12 Flange Types, 13 Common Issues: The Most Comprehensive Flange Basics

This article introduces 12 types of flanges and 13 common problems in flange applications. This should be the most comprehensive introduction to the basics of flanges, and hopefully it will help you better understand flanges.

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Flange Basic Introduction

Flanges, gaskets, and fasteners are collectively referred to as flange joints.

Application of flanges

Flange joints are a widely used component in engineering design. They are essential parts for piping design, pipe fittings, and valves, and are also indispensable components in equipment and equipment parts.

In addition, flange joints are frequently used in other disciplines such as industrial furnaces, thermal engineering, water supply and drainage, heating and ventilation, and automatic control.

Flange Materials

Forged steel, WCB carbon steel, stainless steel, 316L, 316, 304L, 304, 321, chrome-molybdenum steel, chrome-molybdenum-vanadium steel, molybdenum-titanium steel, rubber-lined, fluoropolymer-lined.

Flange Classifications

Slip-on flanges, neck flanges, butt-welding flanges, ring flanges, socket flanges, and blind flanges, etc.

Flange Standards

GB series (national standards), JB series (Ministry of Machinery Industry), HG series (Ministry of Chemical Industry), ASME B16.5 (American Standard), BS4504 (British Standard), DIN (German Standard), JIS (Japanese Standard).

International Flange Standard Systems

Internationally, there are two main flange standard systems: the European flange system represented by German DIN and the American flange system represented by American ANSI.

In addition, there are Japanese JIS flanges, but they are generally only used in public works in petrochemical plants and have little international influence.

12 types of flanges and sealing surface forms

Plate-type Slip-on Flange

Plate-type slip-on flanges (Chemical Standard HG20592, National Standard GB/T9119, Mechanical Standard JB/T81).

Advantages: Easy to source materials, simple to manufacture, low cost, widely used.

Disadvantages: Poor rigidity, therefore it cannot be used in chemical process piping systems with supply and demand requirements, flammable, explosive, or high vacuum requirements, or in highly hazardous or extremely hazardous environments.

Sealing surface types include flat and raised face.

Neck-type Slip-on Flange

Neck-type slip-on flanges belong to the national standard flange system. It is one of the forms of national standard flanges (also known as GB flanges) and is one of the commonly used flanges on equipment or pipelines.

Advantages: Convenient on-site installation, eliminating the need for weld seam patting and polishing.

Disadvantages: The neck height of the neck-type slip-on flange is lower, which increases the flange’s rigidity and load-bearing capacity. Compared with weld neck flanges, the welding workload is larger, welding rod consumption is higher, and it cannot withstand high temperature and pressure, repeated bending, and temperature fluctuations.

Weld Neck Flanges

The sealing face forms of weld neck flanges include: Raised Face (RF), Recessed Face (FM), Raised Face (M), Tongue Face (T), Groove Face (G), and Flat Face (FF).

Advantages: The connection is not easily deformed, the sealing effect is good, and it has a wide range of applications. It is suitable for pipelines with large temperature or pressure fluctuations, or for high-temperature, high-pressure, and low-temperature pipelines. It is also used for pipelines transporting expensive media, flammable and explosive media, and toxic gases.

Disadvantages: Weld neck flanges are bulky, heavy, expensive, and difficult to install and position. Therefore, they are more susceptible to damage during transportation.

Integral Flanges

Integral flanges are a type of flange connection. They are also a type of weld neck steel flange. Materials include carbon steel, stainless steel, and alloy steel.

In various domestic standards, integral flanges are represented by “IF”. They are mostly used in high-pressure pipelines. The manufacturing process is generally casting.

In flange type classifications, “IF” is used to indicate the type of integral flange.

Generally, raised face (RF) is used. However, in flammable, explosive, or highly hazardous operating conditions, sealing surfaces such as raised face (MFM) and tongue and groove face (TG) can be selected in addition to RF.

Socket Welding Flange

A socket welding flange is a flange where one end is welded to a steel pipe and the other end is bolted.

Sealing Face Types: Raised Face (RF), Male Face (MFM), Tongue and Groove (TG), Ring Joint (RJ)

Applications: Boilers, pressure vessels, petroleum, chemical, shipbuilding, pharmaceutical, metallurgy, machinery, stamped elbows, food processing, etc.

Commonly used in pipelines with PN≤10.0MPa and DN≤40.

Threaded Flange

A threaded flange has its inner bore machined with pipe threads and is used to connect to a threaded pipe. It is a non-welded flange.

Advantages: Compared to flat-face or weld neck flanges, threaded flanges are easier to install and maintain, and can be used on pipelines where welding is not permitted on-site. Alloy steel flanges have sufficient strength but are difficult to weld or have poor weldability; threaded flanges can also be chosen in these situations.

