After the flanged butterfly valve
is tightly closed, the thermal expansion of the valve components will increase the pressure between the gate and the valve seat due to changes in the medium or ambient temperature. This force is reflected on the valve stem thread, which makes it difficult to open the valve again, so the torque required to open the valve (that is, the torque applied to the valve stem nut) is generally higher than the torque required to close the valve. come big. In addition, for a pair of sealing surfaces in contact with each other, the coefficient of friction between them is also larger than the coefficient of kinetic friction, so to make them produce relative displacement from a static state, a larger force must be added to overcome the static friction force. This situation is more pronounced in valves that are closed and then reopened. Because the temperature change increases the pressure between the sealing surfaces, the static friction to be overcome also increases, so when the valve is closed and reopened, the torque required to be applied to the stem nut is larger than the torque required to close the valve. , and sometimes much larger. In some gate valve
s or flanged butterfly valves, the stem nut is set on the gate, and the handwheel or electric device drives the valve stem to rotate to make the gate rise and fall, and the valve stem only rotates without lifting. This gate valve is called a rotary stem valve or a dark stem gate valve.
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