Patent GB-736003-A (GB)
Title: Improvements in or relating to pressure-ratio sensitive mechanisms
Inventor: COOK HENRY
IPC: G01L15/00
Filed: 1951-10-09
Abstract: 736,003. Pressure gauges. ROLLS-ROYCE, Ltd. Jan. 8, 1953 [Oct. 9, 1951], No. 23524/51. Class 106 (2). [Also in Group XXIX] A mechanism sensitive under all conditions of ambient atmospheric pressure to the ratio of the absolute pressures at two compressible fluidpressure sources, comprises a conduit connected to one source having a pair of restrictive orifices in flow series with at least the downstream orifice choked, a pressure-responsive device . subjected on one face to a force due to the pressure between the orifices and on its other face to a force due to the absolute fluid pressure at the second source and means actuated by the pressure-responsive device to adjust one of the loads to balance it against the other load. Since in a choked orifice the non-dimensional mass flow through the orifice is constant it may be shown that for a pair of choked orifices in series in which one is fixed and the other variable, the ratio of the pressures on the upstream sides of the two orifices has a specific value for each value of the variable orifice. In one embodiment, Fig. 1, a conduit 10 supplied with gas from a first or higher pressure source and having choked orifices 11, 12, is connected between the orifices to the upper end of a cylinder 15 slidably mounting a floating piston 16 having equal areas on its opposite faces. The piston, subjected on its lower face to the pressure of gas from a second or lower pressure source, actuates a valve member 13 controlling the orifice 12 and mounts a pointer 18 which moves over a scale 19. Should, e.g., the pressure of the gas from the first source increase there will be a proportional increase in pressure between the orifices resulting in a downward movement of the piston 16 and increased opening of the orifice 12 until the pressure above the piston is reduced and the pressures on the two sides of the piston are again in balance. Thus, the piston moves to a new position corresponding to the increased pressure ratio, there being only one position of the piston and hence of the valve 13 and pointer 18 for each pressure ratio. In a modification, the downstream orifice only is choked and fixed in area whilst the area of the upstream orifice is varied by a needle valve operatively connected to the piston so that on an increase in pressure at the first source the valve moves to decrease the area of the upstream orifice. In another embodiment, Fig. 2, a casing 20 divided into chambers 21, 22, 23, by equal area diaphragms 24, 25, connected together by a rod 26 is supplied with high pressure gas and low pressure gas through inlets 29, 34 respectively. The high pressure gas passes through fixed choked orifices 27, 28 and the chamber 21 to an outlet pipe 30 and the low pressure gas passes through a fixed choked orifice 31, the chamber 23 and a variable choked orifice 32 controlled by a needle valve 33 to the pipe 30. The needle valve 33 is connected to a piston 42 having a bleed hole 44 and slidable in a cylinder 41 having a connection 43 to a liquid pressure source, e.g., engine fuel pump, and a connection 40 to a valve block 35 fixed in the chamber 22 and controlling the area of a transverse hole 39 in the rod 26. In the equilibrium position of the mechanism, the valve 33 is adjusted until the pressures in the chambers 21, 23 are equal, the hole 39 then being open sufficiently to maintain the piston 42 in balance. Should, e.g., the pressure of the high pressure gas increase, the pressure in chamber 21 will increase proportionately resulting in rightward movement of the diaphragms and increased opening of the hole 39. Hence pressure to the left of the piston 42 decreases and the piston moves leftwardly so that the valve 33 moves towards closure until the pressures in the chambers 21, 23 are again balanced at a now different ratio of gas pressures and the area of the hole 39 is again sufficient to restore the piston 42 in a new posi. tion of balance. In a practical application, guide vanes 50, Fig. 3, of an axial-flow compressor having a large compression ratio and operable over a large speed range are adjusted so as to prevent stalling at the inlet stages of the compressor as its speed varies. Each vane 50 is mounted on an arm 51 carrying a peg 52 which engages in a corresponding slot in a ring 53 operatively connected to the piston 42 so that for each position of the piston and hence the corresponding pressure ratio the angle of incidence of each vane 50 is adjusted accordingly. In another application, the mechanism of Fig. 2 may act as a governor to control, e.g., the outlet area of a compressor to maintain a desired compression ratio or corrected speed of the compressor independent of intake temperature, inlet pressure, or shaft torque. The piston 42 in this case is not connected to the valve 33 which is manually movable to adjust the orifice. The piston 42 is connected to some device, e.g., a throttle in the compressor outlet to adjust the outlet pressure from the compressor.