Common Pipe Flange Types and Where They Are Used

Pipe flanges are mechanical connections used to join pipe to pipe, pipe to a valve, pipe to a tee, or pipe to equipment in a process system. They are commonly bolted, welded, or threaded into place. A typical flanged joint uses two flange faces, a gasket, and bolts to create a tight seal while still allowing the piping system to be opened later.

That serviceability is the reason flanges are so common. Welding can make a permanent connection, but a flange lets a pump, valve, spool, instrument, or pipe section be removed for inspection or replacement. In many piping systems, flange connections are one of the main joining methods after welding.

Blind flanges

Blind flanges close the end of a pipe, valve, vessel opening, or flange connection. They have no center bore, so they are used when a line needs to be blocked off. A blind flange can make a future expansion point, inspection point, or pressure-test closure easier to manage because the connection can be unbolted later.

Because the blind flange has to hold system pressure without flow passing through it, the pressure class, gasket, bolt pattern, and flange face all have to match the mating connection.

Lap joint flanges

Lap joint flanges are used with a stub end. The flange itself can rotate around the pipe, which helps align the bolt holes during assembly. This makes them useful where the line may be taken apart often or where bolt-hole alignment is difficult.

The stub end provides the sealing face, while the lap joint flange provides the bolting force. That separation is why the stub end and flange have to be selected as a working pair.

Slip-on flanges

Slip-on flanges slide over the pipe and are welded in place. They are easier to position than weld neck flanges and are often used in lower-pressure piping systems. The pipe is inserted into the flange before welding, so the fit and weld location matter.

Slip-on flanges are practical when installation speed and alignment are important, but they still depend on proper welding and the correct flange rating for the system.

Socket weld flanges

Socket weld flanges have a socket where the pipe is inserted before welding. They are generally used on smaller pipe sizes. The socket locates the pipe, and the weld secures the joint.

This type can provide a strong connection in compact piping, but the pipe size, socket depth, and weld quality all affect the finished assembly.

Threaded flanges

Threaded flanges connect to threaded pipe without welding. They can be useful where welding is not practical or where the system needs a threaded connection style. The pipe screws into the flange bore, and the thread form becomes part of the mechanical connection.

Threaded flanges still have limits. The thread, pressure, temperature, material, and sealing method have to fit the service. They should not be treated as a universal replacement for welded or bolted flange designs.

Weld neck flanges

Weld neck flanges have a long tapered hub and are welded to the pipe. The hub helps transfer stress from the flange into the pipe wall. This is why weld neck flanges are common in higher-pressure, higher-temperature, or more demanding service.

The welded joint becomes part of the pipe run, so pipe schedule, weld preparation, and flange rating have to match the system design.

Reducing and orifice flanges

Reducing flanges connect pipe sizes that are not the same. They can simplify a transition where one pipe size has to meet another connection. The reduction must still support the expected flow, pressure, and mechanical load.

Orifice flanges are used with an orifice plate for flow measurement. In that case, the flange is not only a connector. It helps create a controlled measurement point in the piping run.

Flange alignment and gasket loading

A flange joint can leak if the faces are misaligned, the gasket is unevenly compressed, or the bolts do not load the joint correctly. A well-known pipeline release in Danville, Michigan in 2011 was tied to a flange gasket failure and misalignment. The example is a reminder that flange installation is not only a hardware choice.

Before a flange connection is placed in service, the flange type, gasket, bolt pattern, pressure class, pipe size, face style, and alignment all have to match. A flange that looks close can still be wrong if the bore, face, or bolt pattern does not match the mating connection.