Reverse flow prevention apparatus

A reverse flow prevention apparatus comprising:

• an outer box provided with a flow path formed coaxially therewith between an inlet and an outlet, and also provided with a drain outlet in a midstream region of the flow path;
• a diaphragm provided on the inner periphery of the outer box for partitioning the interior thereof into an upstream region of the flow path, and the downstream of the midstream region;
• an inner cylindrical body slidably fitted into the upstream region of the outer box;
• an escape valve provided on a face of an open end of the inner cylindrical body, on the downstream side, so as to be freely attachable to and detachable from a valve seat provided around a valve port formed between the midstream region and a downstream region, and energized in the direction of valve opening by the urging of an escape valve spring;
• and
• first and second check valves provided inside the inner cylindrical body and the downstream region of the outer box, respectively, such that reverse flow of a fluid from the downstream side can be prevented, wherein an effective diameter of the diaphragm and a diameter of the valve seat of the escape valve are set substantially at an equal value so as to maintain a pressure balance between a pressure acting on the escape valve for valve closing and a pressure acting on the escape valve for valve opening while a spring load of a first check valve spring urging the first check valve in the direction of valve closing and that of the escape valve spring are set such that a pressure of the midstream region is maintained always lower by a predetermined value than a pressure on the upstream side.