In refrigeration systems, many ball valves, globe valves, check valves, safety valves, etc. are used. With the prolonged operation of the refrigeration device, the valves in the system may fail. The failure of valves includes sealing failure, action failure, functional failure, etc. Sealing failure refers to the leakage of the valve, including external and internal leakage; Action failure refers to the inability of the valve to operate normally, including drive failure, valve stem jamming, valve core seizure, etc; Functional failure refers to the performance failure of the valve's intended function, such as inaccurate flow regulation, unstable pressure reduction, and safety valve not jumping according to the specified pressure. Different valve types may experience different functional failure states.
So, what aspects should be paid attention to when dealing with daily inspections, maintenance, or malfunctions of these valves?
ⅠThe daily maintenance cycle and inspection items of valves are generally determined based on the characteristics of the production device, medium properties, corrosion rate and operation, as well as the characteristics of the valve itself. The inspection generally includes the following contents:
● Regularly check the lubrication of the valve's oil cup, nozzle, valve stem thread, and valve stem nut. For parts of the valve with exposed valve stems, lubricating grease or protective sleeves should be applied for protection.
● Regularly check the sealing and fastening of the valve, and promptly handle any leaks or looseness found. For example, check whether the connections of the valve are loose, and if there is any looseness, tighten it in a timely manner; Check if the filler is outdated and ineffective, and replace it in a timely manner if there is any damage; Check the wear or change of the valve switch limit position, as well as the wear of the sealing surface, and adjust the limit or replace the sealing ring in a timely manner.
● For valves with pneumatic and hydraulic devices, regularly clean the pneumatic and hydraulic devices.
● Check the anti-corrosion layer and insulation layer of the valve, and promptly repair any damage found.
● Check the rust prevention of flange bolt threads, and apply rust inhibitor for protection if the rust inhibitor layer fails.
● Check the parts on the valve, such as handwheels, handles, etc. If there is damage or loss, they should be assembled as soon as possible.
● For water and steam valves that have been out of service for a long time, if a drain plug is installed at the bottom of the valve, the plug can be opened for drainage.
● Regularly calibrate safety valves in strict accordance with national standards for pressure vessel maintenance and repair.
Ⅱ.Before repairing the valve, the technical information related to the valve and device should be checked first. Prepare the necessary tools, measuring tools, and materials for maintenance. Clean the medium inside the valve according to safety operation regulations. When repairing valves, the following contents are generally included:
● Clean and inspect the valve body and all valve components. Replace and repair damaged valve components.
● Grind the sealing surface. Repair the flange sealing surface. Replace the sealing surface of damaged non-metallic materials.
● Replace or add fillers and replace gaskets.
When repairing valves, a signboard should also be hung on the valve, indicating the maintenance number, working pressure, working temperature, and medium. If there are requirements for direction and position, the disassembled valve components should be checked or marked. Copper pads should be annealed before installation. The bolts should be installed neatly. When tightening the middle flange bolts, the gate valve and globe valve should be in an open state. After each repair of the valve, clear markings should be made on the valve body.
Ⅲ.After the maintenance is completed, check and verify the maintenance results again:
● The valve nameplate is complete, and the lead seal of the safety valve is undamaged.
● The casting of the valve shall be free from defects such as cracks, shrinkage cavities, and slag inclusions.
● The machined surface of the valve forging should be free of defects such as interlayers, double skin, cracks, and scars.
● The welding seam of the valve should be free of defects such as cracks, slag inclusions, air holes, undercuts, and poor forming.
● The valve bolts should be fully tightened without looseness. The transmission system components are complete and easy to use.
● Check the contact surface imprint with a display agent on the sealing surface.
● The connection between the valve seat and valve body should be firm and tight without leakage.
● The valve plate and guide rail are properly matched, and there is no jamming or derailment at any position.
● The flange sealing surface is clean and free from scratches. The packing gland is not damaged or deformed.
● Bolts with tightening torque requirements should be tightened according to the specified torque, and the error in tightening torque should not exceed ± 5%.
● The connection between the valve stem and the opening and closing parts is firm and does not fall off. The handwheel and bearing gland must not be loose.
● The connection between the end of the valve stem and the valve plate should be centered between the valve plate and the valve body when the valve is closed.
● The surface of the valve stem should be free of dents, scratches, and axial grooves. The valve stem head should not be dented or deformed.
● Before installation, valves should be inspected according to corresponding specifications, and shell pressure and sealing tests should be conducted. Valves with upper sealing structures should also undergo upper sealing tests.
Ⅳ.After disassembling and reinstalling the valve into the system, pay attention to keeping the installation direction and method consistent with the original state, especially the following precautions:
1) Valves with specified flow direction should be installed according to the specified flow direction of the valve.
2) Generally, the valve handwheel should not be installed downwards to avoid corrosion of the valve stem.
3) Lift check valves should be installed horizontally, and swing check valve pins should be horizontal.
4) Valves with safety relief devices should have an outlet pipe, and the relief direction should not be facing the operator during installation.