Causes Of Valve Failure And Maintenance
Feb 23, 2022
Leakage at packing
Cause of failure
● The filler is not selected correctly, which is incompatible with the corrosivity, temperature and pressure of the working medium.
● if the packing is not installed correctly, especially if the whole packing is screwed in for standby, it is most likely to cause leakage.
● the filler has exceeded its service life, aged and lost elasticity.
● insufficient number of packing rings.
● the processing accuracy or surface finish of the valve stem is not enough, or there is ovality or scoring.
● improper operation and excessive force.
Maintenance method
● the material and type of filler shall be selected according to the working conditions.
● packing shall be placed and pressed one by one, and the joint shall be 30 ° or 45 °.
● aging and damaged packing shall be replaced in time.
● the packing shall be installed according to the specified number of turns.
● observe the operating procedures and operate with constant speed and normal force except for the impact handwheel.
● gland bolts shall be tightened evenly and symmetrically.
Leakage at gasket
reason
● the gasket is not resistant to corrosion, high pressure, vacuum, high temperature or low temperature.
● unstable operation causes fluctuation of valve pressure and temperature.
● the pressing force of the gasket is not enough.
● the gasket is improperly assembled and the stress is uneven.
● the gasket surface is rough and mixed with foreign matters.
Maintenance method
● gasket materials shall be selected according to working conditions.
● careful adjustment and smooth operation.
● bolts shall be tightened evenly and symmetrically.
● the gasket assembly shall be stressed evenly, and the gasket is not allowed to overlap or use double gaskets.
● pay attention to cleaning when installing gaskets, and clean the sealing surface with kerosene.
Leakage at sealing surface
reason
● the sealing surface shall not be grinded unevenly to form a sealing line.
● the top center of the connection between the valve stem and the closing part is suspended, incorrect or worn.
● the valve rod is bent or improperly assembled, which skews the closing part.
● improper selection of sealing surface materials, resulting in corrosion, erosion and wear of sealing surface.
● the surfacing and heat treatment are not operated according to the regulations, resulting in wear, corrosion, cracks, etc.
● peeling of sealing surface.
Maintenance method
● when grinding the sealing surface, the grinding tools and abrasives shall be selected reasonably. After grinding, the coloring inspection shall be carried out, and the sealing surface shall be free of indentation, crack, scratch and other defects.
● if the top center at the connection between the valve stem and the closing part does not meet the requirements, it shall be repaired. The top center shall have a certain movable clearance, and the axial clearance between the valve stem shoulder and the closing part shall not be less than 2mm.
● straighten and bend the valve stem, and adjust the valve stem, valve stem nut, closing part and valve seat on a common axis.
● select the sealing surface with corrosion resistance, scratch resistance and other properties that meet the working conditions.
● the high-temperature valve will shrink after being closed, and there will be fine cracks. It will be closed again at a certain interval after being closed.
● valves used as shut-off valves are not allowed to be used as throttle valves and pressure reducing valves. The closing parts should be in the fully open or fully closed position. If it is necessary to adjust the medium flow and pressure, throttle valves and pressure reducing valves should be set separately.
● the sealing surface that cannot be adjusted shall be replaced in time.
The closing part falls off
reason
● the closing part is stuck or exceeds the top dead center due to poor operation, and the connection is damaged and broken.
● the connection of the closing part is not firm, loose and fall off.
● the material of the selected connector is incorrect and cannot withstand the corrosion of medium and mechanical wear.
Maintenance method
● operate correctly. Do not use excessive force to close the valve, and do not open the valve beyond the top dead center. After the valve is fully opened, the handwheel should be reversed a little.
● the connection between the closing part and the valve rod shall be correct and firm, and the threaded connection shall have a non-return part.
● the fasteners used for connecting the closing part and the valve rod shall withstand the corrosion of the medium and have certain mechanical strength and wear resistance. Although the falling off of the closing part is rare, it is a very dangerous fault.
Inflexible valve stem
reason
● the processing accuracy of the matching parts of the valve stem and it is low, and the matching clearance is too small.
● the axes of valve stem, valve stem nut, support, gland, packing, etc. are not in a straight line.
● the packing is pressed too tightly.
● the valve stem is bent or damaged.
● the thread is not clean or rusty, and the lubrication condition is poor.
● the nut is loose and the thread is slippery.
● the connection between the valve rod and the transmission device is loose or damaged.
Maintenance method
● improve the machining accuracy and repair quality of valve rod, valve rod nut, etc., so that the clearance shall be appropriate.
● when assembling the valve stem and its connectors, the clearance shall be consistent, concentric and flexible.
● if the packing is too tight, loosen the gland properly.
● the bending of the valve rod shall be corrected. If it is difficult to correct, it shall be replaced. When operating the valve stem, the closing force is appropriate.
● the threads of valve rod and valve rod nut shall be cleaned and lubricated frequently. For high-temperature valves, disulfide pin or graphite powder shall be coated for lubrication.
● if the valve stem nut is loose, it shall be repaired, and if it cannot be repaired, it shall be replaced in time.
● make the oil circuit at the nut unblocked and well lubricated. For valves that are not operated frequently, check and move the valve stem regularly. If there is wear and seizure, repair the valve stem nut, support and other accessories in time.
● the valve shall be operated correctly and the closing force shall be appropriate to avoid deformation and damage to the valve stem.
● when the valve is heated and expanded after the valve is closed, turn the handwheel a little counterclockwise at a certain interval after the valve is closed to prevent the valve stem from jacking.
Leakage of valve body and bonnet
reason
● the valve body has sand holes or cracks.
● the valve body is cracked during repair welding.
Maintenance method
● polish the suspected crack and etch it with 4% nitric acid solution. If there is a crack, it can be displayed.
● repair or replace the cracks.