Disadvantages: Threaded flanges are not recommended for use in pipelines with rapid temperature changes or temperatures above 260℃ or below -45℃ to avoid leakage.

Weld-on-Ring Loose Flange

A weld-on-ring loose flange is a movable flange, typically used with water supply and drainage fittings. Manufacturers usually ship expansion joints with one flange at each end, directly bolted to the pipes and equipment in the project.

Purpose: The purpose of using weld-on-ring loose flanges is generally to save materials. Its structure consists of two parts: one end connects to the pipe, and the other end is formed by a weld-on ring. The flange uses a lower-grade material, while the pipe part uses the same material as the pipe, achieving material savings.

Advantages:

Cost savings. When the pipe material is special and expensive, welding a flange of the same material is costly.

Difficult to weld, difficult to process, or requires high strength. Examples include plastic pipes and fiberglass pipes.

Easy to install. For example, when the flange bolt holes are difficult to align during connection, or to prevent changes in the flange bolt holes when replacing equipment later.

Disadvantages:

Low pressure resistance.

Low strength at the weld ring.

Slip-on Welded Ring Loose Flange

A slip-on welded ring loose flange is a movable flange plate. It is directly bolted to pipes and equipment in the project.

The purpose of using slip-on welded ring loose flanges is generally to save materials. Its structure is divided into two parts: one end of the pipe section connects to the pipeline, and the other end is made into a flange. The flange part fits over the flange.

The flange uses a lower grade material, while the pipe section uses the same material as the pipeline, achieving the purpose of saving materials.

Advantages: Easy to weld, easy to process, or requires high strength, such as plastic pipes, fiberglass pipes, etc.

Easy to construct, such as easy alignment of flange bolt holes during connection, or preventing changes in flange bolt holes when replacing equipment later.

Saving costs when the price is high. When the pipe material is special, welding a flange of the same material is costly.

Disadvantages: Low pressure resistance.

Low strength at the weld ring (especially when the thickness is less than 3mm)

Flange Cover

Also called blind flange or blind plate. A blind flange without a central hole is used to seal pipe ends.

Its function is the same as welded heads and threaded caps, except that blind flanges and threaded caps can be removed at any time, while welded heads cannot.

There are various types of sealing faces, including flat face, raised face, male and female face, tongue and groove, and ring face. Production is mainly concentrated in the Mengcun area.

Flange cover sealing faces:
Flat face (FF), Raised face (RF), Male and female face (MFM), Tongue and groove (TG), Ring face (RJ)

Lined Flange Cover

A lined flange cover is a type of blind flange with stainless steel overlay welded onto the side closest to the medium, forming a single unit. Lined flange covers are used as blind flanges on pipelines carrying corrosive media. The difference from a regular flange cover is the addition of a corrosion-resistant lining on the surface in contact with the medium.

ASME Neck Welded Strap Flange

A neck welded strap flange is connected to the pipe end. It is mainly used to connect pipes to each other. Slip-on-neck flanges have holes for bolts to pass through, allowing two flanges to be tightly connected. A gasket seals between the flanges.

A slip-on-neck flange connection consists of a pair of flanges, a gasket, and several bolts and nuts.

The gasket is placed between the sealing surfaces of the two flanges. After tightening the nuts, the specific pressure on the gasket surface reaches a certain value, causing deformation and filling the unevenness of the sealing surface, ensuring a tight, leak-proof connection. Flange connections are detachable connections.

Based on the components being connected, they can be divided into vessel flanges and standard flanges. Slip-on-neck flanges are suitable for connecting steel pipes with a nominal pressure not exceeding 2.5 MPa.

Slip-on-neck flanges are used for butt welding flanges to pipes. They have a reasonable structure, high strength and rigidity, and can withstand high temperature and pressure, repeated bending, and temperature fluctuations, providing reliable sealing. Slip-on-neck flanges with a nominal pressure of 0.25–2.5 MPa use a raised face sealing surface.

ASME (American Standard) Slip-on-Neck Flange

ASME flanges are parts used to connect pipes to each other, attached to the pipe ends. ASME (American Standard) welding flanges are manufactured using two methods: forging and casting.

ASME welding flanges can be further divided into necked and neckless types, depending on whether they have a neck.

An ASME welding flange consists of two flanges with a gasket, fastened together with bolts. The flange has perforations for the bolts to secure the two flanges together.

A gasket is used for sealing between the flanges.

Flange Basics Q&A

Q: What is a slip-on flange, and where and in what pipe fittings is it used?

