Patent GB-680209-A (GB)
Title: Improvements in and relating to automatic regulating systems
Filed: 1949-05-02
Abstract: 680,209. Gas turbine plant. BRITISH THOMSON HOUSTON CO., Ltd., May 2, 1949 [May 6, 1948], No. 11686/49. Class 110(iii) [Also in Group XXIX] Apparatus for automatically regulating the fuel supply to a thermal power plant in accordance with temperature changes comprises an input member such as the piston 12 responsive to temperature changes, and a balancing lever 55 connected at a first point 32a to the input member, at a second point 35c to a servo member such as the piston 35a, and at a third point 56 to means 34 for controlling the movement of the servo member and also to an output member constituted by the lever 57, the arrangement being such that the point 56 is normally limited to a predetermined small range of movement in which the output lever 57 is not operated but when the temperature changes at more than a predetermined rate or exceeds a predetermined value, the point 56 moves beyond its normal range and the output lever is operative to change the fuel supply. As shown the power plant comprises a gas turbine driving a power shaft 7 and having burners fed through conduits 3, 4, 5 by a pump 2 the displacement of which is regulated by a lever 2a either manually through a cam 59 or automatically according to the temperature in the exhaust hood 6. A long tube 8 containing an expansible fluid such as argon gas is arranged in the hood 6 and connected to a bellows 10 which thus responds to the temperature in the hood 6. The bellows is connected to an intermediate point 31 of a floating lever 30 connected at one end 32 to a pilot valve 14 controlling the double-acting servo-piston 12 connected to the other end of the floating lever through a dowel 33. The balancing lever 55 has a pin 56 engaging a slot in the output lever 57 which is connected to the pump control lever 2a. The lever 57 is biased by a spring 61 against the manually-operated cam 59 and is connected at an intermediate point 58 to a pilot valve 47 controlling the flow through conduits 50a, and 52 to the servopiston 35a. The conduit 52 is unrestricted but the conduits 50a. 50b are connected to a restricting device 36 comprising an outer housing 36b with ports 36d, 36e which are intermittently connected by a groove 36f in a rotary bushing 36a driven by a gear 36c. A double-acting accumulator 54 is connected in parallel with the restricting device or "chopper" valve 36 across the conduits 50a. 50b. The accumulator comprises a spring-centralised piston 54b in a cylinder 54a, the movement of the piston being limited by adjustable stop screws 54e, 54f. The cylinder 54a has unrestricted communication with the conduit 50a but communications with the conduit 50b is limited by a restricting device 54h which may be constituted as a fixed orifice or as a manually adjustable valve or as a rotary " chopper" valve similar to the valve 36. Movement of the pilot valve 47 is limited by an adjustable screw 63 in the upward direction and by contact of an adjustable collar 46 with the end surface 48 of a cylinder 41 in the downward direction. The pilot valves 14, 47 are each constituted as three-land piston valves and operate in ported cylinders 19, 41 which are continuously rotated to prevent sticking through gears 21, 42, 36g, 36c. Movement of the bellows 10 following a temperature change results in an amplified movement of the piston 12. This results in movement of one end of the lever 55 and as the other end is initially stationary, the pilot valve 47 is moved from its neutral position to control the piston 35a. Due to the " chopper valve " 36 the piston 35a moves at a preselected rate. If this rate of movement of the piston 35a is faster than that of the piston 12, the valve 47 tends to be returned to its neutral position and the rate of movement of the piston 35a is slowed down until it equals that of the piston 12. Thus the temperature is permitted to increase at a slow rate without any effect on the output fever 57 provided the temperature remains below a predetermined maximum valve. The latter is actually determined by an adjustable screw stop 53 which limits the total movement of the piston 35a in a direction permitting increase of temperature. Under these conditions, the output lever may be controlled manually by the cam 59 to select any power output desired. If the rate of change of temperature corresponds exactly with the rate of movement of the piston 35a, the operation is as described above but the pilot valve 47 does not shut off the supply of fluid to the piston 35a. If the rate of change of temperature is greater than that corresponding to the rate of movement of the piston 35a, the pilot valve 47 is ultimately carried against one of the stops 48, 63 and the resultant movement of the lever 57 adjusts the fuel pump in a direction to reduce the rate of increase or the rate of decrease of temperature as the case may be. In the latter case the manual control is overcome by the lever 57; in the former the lever 57 merely leaves the cam 59. The stop 53 operates to limit the maximum temperature whether the rate of increase of temperature is above the preselected rate or not. Thus in the latter case the piston 35a is first arrested by the stop 53 and the continued downward movement of the piston 12 causes the pilot valve 47 to be arrested by the surface 48 whereupon the lever 57 is adjusted to decrease the fuel supply. If the rate of temperature increase is above the preselected rate, the collar 46 engages the surface 48 first and the lever 57 will be moved to decrease the fuel supply so as to reduce the temperature change to the selected rate. When eventually the piston 35a engages the stop 53, the lever 57 will be operated to reduce the fuel supply still further so that the maximum temperature is not exceeded. The purpose of the hydraulic accumulator 54 is to permit a certain initial instantaneous change in the temperature before the rate of change control takes effect. The surge of pressure in the conduit 50a or 50b causes the piston 54b to move against one of the stops 54e, 54f which determine the size of the initial instantaneous change permitted before the piston moves at the preselected rate. The rate of change at which these initial movements are permitted may be limited by the restriction 54h. The lost motion at the various pivots, the slight compressibility of the hydraulic fluid, the expansion of the conduits under the surge of pressure, or certain leakage paths in the hydraulic system are all factors which inherently permit a sudden initial change in the temperature condition before the rate of change control takes effect and they may be sufficient to dispense with the accumulator 54. The stop 48 limits the instantaneous increase in the fuel supply which the manual control can call for.