Analysis on sealing principle of flange
The flange connection structure is an assembly, which is composed of a pair of flanges, several bolts, nuts and a gasket. Flange connection has good strength and tightness, and has a wide range of application. It is widely used in the connection between mechanical equipment and industrial pipelines. The flange seal failure caused by the strength damage of the connector or the connected parts is very rare, and most of it is leakage due to poor sealing. Moreover, with the longer production cycle requirements and the more stringent process operation conditions, the flange seal leakage problem becomes more and more obvious. It is common that the equipment is shut down and the pipeline cannot be used due to flange leakage, resulting in economic losses to enterprises, inconvenience to people’s life and waste of resources. In order to effectively solve the problem of flange seal leakage, this paper summarizes the relevant problems.
Sealing principle of flange
Table of Contents
The flange compresses the gasket between the sealing surfaces under the action of bolt preload. The pressure (compression stress) applied to the unit area must reach a certain value to make the gasket deformed and compacted, and the micro gap formed by machining on the sealing surface is filled to form the initial sealing condition. The required compression stress is called gasket sealing specific pressure, which is expressed in Y, and the unit is MPa. The sealing specific pressure mainly depends on the gasket material. When the equipment or pipeline is in the working state, the axial force formed by the internal pressure of the medium causes the bolt to be stretched, and the flange sealing surface moves in the direction of separation from each other, reducing the compression stress between the sealing surface and the gasket. If the gasket has sufficient rebound capacity, so that the recovery of compression deformation can compensate the deformation of bolt and sealing surface, and the specific pressure of pre tightened seal is reduced to at least a certain value (this specific pressure value is called working seal specific pressure), a good sealing state can be maintained between flange sealing surfaces; Otherwise, the seal will fail. Therefore, in order to realize the sealing at the flange connection, the deformation of each part of the sealing assembly must adapt to the sealing conditions under the operating conditions, and even if the sealing element is under the action of the operating pressure, it still maintains a certain residual pressing force. Therefore, the bolt and flange must have enough strength and stiffness to prevent excessive deformation under the axial force formed by internal pressure.
It can be seen that the sealing performance of flange is closely related to the selection of flange and gasket forms and bolt preload.
The common sealing surface types of medium and low pressure flanges include plane, concave convex surface and tenon groove surface.
(a) Flat flange (b) concave convex flange (c) tenon face flange
Figure 2-1 sealing surface type of medium and low pressure flange
The sealing surface of this kind of flange is a smooth plane, and sometimes two concentric grooves (commonly known as waterline) with triangular interface are machined on the sealing surface. As shown in Figure 2-1 (a).
Gaskets suitable for flat flange include various non-metallic flat gaskets, cladding gaskets, metal cladding gaskets and spiral wound gaskets (with inner and outer rings). Because of its simple structure, convenient processing and convenient construction of anti-corrosion lining, it can be used when the nominal pressure P < 2.45mpa. However, the contact area between the sealing surface and the gasket is large (especially the wide face flange for pipeline), and the required pressing force is large, so the gasket should not be positioned during installation. After pre tightening, the gasket is easy to extend or move to both sides. Therefore, the gasket with small friction coefficient such as polytetrafluoroethylene should not be used. In addition, if the spiral wound gasket is used, in order to reuse the gasket, the triangular groove is not machined on the sealing surface.
Concave convex flange
The flange sealing surface is composed of a concave surface and a convex surface, and the gasket is placed in the concave surface, as shown in Figure 2-1 (b). The applicable gaskets are: various metal and non-metal flat gaskets, coated gaskets, metal wrapped gaskets, wound gaskets (basic type or with inner ring), metal corrugated gaskets and metal toothed gaskets.
Compared with the flat flange, the gasket in the concave convex flange is not easy to be extruded, which is convenient for alignment during assembly. The working pressure range is wider than that of the flat flange, which is used in occasions with strict sealing requirements. However, for equipment with high operating temperature and large sealing diameter, the gasket may still be extruded when using this sealing surface.
Tenon face flange
The flange sealing surface is composed of a tenon groove surface and a groove surface, and the gasket is placed in the groove, as shown in Fig. 2-1 (c). Applicable gaskets include: metal and non-metal flat gasket, metal clad gasket, wound gasket (basic type), etc. Like the concave convex flange, the tenon groove flange will not be extruded in the groove, and the compression area is the smallest, which is only 52 ~ 68% of the plane flange and concave convex flange, and the gasket is stressed evenly. Since the gasket is not in direct contact with the medium, the influence of medium corrosion and the penetration of pressure mechanism is the smallest. It can be used in occasions with strict sealing requirements for high-pressure, flammable, explosive and toxic media. The sealing surface gasket is well aligned during installation, and it is difficult to process the sealing surface and replace the gasket.
