Protection and harm of abnormal pressure rise of valves
Time: January 8, 2020 18:38:45 Source: Yonglong Valve Reading: zero second
Protection and harm of abnormal pressure rise of valves
When high temperature or high pressure fluid (liquid or gas) is blocked in the middle chamber of pipeline valve, if the temperature of the upstream side fluid rises, and the volume of the middle chamber cannot be expanded, when the blocked middle chamber fluid changes from cold state to hot state, the liquid may vaporize rapidly, which is likely to cause a sharp rise in pressure. Therefore, when purchasing valves, a certain margin value should be enlarged according to the actual service pressure.
Many industrial systems are caused by abnormal pressure rise of valves, that is, the system medium changes from cold state to hot state after startup; The valve core is closed in cold state and opened in hot state. Therefore, if no measures are taken for the system, the damage of abnormal pressure rise to the system is almost inevitable.
The consequences of valve overpressure operation are more dangerous. In case of abnormal pressure rise in the middle chamber of the valve, the working stress of its pressure bearing parts and the opening and closing parts (such as the service stress of the valve stem and the gate frame) will increase sharply, and the driving force of the driving mechanism will be overwhelmed, or even cannot be started. In serious cases, the valve stem will be pulled off, the gate frame will be broken, and the motor will be burned out. These phenomena are common in many high-pressure large caliber gate valves, Many users often complain that this is the "bite" of the ram. In fact, the real reason for the "bite" is often the abnormal pressure rise of the middle chamber.
Daily protective measures for valves:
1. Pressure relief hole is set inside the valve
The key to solving the abnormal pressure rise in the middle chamber is to balance the pressure in the middle chamber. Opening a pressure relief hole is an economical and effective solution. The pressure relief holes are respectively set at the outer circle of the upstream gate and the inlet valve seat. When the pressure in the middle chamber rises, the pressure in the middle chamber will automatically release to the upstream side, and the pressure in the middle chamber will always be equal to the pressure in the upstream side, so as to avoid the occurrence of abnormal pressure rise.
2. External bypass pressure relief for valve installation
For the ex factory valves, external bypass and external relief valve can be used to reduce the pressure of the middle chamber. The bypass with a stop valve can be used to connect the middle chamber and the upstream side. When the main gate is closed, the stop valve can be closed (depending on the temperature change of the middle chamber, it must be opened if it is too high). When opening the main valve, the bypass stop valve shall be opened first, and the main valve shall be started after the pressure of the middle chamber is reduced.
3. Special relief valve installed outside the valve
In order to set the control pressure range of the pressure relief valve outside the middle chamber of the gate valve, set the discharge pressure of the pressure relief valve as the rated working pressure of the main valve. When the middle chamber is overpressure, it will automatically discharge to the set pressure, so as to maintain the safe operation of the main gate valve.
It is the structural design drawing of the pressure relief valve (for reference). The pressure relief valve is installed in front of Ammonia stop valve To facilitate the adjustment and maintenance of the pressure relief valve. The pressure setting of the pressure relief valve can generally be considered as 1.33PN (PN is the nominal stress of the system valve).
In addition, during valve commissioning, especially electric valve commissioning, attention should be paid to the control of closing stroke and torque, and the closing torque should be adjusted as small as possible to prevent wedged ram; For high-temperature valves, the wedge of the gate caused by the thermal expansion of the valve stem at high temperature should be considered. It is recommended that the valve stem should be properly retracted after the gate is in place during the commissioning of high-temperature and high-pressure large-diameter gate valves to avoid real seizure accidents.
valve Hazards of abnormal pressure rise:
1. Damage to the valve itself
The strength of the valve shell, bonnet and stem parts is generally designed based on the nominal pressure of the valve. When the pressure rises abnormally, the opening pressure will be doubled, resulting in a doubling of the service stress of the relevant parts. When the actual stress of the material exceeds the allowable stress, the high stress parts will break, causing the valve to fail to open, and the whole valve will be damaged or scrapped.
2. Damage to system security
Obviously, it is very dangerous when the pressure bearing parts such as the shell and valve cover are overpressure. Once overpressure occurs, the weak parts may be perforated first, causing medium leakage; Its packing and self sealing ring parts are often flushed out by high-pressure fluid, causing a large number of media leakage. When the medium is high temperature gas, toxic gas and harmful gas, it will be more serious, and even cause equipment and personnel injury.
3. Cause huge losses to the production control process
The normal opening and closing of valves is the key to the control of various industrial processes. Once this control cannot be achieved, the system must be shut down for maintenance, which will cause huge direct or indirect losses.
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