Hazards and protective measures of abnormal pressure rise of gate valves
(1) Opening a pressure relief hole inside the valve
The fundamental solution to abnormal pressure rise in the middle chamber is to balance the pressure in the middle chamber. Opening a pressure relief hole is the most economical and effective solution. As shown in Figure 2, pressure relief holes are respectively opened in the outer circle of the upstream side gate and the inlet side valve seat. When the pressure in the middle chamber rises, the pressure in the middle chamber will automatically release to the upstream side, always maintaining the pressure in the middle chamber equal to the pressure in the upstream side, thereby avoiding the occurrence of abnormal pressure rise.
(2) Valve installation external bypass pressure relief
For ex-factory valves, external bypass during installation and external pressure relief valve can be used to reduce the pressure in the middle chamber, as shown in Figure 3.
A bypass with an external stop valve is 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 in the middle chamber, it must be opened if it is too high). When opening the main gate valve, first open the bypass stop valve, reduce the pressure in the middle chamber, and then start the main gate valve.
(3) Special pressure relief valve installed outside the valve
As shown in Figure 4, a pressure relief valve is installed outside the middle chamber of the gate valve to control the pressure range. The discharge pressure of the pressure relief valve is set as the rated working pressure of the main valve. When the middle chamber is overpressure, it will automatically discharge to the set pressure, thereby maintaining the safe operation of the main gate valve.
Figure 5 Structural design diagram of pressure relief (for reference). 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 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 the closing stroke and torque, and the closing torque should be reduced as much as possible to prevent clogging of the gate; For high-temperature valves, it is recommended to consider the wedge locking of the gate plate caused by the thermal expansion of the valve rod under high temperature conditions. During commissioning of high-temperature high-pressure large-diameter gate valves, it is recommended to properly retract the valve rod after the gate plate is in place to avoid the occurrence of a real bite accident.
Abnormal overpressure is a hidden killer of dual seat gate valves, which seriously threatens the safe operation of equipment and systems, and must be paid attention to; When designing and installing gate valves, especially high temperature, high pressure, and large diameter gate valves, it is necessary to consider the occurrence of abnormal overpressure and take necessary preventive measures.