Abnormal Pressure Increases in Middle Cavities of Gate Valves

Gate valves usually have structural design such as single discs, double discs, wedges and  parallel. Double wedges Z42 and Z62 series and elastic wedge Z40 and Z60 series are the most widely used. The wedge automatically fits the valve seats on both sides when the Z42 and Z62 series valves are closed, automatically compensating the processing position error of the wedge angle, while Z40 and Z60 series rely on the elastic groove in the middle of the wedge and the axial thrust of the valve stem to compensate the processing position error of the wedge angle. They can both achieve the purpose of good sealing. However, the pressure in the middle cavity will increase abnormally in some occasions due to their excellent sealing on both sides. When the high temperature and high pressure fluid is blocked in the middle cavity of the valve, if temperature of the upstream fluid increases, the fluid in the middle cavity will be simultaneously increased by heat transfer. Because the volume of the middle cavity cannot be expanded, when the blocked fluid in the middle cavity changes from a cold state to a hot state, the liquid may quickly vaporize, resulting in a sharp increase in pressure. The increased pressure is often geometric progression.
 
The consequences of overpressure of valves are very serious. When pressure in the valve cavity is abnormally increased, the working stress of the pressure bearing parts and opening and closing parts such as the use stress of the valve stem and wedge frame will increase sharply, and the driving force of the driving mechanism will be overwhelmed, or even unable to start. The valve stem and wedge frame will be broken, and the motor will be burned in a serious situation. These are common in many high-pressure large diameter gate valves.
 
The harm of abnormal pressure increases
The abnormal pressure increase in many gate valves has the following harm in three aspects: 
(1) Damage to the valve itself
The strength of valve housings, bonnets and valve stem parts is generally designed according to nominal pressure of the valve. When the pressure rises abnormally, the opening pressure will increase exponentially, resulting in an exponential increase in the use stress of related parts. When the actual stress of the material exceed the allowable stress, the high stress parts will be broken and damaged, which will cause the valve not to be opened, and the whole valve will be damaged or scrapped.
 
(2) Damage to system security
Obviously, an increase in pressure for pressure bearing parts such as valve housings and bonnets is very dangerous. Once pressure increases occur, the weak parts may be perforated first, causing medium leakage. The packing and self-sealing ring parts are often flushed by high-pressure fluids, causing a large number of medium leakages. When the media are high temperature gas, toxic gas and harmful gas, the situation will be more serious, and even cause damage to equipment and personnel.
 
(3) Great losses to the production control process
The normal opening and closing of valves are the keys to controlling various industrial processes. Once this control cannot be achieved, the system must be shut down for maintenance, otherwise great direct or indirect losses will be caused.
 
Protective measures
Starting from the design, installation and adjustment, it is completely possible to fundamentally eliminate the abnormal pressure increase in the cavity. Three common solutions provided for users are as the following:

(1) Setting a pressure relief hole inside the valve

The fundamental solution to the abnormal pressure rise in the middle cavity is to balance the pressure in the middle cavity. Setting a pressure relief hole is the most economical and effective solution. A pressure relief hole is set at the side wedge of the upstream and on the outer circle of the inlet side valve seat. When pressure in the middle cavity rises, pressure in the middle cavity is automatically discharged to the upstream side, and pressure in the middle cavity is always equal to the pressure on the upstream side, so as to avoid the occurrence of abnormal pressure rise.
 
(2) Installing an external bypass pressure relief valve
For the valves that have been shipped from the factory, the external bypass and pressure relief valve can be used to reduce the pressure in the middle cavity. A bypass with a shut-off valve is used to connect the middle cavity with the upstream side. When the main gate is closed, the shut-off 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, open the bypass stop valve first, and then start the main gate valve after reducing the pressure in the middle cavity.
 
(3) Installing a dedicated pressure relief valve outside the valve
Set a pressure relief valve outside of the middle cavity of gave valves to control the pressure range. Set the discharge pressure of the pressure relief valve to the rated working pressure of the main valve. When there is an increase in the middle cavity, the pressure relief valve will discharge to the set pressure to maintain safe operation of the main gate valve.
 
A globe valve is installed in front of the pressure relief valve to facilitate the adjustment and maintenance of the pressure relief valve. The pressure setting of the pressure relief valve can usually be considered according to 1.33PN (PN is the nominal stress of the system valve).
 
In addition, when debugging the valve, especially the electric valve, pay attention to control of the closing stroke and torque, and reduce the closing torque as much as possible. The thermal expansion of the valve stem at high temperature should be considered for high temperature valves. It is recommended that when the high temperature and high-pressure large diameter gate valve is debugged, the valve stem should be properly retracted after the valve plate is in place to avoid block accidents.
 
Conclusion
Abnormal overpressure is the invisible killer of double seats gate valves, which seriously threatens the safe operation of equipment and systems, and must be paid attention to. The occurrence of abnormal overpressure must be considered for gate valves, especially high temperature, high-pressure and large diameter gate valves concerning designs and installation, and take the necessary precautions.