【Weekly Topic】What considerations are necessary when operating low-temperature valves, normal-temperature valves, and high-temperature valves? (20110718--20110724)

Operation of manual valves Manual valves are operated using handwheels or handles, and they are widely used in petrochemical plants. Operators should pay attention to the following points: ◆ The direction in which the valve is opened – generally, turning counter-clockwise opens the valve, while turning clockwise closes it. Therefore, pay attention to the direction when opening and closing valves. ◆ The size of the valve handle or knob is designed based on the force required to operate the valve manually. Therefore, the valve can be opened and closed simply by turning it manually; generally, the use of levers or wrenches to operate the valve is not allowed. To prevent excessive force from damaging the sealing surfaces and other components: ◆ When closing gate valves and stop valves to their fully closed position, turn them 1/4 to 1/2 turn in order to ensure a better fit of the threads, thus avoiding over-tightening that could damage the valve parts. ◆ Gate valves and stop valves should not be used for throttling purposes, as this can lead to erosion of the valve plates and sealing surfaces, as well as vibration and leakage, which can damage the valves. ◆ When opening steam valves, do so slowly in order to remove any condensate water from the pipes and prevent water hammer effects. Damaged valves and pipes ◆ Large-diameter gate valves, globe valves, and butterfly valves often come equipped with bypass valves, which serve to balance the pressure on both sides of the valve and reduce the force required to open it. Therefore, when starting to use it, the bypass valve should be opened first, followed by the gradual opening of the main valve. When shutting it down, the bypass valve should be closed first, and then the main valve should be closed gradually. ◆ After some valves are shut off, as the temperature drops, the valve components contract and minor leaks may occur at the sealing surfaces; in such cases, it is sufficient to tighten the valve slightly after it has been shut off. Operation of automatic valves ◆ Steam traps Before putting them into use, any dirt or impurities in the pipes must be removed to prevent blockages that could prevent proper operation. During operation, check the performance of the steam trap; if too much steam is discharged, it indicates that the valve is not functioning properly, while if only water is discharged, it means the valve is working correctly. ◆ Pressure reducing valve: Before activating it, open the bypass valve or flushing valve to clean the pipeline. Once the pipeline is clean, close the bypass valve before turning on the pressure reducing valve. Operation of other valves: Other valves are operated using electric, electromagnetic, pneumatic, or hydraulic devices to open and close them. Remote control is achieved through instruments, electric switches, etc. Therefore, operators should have a thorough understanding of the structural features and working principles of these valves, be familiar with the operating procedures, and be able to operate them independently, which facilitates timely emergency handling in case of accidents. Precautions for valve operation: ◆ Prevent the valves from being opened or closed incorrectly, or from leaking when open or closed. Operators must be fully aware of the function of each valve and its location in the process piping to avoid misoperations, as this could lead to accidents such as material leakage, mixing of materials, or pressure buildup. In the case of a certain refinery unit, steam purging was carried out before it was put into operation. One of the vent valves failed to close properly after being purged; as a result, during operation, hot oil slurry at 370°C flowed out through that vent valve, causing a large fire that led to the shutdown of the unit. 　　◆During seasons with low temperatures below 0°C, for valves that are not in use, it is essential to prevent freezing by promptly removing the plug at the bottom of the valve to drain any accumulated water inside. Insulation measures should be taken for valves that cannot be excluded from operation and those that operate intermittently. 　　◆For valves that operate at high temperatures, when the temperature rises above 200°C, the bolts expand due to heat, which can lead to poor sealing of the valve. In such cases, it is necessary to \"heat-tighten\" the bolts; however, this should not be done when the valve is in its fully closed position. To prevent the valve stem from getting stuck and making it difficult to open again. 　　◆When a valve leaks, it is necessary to first identify the location and cause of the leak. Generally, leaks can be categorized as internal or external. External leaks can be further divided, based on the valve’s external structure, into leaks at the packing area, leaks in the valve cover or body, and leaks in the sealing surfaces or lining materials. Operators should make an accurate assessment; any leaks that can be resolved on their own should be dealt with promptly, while those that cannot be fixed should be addressed by maintenance personnel as soon as possible. 　　◆The valve packing gland should not be tightened too much; it is sufficient for the valve stem to rotate freely. Tightening it too much increases the friction on the valve stem and raises the operating torque. When the valve is in operation, it is generally not permissible to replace or add packing under pressure; otherwise, it may lead to medium leakage and pose a danger.

