Issues to consider when using manual valves

Manual valves are control components in fluid conveyance systems, used to alter the cross-sectional area of the passageway and the direction of medium flow. They perform functions such as flow guidance, shut-off, regulation, throttling, check, diversion, or pressure relief. Valves used for fluid control range in nominal diameter from extremely small instrument valves to industrial pipeline valves with diameters up to 10 metres.

I. Opening and closing of manual valves:

1. Manual valves are the most widely used type of valve. Their handwheels or handles are designed for ordinary human strength, taking into account the strength of the sealing surfaces and the necessary closing force. Therefore, long levers or wrenches should not be used to operate them. Some people are accustomed to using wrenches, but strict care must be taken not to apply excessive force, as this may damage the sealing surfaces or break the handwheel or handle.

2. When opening or closing valves, force should be applied smoothly and steadily, avoiding impact. Certain high-pressure valves designed for impact operation have components specifically engineered to withstand such forces, which differ from standard valves.

3. For steam valves, preheating should be performed before opening, and condensate should be drained. When opening, the process should be conducted slowly to prevent water hammer effects.

4. After fully opening the valve, slightly reverse the handwheel to ensure the threads are tightly sealed, preventing loosening or damage. For stem valves, remember the stem position at full open and full closed to avoid colliding with the upper dead point at full open. This also facilitates checking whether the valve is properly closed. If the valve disc falls off or large debris becomes embedded between the valve core and seal, the stem position at full closed will change.

5. When the pipeline is first used, there may be a significant amount of debris inside. The valve can be slightly opened to use the high-speed flow of the medium to flush away the debris, then gently closed (avoid rapid or forceful closing to prevent residual debris from damaging the sealing surface). The valve should be reopened and this process repeated multiple times to thoroughly clean the debris before resuming normal operation.

6. For valves that are frequently open, contaminants may adhere to the sealing surface. When closing, use the aforementioned method to flush them clean before fully closing the valve.

7. If the handwheel or handle is damaged or lost, it should be replaced immediately. Do not use an adjustable wrench as a substitute, as this may damage the valve stem’s square end, causing operational issues and potentially leading to accidents during production.

8. For certain media, after manually closing the valve, cooling may cause the valve components to contract. Operators should close the valve again at an appropriate time to ensure the sealing surface is completely sealed. Otherwise, the medium flowing through the gap at high speed may erode the sealing surface.

9. During operation, if excessive effort is required to operate the valve, the cause should be analysed. If the packing is too tight, it can be loosened appropriately. If the valve stem is misaligned, personnel should be notified for repairs. Some valves, when in the closed position, may experience difficulty opening due to thermal expansion of the closing component. If opening is necessary at this time, the valve cover threads can be loosened by half to one turn to relieve valve stem stress, then the handwheel can be turned.

II. Precautions:

1. For high-temperature valves above 200°C, since they are installed at room temperature and the temperature rises during normal use, the bolts expand due to heat, increasing the gap. Therefore, they must be retightened, known as ‘thermal tightening.’ Operators must pay attention to this procedure; otherwise, leakage may occur.

2. In cold weather, if water valves are left closed for an extended period, the accumulated water behind the valve should be drained. After steam valves are shut off, condensate water should also be drained. If the valve base has a plug, it can be opened to drain water.

3. Non-metallic valves may be brittle or have low strength. When operating them, the force applied to open or close them should not be excessive, especially sudden force. Care should also be taken to avoid collisions with objects.

4. When using new manual valves, the packing should not be compressed too tightly; it should be tight enough to prevent leakage. Over-tightening can cause excessive pressure on the valve stem, accelerating wear and making operation difficult.