A Comprehensive Guide to Flange

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What is a flange?

Brief definition: A flange is a mechanical component used to connect pipes, valves, pumps and other equipment, and is often used to form a sealed connection.

Function: connection, sealing, easy maintenance (detachable), suitable for high-pressure and high-temperature occasions.

Application areas: petroleum and natural gas, chemical industry, electric power, shipbuilding, water treatment, etc.

Common flange types and characteristics

Weld Neck Flange (WN)

A butt-weld flange is a flange with a long tapered neck, the end of which is butt-welded to the pipe. Its neck gradually transitions to the thickness of the pipe, so it has good mechanical properties and stress resistance. Butt-weld flanges are suitable for high-temperature and high-pressure environments and are often used in key piping systems in industries such as oil, gas, and chemicals. This type of flange generally has a raised face (RF) or a ring joint face (RTJ).

Features: With a long tapered neck, it is connected to the pipe by welding.
Application: Pipe systems with high pressure, high temperature, or severe vibration (such as the petrochemical industry, and power industry).
Advantages: Uniform stress distribution and strong fatigue resistance.

Slip-On Flange (SO)

The inner diameter of the flat welding flange is slightly larger than the outer diameter of the pipe. When installed, the flange is inserted into the end of the pipe and welded inside and outside. It has a simple structure and is easy to install. It is suitable for occasions with low pressure and less corrosiveness. Flat welding flanges are usually used in general industrial pipelines, and the joint surface can be flat (FF) or convex (RF).

Features: Double-sided fillet welding after the pipe is inserted into the inner hole of the flange.
Application: Low-pressure, normal temperature system (such as water treatment, low-pressure gas pipeline).
Advantages: Low cost and easy installation.

Threaded Flange

Threaded flanges are connected to pipes with external threads through internal threads, without the need for welding, and are therefore suitable for use in situations where welding is not appropriate (such as inflammable and explosive environments). They are often used in low-pressure, small-diameter piping systems. Threaded flanges are suitable for on-site assembly and disassembly, and are also suitable for systems that are sensitive to temperature fluctuations.

Features: The inner hole of the flange is threaded and connected to the pipe thread.
Application: Low-pressure small-diameter pipelines (such as instrument pipelines, and fire protection systems).
Advantages: No welding required, suitable for flammable and explosive environments.

Blind Flange

A blind flange (also called a ‘closure plate flange’) is installed at the end of a piping system to terminate a pipe. It has no center hole (bore), so there is no flow through the flange. A blind flange may be used for isolating a pipe, valve, or pressure vessel. This type of flange is available in all sizes and classes and may use a flat, raised, or ring-type joint face.

Features: No center hole, is used to block the opening of the pipeline or equipment.
Application: system isolation, pressure testing, or future expansion reservation.
Advantages: strong sealing and high pressure bearing capacity.

Lapped Joint Flange

The loose flange consists of two parts: a stub end and a loose flange ring. The flange ring can rotate freely on the stub end, which is convenient for centering and installation. Since the flange body is not welded to the pipe, it can be reused and is suitable for corrosive media (often used with lined pipes), which can save expensive material costs.

Features: The flange and the pipe are connected through a stub end.
Application: Systems that are frequently disassembled (such as food and pharmaceutical industries).
Advantages: Can be rotated for alignment, reducing installation stress.

Socket Weld Flange (SW)

There is a step on the inner side of the socket welding flange, which is used for inserting the pipe before corner welding. This structure is suitable for small-caliber, high-pressure systems, and has good sealing and strength after installation. Its welding part is not easy to generate eddy current or stress concentration, and is suitable for high-pressure but low-temperature occasions.

Features: The pipe is inserted into the flange socket and then welded on one side.
Application: Small-diameter high-pressure pipelines (such as hydraulic systems and chemical plants)
Advantages: High weld strength, suitable for thin-walled pipelines.

Flange Face Types

Different types of flange faces serve as the contact surface for the gasket material. ASME B16.5 and B16.47 define various types of flange faces, including raised faces, large raised faces, and concave faces, which have the same dimensions to provide a relatively large contact area.

Raised Face(RF)

The most widely utilized type of raised face flanges in process plant applications are easily recognized. Because the gasket surface is elevated above the bolt circle, they are known as raised faces. Different gasket design combinations, such as flat ring plate types and metal composites like spiral wound and double jacketed types, are made possible by this face type.

