Flange types and when to use each one

Flanges join pipe to pipe, pipe to valves and pipe to equipment. The same pressure class can be had in several different flange types, and each carries different strengths, costs and installation needs. Here is how the six common types compare.

Weld neck flanges

A weld neck flange has a long tapered hub that blends into the pipe wall and is joined to the pipe with a single buttweld. The taper moves stress away from the joint and gives the flange the best strength and fatigue performance. It is the standard choice for high pressure, high temperature, cyclic loading and hazardous fluids. The bore matches the pipe, so flow is smooth, and the weld can be fully radiographed.

Slip-on flanges

The pipe slides into the flange and is fixed with two fillet welds, one inside and one outside. Slip-on flanges are cheaper, easier to align and need less precise pipe cutting. They are well suited to moderate pressure water, air and utility lines. Their fatigue life is lower than a weld neck, and they are normally not chosen for severe service.

Blind flanges

A blind flange is a solid disc used to close the end of a line, a nozzle or a valve. It sees the full pressure load across its face, so it is among the thickest flanges for a given size and class. Use it for future connections, for isolation and for inspection openings.

Socket weld flanges

These have a recess for the pipe and a single fillet weld at the outside. They are meant for small bore, high pressure lines. The bore is slightly smaller than the pipe at the shoulder, so smooth flow and flow of solids are not ideal, but the strength is well above a slip-on flange of the same size.

Threaded flanges

Threaded flanges screw onto pipe with tapered threads, so no welding is required. They are useful in areas where hot work is prohibited and in low-pressure utility lines. As with all threaded joints, they are not recommended for severe cyclic or high-temperature service.

Lap joint flanges

A lap joint flange is a loose backing ring used with a stub end that is welded to the pipe. The flange can rotate freely, so bolt holes align easily, which is a real advantage on lines that are dismantled often and on large diameters. It also lets you use a stainless steel stub end with a cheaper carbon steel backing flange, because only the stub end touches the fluid.

Comparison at a glance

Choosing the right type

• Start from the service: pressure, temperature, fluid hazard and cycling
• Choose weld neck whenever the piping class asks for it or the risk is high
• Choose slip-on or socket weld when the class allows and the cost saving matters
• Choose lap joint when alignment or material savings drive the design
• Always match the flange to the pipe schedule at the weld end

Standards and sizes

ASME B16.5 covers pipe flanges and flanged fittings from NPS 1/2 up to 24 in classes 150 to 2500. ASME B16.47 covers large diameter flanges from NPS 26 to 60. Material is commonly forged carbon steel to ASTM A105, low-temperature carbon steel to A350 LF2 or stainless steel to A182 F304 and F316. Each flange should carry its markings and, where required, a test certificate.

Tell us the flange types, sizes, classes and materials on your list and we will send you a quote. Ask us for test certificates when you enquire.