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What is a pressure vessel flange

What is a pressure vessel flange?

Pressure vessel flange is a flange connected between cylinder and head, cylinder and simplification or head and tube sheet, with the same role as the pipe flange, similar in appearance, but not interchangeable because of the different definition of nominal diameter. Its role is to make different pressurized components combined together, while ensuring that the connection part does not produce leakage.

The form of pressure vessel flange sealing surface

The tightness of the flange connection is directly related to the form of the sealing surface of the flange compression gasket, commonly used flange sealing surface form the following three.

Flat Face (FF)

The gasket surface of the flat face flange is in the same plane as the round surface of the bolt. Applications using flat head flanges are usually those where the mating flanges or flange fittings are made of castings.
Flat face flanges must not be bolted to raised face flanges. According to the standard ASME B31.1, when the flat cast iron flange is connected with the carbon steel flange, the raised surface on the carbon steel flange must be removed, and a comprehensive gasket is required. This is to prevent the gap caused by the thin and small cast iron flange being bounced into the raised surface of the carbon steel flange.

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Male-and-Female (M&F)

With this type the flanges also must be matched. One flange face has an area that extends beyond the normal flange face (Male). The other flange or mating flange has a matching depression (Female) machined into it’s face.
The female face is 3/16-inch deep, the male face is1/4-inch high, and both are smooth finished. The outer diameter of the female face acts to locate and retain the gasket. In principle 2 versions are available; the Small M&F Flanges and the Large M&F Flanges. Custom male and female facings are commonly found on the Heat Exchanger shell to channel and cover flanges.

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Tongue-and-Groove (T&G)

The tongue surface and groove surface of the flange must match. A flange face has a raised ring (tongue), which is processed on the flange face, while the mating flange face has a corresponding groove, which is processed on its face.
There are two types of standards for tenon and groove surfaces. They differ from male and female types in that the inner diameter of the tenon does not extend to the underside of the flange, thus retaining the gasket on its inner and outer diameter. This joint usually appears on the pump cover and valve cover.
Another advantage of tenon joints is that they can be aligned automatically and serve as a reservoir of adhesive. The tenon connection keeps the load axis consistent with the connection, and no large processing operation is required.
General flange faces, such as RTJ, T and G and fandm, must not be bolted. The reason is that the contact surfaces do not match, and there is no washer with one type on one side and another type on the other.
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Types of pressure vessel flanges

Pressure vessel flange according to its structure is divided into type A flat welding flange, type B flat welding flange and long neck butt welding flange, of which type A, type B flat welding flange belong to any type of flange.
Type A flat welding flange is directly welded with the vessel, the size of the weld seam is small and cannot ensure that the flange and the vessel are stressed at the same time. Its sealing surface has two forms of flat sealing surface and concave-convex sealing surface, which is suitable for pressure vessels with smaller nominal diameter and smaller nominal pressure.
Type B flat welding flange itself has a thicker short section, so that the flange has a larger rigidity, while directly connected with the vessel cylinder is not the flange but the short section, which can save the cylinder from the additional bending moment brought about by the deformation of the flange under the action of internal pressure. The sealing surface form of type B flat welding flange has three types of flat type, concave and convex type and tongue and groove type, which is suitable for larger nominal diameter and larger nominal pressure of pressure vessel.

  • (1) Type B flange with a short cylinder, so the rigidity is better than type A flange, can be used for higher pressure, larger diameter occasions.
  • (2) The form of weld seam: type A is V-bevel, type B is U-bevel, so type B is easier to weld through, so its strength and rigidity is higher.

Long weld neck flange short section and the flange itself is a whole, eliminating the possibility of residual strain when welding, while the root thickness of the neck is larger, greatly increasing the overall strength and stiffness of the flange, its sealing surface form also has a flat type, concave and convex type and tongue and groove type three. Compared to type A, type B flat welding flange, long neck butt welding flange has the largest scope of application and can be used for medium and high pressure pressure vessels. Long-neck butt-weld flanges are more rigid and suitable for higher pressure due to their long neck and butt-welding.

