| 摘要 |
<p>The valve spool of a three-way pilot valve responds to changes in pressure in a pipeline downstream from a shutoff valve that is biased to close by spring pressure and has a control cylinder to admit pressurized control fluid to open in opposition to the spring pressure. When the pipeline pressure is in the normal range the valve spool 18 at a normal position where it places the pressurized control fluid in communication with the control cylinder of the shut-off valve to keep the shut-off valve open. When the pipeline pressure drops below the normal range the valve spool moves in one direction from its normal position to a first alternate position to cause the shut-off valve to close and when the pipeline pressure rises above the normal range the valve spool moves in the opposite direction to a second alternate position to cause the shut-off valve to close, At each of the two alternate positions of the valve spool, it cuts off the source of pressurized fluid and at the same time vents the control cylinder of the shut-off valve to close the shutoff valve. When the pipeline pressure returns to the normal pressure range, the valve spool returns automatically to its normal position to open the shut-off valve.</p> |
