Valve leakage: internal and external leakage of the main
Valve leakage is divided into internal leakage and external leakage. Internal leakage mainly affects the function of the valve. External leakage not only causes the loss of medium, but also poses a hidden danger to the surrounding equipment and personnel, pollutes the environment, and has a great impact.
There are two main parts of the valve leakage, one is the leakage of the valve body and the valve cover connecting flange, and the other is the leakage of the packing.
The connecting flange between the valve body and the valve cover is sealed by pressing the gasket with fastening bolts. The reasons for leakage are as follows:
Bolts due to thermal shock stress relaxation, resulting in insufficient preload bolts; or uneven screw.
The hardness of the gasket is higher than that of the flange, or the contact between the gasket and the flange is not strict due to aging failure or mechanical vibration.
The accuracy of the contact surface is low (with grooves, chipping, etc.), and it is corroded or penetrated by the medium.
When assembling, the gasket is skewed, and the local preload is excessive, which exceeds the design limit of the gasket, resulting in insufficient local sealing specific pressure.
Treatment: If there are no disassembly conditions on site, all bolts of the flange in the valve body and the valve cover can be loosened, and the fastening bolts can be evenly symmetrical again. Disassemble the valve during maintenance, replace the sealing gasket, reassemble the valve, and tighten the bolts evenly symmetrically according to the specified pre-tightening force.
When designing the valve, the bolt preload force should be calculated, and the bolt preload torque range should be given on the drawing. When assembling the valve, a torque wrench should be used to control the bolt preload force, and the preload force should be applied evenly and symmetrically. Regularly replace the sealing gasket. Improve the processing accuracy of the sealing surface. Multiple sealing structures are used in the design to prevent leakage.
The connection between the valve body and the valve cover is an internal pressure self-sealing structure. Generally, high temperature and high pressure valves use this sealing structure. This structure has the advantages of good sealing performance, good sealing when temperature and pressure change, and the middle flange does not bear the pressure of the working medium and compact structure. There are several reasons for its leakage: insufficient pre-tightening force; damage to the sealing ring and the self-pressure sealing part; deformation of the inner cavity of the valve body.
Treatment: If there is leakage at low pressure and no leakage at high pressure, it may be that the pre-tightening force of the bolts is not enough. Loosen all the bolts and re-tighten the bolts evenly and symmetrically. If leakage occurs at low pressure and high pressure, it may be that the sealing ring and the self-pressure sealing part are scratched, or the inner cavity of the valve body is deformed. It is necessary to disassemble the valve and check the self-sealing part and sealing ring of the valve body. If the self-sealing part of the valve body is damaged, it needs to be ground and repaired. If the self-sealing part of the valve body is not damaged, but the sealing ring is damaged. After replacing the sealing ring, re-assemble.
When designing the valve, the bolt preload force required for low pressure sealing should be calculated. There should be enough preload force. Under the action of preload force, the sealing ring will produce elastic-plastic deformation to seal the middle cavity of the valve. Reduce the number of disassembly and assembly of the sealing ring. Because under the action of test pressure or high temperature and high pressure working medium, the sealing ring will undergo plastic deformation. Each time it is disassembled and assembled, there will be different degrees of scratches on the surface of the self-pressure sealing ring and the self-pressure sealing part of the valve body, which will affect the sealing.
