When the fire valve is closed, the sealing surface can only be sealed by the medium pressure, that is to say, the sealing surface of the ram is pressed to the valve seat on the other side by the medium pressure to ensure the sealing of the sealing surface, which is self sealing. Most of the valves are sealed by force, that is, when the valve is closed, the ram shall be forced against the valve seat by external force to ensure the tightness of the sealing surface.

The gate of the fire valve moves in a straight line with the stem, which is called the rising stem valve (also known as the rising stem valve). Generally, there is trapezoid thread on the lifting rod. Through the nut at the top of the valve and the guide groove on the valve body, the rotating motion is changed into linear motion, that is, the operating torque is changed into the operating thrust.

When opening the valve, when the lifting height of the ram is equal to 1:1 times of the valve diameter, the flow passage is completely unblocked, but this position cannot be monitored during operation. In actual use, the vertex of the valve rod is taken as the mark, that is, the position that can not be opened is taken as its full open position. In order to consider the phenomenon of locking due to the change of temperature, the valve is usually opened to the vertex position, and then turned back 1 / 2-1 circle as the position of fully open valve. Therefore, the full open position of the valve is determined by the position of the RAM (i.e. stroke).