ASME B16.34 Valve Flange Standard Explained for Buyers

Why this matters

If you are buying valves for any high-integrity piping system, the line-item that decides whether the valve is fit-for-service is the ASME B16.34 rating. B16.34 is the umbrella standard that ties pressure class, material group, design temperature and dimensional / NDE / marking requirements together for cast, forged and fabricated steel and alloy valves with flanged, threaded or weld ends. Misreading the pressure-temperature chart is one of the easiest ways to over- or under-buy a valve.

Scope comparison

Classes 150 through 2500 align with the flange classes in ASME B16.5 / B16.47, which is why B16.34 is the natural companion code for flanged valves.

Key differences from a pure flange standard

• B16.34 is a valve standard, not just a flange standard. It covers the body shell thickness, NDE and marking in addition to pressure ratings.
• It defines Standard Class, Special Class and Limited Class valves. Special Class permits a higher rating when supplemental NDE is performed at the foundry/forge.
• For flanged-end valves, the rating ends at 538 degrees C even if the body material allows higher temperature. Above that, weld-end valves are typically used.
• Pressure ratings derate with temperature. As an example commonly cited in industry references, a Class 300 carbon steel valve good for around 740 psig at ambient drops to roughly 535 psig at 700 degrees F. Always read the actual table in the latest published edition.

When to specify B16.34

• All ASME B31.3 process piping valves should reference B16.34 explicitly.
• For sour or cryogenic service, B16.34 is combined with NACE MR0175 / ISO 15156, or with low-temperature material grades like A350 LF2.
• Where a higher pressure rating is needed for the same dimensional envelope, specify Special Class and require the corresponding NDE on the body.

B16.34 valve castings and forgings are commonly mated with our forged flanges and seamless butt-welding pipe fittings on the same skid.

Procurement / spec checklist

1. State the valve type, end connection, size, class, body material and B16.34 reference, e.g. "Gate valve, flanged RF, NPS 6, Class 300, ASTM A216 WCB, ASME B16.34, Standard Class".
2. Always state design pressure AND design temperature. Confirm both fall inside the pressure-temperature chart for the chosen material group and class.
3. If you need any pressure rating above the standard table, request Special Class and define the NDE method.
4. For flanged ends above 538 degrees C, switch to weld-end valves.
5. Cross-check the valve trim and seat material against the process fluid; B16.34 governs the shell, not the trim.
6. Confirm marking per B16.34: manufacturer, material, class, size and rating.

We can supply matching flange-to-valve sets with full MTRs. Send your spec via our inquiry page.

Sources

• https://www.asme.org/codes-standards/find-codes-standards/b16-34-valves-flanged-threaded-welding-end
• https://blog.projectmaterials.com/valves/asme-b16-34-valve-specification/
• https://www.wermac.org/valves/valves_B16_34.html
• https://www.valvemagazine.com/articles/an-overview-of-asme-b16-34-2017