Types of flanges and flange connection

What are flanges?
Flanges, also translated as rims, meaning symmetrical disc-like structures used to connect pipes, containers or fixed-shaft mechanical parts, usually with bolts and threaded structures for fixing . Flanges are disc-shaped parts that are most common in plumbing and flanges are used in pairs.
In plumbing, flanges are primarily used for pipe connections. In the pipelines that need to be connected, a variety of flanges are installed, and the low-pressure pipelines can use wire-bonded flanges, and the welding flanges are used at pressures above 4 kg.
A sealing point is added between the two flanges and then fastened with bolts. Different pressure flanges have different thicknesses and use different bolts. Pumps and valves, when connected to the pipeline, part of these equipment, also made into the corresponding flange shape, also known as flange connection.
Connection parts that are closed at the same time by bolting on both planes are generally referred to as “flanges”. For example, the connection of ventilation ducts may be referred to as “flange parts”.
However, this connection is only a part of the equipment, such as the connection between the flange and the water pump, it is not good to call the pump “flange parts”. Smaller such as valves, etc., can be called “flange parts”.
What is a flange connection?
Flange connection is to fix two pipes, pipe fittings or equipment on a flange, and between the two flanges, with flange mats, bolted together to complete the connection. Some fittings and equipment have their own flanges and are also flanged.
Flange threaded connection (wire connection) flange and welded flange. Low-pressure small-diameter wire-bonded flanges, high-pressure and low-pressure large diameters are all welded flanges. The thickness of the flanges of different pressures and the diameter and number of connecting bolts are different.
Depending on the level of pressure, the flange pads are also available in different materials, from low pressure asbestos pads, high pressure asbestos pads to metal pads.
The flange connection is easy to use and can withstand large pressures.
Flange connections are widely used in industrial piping. In the home, the pipe diameter is small and low pressure, and the flange connection is not visible. If you are in a boiler room or production site, there are flanged pipes and equipment everywhere. Flange connection is to fix two pipes, pipe fittings or equipment on a flange, and between the two flanges, with flange mats, bolted together to complete the connection. Some fittings and equipment have their own flanges and are also flanged.
Pay attention to the fact that all bolts are tight when connecting, especially when the bolts are tightened.
Flange connection is a detachable part consisting of a pair of flanges, gaskets and bolts, nuts, washers and other parts. The main parameters of the standard flange are nominal diameter, nominal pressure and sealing surface type.
Flange implementation standards: GB series (national standard), JB series (mechanical department), HG series (Ministry of Chemical Engineering), ASME B16.5 (American standard), BS4504 (British standard), DIN (German standard), JIS (Japanese standard).
Flange’s international pipe flange standard system: There are two main systems in the international pipe flange standard, namely the European pipe flange system represented by German DIN (including the former Soviet Union) and the Americas represented by the American ANSI pipe flange. Pipe flange system.
In addition, there are Japanese JIS pipe flanges, but they are generally only used for public works in petrochemical plants, and have little impact internationally.
Classification of flanges: flat welded flanges, neck flanges, butt weld flanges, ring joint flanges, socket flanges, and blind plates.
Types of flanges

(1) Pressure vessel flanges are divided into general flanges and reverse flanges according to the overall structure.

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(2) According to the arrangement of the gasket, it is divided into two categories: narrow flange and wide flange.
a) Narrow flange is the flange with the gasket contact surface located within the circumference of the flange bolt hole. It is the most widely used flange.
b) Flange flange refers to the flange on which the gasket contact surface is distributed on both the inner and outer sides of the center circle of the flange bolt. Generally used only for low pressure applications.
(3) According to the integrity of each part of the flange, it is divided into three types: loose flange, integral flange and arbitrary flange. Its characteristics are:
a) Loose flange: Refers to the flange that the flange fails to effectively connect with the container or the joint. In the calculation, it is considered that the cylinder does not bear the flange torque together with the flange ring, and the flange torque is completely absorbed by the flange ring itself.

