Pressure responsive valves in liquid distribution systems can monitor the flow of the liquid and can particularly detect blockages, leaks and pressure variations.
One device for detecting excessive pressure increase is the secondary valve 72 described in U.S. Pat. No. 3,730,297 issued May 1, 1973 to Francis E. Colgan, entitled "Liquid System Function Indicator," and assigned to the assignee hereof. This valve includes a sealed piston in fluid communication with the distribution system and which is restrained against shifting by a wire having a predetermined burst pressure. The piston has a knife edge that bears upon the wire and that eventually cuts the wire when the pressure in the distribution system increases beyond the burst pressure of the wire. The difficulties with this valve are that it is not reusable and must be replaced after use, that it may not be adjusted and that it may be tested only by destructive testing.
Other pressure detecting devices include detent means for restraining a control valve, as described in U.S. Pat. No. 3,088,489. In the device of this patent, a longitudinally extending spring biases a longitudinally movable camming cone to urge detent balls into registered detent notches in the piston and the cylinder. This detent means positions the piston longitudinally along the cylinder. Excess pressure drives the piston and its camming cone to release the balls from the detent notches, whereby the piston moves under pressure. One limitation of this device is that the biasing force of the spring upon the piston cannot be adjusted without dismantling the entire valve. For other detent devices, see U.S. Pat. Nos. 3,685,813 and 3,218,882.