Patent GB-719455-A (GB)
Title: Improvements in or relating to servo fluid flow control systems
Inventor: ANDREWS WALTER AGNEW
Filed: 1950-09-29
Abstract: 719,455. Gas turbine plant. ROLLS-ROYCE, Ltd. Sept. 11, 1951 [Sept. 29, 1950], No. 23953/50. Class 110(3) [Also in Groups XXIX and XXXVIII] In a gas turbine fuel system controlled by a fluidpressure servomotor the latter is controlled in part by a pressure produced by the rapid alternate opening and closing of a valve whose operation is such that the ratio of the time for which it is open to the time it is closed is varied under manual control or in accordance with variations in an operating condition such as exhaust gas or turbine temperature, atmospheric or compressor delivery pressure, or rotational speed of the turbine. As shown in Fig. 1, a spring-closed valve 13 controlling flow of servo fluid in a conduit 10 is opened and closed by the alternate energization and de-energization of a solenoid 15 controlled by a switch 17a which is actuated by the joint effect of two coils 19, 20. The coil 20 carries a pulsating " saw-tooth " current of a frequency of about 20 c.p.s. produced by a circuit 22 and tending to open the switch 17a, whilst the coil 19 carries a variable control current which tends to close the switch and which may be adjusted manually or automatically by the variations of an operating condition. Thus, owing to the " sawtooth " formation of the current in the coil 20, the ratio of the time for which the switch 17a is closed to the time for which it is open depends upon the magnitude of the current in the coil 19. This is varied by a potentiometer 29. Alternatively, in the case of temperature control, a thermocouple may be connected to the coil 19 through a comparator and an amplifier. In the fuel system shown in Fig. 4, an engine driven fuel pump 41 having a swashplate 54 adjustable bya spring-loaded piston 49a is controlled by barometric pressure, fuel pump delivery pressure, and exhaust temperature. Barometric pressure acts on an evacuated bellows 75 which has an abutment on a spring-loaded lever 70 carried by a flexible diaphragm 71, whilst fuel pump delivery pressure, transmitted through a conduit 84, acts on a plunger 81 also having an abutment with the lever 70. A valve member 67 carried by the lever 70 controls a leak-off passage'68, 66 from a chamber 49d at one side of the piston 49a, this chamber having a restricted connection 64 with the fuel pump delivery whilst a chamber 49c at the other side of the piston ,is subject to the full delivery pressure of the pump. The temperature control, which reduces the fuel flow in the event of excessive exhaust temperature, comprises a thermocouple 87 and electrical control apparatus 88 as previously described for imparting to a solenoid 15 a pulsating current and thereby operating a valve 85 controlling a leak-off from the piston 81, the ratio of the time for which the valve 85 is open to the time for which it is closed determining the magnitude of the mean load applied by the piston 81 to the lever 70. The valve 85 is normally open but becomes operative on the attainment of a certain temperature. Specification 577,016, [Group XXIX], is referred to.