Principle: it is a combination of direct action and pilot principle, when there is no pressure difference between the inlet and outlet, after the power, the electromagnetic force directly to the pilot small valve and the main valve closed parts in turn up, the valve is opened.
When the inlet and outlet to start the pressure difference, after the power, the electromagnetic force pilot small valve, the lower chamber pressure of the main valve rises, the upper chamber pressure drops, so as to use the pressure difference to push the main valve upward; When the power is off, the pilot valve uses the spring force or medium pressure to push the closing part, moves down, so that the valve is closed.
