GB762328A - Improvements in or relating to fuel pumping systems for aero engines - Google Patents

Abstract

762,328. Centrifugal pumps; gear pumps; gas turbine and jet propulsion plant. THOMPSON PRODUCTS, Inc. Sept. 2, 1954 [Nov. 19, 1953], No. 25526/54. Classes 110 (1), 110 (2) and 110(3). A fuel pumping system for an aircraft engine having a burner unit comprises a multi-stage pump having a casing formed with an opening by means of which a by-pass duct is connected between the discharge side of the final pump stage and a point of intermediate pressure between successive stages of the pump. Preferably, the pump comprises a centrifugal first stage, followed by a gear, piston or other positive-displacement stage. Any stage may comprise pumps in parallel. The discharge may pass through a heat exchanger to cool oil used to lubricate gear-boxes and bearings. Fig. 3 shows a centrifugal pump 1 feeding gear pumps 2a, 2b, 2c in parallel, the common discharge passing by way of a flow control unit 3 to the burners 4 of an aircraft gas turbine engine. At high altitude, excess fuel is by-passed from the control unit 3 by way of a duct 5 to a point P between the centrifugal and gear pump stages. The gear pumps 2a, 2b have additional by-pass ducts 9, 10 opened at predetermined altitudes by alltitude-responsive valves 11, 12 respectively. The gear pumps 2a and 2b may be of different capacities, to give the assembly some of the advantages of a variable-displacement pump. In Fig. 4, main burners 4 and reheat burners 4a are supplied by separate pump units, the by-pass ducts 5, 5a of which are interconnected at 15 through a constriction 16. This ensures that, even though the flow to the reheat burners 4a is cut off completely at the inlet to the pump 13a, the gear pumps 2a do not run completely dry. Non- return valves 14, 14a are provided. In a modification, Fig. 6 (not shown), the valves 14, 14a are omitted and the reheat fuel pump comprises one gear pump 2a instead of two, the flow through the by-pass duct of the reheat fuel pump being controlled by a valve which opens when the delivery pressure reaches the re-circulating value. In the embodiment shown in Fig. 5, the main and reheat pumps have a common centrifugal first stage 1a which forms part of the reheat pump. The main and reheat pump may be separately engine-driven or incorporated in a single unit having a common drive. Fig. 8 shows a fuel pump comprising a centrifugal first stage, followed by two gear pump stages operating in parallel. The input shaft 38 is splined to the tubular hub 36a of the driving gear 36 of one of the gear pumps, and is internally splined to one end of a solid shaft 40, the other end of which is splined to the tubular hub 34a of the driving gear 34 of the other gear pump. Shear necks 42, 43, 44 permit uninterrupted operation of one gear pump in the event of binding or seizure of the other gear pump. The impeller 23 of the centrifugal first stage is driven from the tubular hub 34a through gears 45, 46, the stub shaft 47 carrying the impeller being journalled on an extension 48 of the tubular hub of the driven gear 35 of the adjacent gear pump. Passages (not shown) are provided to connect the discharge of the centrifugal pump to the inlets of the gear pumps and to connect the outlets of the gear pumps, by way of non-return valves, to a common discharge. Each pump gear 34, 35, 36, 37 is journalled at one end in a bushing 49 which is pressed into sealing engagement with the gear by fuel at the discharge pressure, introduced into a chamber 52 through a passage (not shown). Initial loading of the bushings 49 is provided by springs 54.