Causes and maintenance of valve failure

Leakage at packing
Fault cause
● The filler is not selected correctly and is not suitable for the corrosivity, temperature and pressure of the working medium.
● The packing is not installed correctly, especially the whole packing is put in standby, which is most likely to cause leakage.
● The filler has aged and lost its elasticity after its service life.
● The number of packing rings is insufficient.
● The machining accuracy or surface finish of the valve rod 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.
● The packing shall be placed and pressed one by one, and the joint shall be 30 ° or 45 °.
● The aged and damaged packing shall be replaced in time.
● The packing shall be installed according to the specified number of turns.
● Abide by the operating procedures, and operate with constant speed and normal force except for the impact handwheel.
● The 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.
● The operation is not smooth, causing the fluctuation of valve pressure and temperature.
● The compression force of the gasket is not enough.
● Improper gasket assembly and uneven stress.
● 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 uniformly stressed, and the gasket shall not be overlapped and double gaskets shall not be used.
● Pay attention to cleaning when installing the gasket, and clean the sealing surface with kerosene.
Leakage at sealing surface
Cause
● The sealing surface is uneven and cannot form a sealing line.
● The top center of the connection between the valve rod and the closing part is suspended, out of alignment or worn.
● The valve rod is bent or improperly assembled, which makes the closing part skew.
● Improper selection of sealing surface material causes 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.
● The sealing surface peels off.
Maintenance method
● When grinding the sealing surface, the selection of grinding tools and abrasives shall be reasonable. After grinding, dye inspection shall be carried out. The sealing surface shall be free of indentation, crack, scratch and other defects.
● If the top center of the connection between the valve rod and the closing part does not meet the requirements, it should be repaired. The top center should have a certain movement clearance, and the axial clearance between the valve rod shoulder and the closing part should not be less than 2mm.
● Straighten and bend the valve rod, and adjust the valve rod, valve rod 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.
● High-temperature valve, after being closed, there are fine cracks in cold shrinkage, and it shall 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
● Poor operation causes the closing part to jam or exceed the top dead center, and the connection is damaged and broken.
● The closing part is not firmly connected, loose and falling off.
● The selected connecting piece is made of the wrong material and cannot withstand the corrosion of medium and mechanical wear.
Maintenance method
● To operate correctly, close the valve without too much force, open the valve without exceeding the top dead center, and turn the handwheel back a little after the valve is fully opened.
● The connection between the closing part and the valve rod shall be correct and firm, and the threaded connection shall have a retaining part.
● Fasteners used for connecting the closing parts and valve stems shall withstand the corrosion of media and have certain mechanical strength and wear resistance. Although the closing parts are rare, it is a very dangerous failure.
The valve rod is not flexible
reason
● The machining accuracy of the valve rod and its matching parts is low, and the matching clearance is too small.
● The axes of valve rod, valve rod nut, bracket, gland, packing, etc. are not in a straight line.
● The packing is pressed too tightly.
● The valve rod 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 valve rod and transmission device is loose or damaged.
Maintenance method
● Improve the machining accuracy and repair quality of valve rod, valve rod nut, etc. to make the clearance appropriate.
● When assembling the valve rod and its connecting parts, the clearance shall be consistent, concentric and flexible.
● The packing is pressed too tightly, and the gland is properly loosened.
● The bending of valve rod shall be corrected, and those difficult to be corrected 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 pins or graphite powder shall be coated for lubrication.
● The valve rod nut shall be repaired if it is loose, and it shall be replaced in time if it cannot be repaired.
● Make the oil circuit at the nut smooth and well lubricated. For valves that are not operated frequently, check and move the valve rod regularly. If there is wear and seizure, repair the valve rod nut, bracket and other accessories in time.
● The valve shall be operated correctly and the closing force shall be appropriate to avoid deformation and damage of the valve rod.
● When the valve is closed and the valve parts expand due to heat, after the valve is closed, turn the handwheel counterclockwise a little to prevent the valve stem from sticking.
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
● Grind the suspected crack and etch it with 4% nitric acid solution. If there is any crack, it can be shown.
● Repair or replace the crack.