Orifice Flange RFQ Checklist: Class, Type, Taps and Documents

When you need an orifice flange for flow measurement, the RFQ is not a standard flange request. ASME B16.36 defines the pressure-temperature ratings, materials, dimensions, tolerances, testing, and marking for flanges with orifice pressure-differential connections. But the standard leaves several critical details to the buyer's metering design. This guide gives EPC engineers, procurement managers, and inspectors a practical checklist to avoid costly mismatches.

Understand the ASME B16.36 Scope First

ASME B16.36 covers flanges similar to those in ASME B16.5 but with orifice pressure-differential connections. The standard explicitly limits coverage to:

• Welding neck flanges: Classes 300, 400, 600, 900, 1500, and 2500
• Slip-on and threaded flanges: Class 300 only

This means if your project requires a Class 600 slip-on orifice flange, it is outside the B16.36 scope. You would need to verify with the manufacturer whether they can supply a non-standard design, and you must define all requirements clearly in the RFQ. For welding neck types, the class range is broad, but you still need to confirm the exact class and facing.

ASME B16.5 supplies related flange-system requirements—such as bolting, gaskets, and general flange dimensions—but it does not replace the orifice-connection details in B16.36. Your RFQ should reference both standards: B16.36 for the orifice flange itself and B16.5 for the mating flange system.

Define the Flange Type and Class Explicitly

Your RFQ must state the flange type (welding neck, slip-on, or threaded) and the pressure class. For welding neck, classes 300 through 2500 are available. For slip-on and threaded, only Class 300 is covered. If you need a different combination, you are in non-standard territory—expect additional engineering review and possibly longer lead times.

Also specify the facing: raised face, flat face, ring joint, or other. B16.36 does not automatically include facing details; you must state them. For ring joint flanges, the ring groove dimensions and gasket type must be specified.

Specify the Orifice Connection Details

This is where most RFQs go wrong. The orifice pressure-differential connections—the taps—are not standardized in B16.36. You must provide:

• Tap location: radial taps, flange taps, or corner taps? The standard does not dictate this; it depends on your metering design.
• Tap size and type: NPT, socket weld, or other? Specify the thread size and whether you need shutoff valves or just the tap opening.
• Tap orientation: the angle relative to the pipe axis? This is critical for flow measurement accuracy.
• Number of taps: typically two, but some designs require more.

If you do not specify these, the manufacturer will assume a default that may not match your orifice plate and transmitter setup. Always provide a sketch or reference to your metering standard (e.g., AGA, ISO 5167) to avoid ambiguity.

Do Not Assume Plate, Gasket, or Jack Screws

ASME B16.36 covers the flange itself, not the orifice plate, gaskets, or jack screws. These are separate components that you must specify in the RFQ. Common omissions include:

• Orifice plate: material, thickness, bore size, and beta ratio. The bore is determined by your flow calculation—do not ask the flange manufacturer to calculate it.
• Gaskets: type (spiral wound, ring joint, etc.) and material. The flange facing must match the gasket.
• Jack screws: used to separate flanges for plate removal. Specify if required, including size and thread.

Treat these as explicit RFQ line items. If you leave them out, you will face delays and extra costs later.

Include Required Documents in the RFQ

For any flange order, you need certain documents. For orifice flanges, the list is similar but with extra attention to material traceability and testing. Specify in your RFQ:

• Material test certificates: per EN 10204 3.1 or 3.2, depending on your project requirements. Verify that the material grade matches your service conditions.
• Dimensional inspection report: confirming compliance with B16.36 dimensions and tolerances.
• Pressure test certificate: if hydrostatic testing is required. B16.36 does not mandate it, but many projects do.
• NDT reports: if you require ultrasonic, radiography, or other non-destructive testing on the flange body or welds.

Also ask for the marking details. B16.36 specifies marking requirements, but you should confirm that the flange will be marked with the standard, class, material, and manufacturer's identification.

Common Risks and How to Avoid Them

• Mismatched tap orientation: If the taps are not aligned with your flow direction, the measurement will be wrong. Always provide a clear drawing.
• Incorrect class for slip-on: Many buyers assume slip-on is available in higher classes. It is not per B16.36. Verify before ordering.
• Missing gasket details: The flange facing and gasket must match. Specify both.
• Assuming the standard includes the plate: It does not. Always order the plate separately with your calculated bore.
• Ignoring B16.5 requirements: The mating flange must comply with B16.5. Ensure your RFQ covers both standards.

Procurement Checks for Your RFQ

Before sending the RFQ, run through this checklist:

• [ ] Flange type and class are within B16.36 scope.
• [ ] Facing type is specified.
• [ ] Tap location, size, type, and orientation are defined.
• [ ] Orifice plate, gasket, and jack screws are specified as separate items.
• [ ] Material grade and test certificates are required.
• [ ] Dimensional and pressure test reports are requested.
• [ ] Marking requirements are stated.
• [ ] Reference to B16.5 for mating flange is included.

If any item is missing, clarify with the end user or metering engineer before issuing the RFQ.

Final Thoughts

Orifice flanges are precision components. A vague RFQ leads to delays, rework, and measurement errors. Use this checklist to ensure every critical detail is captured. For a wide range of flanges and fittings, explore our products page and send your detailed inquiry via our contact form.

Sources

• https://www.asme.org/codes-standards/find-codes-standards/b16-36-orifice-flanges
• https://www.asme.org/codes-standards/find-codes-standards/b16-5-pipe-flanges-flanged-fittings-nps-1-2-nps-24-metric-inch-standard