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b) Integral flange: refers to the flange, neck and cylinder can be effectively connected into a whole flange. The three parts share the role of the flange torque.

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The integral flange is a flange connection. It is also a type of neck welded steel pipe flange. Materials are carbon steel, stainless steel, alloy steel, etc. Among the various domestic standards, the IF is used to represent the overall flange. It is mostly used in pipes with higher pressure. The production process is generally cast. In the flange type, an “IF” is used to indicate the type of the integral flange. Generally, it is a facet (RF). If it is used in flammable, explosive, high and extreme hazard conditions, it can be used to seal the convex and concave surface (MFM) and the groove surface (TG) except the RF surface. The form of the face.
c) Arbitrary flange: Refers to a flange with an overall degree between the two. Although the cylinder and the flange ring fail to form a unitary structure, they can act as a structural element to jointly withstand the flange moment. Flat welded flanges are such flanges.

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(4) According to the connection between the flange and the cylinder, it is divided into a loop flange, a threaded flange, a socket weld flange, a flat weld flange and a butt weld flange.

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The loose flange of the welding ring is a movable flange. It is usually matched with the water supply and drainage fittings. When the factory leaves the factory, there is a flange at each end of the expansion joint, which is directly connected with the pipeline and equipment in the project.
Effect:
The purpose of using the butt flange to loosen the flange is generally to save material. The structure is divided into two parts. One end of the pipe is connected to the pipe and the other is made into a butt weld ring. The flanges are made of low-grade materials, and the tubes are made of the same material as the pipes to save material.
Advantage:
save costs. When the material of the pipe is special and expensive, the flange of the same material is costly.
It is inconvenient to weld or inconvenient to process or requires high strength. Such as plastic pipes, glass steel pipes and the like.
Easy to construct. If the flange bolt hole is connected when connecting, it is not easy to find or prevent the flange bolt hole of the equipment to be changed in the future.
Disadvantages:
Understand low pressure.
Low strength at the weld ring (especially when the thickness is less than 3mm)

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The flat weld ring loose flange is a movable flange. Connect directly to the pipes and equipment in the project.
The purpose of using a flat welding ring to loosen the flange is generally to save data. The structure is divided into two parts, one part of the tube is connected to the pipe, one end is made into a flange, and the flange part is placed on the flange.
The flanges use low-level data, and the tubes are used in the same way as the pipes to save data.
Advantage:
Convenient for welding or convenient for processing or high strength, such as plastic tubes, glass tubes and the like.
It is convenient for construction. For example, when the flange bolt hole is connected, it is convenient to correct or prevent the flange bolt hole of the equipment to be changed in the future.
When the price is high, the cost is saved. When the material of the pipe is special, the flange of the same material is costly.
Disadvantages:
Accepting low pressure.
Low strength at the weld ring (especially when the thickness is less than 3mm)

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Threaded flanges are made by machining the inner bore of the flange into a pipe thread and connecting it with a threaded pipe. It is a non-welded flange.
Advantage:
Compared with the flat welding flange or the butt welding flange, the threaded flange has the characteristics of convenient installation and maintenance, and can be used on some pipelines where welding is not allowed on the site. Alloy steel flanges have sufficient strength, but are not easy to weld, or the welding performance is not good, you can also choose a threaded flange.
Disadvantages:
It is recommended not to use a threaded flange to avoid leakage when the pipe temperature changes sharply or the temperature is higher than 260 °C and lower than -45 °C.
When can I use a threaded connection?
When the design pressure is less than or equal to 1.6 MPa and the design temperature is not more than 200 °C, the threaded connection can be adopted in the “welded steel pipe for low-pressure fluid transportation” GB / T 3091.

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The socket weld flange is a flange that is bolted at one end to the other end of the steel pipe.
Sealing surface form:
Projection surface (RF), concave and convex surface (MFM), groove surface (TG), ring joint surface (RJ)
Application range:
Boiler pressure vessels, petroleum, chemical, shipbuilding, pharmaceutical, metallurgy, machinery, stamping elbow food and other industries.
Commonly used in pipelines with PN≤10, 0MPa and DN≤40.

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The Slip-on Flanges belongs to the national standard flange standard system. It is one of the manifestations of the national standard flange (also known as GB flange), which is one of the commonly used flanges on equipment or pipelines.
Advantage:
It is convenient to install on site, and the process of welding seams can be omitted.
Disadvantages:
The neck welded flat flange has a low neck height and improves the rigidity and load carrying capacity of the flange. Compared with the butt welding flange, the welding work is large, the welding rod consumption is high, and the high temperature and high pressure and repeated bending and temperature fluctuation cannot be withstood.

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The sealing faces for Welding Neck Flanges are:
Projection (RF), Concave (FM), Convex (M), Cone (T), Groove (G), Full Plane (FF).
Advantage:
The connection is not easy to be deformed, the sealing effect is good, and the application is wide. It is suitable for pipes with high temperature or pressure fluctuation or pipes with high temperature, high pressure and low temperature, and also for conveying expensive medium, flammable and explosive medium and toxic gas.
Disadvantages:
The neck-welded flange is bulky, heavy, expensive, and difficult to install. Therefore, it is easier to bump during transportation.
Plate flat welding flange

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Plate type flat welding flange (chemical standard HG20592, national standard GB/T9119, mechanical JB/T81).
Advantages: easy to take, simple to manufacture, low cost, widely used
Disadvantages: Poor rigidity, so it should not be used in chemical process piping systems with high and extreme hazard requirements for supply, demand, flammability, explosive and high vacuum.
The sealing surface type has a flat surface and a convex surface.
What is the difference between a welded flange and a flat welded flange?
The butt weld flange is the same as the pipe to be welded at the joint end, and is welded like two pipes.
The flat welding flange is a concave table that is slightly larger than the outer diameter of the pipe at the interface, and the pipe is welded inside.
Butt welding has better welding performance and less corrosion.
Flat welding and butt welding are welding methods when flanges and pipes are connected. When welding flat welding flanges, only one side welding is required, and the inner joints of welded pipes and flanges are not required. The welding of the welded flanges requires flange double. Face welding.
Therefore, flat welding flanges are generally used for low and medium pressure pipes. Butt welding flanges are used for the connection of medium and high pressure pipes. Butt welding flanges are generally at least PN2.5 MPa. Butt welding is used to reduce stress concentration. The welding flange is mostly a neck flange and also a nipple flange. Therefore, the installation cost of the welding flange, labor costs and auxiliary materials costs are higher, because there is more than one process.
For the welding flange, not all of them need to be welded inside and outside. There is no special requirement. Generally, it is only welded outside. I have not seen the 1/2″ soldering flange:), the flat welding is better. Because the tube and flange are right and vertical, the tube will not be inclined.
What are the main principles for selecting a flange?
For pipes with a design temperature of 300 ° C and below and a nominal pressure of less than or equal to 2.5 MPa, flat weld flanges shall be used: for pipes with a design temperature greater than 300 ° C or a nominal pressure greater than or equal to 4.0 MPa, a butt weld flange shall be used.
Socket weld flange

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The socket weld flange is a flange that is bolted at one end to the other end of the steel pipe.
Sealing surface form:
Projection surface (RF), concave and convex surface (MFM), groove surface (TG), ring joint surface (RJ)
Application range:
Boiler pressure vessels, petroleum, chemical, shipbuilding, pharmaceutical, metallurgy, machinery, stamping elbow food and other industries.
Commonly used in pipelines with PN≤10, 0MPa and DN≤40.
Butt ring loose flange

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The loose flange of the welding ring is a movable flange. It is usually matched with the water supply and drainage fittings. When the factory leaves the factory, there is a flange at each end of the expansion joint, which is directly connected with the pipeline and equipment in the project.

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Function: The purpose of loosening the flange of the welding ring is generally to save material. The structure is divided into two parts. One end of the pipe is connected to the pipe and the other is made into a butt weld ring. The flanges are made of low-grade materials, and the tubes are made of the same material as the pipes to save material.
Advantages: cost saving and easy construction.
Disadvantages: low pressure. Low strength at the weld ring (especially when the thickness is less than 3mm)
Flat welding ring loose flange
The flat welding ring loose flange is a movable flange. It is usually matched on the water supply and drainage fittings (the most common on the expansion joint). When the factory leaves the factory, there is a flange at each end of the expansion joint, directly in the project. The pipes and equipment are bolted. The purpose of using a flat welding ring to loosen the flange is generally to save material. The structure is divided into two parts. One end of the pipe is connected to the pipe, one end is made into a flange, and the flange is partially placed on the flange. The flanges are made of low-grade materials, and the tubes are made of the same material as the pipes to save material.

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Flange cover
The flange cover is also called blind flange and blind plate. It is a flange without a hole in the middle for sealing the pipe plug. The effect is the same as that of the welded head and the threaded cap, except that the blind flange and the threaded cap can be removed at any time, while the welded head does not.

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Flange cover sealing surface: plane (FF), protruding surface (RF), concave and convex surface (MFM), grooved surface (TG), ring joint surface (RJ)
Lining flange cover
The lining flange cover is a blind flange that is welded to the side of the medium and is made of stainless steel. The lining flange cover is used as a blind plate on pipes with corrosive media. The difference from the ordinary flange cover is that the anti-corrosion lining is added to the contact surface of the medium.

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American standard neck welding flange
A necked flat weld flange is attached to the pipe end. Mainly the parts that connect the pipe to the pipe. The neck-welded flange has perforations on the flange, which can be bolted so that the two flanges are tightly connected and the flanges are sealed with gaskets.
The neck-welded flange joint refers to a pair of flanges, a gasket and a plurality of bolts and nuts.
The gasket is placed between the two flange sealing surfaces. After the nut is tightened, the specific pressure on the surface of the gasket reaches a certain value and then deforms, and fills the unevenness on the sealing surface to make the joint tight. A flanged joint is a detachable joint. According to the connected parts, it can be divided into container flange and pipe flange. The neck welded flat flange is suitable for steel pipe connections with a nominal pressure not exceeding 2.5 MPa.
The neck flat welding flange is used for the butt welding of the flange and the pipe. The structure is reasonable, the strength and rigidity are large, and it can withstand high temperature and high pressure, repeated bending and temperature fluctuation, and the sealing performance is reliable. The necked flat welding flange with a nominal pressure of 0.25 to 2.5 MPa has a concave-convex sealing surface.

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American standard with strong butt welding flange
The American standard flange is a part that connects the pipe to the pipe and is connected to the pipe end. American standard butt welding flanges are forged and cast in two ways.
According to the neck condition, the American standard butt welding flange can be divided into the American standard welding flange with neck and the American standard welding flange without neck.
The American standard butt welding flange is flanged by two flanges and bolted together to complete the connection. There are holes in the American standard flange, and the bolts make the two flanges tightly connected.

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Material
WCB (carbon steel), LCB (low temperature carbon steel), LC3 (3.5% nickel steel), WC5 (1.25% chromium 0.5% molybdenum steel), WC9 (2.25% chromium), C5 (5% chromium 0.5% molybdenum), C12 (9% chromium 1% molybdenum), CA6NM (4 (12% chrome steel), CA15 (4) (12% chromium), CF8M (316 stainless steel), CF8C (347 stainless steel), CF8 (304 stainless steel), CF3 (304L Stainless steel), CF3M (316L stainless steel), CN7M (alloy steel), M35-1 (Monel), N7M (Hast nickel alloy B), CW6M (Hasta nickel alloy C), CY40 (Inconel nickel alloy) Wait.
(5) The pressure vessel flange standard is divided into three types: flat type flat welding flange, flat type flat welding flange and long neck butt welding flange.

