Causes of flange gasket failure and precautions
Generally, there are four types of leakage phenomenon of flange gasket: interface leakage, penetration leakage, diffusion leakage and damage leakage. Combined with many years of engineering construction, maintenance and manufacturing experience, the author thinks that the main reasons for seal failure are as follows: uneven distribution of compression force, stress relaxation, unsuitable surface roughness of sealing surface and temperature change.
Uneven distribution of pressing force
The uneven distribution of the pressing force will cause the gap of the flange gasket, which will cause the leakage. The uneven compaction force is caused by many factors.
- 1. It is man-made factor, asymmetric pre tightening bolt is easy to cause uneven during construction, which can be eliminated in construction;
- 2. The second is the influence of flange dislocation on the compression force. Theoretically, when the flange is compressed, the sealing surface is absolutely parallel. In fact, the center line of the pipeline cannot be absolutely concentric. Therefore, when the bolt is tightened, the bending moment is generated on the flange, which causes the uneven stress on the flange. This uneven connection causes the sealing surface to deform more or less, which reduces the compression force of the seal. Under the operation load, the sealing surface can be Leakage is easy to occur. The solution is to master the law of flange deformation, evenly compress the force, and add sealant on the gasket to eliminate the gap, which can eliminate the leakage;
- 3. In addition, the influence of bolt arrangement density on the pressure distribution is also obvious. The smaller the bolt spacing, the more uniform the pressure.
Stress relaxation and torque loss
Stress relaxation and torque loss are also the main causes of leakage. After the bolt is tightened on the flange, due to the vibration of the mechanism, the rise or fall of temperature and other factors, the gasket will have stress relaxation during the working process, and the bolt torque will gradually decrease, resulting in torque loss and leakage. Generally speaking, the longer the bolt, the larger the residual torque and the smaller the diameter are, the more favorable it is to prevent torque loss. Therefore, using long and thin bolts is an effective way to prevent torque loss. In addition, the larger the temperature change, the longer the duration, the more serious the torque loss. It is very effective to prevent the loss of torque by heating the bolt for a certain period of time to extend it and then maintaining the given torque. The thinner the gasket, the smaller the torque loss. In addition, the torque loss can be prevented by preventing the strong vibration of the machine and the pipeline itself, eliminating the influence of the vibration of adjacent equipment, unnecessary impact on the sealing surface, and not knocking the bolts after fastening.
Surface roughness of sealing surface
The surface roughness of the sealing surface should be consistent with that of the sealing surface, such as RF sealing surface, ra3.2-6.3 μ M. too high surface roughness value will affect the sealing effect. It is necessary to control the defect degree of flange surface to achieve a successful sealing. Large nicks, grooves, or scratches and other surface defects must be avoided. A greater pressing force must be used to seal it.
Generally speaking, in normal temperature and low pressure, non-metallic soft gasket is selected; in medium pressure and high temperature, metal and non-metallic combined gasket or metal gasket is selected; in case of large fluctuation of temperature and pressure, elastic or self tightening gasket is selected; in low temperature, corrosive medium or vacuum conditions, gasket with special performance shall be considered. What needs to be explained here is the influence of flange condition on gasket selection.
Influence of flange form
Different flange forms require different gaskets. Smooth surface flange is generally only used for low pressure, with soft thin gasket; under high pressure, if the strength of the flange is enough, smooth surface flange can also be used, but thick soft gasket, or spiral gasket with inner reinforcing ring or outer reinforcing ring should be used. In this case, the metal gasket is also not applicable, because the required pressing force is too large, leading to large deformation of the bolt, making the flange not easy to seal. If a metal gasket is to be used, the smooth surface shall be reduced to reduce the contact area with the gasket. In this way, with the same bolt tension, the compression stress of the narrow smooth surface after reduction will increase.
Influence of flange surface roughness
The surface roughness of flange has a great influence on the sealing effect, especially when non soft gasket is used, the large surface roughness is one of the main causes of leakage. For example, when using metal gasket, if the roughness value is large, the gasket can not block the spiral groove formed by the knife pattern. Under the pressure, the medium will leak out along the groove. The soft gasket has a lot of lower requirements on the flange surface finish, because it is easy to deform and can block the processing knife lines, thus preventing leakage. For soft gasket, too smooth flange surface is unfavorable, because the resistance of interface leakage becomes smaller. Therefore, different gasket requires different roughness of flange surface. Table 1 below lists the experience data of flange surface roughness required by various gaskets.
Table 1 surface roughness of flange sealing surface μ M
|Gaskets category||Name of gasket||Surface roughness Ra is less than||Remarks|
|Metal gasket||Ring gasket||0.8||Self tightening gaskets hope that the better the surface is, the better.|
|Half metal gasket||Metal gasket||1.6|
|Spiral wound gasket||12.5|
|Spiral wound gasket||3.2||Gas seal time|
|Asbestos rubber sheet||12.5|
|Asbestos cloth gasket||25|
|PTFE gasket||PTFE plate gasket||12.5|
|Polytetrafluoroethylene packing gasket||12.5|
|Organic mat||oil seal||25|
Hardness difference between flange and gasket the purpose of using gasket is to make gasket produce elastic or plastic deformation to fill the flange surface with small unevenness and prevent leakage. Therefore, the hardness of gasket material should be lower than that of flange material. The greater the difference between them, the easier to achieve sealing. When using metal gasket, in order to ensure the realization of sealing, the softer material should be selected as far as possible to make the hardness of metal gasket lower than that of flange by more than 40hbs.
The decrease of temperature also has a great influence on the sealing effect. When the pipeline is installed under normal temperature, it will expand and compact when the temperature rises; when the temperature drops, the pipeline will contract. The leakage at the flange connection often occurs in the process of temperature drop (cooling). Because the cooling speed of flange and bolt is different when cooling, the compression force of gasket changes after cooling, resulting in stress relaxation. In addition, the cold shrinkage of pipeline produces the force towards the bolt tensile direction, which will promote the leakage. Therefore, when selecting gasket in low-temperature medium, attention should be paid to:
- ① Adopt flexible flange sealing gasket at low temperature.
- ② The thickness of flange sealing gasket shall be as small as possible, and the flange clearance shall be as small as possible.
- ③ Adopt high strength bolt to reduce strain.
Source: China Gaskets Manufacturer – Yaang Pipe Industry (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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