Flanges with other sealing surfaces
In addition to the above three types of sealing surfaces, there are also special sealing surfaces with trapezoidal groove sealing surface, O-ring and lens gasket.
Figure 2-2 sealing surface type of other flanges
Figure 2-2 (a) shows the sealing surface form of rubber O-ring and metal hollow O-ring.
Figure 2-2 (b) shows the sealing surface of trapezoidal groove, which can be equipped with metal octagonal pad and elliptical pad.
Figure 2-3 (c) shows the sealing structure of lens gasket, which is used for the connection of high-pressure pipeline.
The type and material of gasket shall be selected according to the fluid, service conditions (pressure and temperature) and the sealing requirements of flange joint. The type and surface roughness of flange sealing surface shall adapt to the type and material of gasket.
Asbestos rubber gasket
Asbestos rubber sheet is made of asbestos fiber, rubber and other filling materials by mixing, heating and rolling. It can be divided into high, medium, low pressure, oil resistance, acid resistance, alkali resistance and other types, but the sealing performance of this material is not high (its sealing performance is usually 1 × 10-2 ~ 1 x 10-3cm3n / s). The rubber and other components of the gasket are easy to be damaged by temperature and medium (such as solvent), and stress relaxation is easy to occur in use; Asbestos is a kind of fibrous material. There is a small gap in the gasket, resulting in “capillary phenomenon” leakage, which affects its sealing performance and service life. Generally, it is only used in non important equipment occasions with low temperature and pressure.
PTFE gasket is resistant to the corrosion of various chemical media such as acid, alkali and solvent, and has the characteristics of cleanness and no pollution. However, the biggest disadvantage of this material is its cold flow performance and thermal creep. Even at room temperature, its contact stress will decrease with the passage of time, which is easy to produce stress relaxation, thus affecting the sealing life of the gasket. Therefore, the service pressure is generally no more than 4.0Mpa and the service temperature is generally no more than 150 ℃. The sealing surface type should be full plane, concave, convex or tenon surface and groove surface.
Spiral wound gasket
Spiral wound gasket has good elasticity, high strength and is not easy to be damaged. It is suitable for flange connection with temperature and pressure fluctuation and impact and vibration load. The spiral wound gasket is divided into basic type and belt inner and outer locating ring type. Because the non-metallic belt is clamped between the metal belt and clamped between the two flange sealing surfaces during operation, it can still work effectively even if it is dehydrated into powder at high temperature. The expanded graphite spiral wound gasket has good sealing performance, can adapt to the requirements of high temperature and high pressure parameters, has good resilience, and can still maintain good sealing performance when the temperature and pressure change (its sealing performance index can usually reach 1 x 10-4cm3n / s). It can be used under the conditions of water, gas, steam, oil and other media with pressure of 6.4Mpa and temperature below 500 ℃, If chromium nickel alloy is used as the shell, the service temperature can reach 600 ~ 700 ℃.
Other gasket forms
In addition to the above three commonly used gasket forms, metal clad gasket, wave tooth composite gasket and metal gasket are often used in equipment assembly and pipeline engineering.
Bolt fastening requirements
According to the form and performance requirements of flange and gasket, bolts with higher strength grade shall be selected to ensure the stability and reliability of flange connection. The key of flange connection is to ensure that the seal does not leak. To ensure the reliability of the seal, an appropriate preload must be applied to the seal gasket in advance to make the seal gasket produce elastic deformation, so as to achieve the sealing effect. The pretightening force of the bolt must be determined according to the model and type of flange gasket. If the pretightening force is determined from the bolt strength, the sealing gasket will lose elasticity due to excessive pressing force and cannot achieve the purpose of sealing.
There are three methods of bolt tightening: torque method, rotation angle method and yield point method. The most commonly used method for tightening flange connecting bolts is torque method. Select the appropriate torque value according to the flange gasket model, and tighten the bolts in three times of 30%, 60% and 100% according to the “cross method”.
In short, solving the flange seal leakage is a complex problem. As long as we strengthen management and improve the technical level, reasonably select the form of flange and gasket according to the working requirements of the equipment, and fasten the bolts according to the correct torque tightening method, the flange seal leakage problem can be solved more reasonably.
Source: China Flanges Supplier: www.epowermetals.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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