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Flange connection design
Flange design refers to the design of the flange connection, including three parts: gasket design, bolt (stud) design and flange body design. The most important method of flange design is the famous Waters method, which is used in GB 150. The design points are:
(1) gasket design
This is the basis of the flange connection design. According to the design conditions and the medium used, the appropriate gasket type and material should be selected to determine the size of the gasket (inner diameter, outer diameter, thickness), and then the gasket is pre-tightened. And the pressing force in the operating state.
(2) Bolt/stud design
According to the design conditions, bolts of appropriate materials are selected to calculate the bolt area required to satisfy the pre-tightening state of the gasket and the pressing force of the operating state. The actual bolt area should not be less than the calculated area.
The principle of bolt design is to determine the smaller center diameter of the bolt. This can be achieved by selecting the appropriate bolt size and quantity.
(3) Flange body design
The flange design is divided into two cases: internal pressure and external pressure. The flange subjected to external pressure can be designed according to the flange calculation method that bears the internal pressure, except that the operating torque of the flange is slightly different.
The calculation method for narrow-face flanges is divided into two types, namely, loose flange calculation and integral flange calculation. Arbitrary flanges are usually calculated as integral flanges and can be simplified to loose flange calculations under certain conditions.
The loose flange calculation is relatively simple, and the thickness of the flange can be calculated at one time. The design of the integral flange is done by a method of assuming repeated calculations of the structural dimensions of each part.
For the integral flange, first assume the dimensions of the flange cone and flange ring according to the structural conditions of the equipment. Then calculate the moment the flange is subjected to and the various stresses generated. When the difference between the stress and the corresponding allowable stress is large, the original flange size should be adjusted, and the above calculation should be repeated until the stresses are less than the respective allowable stresses and the difference is not reasonable.
The calculation of the wide-faced flange is not divided into forms, and is calculated according to the simplified model of the “simple beam”.

What are the causes of flange leakage?
Flange leaks are more common for the following seven reasons.
Partial mouth
The partial mouth means that the pipe and the flange are not perpendicular and different, and the flange faces are not parallel. Flange leakage occurs when the internal medium pressure exceeds the load pressure of the gasket. This situation is mainly caused during the installation or maintenance process and is easier to find. As long as the actual inspection is completed at the completion of the project, this kind of accident can be avoided.
Misplaced
The wrong mouth means that the pipe and the flange are vertical, but the two flanges are not the same. The flanges are not concentric, causing the surrounding bolts to not penetrate freely into the bolt holes. In the absence of other means, only the reaming or the use of a small bolt into the bolt hole, and this method will reduce the tension of the two flanges. Moreover, the sealing surface of the sealing surface is also deviated, so that leakage is very likely to occur.
Open mouth
Open mouth, means the flange gap is too large. When the gap of the flange is too large and causes an external load, such as an axial or bending load, the gasket will be shocked or vibrated, and the pressing force will be lost, thereby gradually losing the sealing kinetic energy and causing failure.
Wrong hole
The wrong hole means that the pipe is concentric with the flange, but the distance between the bolt holes of the two flanges is relatively large. The wrong hole will cause stress on the bolt, and the force will not be eliminated, which will cause shearing force on the bolt. When the time is long, the bolt will be cut off, resulting in failure of the seal.
Stress influence
When the flange is installed, the two flanges are relatively standardized. However, in the system production, after the pipeline enters the medium, the temperature of the pipeline changes, causing the pipeline to expand or deform, so that the flange is subjected to bending load or shearing force. Causes gasket failure.
Corrosion effect
The gasket chemically changes due to the corrosion of the gasket to the gasket for a long time. The corrosive medium penetrates into the gasket, and the gasket begins to soften, losing the pressing force and causing the flange to leak.
Thermal expansion and contraction
Due to the thermal expansion and contraction of the fluid medium, the bolt expands or contracts, so that the gasket creates a gap and the medium leaks through the pressure.

Source: China Pipe Flange Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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References:

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