Knife gate valve valve stem failure is mostly caused by excessive operation, resulting in damage to the valve stem thread or bending of the valve stem, or due to the imbalance of the valve stem and nut, nut misalignment, thread corrosion, rust, etc. In addition, the selection of materials can also cause valve stem failure, such as the valve stem and valve stem nut being made of the same material, which is easily stuck and causes failure.
Solution: When using a knife gate valve, workers should operate it carefully, strictly control the opening and closing dimensions, and avoid strong switching to prevent bending of the valve stem. Do not fully open the valve to the upper end point, but open it to a suitable degree and then reverse the handwheel by one turn to ensure the tight sealing of the upper side of the thread and prevent medium-induced upward impact on the valve stem. Adjust the position of the nut. If corrosion and rust cannot be prevented, replace the valve stem and nut in time. Choose nuts and valve stems made of different materials.
The main reasons for the insensitivity or jamming of the knife gate valve stem are excessive packing compression, improper filling of the packing into the packing box, improper clearance between the valve stem and the bushing, valve stem bending, rough surface finish of the thread, inappropriate selection of materials for the valve stem and bushing, etc.
Solution: Loosen the packing gland screws appropriately; increase the filling according to the specifications; control the clearance between the valve stem and the bushing within the specifications; control the roughness of the thread's surface finish within the specifications.
Some valve bonnets have gland holes for factory pressure tests, which may cause water leakage during use. The main reason is that the gland hole is not properly tightened.
Solution: Tighten it again, or remove the bolt from the gland hole, take out the copper gasket inside, and then clean the screw hole and screw the bolt back in and weld it with electric welding.
The insufficient or uneven tightening of the bolts at the flange joint causes the flange to tilt, resulting in leakage at the joint between the knife gate valve body and bonnet. Or there is damage to the joint surface between the valve body and bonnet, and the flange gasket is damaged, etc., which causes leakage at the joint.
Solution: The flange should be tightened diagonally; pay attention to maintaining the joint surface during use; choose better quality gaskets. If the contact surface cannot be adjusted due to quality issues, reprocessing can be done on the contact surface.
The leakage of the packing box of the knife gate valve is caused by the incompatibility between the packing and the working medium in terms of corrosion, temperature, and pressure, incorrect filling method, poor surface finish of the valve stem, corrosion, rust, or the aging of the packing due to prolonged use.
Solution: Choose the correct packing method; fill the packing according to the correct method; choose a valve stem with the required surface finish; adopt appropriate protective measures to prevent corrosion, and replace it in time if rust is found.
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