Seal form of pressure vessel

  • 1. Forced sealing, by tightening the end cover and cylinder flange connecting bolts, the sealing surface will be pressed tightly to achieve the purpose of sealing.
  • 2. Self-tight seal. The pressure of medium in the vessel is used to generate pressure on the sealing surface so as to achieve the purpose of sealing. Its sealing force increases with the increase of medium pressure, so it can maintain reliable sealing performance even under higher pressure, such as combination seal, O-ring seal, C-ring seal, B-ring seal, wedge seal, octagonal gasket and oval gasket seal, flat gasket self-seal, wood seal, nitrogen seal, etc.
  • 3. Semi-self-tight seal, which not only uses the pressure of the medium in the container, but also uses the connection of fasteners to generate pressure on the sealing surface, so as to achieve the purpose of sealing. Such as double cone seal.

Sealing gasket of pressure vessel flange

The role of the sealing gasket is in the bolt pressure, filling the flange sealing surface of the unevenness to prevent the vessel media leakage. The sealing gaskets of pressure vessel flanges are non-metallic soft gaskets, wound gaskets and metal covered gaskets.
The material used for non-metallic soft gaskets is oil-resistant asbestos rubber sheet with thickness of 3mm (using temperature ≤ 200℃) and asbestos rubber sheet (using temperature ≤ 250℃). These gaskets can be used on the above three types of flanges, but are not applicable to the sealing surface of tongue and groove type.
Wrap-around gaskets are made of V- and W-shaped steel strips such as 0Cr13 or 0Cr18Ni9 or 08F and filled strips such as asbestos, PVC or flexible graphite wrapped around each other. To prevent loosening, the beginning and end of the metal band are welded together. The outer diameter of these gaskets is the same as non-metallic gaskets, but the width is slightly narrower and can only be used for type B flat-welded flanges and long-neck butt-welded flanges.
Metal clad gaskets are made of asbestos sheet as inner core and thin metal sheet with thickness of 0.2~0.5mm, and the metal material can be aluminum, copper and its alloy, stainless steel or high quality carbon steel. The gasket can also only be used in b-type flat welding flange and long-neck butt welding flange.

Factors affecting flange sealing

There are many factors affecting the flange seal, but in general, there are the influence of the flange connection component’s own condition and the influence of the media, temperature and other external conditions.

The bolt preload force

Proper preload can ensure that the gasket can retain a certain sealing pressure when working, too much preload will crush or extrude the gasket, thus destroying the seal. The distribution of preload on the gasket also affects the sealing performance. The way to ensure the uniform distribution of preload is to increase the number of bolts while meeting the space required for fastening and dismantling the bolts.

Sealing performance of gasket

Gasket is an important component of sealing. The suitable gasket material should reflect the gasket to produce the necessary elastic deformation under the appropriate preload, and not to be crushed or extruded. The distance between the flange sealing surface is stretched when working, and the gasket material should have enough rebound ability to make the gasket surface in close contact with the flange surface to continue to maintain good sealing performance.

Characteristics of flange sealing surface

The type and surface performance of the flange sealing surface play a vital role in the sealing effect. Concave and convex surface and tongue and groove surface should be used in the case of strict working conditions. The flatness of the flange sealing surface, the centerline and perpendicularity of the sealing surface and the flange directly affect the uniformity of the force on the gasket and the good contact between the gasket and the flange. The surface roughness of the sealing surface of the flange should be compatible with the requirements of the gasket, and the surface should not allow radial knife marks or scratches, and surface cracks are not allowed to exist.

Flange stiffness

Insufficient stiffness will cause the flange to produce excessive warpage deformation, resulting in sealing failure. Many factors affect the stiffness of the flange, including increasing the thickness of the flange, increase the outer diameter of the flange and other methods can improve the stiffness of the flange, reduce deformation, so that the bolt force is uniformly transferred to the gasket, to obtain a uniform and sufficient sealing pressure, while improving the seal. Reduce the force arm of the bolt force, can reduce the bending moment of the flange, conducive to sealing.

