A practical buying guide for “pipe flanges” that keeps projects moving

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 misunderstood. For procurement teams, MRO managers, and project engineers, flange errors create schedule delays, change orders, and unplanned shutdown time.

This guide explains how to specify pipe flanges for common U.S. industrial and waterworks applications—using the right standard (ASME B16.5 vs ASME B16.47 vs AWWA), the right pressure class, the right facing, and the right material—so your RFQ is clear and your installation is predictable.

1) Start with the correct flange standard (this prevents “won’t bolt up” surprises)

The word “flange” is not a complete specification. In the U.S., most flange mix-ups trace back to the standard not being stated (or the wrong standard being assumed).

Common standards you’ll see on U.S. projects
  • ASME B16.5 (NPS 1/2 through NPS 24): Covers dimensions, pressure-temperature ratings, materials, tolerances, marking, testing, and also includes recommendations for bolting/gaskets/joints. (asme.org)
  • ASME B16.47 (large diameter flanges, typically NPS 26 through 60, Series A or Series B): Used when you’re outside B16.5’s size range and need large-diameter standardization. (apiint.com)
  • AWWA C207 (waterworks service): Steel pipe flanges for waterworks; commonly used for municipal/utility water transmission and related water infrastructure where AWWA is specified by the owner/engineer. (store.awwa.org)

If your project touches waterworks & irrigation plus industrial piping, it’s especially important to confirm whether the governing spec is AWWA or ASME—they are not interchangeable by default.

2) Pressure class is not “PSI”: how Class 150/300/600 actually works

One of the most expensive misconceptions in flange buying: Class 150 does not mean “150 psi.” ASME pressure classes are tied to pressure-temperature rating tables that vary by material group and decrease as temperature increases. (piping-designer.com)

Example: ASME B16.5-style pressure-temperature behavior (illustrative values for common carbon steel groups)
At lower temperatures (around 100°F), a Class 150 flange can be allowed at a few hundred psi for typical carbon steel material groups, but allowable pressure drops at elevated temperatures. Always confirm the exact table for the material group and design temperature. (piping-designer.com)

Practical buying takeaway: your flange class selection should follow the project’s design pressure + design temperature, plus any code/owner requirements. If your process sees temperature swings, cycling, or upset conditions, avoid “matching the old flange” without verifying the rating table.

3) Face type: RF vs FF vs RTJ (and why gasket choice depends on it)

Facing selection is one of the fastest ways to prevent leaks—because it directly dictates what gasket style is appropriate and how sealing stress is developed.

  • Raised Face (RF): Common on many ASME B16.5 industrial piping systems; widely compatible with common gasket styles when correctly specified.
  • Flat Face (FF): Often used where specified by equipment/system requirements; verify compatibility with mating flange and gasket selection.
  • Ring-Type Joint (RTJ): Used for higher-integrity joints with ring gaskets; more common on severe-service applications and certain ASME B16.47 Series A configurations. (texasflange.com)

4) Material selection: match the environment, not just the legacy part

Flange material is where “low price” can become “high lifetime cost.” Corrosion, temperature, media compatibility, and regional conditions all matter.

Common material families you’ll see on U.S. purchase orders
  • Carbon steel (e.g., A105 family): Common for many industrial services where corrosion is controlled and temperatures are within the selected rating table.
  • Stainless steel (304/316 families): Selected for corrosion resistance (chlorides/chemicals), washdown environments, and long-term reliability in many facilities. (haywardpipe.com)

If you’re buying for MRO: confirm whether your plant standardizes on a specific material group to reduce stocking complexity (gaskets, bolting, and torque practices tend to follow material and facing).

Did you know? Quick flange facts that prevent procurement mistakes

ASME B16.5 has a defined size limit
ASME B16.5 covers NPS 1/2 through NPS 24; larger sizes typically move to ASME B16.47. (asme.org)
Series A vs Series B is not a preference—it changes dimensions
ASME B16.47 Series A and Series B differ in bolt circle, fasteners, and general robustness; they are generally not interchangeable. (apiint.com)
Waterworks flanges commonly follow AWWA
AWWA C207 provides minimum requirements/dimensions for steel flanges for waterworks service (steel pipe and fittings). (store.awwa.org)

Quick comparison table: which flange standard is likely right?

