A seal device that prevents the medium from leaking into the stuffing box when the valve is fully open is referred to as backseat.

When the gate valve, the globe valve and the throttle valve are in the closed state, there is no or little pressure in the body cavity, thus the pressure on stem packing is small.

When it is opened, the packing is subjected to the working pressure. If there is backseat structure, the working medium pressure can be prevented from acting on the sealing packing, and the life of the sealing packing can be prolonged, so that the valve does not have an external leakage.

Another function is that when there is leakage at the sealing packing, the valve can be fully opened to seal the upper sealing portion, so that the packing gland or the packing sleeve can be loosened to replace packing. Therefore, the upper sealing device is required for the gate valve, the globe valve and the throttle valve.