Start with the flange type
The type follows from how the flange attaches to the pipe and how the joint will be inspected and maintained.
- Weld neck: butt-welded, used where fatigue or cyclic loading matters
- Slip-on: slid over the pipe and fillet welded, easier alignment
- Blind: closes off a run or vessel opening
- Socket weld and threaded: usually small bore
- Lap joint with a stub end: allows free rotation for bolt alignment
Then fix the pressure class
ASME B16.5 defines classes 150, 300, 400, 600, 900, 1500 and 2500 for NPS 1/2 through 24. The allowable pressure of a class falls as temperature rises, and depends on the material group, so read the pressure–temperature rating table for your specific material.
Facing and surface finish
Raised face (RF) is the most common facing; flat face (FF) is used against cast iron or other brittle flanges; ring type joint (RTJ) is used at higher pressures with a metal ring gasket. The gasket seating surface finish must suit the gasket selected.
Bolting and dimensions
For each class and size the standard fixes the bolt circle diameter, number of bolts and bolt hole diameter, so mating flanges of the same class and size always line up. Confirm bolt length against the assembled stack of flanges, gasket and any spacers.
What to specify on an enquiry
Size, class, type, facing, material specification, bore or schedule for weld neck flanges, quantity and any documentation requirement. Sending the full string — for example 'NPS 6 Class 300 WNRF SCH 40 ASTM A105' — makes quoting straightforward.
Common questions
- Will a Class 150 flange bolt to a Class 300 flange?
- No. The bolt circle diameter, bolt count and bolt hole size differ between classes, so the flanges do not mate. Both sides of a joint must be the same class and size.
- Does Class 150 mean 150 psi?
- No. The class is a rating designation. The allowable working pressure depends on the material and the temperature, and at ambient temperature it is generally higher than 150 psi.
- What bore should I give for a weld neck flange?
- The bore must match the schedule or wall thickness of the connecting pipe, so state the pipe schedule along with the size and class.
Have a requirement based on this guide? Send the details and we'll quote against your specification.