　(1) High-temperature butterfly valve. Butterfly valves with t＞450°C (2) Medium-temperature butterfly valves. Butterfly valves with 120 C＜t＜450 C (3) Butterfly valves for normal temperature conditions. 1 40C＜t

For valves operating at high temperatures, the bolts expand due to heat, and it is necessary to perform \"thermal tightening\" of these bolts; for valves operating at low temperatures, the bolts contract due to cold, and \"cold tightening\" of the bolts is required; tightening is not needed when the valves are not in use at normal temperatures

1. When operating the valve, pay attention to the direction of opening and closing, as well as to using appropriate force – especially when using tools such as an F wrench – to avoid closing or opening the valve too tightly； Or damage the handwheel or valve handle. 2. After the valve is switched into position, pull it back slightly to prevent excessive opening or closing of the valve. 3. After closing the low-temperature valve, open it slightly after some time and close it again, to prevent the valve from contracting due to the cooling of the material, which could result in the valve not sealing properly. 4. When a high-temperature valve is used for the first time, the bolts should be “heat-tightened” to prevent leakage. 5. When putting high-temperature valves into use, it is important to open them slowly and preheat the pipeline downstream of the valve. 6. When the valve is in operation, it must not be replaced or have its packing changed under pressure, nor should it be struck, as this could lead to medium leakage and pose a danger.

For valves operating at low temperatures, it is necessary to check whether there is any leakage in the packing after they are put into use. Valves operating at high temperatures are closed tightly when shut down, but they become looser after cooling over time

Operate the valve in accordance with its characteristics; be careful to retract it slightly after it has been fully opened or closed to prevent excessive movement of the valve. After the low-temperature valve is closed, open it slightly after some time and close it again, to prevent the valve from contracting due to cooling, which could result in it not closing properly. When high-temperature valves are put into use for the first time, the bolts should be \"heat-tightened\" to prevent leakage. When putting high-temperature valves into use, care should be taken to open them slowly and preheat the pipeline downstream of the valve. When the valve is in operation, it must not be replaced or have its packing changed under pressure, nor should it be struck, as this could lead to medium leakage and pose a danger to high-temperature butterfly valves. Butterfly valves with t＞450°C (2) Medium-temperature butterfly valves. Butterfly valves with 120 C＜t＜450 C (3) Butterfly valves for normal temperature conditions. 1 40C＜t

Reply to 1# security: 1. For high-temperature 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 between them. Therefore, it is necessary to tighten them again; this process is known as \"thermal tightening\". Operators must pay attention to this step, as otherwise leaks can occur easily. 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 prevent the items from being bumped. 4. When using a new valve, do not compress the packing too tightly – just enough to prevent leakage. This is to avoid excessive pressure on the valve stem, which could accelerate wear and make it difficult to open and close the valve

When replacing bolts on low-temperature valves, it is essential to be careful to avoid using ordinary bolts, as this could cause the valves to become brittle and break. Also, if the valves can be easily opened and closed by hand, do not use an F wrench for that purpose, as this can lead to leaks at the sealing surfaces.

Coming here is to learn*, what was said upstairs is spot-on

For valves operating at high temperatures, the bolts expand due to heat, and it is necessary to perform \"thermal tightening\" of these bolts; for valves operating at low temperatures, the bolts contract due to cold, and \"cold tightening\" of the bolts is required; tightening is not needed when the valves are not in use at normal temperatures