A slip-on flange is a movable flange plate, typically used with water supply and drainage fittings (commonly found on expansion joints). Expansion joints usually come with a flange at each end, directly bolted to the pipes and equipment in the project.

Q: How is a flange connected to the pipe? Where is it welded?

There are many types of flanges. Commonly used types include slip flanges, welding flanges (long neck flanges), and slip-on flanges.

Slip-on flanges are used for medium and low pressure applications, using insert welding, with welding on both sides. The weld depth is 2-5mm from the flange sealing surface. Welding flanges are directly welded, while slip-on flanges are bolted without welding.

Q: What does “flange type” mean?

A flange is a connecting piece used to connect valves to pipes. The connection between valves and pipes or machinery/equipment uses a flange connection; this connection method is commonly called a flange type.

Q: What is a flange?

A flange is a disc-shaped part commonly found in piping engineering. Flanges are always used in pairs. Any connecting part that uses bolts to connect and seal two flat surfaces around their perimeter is generally called a “flange”.

Q: What kind of connection is a flange?

A flange connection involves first fixing two pipes, fittings, or equipment to separate flanges, then placing a gasket between the two flanges and tightening them together with bolts to complete the connection.

Flanges are divided into threaded flanges and welded flanges. Low-pressure, small-diameter flanges are used, while high-pressure and low-pressure, large-diameter flanges are used. The thickness of the flange and the diameter and number of connecting bolts differ depending on the pressure rating.

Q: What is the purpose of a flange?

A flange is used for positioning.

Q: What are the differences between the connection methods of orifice flanges and ordinary flanges?

An orifice flange connects the orifice plate to the pipeline on an orifice flow meter. Two flanges fix the orifice plate in the middle, and pressure is measured through pressure taps on the flanges to a differential pressure transmitter.

Q: To ensure that the installed flange does not leak oil, what issues should be considered during installation?

The flange should have sufficient strength and should not deform during tightening. The flange sealing surface should be flat and clean; oil and rust should be carefully removed during installation.

The gasket should have good oil resistance and anti-aging properties, as well as good elasticity and mechanical strength.

The gasket with different cross-sections and sizes should be selected according to the shape of the connection and placed correctly. The flange tightening force should be uniform, and the gasket compression should be controlled at about 1/3.

Q: What are the differences between butt-welded flanges and slip-on flanges?

Use a table to illustrate.

Q: What are the differences between flanges and valves?

Use a table to illustrate.

Q: What are the main principles for selecting flanges?

For pipelines with a design temperature of 300℃ or below and a nominal pressure less than or equal to 2.5MPa, slip-on flanges should be used; for pipelines with a design temperature greater than 300℃ or a nominal pressure greater than or equal to 4.0MPa, butt-welded flanges should be used.

Q: Under what circumstances can threaded connections be used?

When the design pressure is less than or equal to 1.6MPa and the design temperature is not greater than 200℃, threaded connections can be used according to GB/T 3091 “Welded Steel Pipes for Low-Pressure Fluid Transportation”.

What are the causes of flange leakage?

Flange leaks are commonly caused by the following seven reasons:

Misalignment: The pipe and flange are not perpendicular or concentric, and the flange faces are not parallel. Leakage occurs when the internal medium pressure exceeds the gasket’s load capacity.

Misalignment: The pipe and flange are perpendicular, but the two flanges are not concentric. This misalignment prevents bolts from freely entering the bolt holes. In the absence of other solutions, the holes must be enlarged or smaller bolts used, which reduces the tightening force of the flanges. Furthermore, the sealing surfaces are misaligned, making leaks highly likely.

Excessive Flange Clearance: An excessively large flange clearance causes external loads, such as axial or bending loads, which impact or vibrate the gasket, causing it to lose its clamping force and gradually lose its sealing energy, leading to failure.

Misaligned Bolt Holes: The pipe and flange are concentric, but the distance between the bolt holes on the two flanges deviates significantly. Misaligned holes can cause stress on the bolts. If this stress is not relieved, it will exert shear force on the bolts, eventually cutting them off and causing seal failure.

Stress Effects: During flange installation, the connection between the two flanges is generally standardized. However, during system production, the introduction of media into the pipeline causes temperature changes, leading to expansion or deformation of the pipeline. This subjects the flanges to bending loads or shear forces, easily causing gasket failure.

Corrosion Effects: Prolonged erosion of the gasket by corrosive media causes chemical changes. The corrosive media penetrates the gasket, causing it to soften and lose its clamping force, resulting in flange leakage.

Thermal Expansion and Contraction: Due to the thermal expansion and contraction of the fluid medium, the bolts expand or contract, creating gaps in the gasket and allowing the medium to leak under pressure.