A procurement-ready guide for engineers, MRO teams, and project buyers
Pipe flanges look straightforward—until a shipment arrives and the bolt circle doesn’t match, the facing is wrong for the gasket, or the pressure class is “right” on paper but wrong at operating temperature. For U.S. projects spanning industrial plants, municipal waterworks, irrigation districts, and fire protection infrastructure, flange details affect schedule, safety, and total installed cost.
Below is a practical, spec-first breakdown of how to select and communicate flange requirements clearly—especially when multiple stakeholders (engineering, purchasing, installers, QA/QC, and logistics) touch the same order.
Why “pipe flanges” is not a complete spec
The word flange can mean dozens of combinations. To get the right part the first time, you typically need:
• Standard (ASME vs AWWA, etc.)
• NPS size and schedule / pipe OD basis
• Pressure class (and temperature basis)
• Flange type (WN, SO, threaded, blind, lap joint)
• Facing (RF, FF, RTJ) and finish requirements
• Material specification (A105, stainless, duplex, etc.)
• Compliance / traceability (MTRs, heat numbers, marking)
The most common standards you’ll see in U.S. purchasing
ASME B16.5 is the everyday workhorse for many industrial systems (generally up to NPS 24), covering dimensions plus guidance on bolting, gaskets, and joints. (asme.org)
For large diameter steel flanges, ASME B16.47 is commonly referenced (NPS 26 through NPS 60) and includes pressure–temperature ratings, materials, dimensions, and more. (reliavalves.com)
In municipal waterworks service, buyers frequently encounter AWWA C207, which sets minimum requirements and dimensions for steel flanges used with steel water pipe and appurtenances in water utility conditions. (store.awwa.org)
Pressure class: what it means (and what it doesn’t)
A frequent source of mistakes is treating “Class 150” or “Class 300” as a simple PSI rating. In ASME systems, the class designation is not the same as allowable pressure—the actual pressure rating depends on material group and temperature. (midstreamcalculator.com)
Practical takeaway for procurement teams: if engineering gives you only “Class 300,” confirm the design temperature and flange material spec. That’s what drives the allowable pressure at temperature.
| Standard / Use Case | Common Class/Rating Labels | Best For | Watch Outs |
|---|---|---|---|
| ASME B16.5 (asme.org) | 150, 300, 400, 600, 900, 1500, 2500 (midstreamcalculator.com) | Most industrial PVF packages, fire pump rooms, mechanical/process piping | Pressure rating varies by material/temperature; confirm facing and gasket style (midstreamcalculator.com) |
| ASME B16.47 (large diameter) (reliavalves.com) | Commonly referenced up to Class 900 (piping-world.com) | Large OD pipelines, transmission, big pump stations | Series and bolt patterns matter—confirm exact standard callout and mating dimensions (reliavalves.com) |
| AWWA C207 (waterworks steel flanges) (store.awwa.org) | Classes B, D, E, F (water utility service) (apiint.com) | Municipal distribution, treatment plants, pump stations | AWWA pressure classes are not ASME classes; confirm bolt pattern and mating equipment (apiint.com) |
Material + temperature: where specs get real
Even with the right standard and class, material selection changes performance and compliance. ASME flange ratings are tied to material groups and pressure–temperature tables (not just the class number). (piping-world.com)
If your team is operating in high ambient heat, steam service, or hot process lines, the allowable pressure may drop significantly as temperature rises. That’s why your PO should reference both the material specification and the design temperature basis, not only “Class 150.”
Procurement tip: If you’re buying carbon steel flanges for industrial service, “A105” is common—but don’t stop there. Clarify heat traceability (MTR), impact testing requirements (if any), coating/finish, and whether the flange must be forged vs plate/other manufacturing route per the governing standard. (asme.org)
Step-by-step: a clean flange specification checklist (ready for RFQs)
1) Start with the mating component
Confirm what the flange must mate to: valve, strainer, hydrant accessory, pump nozzle, hose connection, or spool. Matching bolt circle, hole count, and facing is the non-negotiable.
