A flange is a protruding rim, lip, or ridge used to connect pipes, valves, pumps, and other equipment to form a piping system. Flanged connections allow easy assembly and disassembly of the pipeline for inspection, cleaning, repair, or maintenance without cutting the pipe. They are specified in process plants, refineries, power stations, and chemical facilities wherever bolted joints are required to form pressure-tight connections that can be broken and remade throughout the plant’s operating life.

A weld neck flange has a long tapered hub that is butt-welded to the pipe. The gradual taper distributes stress evenly from the flange to the pipe wall, making it the strongest and most fatigue-resistant type — ideal for high-pressure, high-temperature, and cyclic-load applications. A slip-on flange has a bore slightly larger than the pipe OD, allowing it to slide over the pipe before fillet welding inside and outside. It is simpler and less expensive than a weld neck but rated lower in pressure and fatigue strength. Slip-on flanges should not be used in severe cycling, high-temperature service above approximately 300°C, or applications where the weld neck is mandated by the design code.

ASME B16.5 is the primary American standard governing pipe flanges and flanged fittings in sizes NPS 1/2 through NPS 24. It specifies pressure-temperature ratings across six pressure classes (150, 300, 600, 900, 1500, and 2500), dimensions, tolerances, materials, bolt hole spacing, facing finishes, and marking requirements. The pressure rating is material- and temperature-dependent — a Class 300 flange in carbon steel A105 has a different allowable pressure at high temperature than a Class 300 flange in stainless steel F316L. ASME B16.47 covers larger flanges from NPS 26 to NPS 60.

A raised face flange has a circular raised area around the bore on the mating face — raised 1.6 mm for Class 150 and 300, and 6.4 mm for Class 600 and above. The gasket seats on this raised area only, concentrating bolt load on a smaller contact area for higher seating stress and better sealing at the same bolt load. Raised face is the most common face type in process plant service and is compatible with spiral wound, ring, and sheet gaskets. It is not used when connecting to cast iron or ductile iron flanges (use full face instead) or when an RTJ connection is specified for very high-pressure service.

A blind flange is a solid disc with bolt holes but no central bore. It is used to permanently or temporarily close the open end of a pipe, pressure vessel nozzle, or valve. Common applications include: blanking pipe ends during hydrostatic pressure testing, capping unused nozzles on pressure vessels, creating cleanout or future tie-in access points, and isolating sections of piping during maintenance. Despite having no bore, a blind flange is a pressure-rated component that must be specified to the same pressure class as the rest of the flanged system.

A spiral wound gasket consists of a V-shaped metallic strip wound in a spiral with soft filler material (graphite or PTFE), contained within an outer guide ring and an inner centering ring. It is used on raised face flanges in moderate to high-pressure process service and is the most common gasket type in process piping. A ring-type joint (RTJ) gasket is a solid metal ring (oval or octagonal cross-section) that seats in a machined groove cut into the RTJ flange face. When bolts are tightened, the ring cold-deforms and creates a reliable metal-to-metal seal at very high pressures and temperatures. RTJ is used in Class 600 and above for oil and gas wellhead, high-pressure steam, and other severe service where spiral wound performance is insufficient.