The emergency shut-off valve is one of the hydraulic control valves. It is composed of main valve, throttle valve, pilot valve, pressure gauge and other pipe fittings. When the inlet pressure of the main valve is lower than the pressure set by the pilot valve, the pilot valve is in the open state at this time, the pressure in the upper chamber of the main valve is equal to the outlet pressure of the main valve, and there is a pressure difference between the inlet and the outlet, and the water flows out through the valve port through the outlet. . When the fire pump starts, or the pressure of the variable frequency pump rises and exceeds the set pressure of the pilot valve, the pressure of the control pipe is increased by the main valve inlet and the pilot valve, and the pressure acting on the spring is increased, and the pressure is pressed downward to drive the spool. Move down to close the pilot valve. At this time, the main valve inlet pressure is continuously replenished to the upper chamber of the main valve cover through the throttle valve. Since the downward pressure of the valve flap is greater than the upward thrust, the main valve is closed. The key point to note is that the inlet pressure is less than the set pressure and the emergency shut-off valve is always open. The inlet pressure is greater than the set pressure and the valve is closed.