RF flanges are designed to increase the joint’s pressure-carrying capacity by applying greater pressure to a smaller gasket area. ASME B16.5 defines height and diameter according to pressure class. The height of the elevated face is determined by the flange’s pressure class.

ASME B16.5 RF flanges typically have a surface polish of 125 to 250 µ in Ra (3 to 6 µ m Ra).

Flat Face(FF)

The bolt circle surface and the gasket surface of a flat face flange are in the same plane. Flat face flanges are usually utilized in applications where the flange fitting or mating flange is cast.

Never fasten a flat face flange to a raised face flange. According to ASME B31.1, a full face gasket is needed and the raised face on the carbon steel flange must be removed when joining a flat face cast iron flange to it. This is to stop the thin, delicate cast iron flange from bouncing into the space made by the carbon steel flange’s raised face.

Ring Type Joint(RTJ)

For high-temperature service above 800°F (427°C) and/or high pressure (Class 600 and above), ring-type joint flanges are commonly utilized. Steel ring-shaped gaskets can be received in grooves on the flange surface. A metal-to-metal seal is created when the bolts are tightened, compressing the gasket between the flanges into the grooves and def orming (or coining) it to achieve close contact.

An annular groove may be machined into the raised face of RTJ flanges. The sealing device does not use this raised face in any way. The raised faces of the connecting and fastening flanges may come into touch with one another in RTJ flanges that are sealed with ring-shaped gaskets. In this scenario, vibration and movement cannot further crush the compression gasket and release the connection tension, nor will it be subjected to weights over the bolt tension.

Tongue and Groove(TG)

This flange’s tongue and groove faces need to line up. One flange face has a raised ring (tenon) carved into it, and the corresponding flange face has a matching depression (groove) drilled into it.

There are two varieties of tongue and groove faces: large and small. Both are standardized. They secure the gasket to its ID and OD because, unlike male and female flanges, the ID of the tongue and groove face does not extend into the flange base. These are frequently seen on valve bonnets and pump covers.

Another benefit of tongue and groove joints is that they serve as an adhesive reservoir and self-align. Miter joints don’t need extensive machining and maintain the load axis in line with the joint.

It is never advisable to bolt together general flange faces like RTJ, TandG, and FandM. The contact faces are mismatched, and there isn’t a gasket with one type on one side and another on the other.

Male and Female(FM)

The flanges of this type must also match. There is an area on one flange face that is larger than the typical flange face (convex face). A concave face, or matching depression, is machined into the surface of the opposite flange, also known as the mating flange.

Both the convex and concave faces are sanded and measure 1/4 inch high and 3/16 inch deep, respectively. The gasket is located and fastened using the concave face’s outer diameter. Theoretically, there are two types of M&F flanges: small and large. Heat exchanger shells frequently have convex and concave faces that are custom-made to cover and guide the flanges.

Flange material selection

Malzeme	Feature	Uygulama
Carbon steel	Low cost, good mechanical properties	Normal temperature and pressure water and steam system
Stainless steel	Strong corrosion resistance	Corrosive environments such as chemical, food, and medicine
Alloy steel	Excellent performance at high temperature and high pressure	High-intensity working conditions such as petrochemical and power plants
Copper, aluminum, titanium, Hastelloy, etc.	Special performance materials	Special media or light demand platform

Material selection suggestions:

Strongly corrosive media (such as acid and alkali): 316 or Hastelloy is recommended
High-temperature steam: Cr-Mo alloy steel is recommended
Drinking water or food pipelines: Food-grade stainless steel (such as 304L) is recommended

How to choose the right flange

Choosing the right flange requires comprehensive consideration of system parameters, economic costs, maintenance difficulty and other factors:

1. According to pressure and temperature

High-pressure and high-temperature systems should use butt-weld flanges or socket-weld flanges
Medium and low-pressure systems can choose flat-weld flanges or threaded flanges

2. According to the type of medium

Stainless steel or special alloy materials are used for highly corrosive media
Carbon steel can be used for ordinary water systems

3. According to installation method and maintainability

Loose flanges can be used in occasions where frequent disassembly and assembly are required
Butt-weld flanges are firmly welded and suitable for systems with long-term stable operation

4. According to international/regional standards

Export products must match the standards of the target market (such as ASME, DIN, and JIS)
Standards should be unified within the same project to avoid incompatible joints

5. Other factors