Manual valves are control components in fluid transport systems, used to change the cross-section of the passage and the direction of medium flow, with functions such as guiding flow, shutting off, regulating, throttling, preventing backflow, diverting, or relieving pressure. Valves used for fluid control can range from very small instrument valves to industrial pipeline valves with a nominal diameter of up to 10m.

1. Opening and closing of manual valves:

1. Manual valves are the most widely used valves, and their handwheels or handles are designed for ordinary human strength, taking into account the strength of the sealing surface 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 it is important to be careful not to apply excessive force, as this can easily damage the sealing surface or break the handwheel or handle.

2. When opening and closing the valve, the force should be steady and not abrupt. Some high-pressure valves designed for impact opening and closing have components that account for this impact force, which cannot be compared to general valves.

3. For steam valves, preheating should be done before opening, and condensate should be drained. When opening, it should be done slowly to avoid water hammer.

4. After the valve is fully open, the handwheel should be turned back slightly to tighten the threads and prevent loosening and damage. For rising stem valves, remember the position of the stem when fully open and fully closed to avoid hitting the dead point when fully open, and to facilitate checking if it is fully closed normally. If the valve falls off or if large debris is embedded between the valve core seals, the position of the stem when fully closed will change.

5. When the pipeline is first used, there may be a lot of dirt inside. The valve can be slightly opened to use the high-speed flow of the medium to flush it out, then gently closed (do not close quickly or forcefully to prevent residual impurities from damaging the sealing surface), and opened again. This process can be repeated multiple times to clean out the dirt before returning to normal operation.

6. For normally open valves, dirt may adhere to the sealing surface, and when closing, the above method should also be used to flush it clean before officially sealing it.

7. If the handwheel or handle is damaged or lost, it should be replaced immediately and should not be substituted with an adjustable wrench to avoid damaging the square of the stem, which could lead to malfunction and accidents during production.

8. For certain media, after the manual valve is closed and cools, the valve components may contract. The operator should close it again after a suitable time to ensure that there are no small gaps in the sealing surface; otherwise, the medium flowing at high speed through the small gap can easily erode the sealing surface.

9. During operation, if it is found that operation is too strenuous, the cause should be analyzed. If the packing is too tight, it can be appropriately loosened. If the stem is misaligned, personnel should be notified for repair. Some valves may experience thermal expansion of the closing component when in the closed state, making it difficult to open; if it must be opened at this time, the valve cover threads can be loosened half a turn to a full turn to relieve stem stress, and then the handwheel can be operated.

2. 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 tightened again, known as 'thermal tightening.' Operators should pay attention to this work, or leaks may easily occur.

2. In cold weather, if water valves are closed for a long time, the water behind the valve should be drained. For steam valves, condensate should also be drained after stopping steam. If there is a drain plug at the bottom of the valve, it can be opened to drain water.

3. Non-metallic valves may be hard and brittle or have lower strength. When operating, the opening and closing force should not be too great, especially not with excessive 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, just enough to prevent leakage, to avoid excessive pressure on the stem, which can accelerate wear and make opening and closing difficult.