Main factors affecting sealing performance of flange

Bolt preload

It is an important factor that affects the sealing. The preload must compress the gasket to achieve the initial seal. The sealing capacity of the gasket can be increased by properly increasing the bolt preload, because increasing the preload can make the gasket retain a larger specific pressure of the contact surface under normal working conditions.

But the preload should not be too large, otherwise the gasket will yield and lose its resilience, and even extrude or crush the gasket. In addition, the preload should act on the gasket as evenly as possible. In order to improve the sealing performance, measures such as reducing the diameter of bolts, increasing the number of bolts and adopting appropriate pre tightening methods are usually taken.

Gasket performance

Gasket is an important component of sealing, and its main types are shown in the figure below.
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The function of the gasket is to seal the gap between the two flange sealing surfaces to prevent fluid leakage. The types of gasket are nonmetal gasket, nonmetal and metal combined gasket and metal gasket.
The proper gasket material requires that the gasket can produce the necessary elastic deformation under the action of proper preload without being crushed or extruded; When working, the distance between the flange sealing surface is enlarged, and the gasket material should have enough resilience to make the gasket surface in close contact with the flange surface, so as to maintain good sealing performance.
The working medium and temperature should also be considered when choosing gasket materials. The width of the gasket is also an important factor affecting the sealing. The wider the gasket is, the greater the preload is required, and the larger the size of the bolt and flange is required.
① Rubber, asbestos rubber, polytetrafluoroethylene and other non-metallic gaskets are commonly used on medium and low pressure equipment and pipeline flange. They have good corrosion resistance and softness, but poor strength and temperature resistance. They are usually cut from the whole gasket plate. The shape of the whole gasket is a ring and the section is rectangular.
② In order to improve the strength and heat resistance of the gasket, the spiral wound gasket is made of thin steel strip and asbestos tape (or polytetrafluoroethylene tape or flexible graphite tape), or the metal wrapped gasket is made of metal sheet wrapped with asbestos or other non-metallic materials, which has multi-channel sealing effect and good resilience. It can be used in higher temperature and pressure range, and can be used in the pressure, pressure and other fields It is widely used to keep good seal under the condition of temperature fluctuation.
Spiral wound gasket is made of steel strip and asbestos or PTFE or flexible graphite filled tape. In order to prevent looseness, the beginning and end of the metal strip are welded. In order to increase the elasticity and resilience of the gasket, the metal strip and non-metal strip are rolled into waveform. There are V-shaped and W-shaped waves. As shown in the figure below, it is V-shaped and has four structural types.
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  • Type A — also known as basic type, without reinforcing ring, used for mortise sealing surface.
  • Type B – with inner reinforcing ring, used for concave convex sealing surface.
  • Type C – with external strong ring, used for plane sealing surface.
  • Type D – both inner and outer reinforcing rings are used for plane sealing surface.

③ The metal clad gasket is composed of asbestos rubber sheet as the inner core and 0.2-0.5mm thick thin metal sheet as the outer cover (as shown in the figure below),
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The material of metal plate can be aluminum, steel and its alloy, or stainless steel or high-quality carbon steel. The metal clad gasket is also only used for type B flat welding and long neck butt welding.
④ In high pressure equipment and pipeline flange, commonly used metal gasket, materials are soft aluminum, copper, mild steel and stainless steel. In addition to the metal gasket with rectangular section, there are also metal ring gaskets with oval or octagonal section and other special shapes.
When the operating pressure is very high or the leakage rate is very strict, and the temperature is very high or the corrosion is very strong, the metal gasket can be used. The specific pressure value of the metal gasket is very large. In order to reduce the bolt force, the pressing surface must be very narrow. To maintain a good seal by relying on the very narrow pressing surface, it must have a small surface roughness, RA ≤ 2.5 μ M to RA ≤ 0.63 μm.

Types of sealing surface

The contact surface between the flanges is called flange sealing surface or pressing surface. The selection of sealing surface type is related to operating conditions, leakage consequences and gasket properties. The most common structural types are as follows.
① Plane sealing surface: its structure is shown in figure (a) below. The sealing surface is not a smooth plane, and there are 2 ~ 4 concentric circular triangular cross-section grooves (i.e. flange water line) on the plane.
The plane sealing surface has the advantages of simple structure, convenient manufacture and anti-corrosion lining. Secondly, the sealing surface of this structure is wide, so non-metallic or metal soft gasket is often used in use. However, after the bolt is tightened, the gasket material is easy to extend to both sides. It is used in the occasion where the pressing force is not high and the medium is non-toxic.
② Concave convex sealing surface: the structure of the sealing surface is shown in (b) below, which is equivalent to that on a pair of flat sealing surface flanges, one of which is made into a pressing surface with a raised platform, and the flange is called a convex flange, the other is called a concave flange, and the gasket with exactly the same size as the concave is embedded in it, so that the gasket is easy to center.
The height of the convex plane is slightly greater than the depth of the concave surface, and the sealing effect is achieved by pressing with bolts.
This structure can limit the radial deformation of the gasket, prevent the gasket from being extruded, and improve the sealing performance to a certain extent. It is suitable for high pressure occasions.
③ Mortise and groove sealing surface: in the middle of the width direction of a pair of plane sealing surfaces, one of them is made into a section such as mortise and the other is matched with the pressing surface of groove, as shown in the following figure (c), the former is called tenon face flange, and the latter is called groove face flange.
The groove type pressing surface can limit the radial deformation of the embedded gasket, which has good sealing performance, and the gasket can be less eroded and corroded by the medium. But the tenon surface is easy to be damaged. Commonly used in flammable, explosive, toxic media and high pressure occasions.
In addition, there are trapezoidal groove sealing surface and conical sealing surface. The former uses annular metal gasket with oval section, and the latter uses lens type annular metal gasket. The above two structures are forced sealing, which are commonly used in high-pressure pipelines.
The form and surface performance of flange sealing surface play an important role in the sealing effect. The flatness of flange sealing surface and the perpendicularity of sealing surface and flange center line directly affect the stress uniformity of gasket and the good contact between gasket and flange.
The roughness of flange sealing surface shall match the requirements of gasket. The surface is not allowed to have radial knife marks or scratches, let alone surface cracks.
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Flange stiffness

The excessive warpage caused by insufficient flange stiffness (as shown in the figure below) is often one of the main reasons for the failure of bolted flange connection seal in actual production. The flange with large stiffness has small deformation, which can transfer the bolt preload to the gasket evenly, so as to improve the sealing performance of the flange.
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The flange stiffness is related to many factors, among which the flange stiffness can be improved by appropriately increasing the thickness of the flange ring, reducing the diameter of the bolt center circle and increasing the outer diameter of the flange ring. The bending capacity of the flange can be significantly improved by using the neck flange or increasing the size of the cone neck part. But if the rigidity of flange is increased without principle, the flange will become bulky and the cost will be increased.

Operating conditions

The influence of pressure, temperature and physical and chemical properties of medium on the sealing performance is very complex. The influence of simple pressure and medium on the sealing performance is not significant, but under the combined action of temperature, especially under fluctuating high temperature, the sealing performance will be seriously affected, and even the seal will be completely invalid due to fatigue.

Because at high temperature, medium viscosity is small, permeability is large, easy to leak; The corrosive effect of medium on gasket and flange increases the possibility of leakage; flange, bolt and gasket will produce high temperature creep and stress relaxation, which will lead to seal failure; Some non-metallic gaskets will accelerate aging, deterioration, and even burn.

Source: China Flanges Manufacturer – Yaang Pipe Industry (www.steeljrv.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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