Patent ID: 7434780

Claim:
In a pilot-type two-port valve having a main valve mechanism including a first port and a second port serving as an inlet and an outlet for liquid, respectively, a valve seat formed in midstream of a main flow passage connected between both the ports, and a valve member operative to open or close the valve seat; and a valve drive mechanism including a piston, connected to the valve member via a rod, and first and second piston drive chambers formed on both sides of the pistons and operative such that supplying pilot fluid to or discharging the same from at least one of the first and second piston drive chambers allows the piston to operate for thereby opening or closing the valve member: the two-port valve comprising a water hammer preventing mechanism, operative to prevent the occurrence of water hammer resulting from the valve member closing the valve seat, which has an orifice formed in the first piston drive chamber, from which fluid is discharged when the piston drives the valve member in a direction to close a valve, for limiting the flow rate of discharge fluid whereby when the valve member comes close to the valve set, the flow rate of the discharge fluid is restricted to the flow rate of fluid admitted through the orifice and, wherein the water hammer preventing mechanism comprises a one-way seal member mounted on the piston and operative such that when the piston moves to a position to allow the valve member to get close to the valve seat, the seal member blocks a portion of a flow path for the discharge fluid in a discharge direction to cause the discharge fluid to be discharged under a status where the flow rate of the discharge fluid is restricted.