parts fall under other referenced specifications.

Duplex and super duplex

Where chlorides, produced water or marine atmospheres are present, duplex grades such as UNS S31803/S32205 and super duplex such as S32750 and S32760 combine chloride stress corrosion resistance with strength the austenitic grades cannot match at the same size. Weld procedure qualification matters more here, so confirm the grade against the qualified WPS before releasing the order.

Nickel alloys and super-austenitics

For hydrochloric acid, concentrated sulphuric acid, chlorinated organics and similarly aggressive streams, 6% molybdenum super-austenitics and nickel-base alloys become the material of construction. That is a corrosion-engineering decision based on concentration, temperature and contamination, not a catalogue table, so the inquiry should carry fluid data rather than a grade name.

Configuration Comparison

The table summarises how the practical options differ — a starting point for a conversation with the instrument engineer, not a substitute for the project specification.

Specification Checklist Before You Send an RFQ

• Size, pipe schedule and pressure class, plus the standard edition applied.
• Flange type — weld neck, slip-on or threaded — and facing.
• Tap arrangement and size, clocking angle, and whether taps ship plugged or open.
• Jack screw requirement, number of screws and thread.
• Material grade with governing specification, and whether a test report is required.
• Whether the order is a matched pair, and the flow element tag number.
• Gasket type and thickness, to check the assembled face-to-face dimension.

Frequently Asked Questions

Can a standard ASME B16.5 flange be drilled and used as an orifice flange?

It can be drilled, but rarely acceptably. Drilling after manufacture moves the tap out of the controlled geometry, removes the jack screw provision, and leaves the part outside the orifice flange standard. The result holds pressure but cannot be shown to meet the design's measurement basis.

What is the difference between an orifice flange and an orifice plate?

The plate is the flow restriction — a thin disc with a machined bore, clamped between the two flanges. The flanges are the holders and pressure tap carriers. They are separate purchase items: the bore is sized by the flow calculation, the flange by the line and tapping arrangement.

Which pressure classes does ASME B16.36 cover?

Welding neck orifice flanges in Classes 300, 400, 600, 900, 1500 and 2500, and slip-on and threaded orifice flanges in Class 300 only.

Are jack screws always required?

They are strongly recommended wherever the plate will ever be removed. If the joint is welded in and the plate stays for the life of the line the value is lower, but almost every plate is inspected at some point.

Can orifice flanges be supplied in duplex or nickel alloy grades?

Yes. Orifice flanges follow the same forged material specifications as other flanges. State the grade and governing specification together, and confirm the welding procedure for the line covers it.

Conclusion

Orifice flanges sit between two disciplines: pressure components governed by a product standard, and measurement components governed by a flow element design. Treated purely as flanges they produce a joint that holds but does not measure; treated purely as instruments they produce an order that cannot be manufactured to a standard.

If you are specifying orifice flanges for a new line, or replacing a set damaged in service, send size, class, facing, tap arrangement and fluid data together and we will confirm the configuration against the applicable standard before quoting. Browse the flange range, view an example duplex orifice plate flange, or contact our sales engineers with your datasheet.

Sources and further reading: ASME B16.36 — Orifice Flanges; ASTM A182/A182M — Forged or Rolled Alloy and Stainless Steel Pipe Flanges; ASME B31.3 — Process Piping.

This article provides general technical information only. Final selection must be based on the applicable codes and standards, the project specification, and confirmation by qualified engineering personnel.