Discussion on the torque of the valve

     When the gate is closed, due to the different sealing methods of the sealing surface, different situations will occur. For automatic sealing gate valves (including flat gate valves), when the valve is closed, the sealing surface of the gate exactly aligns with the sealing surface of the valve seat, which is the fully open position of the gate valve. However, this position cannot be monitored under the operating conditions of the gate valve, so in actual use, the position where the gate valve closes to the dead point is regarded as the gate valve fully closed position. It can be seen that the fully closed position of the automatic sealing gate valve is determined according to the position of the gate (that is, the stroke). For a gate valve with forced sealing, the gate must be pressed to the valve seat when it is closed. This pressure can ensure that the sealing surface between the gate and the valve seat is tightly sealed, which is the sealing force for forcibly sealing the gate valve. This sealing force will continue to work due to the self-locking of the stem nut. Obviously, in order to provide sealing force to the gate, the torque transmitted by the stem nut is greater than the torque during valve operation. It can be seen that for a gate valve with a forced seal, the fully closed position is determined by the torque applied to the stem nut.

     When the opening of the gate valve is above 10%, the axial force of the gate valve, that is, the operating torque of the gate valve does not change much. When the opening of the gate valve is less than 10%, the pressure difference between the front and rear of the gate valve increases due to the throttling of the fluid. This pressure difference acts on the gate, so that the valve stem needs a greater axial force to drive the gate. Therefore, within this range, the operating torque of the gate valve changes significantly. The gate valve of the elastic gate requires a larger operating torque when it is close to closing than the rigid gate.

     After the gate valve is closed, due to the change of the medium or the ambient temperature, the thermal expansion of the gate valve components will increase the pressure between the gate plate and the valve seat, which is reflected on the stem nut, and it is difficult to open the gate valve again. Therefore, the torque required to open the gate valve is greater than the torque required to close the gate valve. In addition, for a pair of mutually contacting sealing surfaces, the static friction coefficient between them is also greater than the dynamic friction coefficient. Movement requires a greater force to overcome the static friction; due to temperature changes, the pressure between the sealing surfaces increases, and the static friction that needs to be overcome also increases, so that the torque applied to the stem nut when the gate valve is opened Sometimes it will increase a lot.