A flange is a forged or cast ring bolted to the end of a pipe, valve, pump or pressure vessel to join components in a piping system. Two mating flanges are bolted together with a gasket sandwiched between the sealing faces, creating a leak-tight connection that can be disassembled for inspection, cleaning or modification.
Flange Definition in Plain Terms
Think of a flange as the “rim” that lets two sections of pipe bolt together. The word covers both the ring welded (or screwed) onto the pipe and the mating face on equipment nozzles. In piping work, “pipe flange” almost always means the bolted ring type defined by standards like ASME B16.5, DIN 2576 or EN 1092-1.
What Does a Flange Do?
- Connects pipes to pipes, valves, pumps, strainers and vessels
- Seals the joint with a compressed gasket between faces (RF, FF, RJ and more)
- Allows disassembly – bolted joints can be opened for maintenance without cutting pipe
- Provides access for instrumentation, cleaning and end-of-line blanking (blind flanges)
Main Parts of a Flanged Joint
- The two flange rings, each with a drilling of bolt holes on a bolt circle
- A gasket sized to the sealing face
- Stud bolts and nuts, tightened to a specified torque in a star pattern
Common Flange Types at a Glance
The most common types are weld neck, slip-on, blind, socket weld, threaded and lap joint flanges – each suits different pressure, size and maintenance needs. See all types with photos on our flange types page.
How Are Flanges Specified?
A flange callout includes: nominal size (NPS/DN), pressure class (Class 150-2500 or PN2.5-PN400), type (WN/SO/BL/SW/TH/LJ), facing (RF/FF/RJ…), material (e.g. A105, A182 F304/F316, A350 LF2) and standard (ASME, DIN, EN, JIS, GB). Our size charts list dimensions for every class.
Flange vs Threaded vs Welded Connections
Welding gives the strongest permanent joint; threading suits small, low-vibration lines; flanging wins where equipment must be removed, inspected or replaced – which is why flanges dominate pump, valve and vessel connections in industrial plants.
FAQ
What is the difference between a flange and a coupling?
A coupling joins two pipe ends permanently or semi-permanently; a flanged joint is bolted and designed to be opened. Flanges also connect pipe to equipment nozzles, which couplings cannot.
What does “flanged” mean on equipment?
It means the equipment nozzle ends in a standard flange face (e.g. RF Class 300) ready to bolt to matching pipework.
Which flange type should I choose?
For high pressure/temperature: weld neck. For low-cost low-pressure: slip-on. To blank a line: blind. For small-bore high pressure: socket weld. For no-weld assembly: threaded. See our type-by-type guide.