GB1249864A - Liquid pumping apparatus for supplying fuel to a gas turbine engine - Google Patents

Abstract

1,249,864. 864. Centrifugal pumps; gas turbine engine fuel systems. DOWTY FUEL SYSTEMS Ltd. Jan.23, 1969 [Jan.23, 1968], No.4316/68. Headings F1C and F1G. Liquid fuel is supplied to the burners of a gas turbine engine by a centrifugal pump followed by a positive displacement pump, control means responsive to the flow of fuel to the burners being operable to regulate the effective delivery of the positive displacement pump when the flow demand is less than that which the positive displacement pump is capable of delivering at the speed at which it is driven, and the centrifugal pump being driven at a variable speed which is responsive to the flow to the burners whereby when the flow demand exceeds that which the positive displacement pump is capable of delivering, the centrifugal pump is driven at a speed at which it develops the pressure required to sustain the flow demand, the excess of demanded flow over displacement of the positive displacement pump being bypassed across the latter. Fuel is supplied to burners 7, Fig.1, by a centrifugal pump 2 driven by an air turbine 71, a line 3, an engine-driven positive displacement pump 4 and a line 5 containing a manual throttle valve 38. At low engine speeds, as when the engine is being started or run at low power on the ground, the engine compressor delivery pressure is insufficient to drive the air turbine 71 at a speed at which the centrifugal pump 2 develops the required delivery pressure, the remainder of the required delivery pressure being developed by the positive displacement pump 4. Under these conditions the displacement of the pump 4 exceeds the controlled flow, the excess flow being recirculated to the line 3 by a by-pass valve 22 which is regulated by the pressure difference across the centrifugal pump 2. At higher engine speeds and high flow demand, the controlled flow exceeds the displacement of the positive displacement pump 4, but by then the compressor delivery pressure is sufficient to drive the centrifugal pump 2 at a speed at which it develops the required pressure. The centrifugal pump pressure is then high enough to close the by-pass valve 22 fully, and the excess of centrifugal pump delivery over the displacement of the positive displacement pump 4 is by-passed across the latter through a non-return valve 25. At high altitude and high engine speed, the flow demand is reduced below the displacement of the pump 4, and a servovalve 47 responsive to the pressure drop created across the throttle valve 38 by the flow in the delivery line 5 acts through a servopiston 15 to move a butterfly valve 74 to reduce the speed of the air turbine 71, whereby the pressure developed by the pump 2 is reduced to a level at which it again regulates the opening of the by-pass valve 22. In another embodiment, Fig.2, the excess delivery of the positive displacement pump 119 occurring at low engine speeds is recirculated to the delivery line 121 of the centrifugal pump 110 via a by-pass valve 127, 128. The flow passes to the burners via a valve 125, 126 and a manual metering valve 132, 134 in series. The valves 125, 126 and 127, 128 are combined, with a common spool 124 which is loaded downwardly by a spring 137 seated against the metering valve plug 132 and upwardly by the pressure drop across the metering valve 132, 134, this pressure drop appearing as a pressure difference between the chambers 135 and 139. The valve spool 124 is therefore sensitive to the flow through the metering valve 132, 134 and the position of the metering valve plug 132. The combined pressure drop across the valves 125, 126 and 132, 134 acts on a piston 117 to tend to close the air admission valve 153 of the air turbine 151 during the pump 110. As in the previous embodiment, a non-return valve 141 opens when the flow exceeds the displacement of the positive displacement pump 119.