2) Lock the governing standard
Use ASME B16.5 for many industrial sizes and ASME B16.47 for large diameter steel flanges. (asme.org) For municipal waterworks steel flanges, confirm whether the project is built around AWWA C207. (store.awwa.org)
3) Specify class with temperature basis
Don’t assume the class number is the PSI. Ratings depend on material and temperature, and pressure–temperature ratings apply when bolting/gasket limits and good assembly practices are met. (midstreamcalculator.com)
4) Call out facing + gasket style
Raised face vs flat face vs RTJ drives gasket selection and sealing stress. If the gasket is already selected (spiral wound, full-face, etc.), include it in the RFQ notes so the supplier can confirm compatibility.
5) Add the “buyers always forget this” details
Quantity + spares
Include spares if the shutdown window is tight or if the line is critical.
Include spares if the shutdown window is tight or if the line is critical.
Certs & traceability
Request MTRs, heat numbers, and marking requirements upfront.
Request MTRs, heat numbers, and marking requirements upfront.
Coating / corrosion plan
Clarify bare, painted, galvanized, or coated—and packaging for transit.
Clarify bare, painted, galvanized, or coated—and packaging for transit.
U.S. buying and logistics realities (especially for project procurement)
For projects across the United States, flange purchasing is often a coordination problem—not a product problem. The fastest path is aligning engineering and procurement language so vendors can quote correctly and ship without clarification delays.
IFW Supply supports procurement teams and project engineers sourcing PVF and related products with a practical focus on availability, specification matching, and documentation readiness—especially when your order includes mixed categories (industrial + waterworks + fire protection + safety) that must arrive together for a scheduled install.
Common “save-a-week” move: Send your flange line items as a short spec block (standard + size + class + type + facing + material + quantity). If you also attach the isometric/P&ID excerpt for the relevant node, you reduce RFIs and lower the risk of a bolt-pattern mismatch on site.
Need a quote or spec check on pipe flanges?
Share your standard, class, facing, and material requirements (or send your BOM). IFW Supply can help confirm compatibility, documentation needs, and lead times.
FAQ: Pipe flanges
Is “Class 150” the same as 150 psi?
No. In ASME systems, the class is a designation and the allowable pressure depends on temperature and material group. Always confirm design temperature and material when specifying class. (midstreamcalculator.com)
When should we use ASME B16.47 instead of ASME B16.5?
ASME B16.5 is widely used for common flange sizes, while ASME B16.47 covers large diameter steel flanges (NPS 26 through NPS 60) with its own dimensional requirements. (asme.org)
Are AWWA C207 flange classes interchangeable with ASME classes?
Not automatically. AWWA C207 uses waterworks-oriented classes (B, D, E, F), and interchangeability depends on bolt patterns and application details. Confirm mating equipment and project standard before ordering. (apiint.com)
What’s the minimum info purchasing should put on an RFQ for pipe flanges?
At minimum: standard (ASME/AWWA), NPS size, class, flange type, facing, material spec, quantity, and certification requirements (MTR/marking). If you have gasket requirements or coating requirements, include them too.
Glossary
NPS: Nominal Pipe Size; a naming system used in North America. The actual outside diameter is standardized, while the wall thickness varies by schedule.
Pressure–temperature rating: The maximum allowable working pressure at a specified temperature for a component, as defined by the governing standard and material group. (wermac.org)
RF (Raised Face): A common flange facing with a raised sealing surface used with many industrial gaskets.
FF (Flat Face): A flat sealing surface often used in waterworks service and with certain equipment/gasket configurations.
RTJ (Ring Type Joint): A groove-and-ring sealing style used in higher pressure services.
WN (Weld Neck): A flange type with a tapered hub welded to the pipe; often selected for higher stress services.
MTR: Material Test Report; documents chemical/mechanical properties and traceability for the supplied material.