ASME B16.36 and ASME B16.5 Orifice Flanges： How They Work, How They Are Made, and How to Read the Markings

Orifice flanges are specialized flanges used for flow measurement in pipelines. Unlike standard weld neck or slip-on flanges, each orifice flange is equipped with pressure tap holes and jack screws, allowing an orifice plate to be installed, removed, and inspected without cutting the pipeline. They are widely used in oil and gas, chemical processing, power generation, steam systems, and natural gas metering applications.

Typical standards include ASME B16.36 for orifice flanges and ASME B16.5 for flange dimensions, pressure ratings, and marking requirements.

What Makes an Orifice Flange Different?

An orifice flange set normally consists of:

A pair of orifice flanges
An orifice plate
Gaskets
Bolts and nuts
Jack screws
Pressure tap plugs or fittings

The key design features are:

Pressure tap holes: Usually two radial tapped holes at 180°, used to connect differential pressure instruments.
Jack screws: Allow the flanges to be separated safely for orifice plate replacement.
Matched bore: The bore is aligned with the pipe ID to ensure accurate flow measurement.
Raised face or RTJ face: RF and RTJ faces are commonly available depending on pressure class and service conditions.

Manufacturing Process of Orifice Flanges

The quality of an orifice flange depends not only on final machining, but also on raw material control, forging, heat treatment, and inspection.

Step 1: Raw Material Selection

Production begins with qualified forged billets or bars. Common materials include:

Carbon steel: ASTM A105 / A105N
Low-temperature carbon steel: ASTM A350 LF2
Stainless steel: ASTM A182 F304, F316, F316L
Alloy steel: ASTM A182 F11, F22
Duplex and special alloys on request

Material certificates such as EN 10204 3.1/3.2 are usually required for traceability.

Step 2: Cutting and Heating

The raw material is cut to length and then heated to the required forging temperature. Uniform heating helps improve grain structure and reduce internal defects.

Step 3: Forging

Forging is the core process that determines the mechanical strength of the flange. Depending on size, the flange may be produced by die forging or open-die forging. Proper forging ensures dense grain flow and better fatigue resistance.

Step 4: Heat Treatment

After forging, heat treatment is carried out to relieve internal stress and meet mechanical property requirements. Carbon steel flanges are commonly normalized, while stainless steel and alloy flanges may require solution annealing or other treatments according to the applicable standard.

Step 5: CNC Machining

CNC machining defines the final dimensions, including:

Outer diameter
Bore size
Bolt holes
Raised face or RTJ groove
Pressure tap holes
Jack screw holes

For orifice flanges, the position and alignment of the pressure taps are especially important, because they directly affect differential pressure measurement accuracy.

Step 6: Inspection and Testing

Typical inspections include:

Dimensional inspection
Visual inspection
PMI / chemical analysis
Mechanical property testing
Hardness test
NDT such as UT, MT or PT
Pressure test when required

All results should be recorded and traceable to the heat number.

Step 7: Marking, Surface Protection and Packing

After inspection, each flange is permanently marked, protected against corrosion, and packed for shipment. For export orders, seaworthy wooden cases or plywood pallets are commonly used.

[Insert photo: Finished orifice flanges packed in wooden case]

How to Read Orifice Flange Markings

Flange markings are important for identification, inspection, and long-term traceability. Typical markings include:

Manufacturer name or trademark
Material grade, such as A105, LF2, F316
Standard, such as ASME B16.5
Flange type, such as WN or SO
Size, such as NPS 4" or DN100
Pressure class, such as Class 300, Class 600
Face type, such as RF or RTJ
Heat number
Bore diameter, which is especially required for weld neck orifice flanges

A typical marking may look like:

ORIFICE WN 4" CL300 RF A105 ASME B16.36 / B16.5 HEAT NO. XXXXX BORE XX.X mm

Modern production lines may use laser marking or steel stamping. Laser marking is increasingly popular because it provides clear, durable, and low-stress identification, especially for stainless steel and alloy flanges.

Why Choose Our Orifice Flanges?

Manufactured according to ASME B16.36, ASME B16.5 and other international standards
Available in weld neck, slip-on, and threaded types
Materials include A105, LF2, F304, F316, F11, F22, duplex and special alloys
Pressure classes from Class 150 to Class 2500
RF and RTJ faces available
Full traceability with EN 10204 3.1/3.2 MTC
OEM and non-standard customization supported
Backed by China’s complete industrial supply chain and abundant energy resources, ensuring stable capacity and competitive pricing
Strategically located in Hebei with convenient logistics near Tianjin and Huanghua ports for efficient global delivery

If you need orifice flanges for flow measurement systems, send us 👉 [Sales@hysteelpipefitting.com] your size, pressure class, material, face type, and standard. We can provide drawings, marking confirmation, and quotation for your review.