What Is an Orifice Flange?

Quick Answer

An orifice flange is a weld neck flange with two radial tap holes drilled through the hub. These taps connect to a differential pressure transmitter that measures flow rate. Orifice flanges are always installed in pairs with an orifice plate sandwiched between them. The governing standard is ASME B16.36.

How Orifice Flanges Work

An orifice plate—a thin disc with a precisely sized bore—is clamped between two orifice flanges. Fluid flows through the plate’s bore, creating a pressure drop. The upstream tap measures the higher pressure; the downstream tap measures the lower pressure. A DP transmitter calculates flow rate from the differential pressure using Bernoulli’s principle.

Property	Detail
Standard	ASME B16.36
Size range	NPS 1 to NPS 24
Pressure classes	300, 600, 900, 1500, 2500
Tap holes	Two 1-inch NPT radial bores, 180deg apart
Tap location	Flange taps (1 inch from face, per ASME standard)
Face types	RF, RTJ
Common materials	A105 (CS), A182 F316 (SS)
Jack screws	Two threaded holes for spreading flanges to insert/remove plate

Key Features

Orifice flanges include two features that standard weld neck flanges lack:

1. Pressure tap holes: 1-inch NPT threaded holes drilled radially through the hub, positioned to align with the upstream and downstream faces of the orifice plate
2. Jack screw holes: Threaded holes in the flange rim that accept bolts to spread the flanges apart for orifice plate insertion, removal, or inspection—without removing all stud bolts

Pro Tip

Orifice flanges are sold in matched pairs. Always order them as a set. The upstream and downstream flanges must have identical bore, class, and face finish to ensure accurate measurement.

Orifice Flange vs Standard Weld Neck

Feature	Orifice Flange	Standard WN Flange
Tap holes	Two 1” NPT	None
Jack screws	Yes	No
Standard	ASME B16.36	ASME B16.5
Purpose	Flow measurement	General piping connection
Sold as	Pairs	Individual
Cost	2-3x a standard WN	Baseline

Installation Considerations

Orifice runs require straight pipe lengths upstream and downstream of the plate to ensure laminar flow. ASME MFC-3M and ISO 5167 specify these lengths based on the upstream fitting type. A general rule: 20 pipe diameters upstream, 5 diameters downstream.

The orifice plate itself is specified by process instrumentation engineers based on flow range, fluid properties, and required accuracy. The flange pair is specified by the piping engineer.

Read the full guide to flanges