Flanges

The class system

Flanges are rated by class — 150, 300, 600, 900, 1500, 2500 — not by a pressure. The class is a reference number; the actual allowable pressure depends on the material group and the temperature, and is read from the pressure-temperature tables in ASME B16.5. A Class 150 carbon steel flange is good for around 19.6 bar at ambient but only about 10 bar at 300 °C. Quoting “150#” as though it meant 150 psi at any temperature is a mistake that reaches site more often than it should.

Two flanges of the same class and NPS to B16.5 will always bolt together, regardless of type or material, because bolt circle, bolt count and bolt size are fixed by the standard.

Choosing the type

• Weld neck — the long tapered hub transfers stress gradually into the pipe, so it has by far the best fatigue life. Specify it for cyclic, high-pressure, high-temperature and sour service. It is the default on critical lines.
• Slip-on — the pipe slides through and is filleted inside and out. Cheaper, more forgiving of length error at fit-up, but roughly a third of the fatigue life of a weld neck. Good for utility and low-pressure duty.
• Blind — a solid disc closing a line, vessel nozzle or future tie-in. Remember that a blind under pressure is a stressed component and must be the right class.
• Socket weld — the pipe seats in a counterbore and is filleted externally. Used below NPS 2 at high pressure. Leave the specified expansion gap at the bottom of the socket, or thermal expansion cracks the fillet.
• Threaded — no welding. Used on utility and instrument lines, in galvanised systems, and where hot work is not permitted. Not for cyclic or high-vibration service.
• Lap joint — used over a stub end so it can rotate for bolt alignment, and so only the stub end needs to be in the expensive alloy.

Facing and finish

Raised face (RF) is standard and concentrates the gasket load on a smaller area. Flat face (FF) is used against cast iron and other brittle equipment flanges, where a raised face would bend the flange and crack it. Ring type joint (RTJ) uses a metal ring in a machined groove and is used at high pressure and temperature.

The serrated finish on the face is not a defect — it is specified, normally 125–250 AARH, and it is what grips a soft gasket. A face machined too smooth can leak. Protect faces in transit and storage; a scored face is scrap.

Hydraulic and SAE J518 flanges

Hydraulic power connections use a different system entirely: SAE J518 four-bolt split flanges, in Code 61 (3000 psi) and Code 62 (6000 psi). Sealing is by an O-ring in a groove in the flange head, not by a gasket between faces, and the two codes are not interchangeable — the bolt patterns and flange head dimensions differ. Our SAE flanges are stamped with the size, code and material, for example 1 1/4" SAE FLANGE 3000# SAE J518 CODE 61 MAT.316. Read the stamping rather than trusting the box.

Bolting

Bolting is part of the joint, not an afterthought. The standard flange set is ASTM A193 B7 stud bolts with A194 2H nuts for carbon service, and A193 B8M with A194 8M for stainless. For low temperature use A320 L7. In sour service the bolting must meet NACE hardness limits too. Bolt circle, count and diameter for Class 150 and 300 are tabulated on our flange dimensions page.

What to tell us when you enquire

Type, NPS, class, facing, grade and standard, and quantity. Add the gasket type if you want us to check compatibility, and specify NACE or low-temperature impact testing if either applies.