Emergency shut-off valve is one of the hydraulic control valves. It is composed of main valve, throttle valve, pilot valve, pressure gauge, connecting pipe and other pipe fittings. When the inlet pressure of the main valve is lower than the set pressure of the pilot valve, the pilot valve is now in the open state, the pressure in the upper cover cavity of the main valve is equal to the outlet pressure of the main valve, there is a pressure difference between the inlet and the outlet, and the water flows out from the outlet through the valve port. When the fire pump is started, or the pressure of the variable frequency pump increases and exceeds the set pressure of the pilot valve, the pressure from the inlet of the main valve to the control pipe of the pilot valve increases, the pressure acting on the spring increases, and the valve core is driven downward to close the pilot valve. At this time, the inlet pressure of the main valve is continuously added to the upper cavity of the main valve cover through the throttle valve, because the downward pressure on the valve disc is greater than the upward thrust , the main valve is closed. The key point for your attention is that the emergency shut-off valve is always in the open state when the inlet pressure is lower than the set pressure, and the valve is in the closed state when the inlet pressure is higher than the set pressure.