| 摘要 |
<p>A valve (10) for controlling the flow of fluid from two alternative supplies in such a manner that reduction or cessation of one supply automatically causes the valve to switch to the other supply comprises a cylinder having a piston (26) slidable therein in a region between a first fluid inlet (17) and a second fluid inlet (20). The piston has working surfaces (27A, 27B, 28A) of differential area such that equal pressure on both sides of the piston biases the piston (26) to one end of the cylinder. The valve has a fluid outlet (16) from the valve, a first fluid flow path from the first fluid inlet to the fluid outlet, and a second fluid flow path from the second fluid inlet to the fluid outlet and including an axial outlet bore (29). The second fluid flow path is closable by an end (28) of the piston abutting against a seal (32) in the cylinder and is opened upon axial movement of the piston towards the first fluid inlet caused when the supply of fluid from the first fluid inlet fails, is arrested or falls below a predetermined limit. The first fluid flow path is closable by the other end (34) of the piston sealing abutting the cylinder. The first working area of the piston presents to the first fluid inlet, depending upon the axial position of the piston in the cylinder either a minor working area (27A) less than the second working area (28A) presented to the second fluid inlet or a major working area (27A, 27B) greater than the second working area (28A) presented to the second fluid inlet. Both fluid paths may include the piston axial bore (29) or they may be separate and parallel.</p> |
