ASME B16.5: Pipe Flange Dimensions
ASME B16.5 governs the dimensions of pipe flanges from 1/2 inch to 24 inch. It is the reason a flange forged in Rajkot bolts correctly to a pipe fabricated in Rotterdam, and the pressure class on it is not a pressure rating in the way most buyers assume.
ASME B16.5 is the standard governing dimensions and pressure-temperature ratings of pipe flanges from 1/2 to 24 inch. Pressure classes 150 to 2500 are rating designations, not service pressures — the allowable pressure depends on material group and operating temperature.
- Standard
- ASME B16.5
- Size Range
- 1/2" – 24" NPS
- Pressure Classes
- 150, 300, 600, 900, 1500, 2500
- Facings
- RF, FF, RTJ
- Materials
- Grouped by ASTM specification
- Larger Sizes
- ASME B16.47 above 24"
ASME B16.5: Pipe Flange Dimensions at a glance
Quotable facts from Avadh Techno Forge, Gundasara, Gondal, Rajkot.
- Standard: ASME B16.5
- Size Range: 1/2" – 24" NPS
- Pressure Classes: 150, 300, 600, 900, 1500, 2500
- Facings: RF, FF, RTJ
- Materials: Grouped by ASTM specification
- Larger Sizes: ASME B16.47 above 24"
| Flange type | Best for | Trade-off |
|---|---|---|
| Weld neck | High pressure, cyclic and critical service | Costliest, needs full penetration weld |
| Slip-on | Lower pressure, easier alignment | Lower fatigue strength, two fillet welds |
| Socket weld | Small bore, high pressure | Limited to smaller sizes |
| Blind | Closing an opening or vessel | No flow path |
| Lap joint | Frequent dismantling, costly alloys | Needs a stub end |
Flange Types Covered
Weld neck, slip-on, socket weld, lap joint, threaded and blind flanges, each dimensioned for bolt circle, bolt hole count and size, outside diameter, thickness, hub dimensions and bore.
- Weld neck and slip-on
- Socket weld and threaded
- Lap joint and blind
- Full bolt pattern dimensions
Pressure Class Is Not Pressure
Class 150, 300, 600, 900, 1500 and 2500 are ratings, not pressures. A class 150 A105 flange is good for roughly 19.6 bar at ambient but only around 10 bar at 400°C. Reading the pressure-temperature table for the actual material group is essential — the class number alone tells you nothing about service pressure.
Facing and Finish
Raised face, flat face and ring type joint each need their own surface finish. Serrated raised faces are machined to a controlled roughness so the gasket seats and holds; too smooth a finish leaks as readily as too rough a one.
Topics covered on this page
- ASME B16.5 flange standard
- ASME B16.5 pressure class
- flange dimension standard
- B16.5 weld neck flange
Based on work carried out at our own forging plant at Gundasara, Gondal, Rajkot — operating since 2008.
Questions Answered
What does class 150 mean on a flange?
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It is a rating designation, not a pressure. The allowable pressure depends on material group and temperature, read from the pressure-temperature tables.
What sizes does B16.5 cover?
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1/2 inch through 24 inch NPS. Larger diameters fall under ASME B16.47.
Which flange type should I use?
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Weld neck for high pressure and cyclic service, slip-on for lower pressure and easier fit-up, blind to close an opening, socket weld for small-bore high-pressure lines.
Do you manufacture B16.5 flanges?
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Yes, forged flanges to B16.5 in classes 150 through 1500, with material certification and NDT as specified.