Factors affecting valve sealing performance

The sealing performance of the valve is one of the main indicators to evaluate the quality of the valve. Valve sealing performance mainly includes two aspects, namely inner leakage and outer leakage. Endoleak refers to the degree of sealing between the valve seat and the closing member to the medium. There are two standards in the assessment of leakage in China. One is the National Standard GB/T13927-1992 "General Valve Pressure Test" issued by the State Bureau of Technical Supervision in December 1992 and implemented on June 1, 1993. This standard is based on the use of the international standard ISO5208-1982 "pressure test for industrial valves and valves", and the other is the JB/T9092-1999 "Test and Inspection of Valves" issued by the former Ministry of Machinery Industry. This standard refers to API 598- 1986 "Check and test of valves". GB/T13927-1992 applies to the inspection of general industrial valves; JB/T9092-1999 applies to the inspection of petroleum industry valves. Leakage is the leakage of the valve stem packing, the leakage of the gasket in the middle flange, and the leakage of the valve body due to casting defects. The leakage is not allowed at all. If the medium is not allowed to vent to the atmosphere, the seal of the leak is more important than the seal of the inner leak. Therefore, the sealing performance of the valve should attract the attention of the manufacturer.

The factors affecting the sealing performance of the valve are: 1 the quality of the sealing surface; 2 the width of the sealing surface; 3 the pressure difference between the front valve and the rear valve; 4 the sealing surface material and the processing state; 5 medium properties; 6 surface hydrophilicity; 7 sealing The presence of oil film; 8 closed parts of the rigidity and structural characteristics.

(1) Influence of sealing surface quality on valve sealing performance The sealing surface of a valve refers to the part where the valve seat and the closing member contact each other and close. When the specific pressure on the sealing surface is below 40 MPa, the quality of the sealing surface plays a decisive role in the sealing performance of the valve. This is because the amount of leakage rapidly increases when the specific pressure on the sealing surface is small and the surface roughness is low. When the specific pressure on the sealing surface is large, the influence of the surface roughness on the amount of leakage is significantly reduced.

(2) Influence of sealing surface width on valve sealing performance The width of the sealing surface determines the length of the capillary hole. As the width increases, the fluid travels longer along the capillary, increasing the resistance to movement. Increasing the width of the sealing surface can reduce the erosion wear in the high pressure valve. Increasing the width of the sealing surface will cause the leakage stroke to increase in proportion to the length of the leakage stroke, thus reducing the leakage proportionally. However, the width of the sealing surface is increased. Under the same sealing force, the sealing specific pressure is reduced, which in turn increases the possibility of leakage. Therefore, the sealing surface width cannot be increased infinitely.

(3) The influence of the pressure difference between the front valve and the rear valve on the sealing performance of the valve Theoretically analyzed, there is a positive relationship between the pressure difference and the leakage amount between the front valve and the rear valve. However, tests have shown that under the same conditions, the increase in leakage exceeds the pressure difference. The relation between the leakage amount and the pressure difference can be approximated by the following formula: G = M(N?P2 + S?P) where: M, N, S - constant coefficients, these coefficients depend on the material, the surface of the sealing surface Processing quality, specific pressure on the sealing surface and other conditions; â–³ P - differential pressure.

(4) The influence of the sealing surface material and its treatment status on the sealing performance of the valve The sealing surface material and its processing status have a great influence on the valve leakage. Since the size of the remaining gap between the sealing surfaces depends on the microscopic unevenness of the sealing surface, if a seal ring of steel material is used, resulting in the same degree of sealing, there must be a greater specific pressure, which must exceed that of brass. The pressure ratio of the manufactured seal. Seal-related surface treatment conditions, such as peak deformation, size and seal gap changes, and other phenomena occur on the metal surface. Obviously, the performance of the surface layer is significantly different from that of the substrate material. Changes caused by processing can affect the surface thickness of 50μm. When grinding, the base metal is not exposed. Work surface organization is different from metal matrix organization. Material properties have little effect on geometry and micro geometry. The difference in metal properties is usually less than the effect of other factors. This situation is even more pronounced when the sealing surface is operated under low pressure conditions. When the specific pressure is higher than 40 MPa, the influence of the surface roughness on the sealing performance is reduced, and the influence of the material is increased.

(5) The influence of the nature of the medium on the sealing performance of the valve The influence of the liquid medium on the leakage amount is basically determined by the viscosity in the same sealing valve. Under the same conditions, the medium with a large viscosity has a smaller leakage than the medium with a small viscosity. many. The difference between the gas medium and the liquid medium is more pronounced (except for saturated steam, which is easily ensured by the saturated steam).

(6) Influence of surface hydrophilicity on valve sealing performance The hydrophilicity of the surface affects the leakage amount due to the capillary pore characteristics. Whenever there is a thin film of oil on the sealing surface, the pressure of the water passing through the gap needs to be increased. Due to the good hydrophilicity of the metal surface, kerosene can easily penetrate the gap between the casting and the sealing connection. Therefore, in some of the most critical situations, kerosene is used for sealing hydraulic tests. The method of sealing with kerosene is used to perform the sealing test, which is equivalent to the water tightness test under the pressure of 0.3~0.4MPa.

(7) Influence of the presence of the sealing oil film on the sealing performance of the valve The sealing oil film between the sealing surfaces has a significant influence on the sealing performance. When there is a sealing oil film on the surface, the hydrophilicity between the contact surfaces is destroyed, so that a large pressure difference is required to allow the medium to pass through the pores. In addition, there is a thick seal oil film on the surface that can block the passage of the media, improving the sealability of the connection. When using an oil film seal, it should be noted that when the oil film is reduced during operation, the thickness of the oil film should be restored. The grease used in the valve is not allowed to dissolve in the medium, nor should it evaporate, harden or have other chemical changes.

(8) Rigidity and structural characteristics of the closure The effect on the sealing performance of the valve The rigidity of the closure and the influence of the structure are due to the elasticity of the part. Since the closing part of the closed-circuit valve is not absolutely rigid but has a certain elasticity, the dimensions are varied under the influence of the medium-dependent pressure, which also causes a change in the interaction of the sealing surface forces. In order to compensate for the effect of these changes on the sealing performance of the closure, it is preferable to make the sealing surface less rigid, ie, the elastic deformation is as large as possible. It is expected that the valve manufacturers will attach importance to the factors that affect the sealing performance of the valve, minimize the leakage caused by the valve sealing performance, and prevent the internal leakage and external leakage of the valve.