Orifice flanges are specialized flanges designed for differential pressure flow measurement. They incorporate integral pressure tap holes and jacking screws for orifice plate installation and removal, making them essential components for custody transfer metering and process control applications.
What Is an Orifice Flange?
An orifice flange is a specialized flange pair designed to accept and locate an orifice plate between them for differential pressure (DP) flow measurement. Unlike standard flanges governed by ASME B16.5, orifice flanges are covered by the separate ASME B16.36 standard. They feature two radial pressure tap holes on opposite sides of the flange body for connecting to a differential pressure transmitter. Jacking screws, typically positioned at 90 degrees to the tap holes, allow easy separation of the flange pair for orifice plate inspection, replacement, or maintenance.
Design per ASME B16.36
ASME B16.36 covers orifice flanges from NPS 1 through 24. The standard specifies two 1/2 inch NPT pressure tap holes located on opposite sides of the flange, positioned radially to communicate with the pipe bore at the flange face. Jacking screws are provided at 90 degrees to the tap holes for plate separation. The flange bore thickness is reduced compared to standard B16.5 flanges to accommodate the orifice plate. Flange face parallelism is critical for accurate flow measurement, and ASME B16.36 specifies tight tolerances for face alignment.
| NPS | Tap Hole Size (NPT) | Number of Jacking Screws | Class Range | Standard |
|---|---|---|---|---|
| 1 - 1-1/2 | 1/2 | 2 | 300 - 2500 | ASME B16.36 |
| 2 - 3 | 1/2 | 2 | 300 - 2500 | ASME B16.36 |
| 4 - 6 | 1/2 | 2 | 300 - 2500 | ASME B16.36 |
| 8 - 12 | 1/2 | 4 | 300 - 2500 | ASME B16.36 |
| 14 - 24 | 1/2 | 4 | 300 - 2500 | ASME B16.36 |
Orifice Plate Types
Three standard orifice plate designs are available for different flow measurement applications. Concentric orifice plates have a sharp-edge bore centered on the pipe axis and are the most common type for clean fluid measurement. Eccentric orifice plates have the bore offset from center for measuring dirty or two-phase fluids, allowing particles to pass through without accumulating. Segmental orifice plates have a segment-shaped opening for low flows or slurries. Orifice plate material is typically 316 stainless steel as standard, with other alloys available for corrosive services. Plate thickness ranges from 1/8 to 1/4 inch depending on the differential pressure.
Pressure Ratings
Orifice flanges are available in Class 300 through 2500. Class 150 orifice flanges are not standard because flow measurement applications typically require higher pressure integrity. The pressure rating is based on the flange material and temperature limits following the same pressure-temperature curves as ASME B16.5 for equivalent materials. Higher classes require thicker orifice plates to withstand the differential pressure without deflecting. The tap hole integrity is verified by hydrostatic testing of the assembled flange pair.
Materials
Orifice flanges are manufactured from the same material range as standard ASME B16.5 flanges. ASTM A105 carbon steel is standard for general metering stations. ASTM A182 F304 and F316 stainless steels are used for corrosive gas or chemical service. ASTM A350 LF2 low-temperature material is available for low-temperature hydrocarbon measurement. Duplex grades are specified for sour gas service per NACE MR0175. The orifice plate material is independent of the flange material and is selected based on the fluid properties and differential pressure conditions.
Applications
Orifice flanges are essential for custody transfer metering in oil and gas pipelines where flow measurement accuracy directly affects revenue. Process control flow measurement in refineries and chemical plants uses orifice flanges for mass balance and process optimization. Steam flow measurement in power plants relies on orifice flanges for efficiency monitoring. Natural gas distribution and transmission metering stations extensively use orifice flanges. Chemical injection rate monitoring in process plants also uses this technology.
Installation Requirements
Accurate flow measurement requires specific installation conditions. Straight pipe runs are required upstream (10-20 pipe diameters) and downstream (5 pipe diameters) of the orifice flange to ensure a fully developed flow profile. The orifice plate must be centered in the flange bore with the sharp edge facing upstream. Gaskets with a matching bore diameter must be used to avoid flow disturbance at the flange joint. The pressure taps connect directly to the differential pressure transmitter via impulse lines. Regular inspection of the orifice plate edge condition is essential for maintaining measurement accuracy over time.
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