Operation of Valves (III)

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For valves, one must not only know how to install and maintain them, but also how to operate them. (1) Opening and closing of manual valves: Manual valves are the most widely used type of valves. Their handwheels or handles are designed to be operated using ordinary human strength, taking into account the strength of the sealing surfaces as well as the necessary force required for closing. Therefore, it cannot be moved using a long lever or a long wrench. Some people* are used to using wrenches, and they must be careful not to apply too much force; otherwise, it is easy to damage the sealing surface, or the wrench may break the handwheel or handle. When opening and closing the valve, apply force smoothly without any impact. The various components of certain high-pressure valves that are operated by impact have taken into account this impact force, which is different from that of ordinary valves. For steam valves, they should be preheated and of the condensed water removed before being opened. The opening process should be carried out as slowly as possible to avoid water hammer. After the valve is fully open, turn the handwheel slightly in the opposite direction to ensure a tight fit between the threads, thereby preventing loosening or damage. For straight-stem valves, remember the position of the stem when the valve is fully open and fully closed, to avoid hitting the upper dead center when it is fully open. It also facilitates checking whether it is functioning properly when fully closed. If the valve seat comes loose, or large debris gets trapped between the valve core seals, the position of the valve stem when the valve is fully closed will change. When the pipeline is used for the first time, it contains a lot of dirt inside. The valve can be slightly opened to allow the high-speed flow of the medium to wash away this dirt, after which it should be closed gently (not quickly or forcefully, to prevent residual impurities from damaging the sealing surface). The valve should then be opened again, and this process repeated several times until all the dirt is removed, after which normal operation can resume. For normally open valves, dirt may be stuck on the sealing surface; when closing them, it is necessary to clean them using the aforementioned method before closing them properly. If the handwheel or handle is damaged or lost, it must be replaced immediately; adjustable wrenches cannot be used as a substitute, to avoid damaging the square shape of the valve stem, which could result in poor operation of the valve and lead to accidents during production. In some media, cooling after the valve is closed causes the valve components to contract; therefore, the operator should close it again at an appropriate time to ensure that no gaps remain in the sealing surfaces. Otherwise, the medium will flow rapidly through these gaps, easily eroding the sealing surfaces. During operation, if it is found that the task is too difficult to carry out, the reasons should be analyzed. If the packing is too tight, it can be loosened appropriately; if the valve stem is skewed, personnel should be notified for repair. In some valves, when they are closed, the closing element expands due to heat, making it difficult to open them ； If it must be opened at this time, loosen the valve cover threads by half a turn to one full turn to relieve stress on the valve stem, and then turn the handwheel. (II) Precautions: 1. For valves operating at temperatures above 200°C, since they are installed at room temperature, the temperature rises during normal use, causing the bolts to expand due to heat and increasing the gaps. Therefore, it is necessary to tighten them again; this process is known as \"thermal tightening\". Operators must pay attention to this step, as failure to do so can lead to leaks. 2. When the weather is cold and the water valve remains closed for an extended period, the water accumulated behind the valve should be drained. After the steam valve stops supplying steam, the condensate also needs to be drained. There is a plug at the bottom of the valve, which can be opened to drain water. 3. Non-metallic valves: some are hard and brittle, while others have lower strength. When operating them, the force used for opening and closing should not be too great, especially violent force should be avoided. Be careful to avoid the objects from being bumped. 4. When using a new valve, do not compress the packing too tightly; ensure there is no leakage. This is to prevent excessive pressure on the valve stem, which could accelerate wear and make it difficult to open and close the valve.