A flange is a forged or cast, disk-shaped mechanical joint that lets two pipe sections, a pipe and valve, or a pipe and vessel be bolted together with a gasket, rather than permanently welded [S2]. The connection is reversible: remove the bolts, service the equipment, and bolt it back up without cutting pipe, which is the single property that makes flanges indispensable in process piping [S2].
The six most common industrial flange types are weld neck, slip-on, socket weld, lap joint, threaded, and blind [S1][S4][S5]. These types are standardized under ASME B16.5 for sizes NPS 1/2 through 24, with seven pressure classes (150, 300, 400, 600, 900, 1500, 2500) that govern dimensions, bolting, materials, and pressure-temperature ratings [S3]. Larger sizes fall under ASME B16.47, wellhead and Christmas-tree service under API 6A, and waterworks under AWWA C207 [S2].
Weld Neck Flanges: The High-Pressure Workhorse
Weld neck flanges have a long, tapered hub that transfers stress from the flange face to the pipe wall, distributing mechanical load across the butt weld [S1]. This geometry makes them the default for high-pressure and high-temperature service, including process piping in refineries and chemical plants [S1]. The butt-weld end is the same schedule as the mating pipe, which is why weld necks are the first pick when thermal cycling, fatigue, or pressure transients dominate the duty [S1].
The full-penetration butt weld means weld neck flanges are also the first choice for NACE MR0175 sour-service hydrogen sulfide environments, where weld integrity, hardness control, and stress distribution all matter [S2]. A mismatch between pipe schedule and flange bore is a common field error; the bore must match the pipe exactly to avoid a step that creates turbulence and an erosion site [S1].
Slip-On, Socket Weld, and Threaded Flanges: Lower-Pressure and Specialty Service
Slip-on flanges slide over the pipe OD and are secured with one fillet weld on the inside top and one on the outside, which makes installation easier than a butt weld but limits them to low-pressure and low-temperature service [S1][S5]. They cost less than weld necks because no precise pipe-end bevel is required, and they are common in firewater, cooling water, and low-pressure utility lines [S5].
Socket weld flanges accept the pipe into a socket, with a 1/16 inch (1.5 mm) expansion gap left between the pipe end and the socket shoulder before the single fillet weld is applied [S4]. That gap accommodates thermal expansion and reduces weld-root stress, but it also creates a crevice that is vulnerable to crevice corrosion, so socket welds are not specified for highly corrosive or erosive service [S4]. Typical uses are small-diameter, high-pressure hydraulic lines, generally NPS 2 and below [S4].
Threaded flanges, also called screwed flanges, have female NPT or BSP threads that mate to the male-threaded pipe end, so no welding is required [S1][S4]. They are restricted to low pressure and low temperature, and are favored in flammable or hazardous services where welding hot work is undesirable, or in tight spaces where a weld cannot be made [S4]. Typical uses are air and water utility services, small-bore instrument lines, and plant air [S1][S4].
Lap Joint Flanges and Backing Rings: Designed for Frequent Dismantling
Lap joint flanges are a two-piece assembly: a stub end butt-welded to the pipe and a loose backing flange (also called a backing ring) that slips over the stub end and is free to rotate [S1][S4]. The backing flange never contacts the process fluid, which means it can be made from a cheaper carbon steel while the wetted stub end is a corrosion-resistant alloy, a common pattern in stainless or alloy process lines [S4].
The rotating backing flange is also a practical advantage when bolt holes must be aligned on site, since the flange can be spun to match the mating flange without twisting the pipe [S1][S4]. Lap joints are not recommended for high-pressure or high-temperature service, because the joint relies on the stub-end fillet weld and the gasket seal under a less-rigid flange face [S4].
Reducing flanges, expander flanges, nipoflanges, and weldoflanges are specialty variants used to transition between pipe sizes or to combine a flange with a pipe-fitting geometry in a single forging [S1][S2]. For routine spec work, see our reference on flange types and classifications for the standard part-numbering and material codes used in procurement.
Blind Flanges and Pressure-Class Selection
Blind flanges are solid disks with bolt holes, used to close off the end of a pipe, valve, or pressure vessel, or to isolate a section for pressure testing [S1]. Because they carry full bore pressure on a flat plate, they are rated to the same class as the mating flanges in the line, and they are the standard means of providing line-end access that can be removed for inspection [S1].
ASME B16.5 pressure classes (150, 300, 400, 600, 900, 1500, 2500) do not directly map to psi: a Class 600 carbon-steel flange is not a 600 psi flange [S3]. The class is a temperature-and-material dependent rating, and the allowable pressure drops as temperature rises; a carbon-steel Class 150 flange rated for about 285 psi at 100 degrees F is rated for roughly 150 psi at 600 degrees F [S3]. A 316 stainless-steel flange of the same class will have different pressure-temperature values because of different allowable stress [S3].
