Orifice vs Standard Flange
Definition
A standard flange per ASME B16.5 connects two pipe sections or a pipe to a valve/equipment. An orifice flange per ASME B16.36 is a modified version of a standard weld neck or slip-on flange that includes radial tap holes for pressure measurement. Orifice flanges are always installed in pairs, with an orifice plate sandwiched between them, to measure flow rate using differential pressure.
When Each Is Used
Standard flanges are used throughout a piping system for general connections. They have no provisions for pressure taps and cannot be used for orifice-based flow measurement.
Orifice flanges are used exclusively at flow metering stations. The pair of flanges holds an orifice plate (a thin disc with a precisely machined bore) between them. Pressure taps drilled through the flange body connect to impulse tubing and a differential pressure transmitter that calculates flow rate.
Specifications Table
| Feature | Standard Flange (ASME B16.5) | Orifice Flange (ASME B16.36) |
|---|---|---|
| Standard | ASME B16.5 | ASME B16.36 |
| Tap holes | None | 2 radial tap holes (1/2” NPT or 1” NPT) |
| Tap hole location | N/A | Flange face, at pipe bore level |
| Jack screws | Not included | 2 jack screw holes for plate removal |
| Orifice plate | Not applicable | Held between the two flanges |
| Raised face | Standard RF per B16.5 | Modified RF with recessed face for plate |
| Pressure classes | 150 to 2500 | 300 to 2500 (not available in Class 150) |
| Size range | NPS 1/2 to NPS 24 | NPS 1 to NPS 24 |
| End connections | WN, SO, SW, THD, LJ, BL | WN or SO only |
| Weight | Standard | Slightly heavier (tap boss material) |
| Cost (per flange) | Lower | 30-50% more than equivalent standard flange |
Comparison
Pressure taps. The defining feature of orifice flanges is the pair of machined tap holes that penetrate the flange body from the outside diameter to the inside bore. These taps are precisely positioned at the flange face (flange taps) to measure the upstream and downstream pressures across the orifice plate. Standard flanges have no provision for this.
Jack screws. Orifice flanges include tapped holes for jack screws that allow the flanges to be spread apart for orifice plate insertion and removal without disconnecting the piping. This is required for periodic plate inspections and bore recalibration.
Gaskets. Orifice flange sets use special orifice gaskets with a tab that extends beyond the flange OD. The tab allows quick identification and provides a handle for gasket removal. The gasket must have cutouts aligned with the tap holes to avoid blocking pressure readings.
Pressure class. Orifice flanges start at Class 300 (not Class 150). This is because the tap hole penetrations reduce the structural integrity of the flange body, requiring a higher base pressure rating for safety margin.
For complete pressure-temperature ratings, see the ASME flange rating reference.
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