Failure of flange seal
One of the most effective sealing forms for coal chemical enterprises is flange sealing. This paper introduces the principle of flange sealing, analyzes the factors leading to flange sealing failure, and puts forward the measures to improve the sealing performance of flange.
In coal chemical plant, pipes, fittings, valves, instruments and equipment are connected by flange. The flange connection mode has the advantages of reliable, convenient disassembly and no hot work. Coal chemical production process often involves high temperature, high pressure and other processes. The flowing medium in equipment and pipeline has the characteristics of high temperature, high pressure, toxic, inflammable, explosive and easy to corrode, which requires high sealing performance of flange connection. However, various factors often lead to flange leakage, such as bad external conditions, internal medium with high temperature and pressure characteristics, improper installation and disassembly, etc. If the flange leaks, it will affect the normal production, and even cause heavy economic losses, even casualties and environmental pollution. Therefore, it is of great practical significance to study the factors affecting the sealing performance of flange.
The flange, bolt and gasket connect the pipeline and realize the sealing of the pipeline. The gasket is the main sealing element of the flange connection interface. Under the action of bolt pre tightening force, enough pressure is generated between gasket and flange sealing surface, and the uneven micro geometric clearance of flange surface is filled by gasket to realize sealing. The schematic diagram of flange sealing structure is shown in Figure 1.
Fig.1 Schematic diagram of flange sealing structure
There are two main forms of gasket leakage: interface leakage and seepage leakage. The leakage between flange and gasket is interface leakage, and the interface leakage is related to the size of interface gap; the leakage of fluid through gasket capillary is seepage leakage, which is related to gasket structure material, medium temperature and pressure.
Analysis of sealing failure factors
The main factors affecting flange sealing are as follows:
- (1) The characteristics of flange, bolt and gasket are the condition of flange connecting components;
- (2) Medium, pressure, temperature, external environment, i.e. operating conditions;
- (3) Installation, disassembly and operation, i.e. human factors.
The influence of flange on sealing performance is reflected in the surface roughness of flange. Under the same working condition, the leakage rate is different with different flange surface roughness. The smaller the flange surface roughness is, the smaller the leakage rate is. The surface roughness of flange should match the gasket, the sealing surface roughness of non-metallic gasket can be lower, and the sealing surface precision of metal gasket is relatively high. In addition, the flange grade will also have an impact on the sealing. When the flange deforms greatly under the action of force and torque, it is easy to leak. The grade of flange should be improved, and the grade of flange should meet certain strength and stiffness requirements. The flange with high stiffness has small deformation and uniform bolt pre tightening force, so the sealing performance can be improved. Increasing flange outer diameter, decreasing bolt center circle diameter and increasing flange thickness can improve flange stiffness.
The influence of bolt on sealing performance is mainly reflected in the pre tightening force. If the preload is too small and the sealing is not tight, it is easy to leak; if the preload is too large, the flange root is easy to crack or the gasket will deform, which will also cause leakage. Therefore, the preload should be moderate and the sequence of preloading should be paid attention to. There are many factors affecting the preload, such as elastic deformation and plastic deformation of flange, cleanliness of thread, plastic deformation of gasket, changes of system temperature and pressure, quality and lubrication degree of bolts and nuts. Proper deformation and resilience of gasket are necessary conditions for sealing. The deformation of gasket is divided into plastic deformation and elastic deformation. If the gasket material has good flexibility, the gasket can be well matched with the sealing surface, and the sealing effect will be good. The springback of gasket can compensate the separation of sealing surface in time and keep enough pressing stress to maintain the sealing. The larger the spring back of the gasket is, the greater the residual compression stress is, and the better the sealing performance of the gasket is.
The different physical and chemical properties of the media in the pipeline have different effects on the flange sealing, such as the corrosiveness and permeability of the fluid. Under the same sealing connection form and working conditions, the leakage rate of gas is greater than that of liquid, the leakage rate of strong corrosive medium is greater than that of weak corrosive medium, and the leakage rate of strong permeability medium is greater than that of weak permeability medium. The influence of working conditions such as pressure and temperature of medium on the sealing performance of flange is also an important factor that can not be ignored. The greater the pressure and temperature of the medium, the easier the flange seal will leak. Under high pressure and high temperature conditions, the physical and chemical properties of flange, bolt and gasket materials will change. For example, under high temperature, the resilience and stress of gasket will decrease, and the gasket material will be aged. Therefore, compared with normal temperature sealing, high pressure and high temperature sealing are much more difficult. External load is also an important factor affecting the sealing performance of flange. Due to the suspended support of pipeline, the stress of flange joint is more complex. Under the high temperature and high pressure conditions, the thermal expansion deformation of the pipeline results in the additional thermal stress due to the constraint of the external support; at the joint, due to the uneven flow rate, vibration, pipe quality and other factors, the pipeline stress distribution is not uniform, and external load can be generated. When the external load is too large, it may lead to the flange seal failure and leakage; under certain conditions, the appropriate external load can play a role in strengthening the seal. In addition, the corrosion of the external environment of the pipeline has a great impact on the sealing performance of the flange, bolt and gasket, such as water corrosion, atmospheric corrosion and soil corrosion. With the extension of service time, the corrosion of flange and bolt leads to the serious deterioration of sealing performance and even fracture. For the above fasteners, atmospheric corrosion, especially severe corrosion in industrial atmosphere, is the main factor.
The flange sealing failure caused by the poor professional quality, careless operation and improper use of tools is more prominent than the above two reasons. The common error operations are as follows:
- (1) During installation, the two flange sealing surfaces of flange connection are not parallel or different shafts, the flange surface roughness is not appropriate, the flange face boss is not aligned, the gap between the two flange faces is too large, and the flange sealing surface is damaged. Flange sealing surface is not clean, not smooth, there are mechanical damage scratches, serious corrosion. There are foreign matters between the flange sealing surface and the gasket, resulting in false firmness.
- (2) If the coarse tooth bolt is used instead of the fine tooth bolt or there is a large gap between the bolt and the bolt hole, the bolt will become loose or broken. When the nut is thickened, the thicker the nut, the more uneven the load distribution is, resulting in the looseness of the connector. There are burr on bolts and nuts, shortage of thread, bending of bolts, loose or uneven bolts, over tightening of bolts, resulting in plastic deformation of bolts, gaskets or flanges and loss of elasticity.
- (3) If the bolt is too long and many gaskets are installed, the stress on the gasket is uneven, the bolt preload is reduced and the eccentric load is produced, which makes the performance of bolt connection decline. Improper use of gaskets, the use of large gaskets leads to the decrease of gasket bearing pressure and looseness of bolts. When pressing the gasket, do not know the designed pre tightening force value and tightening procedure, and consciously increase the tightening force to make the bolt slip. In order to facilitate the use of spanner, torque wrench was not used when tightening special pipe flange; hydraulic tightening machine or hydraulic stretcher was not used for large bolts and high strength bolts.
To sum up, the flange seal failure is caused by many reasons. To further improve the sealing of flange, firstly, the professional quality of relevant personnel should be improved, such as improving the understanding of the correct operation of flange sealing; secondly, the correct flange, gasket and bolt should be selected according to the nature of the medium in the pipeline; thirdly, the routine maintenance of the flange, bolt and gasket should be done well; thirdly, the newly installed or replaced flange bolt should be well protected against corrosion, which causes serious corrosion and rust The flange bolts should be replaced in time.
Author: Song Xiaopan, Liu Xiaoli, Wang pan, Gao Jiguang
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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