But here's the catch many buyers miss: a valve stamped with a pressure class or "B16.34" on the nameplate isn't automatically correct for your job. That marking still has to line up with your actual service temperature, fluid, material group, and end connections.
This guide walks through what B16.34 actually covers, how pressure-temperature ratings work in practice, the difference between Standard, Special, and Limited Class valves, how the standard relates to B16.5 and API 6D, and a documentation checklist you can use before signing off on a purchase order.
Key Takeaways
- ASME B16.34 sets valve design, material, testing, and marking requirements — it doesn't size valves for your process
- A single pressure class (like Class 300) allows different pressures depending on material and temperature
- Standard, Special, and Limited Class carry different construction and marking rules, not just different labels
- Flanged valves fall under Standard Class; Special and Limited Class apply to threaded and welding-end valves
- Always request drawings, material certs, and test records instead of trusting a catalog listing alone
What ASME B16.34 Covers
ASME B16.34 is the standard for flanged, threaded, and welding-end industrial valves. The current edition, ASME B16.34-2025, applies to new construction and covers cast, forged, and fabricated valves made from steel, nickel-based alloys, and other listed materials, according to ASME's B16.34 scope.
Wafer-style and flangeless bolted valves are treated as flanged-end valves under this standard, which matters if you're comparing a wafer check valve against a traditional flanged body.
The requirement areas buyers will run into most often include:
- Pressure-temperature ratings: allowable pressure by material and temperature
- Materials: approved steel and alloy groups for pressure-retaining parts
- Dimensions and tolerances: size, thickness, and fit requirements
- Nondestructive examination (NDE): inspection requirements where applicable
- Testing: shell and seat test procedures and acceptance criteria
- Marking: what must appear on the nameplate or body
What the Standard Doesn't Do
B16.34 doesn't select your valve type, calculate flow or control performance, or confirm fluid compatibility for every service. That responsibility sits with the purchaser and the design engineer, not the standard itself.
This distinction matters in US power, refinery, chemical, boiler, and HRSG plant purchasing, where a project spec often layers B16.34 requirements on top of plant-specific or insurance-driven rules. If your project specification references additional standards, those govern alongside B16.34, not instead of it.
Requirements Buyers Must Verify Before Ordering
A valve passing B16.34 doesn't mean it passes your application. Four checks separate a correct order from an expensive mistake.
Reading Pressure-Temperature Ratings Correctly
Start with the material group and pressure class, then check the rating at your actual design temperature — not room temperature. Every class has notes and limitations tied to specific materials.
Don't assume Class 150, 300, or 600 means one fixed pressure across all temperatures. Velan's rating table, based on ASME B16.34-2025, shows exactly why:
| Material/Class | Temperature | Allowable Pressure |
|---|---|---|
| A105/WCB, Class 300 | -29°C to 38°C (-20°F to 100°F) | 51.0 bar (~740 psi) |
| A105/WCB, Class 300 | 200°C (392°F) | 43.8 bar (~635 psi) |
Same valve, same class, roughly 14% less allowable pressure at the higher temperature. Order on the room-temperature number and you may be operating past the real rating.
Materials, Dimensions, and Testing
Check the nameplate, material list, and mill documentation for body, bonnet, trim, and bolting materials. For steam, high-temperature gas, or corrosive service, confirm compatibility isn't assumed from the pressure class alone.
Also confirm:
- Nominal pipe size and end type (flanged, threaded, welding)
- Face-to-face or end-to-end dimensions and actuator envelope
- Installation orientation, especially for control or check valves
- Shell and seat test results, leakage criteria, and test medium
- NDE reports when the class or service requires them
- Nameplate marking: manufacturer, size, class, material ID, and standard edition
Skipping any one of these is how a "compliant" valve ends up wrong for the line it's installed on.
How Valve Classes Affect Purchasing
B16.34 defines three classifications (Standard, Special, and Limited) and this is separate from the numerical pressure class (150, 300, 600, and so on). Confusing the two is a common buyer mistake.
Standard Class
Standard Class is the baseline: B16.34 compliance without the extra conditions attached to Special or Limited Class. Flanged valves fall into this category by default, according to the Ladish Valves explainer.
Don't treat "Standard Class" as a universal green light, though. Still verify pressure-temperature rating, end connection, material group, and service conditions against your actual job. Confirm exact marking and testing provisions in the current standard before finalizing a spec.
Special Class
Special Class applies to threaded- or welding-end valves that meet Standard Class requirements plus additional Section 8 nondestructive examination requirements. These are marked SPL and rated against a separate pressure-temperature table.