Operating conditions

Operating temperature, pressure and the chemical and physical properties of the medium also affect the reliability of the flange connection. Temperature increases, the medium viscosity becomes smaller, the tendency of leakage increases; high temperature makes the medium chemical and physical activity increases, producing corrosion and dissolution of the gasket and flange; high temperature can also make the cuttlebone, flange and gasket creep and stress relaxation.

The working principle of pressure vessel flange

When the preload force is applied to the flange bolts, the cuttlebone force compresses the gasket through the flange ring, forcing the flange gasket to produce compression deformation. When the cuttlebone force reaches a certain value, it forces the flange sealing surface and the four convex planes on the gasket to fill up with the deformation of the gasket, which creates the initial sealing conditions to stop the leakage of fluid medium. At this time, the compression force per unit effective sealing area of the gasket is called the initial sealing pressure of the gasket. When the equipment or pipeline is under medium pressure, the bolts are elongated by tensile stress, the flange sealing surface moves in the direction of separation from each other, the compression force between the sealing surface and the gasket decreases, the compression of the gasket decreases, and the preload sealing pressure decreases. If the gasket has enough resilience, the deformation of the gasket can compensate the separation value of the bolt and the sealing surface, and the preload sealing pressure only decreases to not less than the working sealing pressure, the flange still maintains a good sealing state. On the contrary, if the resilience of the gasket is not enough, the sealing pressure drops to below the working sealing pressure, and even the gap reappears at the sealing mouth, the seal fails.

How to connect butt welding pressure vessel flange and pipe?

Pressure vessel flange and pipe connection and butt welding welding method, pressure vessel flange welding and single-sided welding does not require welding pipe and flange connection in the mouth, butt welding flange installation requires flange double-sided welding. So pressure vessel flanges are generally used for low and medium pressure pipelines, for medium and high pressure pipeline butt welding flanges, flange butt welding is usually at least PN2.5Mpa, butt welding is used to reduce stress concentration.

How to choose the correct pressure vessel flange?

The correct choice of pressure vessel flange is very important, why so? In engineering applications, standard flanges are usually used (in addition to special working parameters and structural requirements of the flange), which can reduce the calculation of pressure vessel design, increase the interchangeability of flanges, reduce costs and improve manufacturing quality.
Selection of pressure vessel flanges need to determine four parameters.

Can not be directly selected, can be calculated using SW6. Tower pressure vessel flange in the selection must be calculated, because there are wind and earthquake load.
Pressure vessel flange selection steps are as follows:

  • 1. According to the nominal pressure level in the flange standard and the design pressure of the vessel, according to the principle that the design pressure is less than or equal to the nominal pressure, close to a nominal pressure in the vicinity; if the design pressure is very close to this nominal pressure, the design temperature is higher than 200, you can increase a nominal pressure level in the vicinity, so as to initially determine the nominal pressure of the flange.
  • 2. According to the nominal diameter of the flange, the design temperature of the vessel and the nominal pressure mentioned above, check the corresponding standard and initially determine the type of flange.
  • 3. Determine the form of sealing surface according to the characteristics of the working medium; flange materials are determined by the common flange materials specified in the medium characteristics, design temperature and vessel material control standards.
  • 4. According to the flange type, material, working temperature and initial nominal pressure, check the corresponding standards and get the maximum allowable working pressure.
  • 5. By comparison, if the maximum allowable working pressure obtained is greater than or equal to the design pressure, the original nominal pressure is the nominal pressure of the selected flange; if the maximum allowable working pressure is less than the design pressure, replace the high quality material or increase the nominal pressure level so that the maximum allowable working pressure is greater than or equal to the design pressure, and finally determine the nominal pressure and type of the flange.
  • 6. Determine the material of the gasket, stud and nut is based on the flange type and working temperature to check the relevant standards.
  • 7. The specific dimensions of the flange are determined by reference to the corresponding flange type, nominal diameter and nominal pressure standards.

Source: China Flanges Supplier:

(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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