For pump applications, suction and discharge pressures and temperatures drive the class: low-head general-service pumps are commonly built with 125/150 pound flanges, while multistage high-head pumps in boiler-feedwater or mine-dewatering service use 600 pound or 900 pound discharge flanges to handle the higher pressure-temperature combinations [S3]. Class 1500 and 2500 are reserved for high-pressure steam, high-pressure chemical, and hydrocarbon service where standard Classes 300 and 600 are not adequate [S2][S3].
Flange Faces, Gaskets, and Material Selection
Flange face, the mating surface that contacts the gasket, is a separate selection from flange type. The five standard face types are flat face (FF), raised face (RF), ring joint (RTJ), male-and-female (M&F), and tongue-and-groove (T&G) [S1]. Raised face is the most common in process piping, with a serrated spiral finish (typically 125 to 250 micro-inches Ra) that grips the gasket and resists blowout [S1]. Ring-type joint (RTJ) faces use a metal ring gasket in a machined groove and are used for high-pressure Class 600 and above [S2]. Flat face is paired with full-face gaskets and is used for cast-iron flanges, where the full face distributes bolt load across the brittle flange body [S1].
Common forged materials are ASME SA-105 carbon steel, ASME SA-182 F304/L and F316/L stainless, F11/F22/F91 chrome-moly for elevated temperature, Alloy 625/825 and Hastelloy for corrosion service, and 2205/2507 duplex for chloride-bearing process streams [S2]. Cast iron and ductile iron flanges are widely used in waterworks, firewater, and low-pressure utility service where their lower cost outweighs the lower pressure-temperature rating [S2]. Selection pivots on three coupled questions: what is the design pressure and temperature, what is the process fluid and its corrosion profile, and how often does the joint need to be broken [S2][S4].
For a deeper dive into how flange specs line up with broader mechanical equipment, see our reference on lamps and light fittings for related industrial component classification logic, and our guide to construction machinery and equipment for the heavy-machinery side of bolted-joint assemblies.
Limitations, Failure Modes, and Selection Trade-offs
Threaded and socket-weld flanges both have crevice-related failure modes: threaded flanges can leak at the threads under thermal cycling, and socket-weld flanges are vulnerable to crevice corrosion at the 1/16 inch expansion gap [S4]. Neither is acceptable for highly corrosive or high-temperature service, and ASME B31.1 and B31.3 restrict socket-weld use to non-corrosive, non-cyclic service [S4]. Lap-joint joints have a lower fatigue rating than weld-neck joints because the stub-end fillet weld concentrates stress, so lap joints are not used on high-cycle or vibration-loaded lines [S4].
Flat-face flanges paired with raised-face gaskets are a common field-mismatch error: a flat face concentrates bolt load on a small gasket area, which can crush the gasket and leak, or crack a cast-iron flange [S1]. Always match face type across the joint. Reducing flanges used as expanders (expander flanges) concentrate stress at the bore transition and are limited in pressure class compared with straight flanges of the same class [S2].
For personal protective equipment that lines up with the same spec-first selection logic, see the reference on safety helmet types and classifications, and for process-side equipment selection adjacent to flanged piping, our guide to flow meter selection covers the bolted spool-piece geometry many flow technologies require.
Standards, Sourcing, and Specification Stack
ASME B16.5 is the primary standard for forged flanges NPS 1/2 through NPS 24 in Classes 150 through 2500, covering dimensions, tolerances, materials, marking, and pressure-temperature ratings [S3]. ASME B16.47 covers larger sizes (NPS 26 through 60) in Series A and Series B geometries, with Series A matching MSS SP-44 and Series B matching API 605 [S2]. API 6A governs wellhead and Christmas-tree flanges in oil and gas upstream, with pressure ratings expressed in psi or MPa and specific material and NACE MR0175 requirements for sour service [S2]. AWWA C207 Class B, D, E, and F covers steel waterworks flanges for potable-water and raw-water service [S2].
Material traceability, mill cert, and marking per ASME B16.5 are required for code-stamped process piping, and the standard class-and-material mark on the flange hub is what procurement and QA inspect against [S2]. For sour service per NACE MR0175, hardness testing, material certification, and weld-procedure qualification add a parallel documentation layer on top of the dimensional standard [S2].
Two near-term signals to track: ASME B16.5 updates on dual-certified material marking for stainless F304/F304L and F316/F316L procurement, and the increasing use of duplex 2205 and super-duplex 2507 in mid-range Class 300 and 600 chemical and offshore service, where the alloy combines higher strength with chloride resistance [S2]. Both will reshape the material column in many plant flange specifications over the next procurement cycle.