What are pipe flanges and what types are commonly used?

A pipe flange is a connecting component used to join pipe to pipe, or pipe to a valve, pump, vessel or heat exchanger. Two flanges are bolted together with a gasket or sealing ring held between their faces, and because the joint can be opened again it is easy to inspect, repair or extend a piping system without cutting the line. Choosing the right pipe flange is about more than size; the hub style, facing, pressure class, material and joining method together decide whether a joint can be assembled, sealed and maintained safely through its whole service life. This guide explains the common flange types, their faces, standard ratings and typical materials so you can specify the right one for your project.

What Is a Pipe Flange?

In simple terms, a flange is the raised rim at the end of a pipe or fitting that carries bolt holes and a sealing face. Flanges turn a permanent weld joint into one that can be dismantled, which is why they appear on almost every piece of rotating equipment, on vessels, in valve connections and at pipeline tie-ins. A complete flanged joint usually needs more than the matching pair of flanges: a gasket seals the faces, and stud bolts and nuts clamp them together. That is why a proper contractor typically specifies flanges, gaskets and fasteners together as one package rather than buying them separately.

Flanges are governed by published standards so that one manufacturer's flange can be bolted to another's. ASME B16.5 is the most widely used standard for flanges and flanged fittings from NPS 1/2 up to NPS 24, covering pressure-temperature ratings, materials, dimensions, tolerances, marking and testing. For large-diameter steel flanges, ASME B16.47 covers sizes from NPS 26 through NPS 60, while EN 1092-1 is the commonly specified European standard using PN designations.

Common Types of Pipe Flanges

The six flange designs that buyers meet most often are weld neck, slip-on, blind, socket weld, threaded and lap joint. Each suits a particular pressure level, temperature and installation situation, so the decision usually starts with function.

Weld Neck Flange

A weld neck flange carries a long tapered hub that is butt-welded to the pipe. The taper gives a gradual transition from the thick flange ring down to the pipe wall, which spreads stress and makes this the standard choice for high-pressure, high-temperature and cyclic service. The bore must be matched to the pipe schedule so the internal profile stays smooth.

Slip-On Flange

A slip-on flange slides over the pipe and is fillet-welded at the hub and at the pipe end. It is quicker to align during fabrication than a weld neck flange and is common in utility, water and general process lines operating at low to moderate pressure.

Blind Flange

A blind flange has no bore. It closes the end of a pipe, nozzle or valve connection and can be removed later for inspection, maintenance or future extension. Because internal pressure acts over the full face, a blind flange can carry significant bending loads and must be selected against the correct pressure-temperature rating.

Socket Weld Flange

Socket weld flanges have a recessed socket that the pipe slides into before fillet welding. They are associated with small-bore, higher-pressure piping and, when installed correctly, require a small expansion gap at the socket shoulder to avoid excessive stress.

Threaded Flange

A threaded flange screws onto an externally threaded pipe and needs no welding, which is useful where hot work is restricted or for selected low-pressure services. The thread form, sealing method and service limits must match the application, because threaded joints are not suitable for every fluid, vibration level or temperature.

Lap Joint Flange

A lap joint uses a backing flange combined with a stub end. The backing flange is free to rotate, which simplifies bolt-hole alignment, and the arrangement can be economical in corrosion-resistant systems because only the wetted stub end needs the expensive alloy while the backing ring can be a lower-cost material.

Specialized Flange Designs

Beyond these six, long weld neck flanges are used as vessel and equipment nozzles, spectacle blinds and spade arrangements give visually confirmable line isolation, and orifice flanges include pressure taps for flow measurement. These are normally chosen as part of the complete piping design rather than as catalogue items.

Flange Faces: RF, FF and RTJ

The flange type describes the connection geometry, and the face describes the sealing surface; the two must be specified together. A raised face (RF) has a machined raised area that the gasket sits on and is the most common facing in process piping. A flat face (FF) provides full-face contact and is used when mating with cast-iron or non-metallic equipment. A ring-type joint (RTJ) face carries a machined groove for a metal ring gasket, giving a robust seal for demanding pressure and temperature service.

Pressure Classes and Flange Standards

ASME classes such as 150, 300, 600, 900, 1500 and 2500 are rating designations rather than direct pressure figures. The allowable pressure depends on the material group and temperature, so a Class 300 flange does not simply mean 300 psi at every temperature. Alongside ASME B16.5 and B16.47, ASME B16.36 covers orifice flanges used with differential-pressure flow meters, and project specifications often add API requirements for oilfield and pipeline service.

A manufacturer that supports an international project range will typically offer flanges to ASME, EN, ISO, JIS and DIN alongside domestic standards such as GB/T, so that buyers can source exactly the code they are working to.

Common Flange Materials

The flange material should be compatible with the connected piping, the design code and the operating environment. The usual families are carbon steel, stainless steel, alloy steel and copper-nickel, and the right choice depends on corrosion, temperature, pressure, weldability and media composition.

Carbon steel flanges are the economical default for general process, oil and gas and water lines. Stainless steel flanges are chosen where corrosion resistance and cleanliness matter, and alloy steel flanges suit high-temperature and high-pressure boiler and power service. For seawater, marine and heat-exchanger duty, copper nickel flanges are widely selected because they resist seawater corrosion and biofouling, and the 90/10 and 70/30 Cu-Ni grades are the two most common families for this type of duty.

How to Choose the Right Flange Type

A practical way to choose a flange is to work from the outside in. Start with the governing piping code and flange standard, then confirm the design pressure and temperature to ｓｅｌｅｃｔ the pressure class from the correct material rating table. Next match the pipe size and schedule so the bore and weld preparation align, choose the material for the actual environment, and finally specify the facing and gasket system. It is also worth thinking about maintenance, because blind and lap-joint arrangements make isolation or dismantling easier, while permanent welded joints prioritise integrity.

Where Flanges Are Used in Industry

Flanges appear across oil and gas production, terminals and refineries, petrochemical plants, power generation and high-temperature steam systems, water treatment, marine and shipbuilding offshore work, and general industrial utility lines. For an EPC contractor or an equipment manufacturer, buying flanges together with the matching gaskets, stud bolts and nuts in one bundle simplifies procurement, keeps the sealing system consistent and shortens delivery time.

As a manufacturer and integrated supplier active since 1994, EZ Steel Industrial Co., Ltd. combines pipe and tube production with a full line of flanges, gaskets, stud bolts, nuts and fittings for project and industrial applications. With more than 500 employees, a stated annual production capacity above 480,000 units, and forming, CNC machining, welding and non-destructive testing capability across its plants, the company supports carbon steel, stainless steel, alloy steel and copper-nickel flange requirements and can bundle a complete flanged package for a single project. Standards such as ISO 9001, API 5L, API 5CT and PED underpin the quality system, with hydrostatic testing, ultrasonic testing, positive material identification and mill test certificates supplied as required.