If your job is… Common standard callout What to confirm on the RFQ
Industrial process piping within NPS 1/2–24 ASME B16.5 (asme.org) Class, facing (RF/RTJ/etc.), material spec, bolting/gasket expectations
Large diameter flanges (typically above NPS 24) ASME B16.47 (Series A or Series B) (apiint.com) Series A vs B, class, facing, and interchangeability with existing bolt patterns
Municipal/utility water transmission or waterworks service AWWA C207 (store.awwa.org) AWWA flange type/class per project spec, coating/linings, compatibility with pipe/fittings

How to write a flange RFQ that suppliers can quote accurately (step-by-step)

Step 1: Name the governing standard and size

Example fields: ASME B16.5, NPS 6 (or DN equivalent if your project documents use DN), and whether the flange is a standard bore or reducing configuration.

Step 2: Specify pressure class and design conditions

Include Class (150/300/600/etc.) plus your design pressure and design temperature so the correct rating table can be validated for the material group. ASME B16.5 explicitly addresses pressure-temperature ratings and related requirements. (asme.org)

Step 3: Choose flange type and facing

Common types include weld neck, slip-on, blind, threaded, socket weld, and lap joint (depending on the standard and system). Match facing to your gasket strategy (RF/FF/RTJ).

Step 4: Call out the material spec and any testing/documentation

State the flange material (e.g., carbon steel vs stainless families), any impact test requirements, and documentation needs (MTRs, PMI, etc.) aligned with your quality plan.

Step 5: Don’t forget bolting, gaskets, coatings, and packing

Even if you buy them separately, mention gasket type expectations and bolting material if your project standard requires it. For waterworks, include coating/lining requirements and how parts should be crated for transit.

U.S. buyer angle: why “standardization” matters across multiple cities and sites

Many procurement teams support multiple facilities or projects across the United States (often across different owner specs). A practical way to reduce rework is to standardize how you request flanges:

  • Use a consistent RFQ template (standard, size, class, facing, material, quantity, documentation).
  • Identify “AWWA vs ASME” early for waterworks and irrigation packages to avoid dimensional mismatches.
  • For large diameters, always specify ASME B16.47 Series A or Series B—that single line item prevents bolt-pattern errors. (apiint.com)

Need help sourcing pipe flanges with the right standard, documentation, and logistics?

IFW Supply supports contractors, distributors, and end users with industrial PVF sourcing, waterworks & irrigation components, and export-ready packaging and documentation. If you want a second set of eyes on a flange callout (B16.5 vs B16.47 vs AWWA), send your spec and we’ll help you tighten the RFQ to reduce lead-time risk and field rework.

FAQ: Pipe flanges

Are ASME B16.5 and ASME B16.47 flanges interchangeable?
Not automatically. ASME B16.5 covers NPS 1/2–24, while ASME B16.47 addresses large diameters (commonly NPS 26–60) and has Series A/Series B dimensional differences. Always match the governing standard and series to your mating flange. (asme.org)
Does “Class 150” mean 150 psi?
No. ASME class designations correspond to pressure-temperature rating tables that vary by material group and reduce allowable pressure as temperature rises. (piping-designer.com)
What’s the difference between ASME B16.47 Series A and Series B?
They differ in dimensions (bolt circle and fasteners) and are generally not interchangeable; Series A is typically heavier/robust, while Series B is more compact. Always specify Series A or Series B on large-diameter flange orders. (apiint.com)
When should a project use AWWA C207 flanges?
AWWA C207 is used for steel pipe flanges intended for waterworks service when the owner/engineer specifies AWWA standards for municipal or utility water systems. (store.awwa.org)
What details should I include to get an accurate flange quote?
At minimum: standard (ASME B16.5 / B16.47 Series A/B / AWWA), NPS/DN, class, type (WN/SO/BL/etc.), facing (RF/FF/RTJ), material specification, quantity, documentation needs, and any coating/packing requirements.

Glossary (helpful terms for non-specialists)

NPS: Nominal Pipe Size (a standardized sizing designation used in North America).
Pressure Class (ASME): A class designation (e.g., 150, 300, 600) tied to pressure-temperature ratings by material group—not a direct psi value. (asme.org)
RF / FF / RTJ: Raised Face / Flat Face / Ring-Type Joint—common flange facing types that influence gasket choice and sealing behavior.
ASME B16.5: Standard covering pipe flanges and flanged fittings within NPS 1/2 through NPS 24. (asme.org)
AWWA C207: Standard covering steel pipe flanges for waterworks service applications. (store.awwa.org)

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