To make liquid flow in only one direction when it exceeds threshold pressure at the inlet of a valve by providing with a device for making eccentrically a valve main body while approaching on a valve seat, which is acted so as to close a valve yarn when back pressure is acted on the valve main body as well as opening the valve corresponding to prescribed pressure at an inlet.

In a gasification system provided with a pump assembly connected to a sump and a gasification device assembly, a valve V is arranged on the inlet and the outlet of each pump of the pump assembly. Each valve v is provided with a valve seat 78 provided in a passage 76 at an outlet 74, and provided with a valve main body 80 which is brought into separatable contact with the valve seat 78. The valve main body 80 is driven by a device which contains a piston 84 and an air actuator head 90, energized in a valve closing direction by a compression spring 94, and a bellow seal 96 is arranged so as to separate liquid in a valve passage 76 from an operating position. The valve element 80 is opened through the actuator header 90 when pressurized air is led into an air chamber through an air line 97.