Choosing a flange by nominal pipe size alone is not enough. A connection can look close on a drawing and still fail at installation if the pressure class, facing, bore, bolt circle, or bolt-hole size is wrong. This guide explains how to know if flange size is correct before material is released for production.
Start with the complete flange identification
For ANSI standard flanges, record more than “6 inch flange.” A complete specification normally includes nominal size, ASME/ANSI standard, pressure class, flange type, facing, material grade, bore or schedule, and any coating or testing requirement. For example: NPS 6, ASME B16.5, Class 150, weld neck, raised face, ASTM A182 F316L, Sch 40 bore. Each item affects fit or service suitability.
How to know correct flange size
Use four checks together:
- Nominal pipe size (NPS): Match the pipe designation, not a measured outside diameter alone.
- Pressure class: Class 150, 300, 600 and higher classes have different thicknesses, bolt patterns, and ratings. A Class 150 flange is not interchangeable with a Class 300 flange of the same NPS.
- Pipe bore and wall: Weld neck and socket weld flanges must match the pipe schedule or specified inside diameter.
- Facing and gasket: Raised face, flat face and ring-type joint connections need compatible mating surfaces and gasket systems.
Verify the ANSI flange bolt pattern
The ANSI flange bolt pattern is the fastest practical confirmation after nominal size and class. Compare the bolt-circle diameter, number of bolt holes, hole diameter, bolt diameter, and bolt-hole spacing against the applicable standard. Never rely on a supplier photo or a “similar” flange. A mismatch of even one bolt-hole dimension prevents assembly and can delay a shutdown.
For ANSI standard for flanges, obtain the current dimensional table for ASME B16.5 or ASME B16.47 where applicable. ASME B16.47 covers many large-diameter applications and uses Series A and Series B dimensions; these series are not automatically interchangeable.
Check fixed flanges and mating parts
“ANSI fixed flanges” is often used when a flange is permanently welded or otherwise fixed to a pipe spool, vessel nozzle, or equipment connection. Before fabrication, confirm the companion flange, gasket, stud length, nut type, and accessible wrench clearance. For weld neck flanges, also verify hub dimensions and schedule. For slip-on flanges, confirm the required weld detail and bore clearance.
A practical pre-order checklist
- Confirm NPS and pipe schedule from the piping isometric.
- Confirm ASME/ANSI standard and pressure-temperature rating.
- Confirm flange type: weld neck, slip-on, blind, threaded, socket weld, lap joint, or special design.
- Match RF, FF, or RTJ facing to the mating connection.
- Cross-check the ANSI flange bolt pattern with the relevant dimensional table.
- Specify material, corrosion allowance, inspection, and certification requirements.
- Send a drawing for non-standard, equipment, or replacement connections.
Final advice
If any dimension is uncertain, do not guess from an old flange. Measure the outside diameter, bore, flange thickness, facing, and bolt circle, then compare them with the specified standard and pressure class. A clear data sheet or drawing allows the supplier to verify the connection before manufacture. SONGHAI can support drawings and technical confirmation for sanitary and industrial stainless steel flange applications.