Special Class isn't automatically "better." It's a different construction and rating path, suited to specific pressure-temperature and inspection demands. Selection should follow your design basis and documented compliance needs, not a general assumption that more testing equals more safety margin.
Limited Class
Limited Class covers threaded- or welding-end valves NPS 2½ and smaller, marked B16.34LTD. It doesn't apply to flanged valves at all.
Because of the restricted size and connection rules, never substitute a Limited Class valve into a spec calling for another class without engineering review. The marking and rating basis are different enough that a drop-in swap can quietly violate your design intent.
ASME B16.34 Compared With Related Valve Standards
Buyers often see B16.34 referenced alongside B16.5 and API 6D on the same project. They're not interchangeable, and knowing the boundary saves confusion during submittal review.
B16.34 vs. B16.5: B16.34 is a valve standard — construction, materials, ratings, and testing for the valve body itself. B16.5 covers pipe flanges and flanged fittings (NPS ½ through 24), including their own pressure-temperature ratings, dimensions, materials, and marking. On a flanged-valve installation, both standards typically apply together: B16.34 to the valve, B16.5 to the mating flange.
B16.34 vs. API 6D: API 6D is the pipeline-valve product specification, covering design, manufacturing, and testing for ball, gate, check, plug, and axial valves used specifically in pipeline service. B16.34 has broader industrial valve coverage across power and process applications. If your project sits at the pipeline/plant boundary, check which standard the specification actually invokes — don't assume overlap.
Other references to watch for:
- MSS practices for specific valve types
- Fire-safe and emissions requirements
- Owner or EPC specifications that layer on additional testing
- Piping codes (B31.1, B31.3) governing the overall system
When these documents conflict, your project specification should state which one governs. If it doesn't, that's a question for the engineer before you place the order.
A Practical ASME B16.34 Buying Checklist
Before you commit to a purchase order, run through this sequence:
- Define service conditions: pressure, temperature, fluid, flow rate, corrosion concerns, and shutoff requirements
- Identify valve type and end connection: flanged, threaded, or welding-end
- Match material group to your temperature and fluid
- Confirm class and size against the rating table for that exact material
- Verify dimensional fit against your piping and actuator envelope
Documents to Request Before Approval
- Current datasheet and certified dimensional drawing
- Pressure-temperature rating basis for the specific material and class
- Bill of materials
- Material certificates (MTRs)
- Pressure-test records
- NDE or inspection reports where required
- Marking details, including class designation
- Compliance statement identifying the applicable B16.34 edition
A catalog claim isn't a substitute for any of these. If a supplier can't produce them, that's worth noting before the PO goes out.
For power and process plants managing steam conditioning, this same verification applies to desuperheaters and attemperators. B-Tech Valve LLC builds application-sized LPD and SPAD desuperheater assemblies in ASME B16.34 pressure classes 150 through 2700, rated for temperatures up to 1,200°F (650°C).
The LPD suits steam lines 6 inches and larger through a 90-degree flanged insertion. The SPAD fits 4-inch and smaller lines, clamping between pipe flanges and drawing water from a separate temperature-control valve.
Power plants, refineries, process facilities, outage contractors, and valve distributors can request LPD and SPAD brochures and a sizing data sheet directly from B-Tech Valve. Confirm the specific unit's rating, materials, dimensions, and test documentation match your service before ordering.
Frequently Asked Questions
What is ASME B16.34?
ASME B16.34 is the standard covering flanged, threaded, and welding-end valves, addressing pressure-temperature ratings, materials, dimensions, testing, and marking. Always confirm the current edition cited in your project specification.
What is the difference between ASME B16.34 Standard Class and Special Class valves?
Standard Class is baseline B16.34 compliance, typical for flanged valves. Special Class applies to threaded or welding-end valves with additional Section 8 NDE requirements and a separate rating table, marked SPL.
What are the key differences between ASME B16.34 and B16.5 standards?
B16.34 governs valve construction, ratings, and testing. B16.5 governs pipe flanges and flanged fittings, including their own dimensions and ratings. Both typically apply together on a flanged-valve installation.
What should a valve buyer verify before ordering an ASME B16.34 valve?
Confirm service pressure and temperature, material group, pressure class, end connection, dimensions, test records, and nameplate markings. Match all of these against your actual operating conditions, not just the catalog listing.
Does ASME B16.34 specify the correct valve for every application?
No. It sets valve construction and rating requirements but doesn't replace process sizing, fluid compatibility analysis, or engineering judgment. Those decisions still rest with the purchaser and design engineer.
