Sep.19.2024
For a rising-stem wedge gate valve, the connection between the valve stem and the valve core can fail due to corrosion, wear, or improper operation during opening and closing. This results in the valve core falling out, which happens every year. When the valve is closed and cannot be opened, it needs to be cut out of the system and replaced. If it cannot be cut out of the system, an alternative flow or shutdown of the device is required.
1. Repair Method:
When the valve core falls out and cannot be removed, with the medium sealed inside, without moving the valve body, valve core, and valve seat, change the assembly of the valve bonnet, valve stem, copper sleeve, gland, and gland nut. Expose the valve core, then reconnect the valve stem with the valve core using conventional methods, ensuring the three seals of the valve, and reopen the valve. After removing the valve bonnet, except for special valves, the wedge angle of the valve core remains within the friction angle range. Regardless of the pressure changes at both ends of the valve core, under the self-locking condition, the valve core will not be pressed out from the valve seat, especially for flat valve cores, there is no possibility of being squeezed out.
2. Operational Steps:
(1) Taking the rising-stem wedge gate valve as an example, when the connection between the valve stem and the valve core fails, the handwheel can still rotate. Use the handwheel to push the valve core back into the valve seat to ensure the sealing between the two sealing surfaces of the valve core and the valve seat, which is a prerequisite for applying this repair method. First, unscrew the gland nut of the valve and loosen the gland. Then, gradually loosen all the nuts on the bolts of the valve bonnet without completely removing them.
(2) Continue to rotate the handwheel in the closing direction. At this time, the valve stem will not move anymore. Instead, the upper part of the valve bonnet will separate from the valve body as a whole. This step is to observe whether the sealing between the two sealing surfaces of the valve core and the valve seat is tight. If there is slight leakage, as long as it doesn't affect the on-site construction in a short time, you can completely remove all the nuts on the bolts, remove the upper part of the valve bonnet and the valve stem, expose the top of the valve core in the valve seat, check the original connection method between the valve stem and the valve core, and identify the cause of the failure to formulate a solution.
(3) Gently rotate the valve stem in the upper part of the valve bonnet to exit from the stem nut and the packing box. At the same time, unscrew the locking nut of the stem nut, remove the handwheel and the stem nut. It can freely rotate without limitation.
(4) Reconnect the removed valve stem with the valve core. Generally, the connection between the valve stem and the valve core is restored using welding, which is well-applied to water lines, air lines, and steam lines where the valve core is prone to falling off. Even if the valve core and valve seat are not tightly sealed, with slight leakage, as long as it doesn't affect construction, it can be used. When repairing valves for oil and gas media, direct welding is unsafe. You can first process the end of the valve stem into the required special shape, and use high-strength adhesive to treat the contact surface between the valve stem and the valve core, or drill and tap the valve core at the top on-site, process the connection end of the valve stem separately, and then connect them. It should be noted that after reconnecting the valve stem and valve core, the installation plane of the valve stem and valve bonnet should be perpendicular and well-centered; otherwise, the valve stem may bend after repair, making it difficult to open and close the valve.
(5) After connecting the valve stem, put the valve bonnet, part of the packing and gland, and the stem nut back onto the valve stem and screw them in. Replace the sealing gasket as needed to prevent leakage after using this sealing surface. After the valve bonnet is in place, align it with the screw holes on the valve body and tighten the bolts symmetrically. If necessary, add some packing to the packing box and press the packing gland appropriately. Finally, put the stem nut back on the handwheel, tighten the locking nut, and finish the repair work.
