At the stage when the shut-off valve is closed from the fully open position, as the valve flap descends, the fluid creates a pressure differential between the flaps to prevent the flap from falling, and this resistance increases rapidly as the flap falls. When the shut-off valve is fully closed, the pressure difference between the disc and the valve is equal to the working pressure of the medium, and the resistance is maximum. The forced sealing force is added to increase the operating force at the moment when the shut-off valve is closed. During the opening of the valve, the thrust caused by the medium pressure or the differential pressure between the flaps helps to open the valve. It should be noted that the moment at the moment of valve opening may exceed the moment when the valve is closed, because a large static friction force is to be overcome at this time.
When the shut-off valve is opened, when the opening height of the valve flap reaches 25% to 30% of the nominal diameter of the shut-off valve, the flow rate has reached the maximum, which means that the shut-off valve has reached the fully open position, so the full open position of the shut-off valve should be the valve flap. The itinerary is determined. The situation when the shut-off valve is closed and the case of being closed again after closing is similar to that of the forced-sealed gate valve. Therefore, the closed position of the shut-off valve should be determined by increasing the operating torque to a prescribed value.