When the wedge-type double gate gate valve is closed, the gate plates automatically match the valve seats on both sides and automatically compensate for the processing position error of the wedge angle; the wedge-type elastic gate gate valve relies on the elastic groove in the middle of the gate plate and the axial thrust of the valve stem to compensate for the wedge. Corner processing position error, both achieve better sealing effect.
It is precisely because of its excellent sealing on both sides that the pressure in the middle cavity will rise abnormally in some situations. That is, when high-temperature and high-pressure fluid (liquid or gas) is blocked in the middle cavity of the valve, if the temperature of the upstream side fluid increases , the fluid in the middle cavity will be raised simultaneously by heat transfer. Since the accumulation in the middle cavity cannot be expanded, when the fluid in the blocked middle cavity changes from a cold state to a hot state, the liquid may vaporize rapidly, resulting in a sharp increase in pressure. The increased pressure is often Geometric series.
The consequences of valve overpressure operation are very serious. When the pressure in the central cavity of the valve increases abnormally, the working stress of its pressure-bearing parts and opening and closing parts (such as the service stress of the valve stem and gate frame) will increase sharply, and the driving force of the driving mechanism will be overwhelmed and may even fail to start, causing serious problems. When the valve stem is pulled off, the gate frame is broken, and the motor is burned out, these phenomena are common in many high-pressure and large-diameter gate valves. Many users often complain that this is the gate “seizing”. In fact, the real reason for “seizing” is often Abnormal pressure increase in the middle cavity is the “invisible killer”.
Typical case:
When the wedge double gate valve is used in the water supply system and its bypass of thermal power plants, this type of valve is usually closed after a hydraulic pressure test in a cold state. When the unit is started, when the system temperature rises to 250~300°C, due to the sharp temperature rise, the cold water temperature in the closed middle will rise sharply and vaporize simultaneously, causing the fluid volume to increase and the pressure to rise. At this time, if you want to open the valve, the driving torque must be large enough, or the valve stem assembly must be strong enough, otherwise the valve will often fail. The rod is broken, the gate frame is broken, and the T-slot of the gate is bloomed and broken, which makes the feed water pump unable to start, leading to serious boiler shutdown accidents.
The dangers of abnormal pressure increase:
1) Damage to the valve itself
The strength of the valve housing, valve cover and valve stem parts is generally designed based on the nominal pressure of the valve. When the pressure increases abnormally, the opening pressure will increase exponentially, causing the service stress of the related parts to increase exponentially. When the actual stress of the material When the allowable stress is exceeded, the high-stress parts will fracture and damage, resulting in the valve being unable to open and the entire valve machine being damaged or scrapped.
2) Damage to system security
Obviously, it is very dangerous when pressure-bearing parts such as casings and valve covers are over pressured. Once over pressured, their weak parts may be perforated first, causing leakage of media; their packing and self-sealing rings are often flushed out by high-pressure fluid. , causing a large amount of media leakage. When the medium is high-temperature gas, toxic gas, or harmful gas, it will be more serious and may even cause damage to equipment and personnel.
3) Cause huge losses to the production control process
The normal opening and closing of valves is the key to controlling various industrial processes. Once this control cannot be achieved, the system will be paralyzed and must be shut down for maintenance, which will cause huge direct or indirect losses.
Protective measures:
1) A pressure relief hole is provided inside the valve
The fundamental solution to the abnormal pressure increase in the middle cavity is to balance the pressure in the middle cavity, and opening a pressure relief hole is the most economical and effective solution. Pressure relief holes are opened on the outer circumference of the upstream side gate plate and the inlet side valve seat. When the pressure in the middle cavity increases, the pressure in the middle cavity will automatically release to the upstream side, always keeping the pressure in the middle cavity equal to the pressure on the upstream side, thus Avoid abnormal pressure rise.
2) Install external bypass pressure relief of valve
For valves that have left the factory, external bypass and external pressure relief valves can be used to reduce the pressure in the middle cavity during installation. A bypass with a peripheral stop valve is used to connect the middle cavity and the upstream side. When the main gate is closed, the stop valve can be closed (depending on the temperature change of the middle cavity, it must be opened when it is too high). When opening the main gate valve, the bypass stop valve should be opened first to reduce the pressure in the middle chamber before starting the main gate valve.
3) Install a special pressure relief valve outside the valve
A pressure relief valve is installed outside the middle cavity of the gate valve to control the pressure range. The discharge pressure of the pressure relief valve is set to the rated working pressure of the main valve. When the middle cavity is overpressured, it is automatically discharged to the set pressure, thereby maintaining the safe operation of the main gate valve.
A stop valve is installed in front of the pressure relief valve to facilitate adjustment and maintenance of the pressure relief valve. The pressure setting of the pressure relief valve can usually be considered as 1.33PN (PN is the nominal stress of the system valve).
In addition, when debugging valves, especially electric valves, attention should be paid to the control of the closing stroke and torque, and the closing torque should be adjusted as small as possible to prevent the gate from being wedged; for high-temperature valves, thermal expansion of the valve stem under high-temperature conditions should be considered It is recommended that when debugging high-temperature and high-pressure large-diameter gate valves, the valve stem should be properly retreated after the gate is in place to avoid a real jamming accident.
Conclusion:
