CONSTANT PRESSURE CONTROL VALVE

PROBLEM TO BE SOLVED: To control hydraulic pressure to prescribed feed water pressure without providing the back tube of a pressure controller, to eliminate the problem of the high pressure feed water to be used after a conventional pump is stopped and to always control a piping system to constant pressure by determining the opening of a main valve element and a valve port by the pressure difference of the pressure applied on back surface of a piston and the pressure of a secondary side piping system. SOLUTION: Because the pressure within a piping system rises higher than the air pressure applied on the piston 61 of a piston room 54 when the use of water of a secondary side water tap is successively reduced, a main valve element 59 moves the side closing a valve port 86, approaches the slide cylinder 89 in an elongation state and the opening with the slide cylinder 89 is reduced. When the pressure of a secondary side piping pressure gradually approaches pilot pressure, the main valve element 59 further approaches the side of the valve port 86 and the opening between the main valve element 59 and the slide cylinder 89 is reduced. Thus, the opening between the slide cylinder 89 and the main valve element 59 is determined by the pressure difference of the pilot pressure of air sealed in the piston room 54 and the secondary side pressure according to the use flow rate and the pressure within the secondary piping system is controlled to constant pressure.