GB2523196A

GB2523196A - Gear pump

Info

Publication number: GB2523196A
Application number: GB1402872.4A
Authority: GB
Inventors: David Porter
Original assignee: Rolls Royce Controls and Data Services Ltd
Current assignee: Rolls Royce Controls and Data Services Ltd
Priority date: 2014-02-18
Filing date: 2014-02-18
Publication date: 2015-08-19
Also published as: GB201402872D0

Abstract

A gear pump comprising a plurality of gear pairs 30, each pair having a respective input port and output port 34, and a connector device 36 to changeably select which output port(s) is connected to a base load fluid line and connect the other output port(s) to an additional load fluid line. Preferably the pairs are equally sized and are angularly spaced around a central connector device which is rotatably operable to changeably select the output ports. Preferably an indexing actuator operates the connector device and sequentially selects the output port(s) connected to the base load fluid line after each period of operation of the pump. Also claimed is an aero engine utilising the pump to pump fuel to a control arrangement which returns excess fuel received from the additional load fluid line back to the pump. By switching the output ports between the base load fuel line and the additional load fuel line intermittently, the gears wear at substantially the same rate and service life of the pump is extended.

Description

GEAR PUMP

Field of the Invention

The present invention relates to a gear pump, and particularly, but not exclusively, to a gear pump of a fuel pumping unit of an aero-engine.

Background of the Invention

A typical fuel pumping system for an aero-engine comprises a low pressure (LF) pumping stage operable to draw fuel from a fuel tank, and supply the fuel at boosted pressure to the inlet of a high pressure (HP) pumping stage. The LP pumping stage may comprise a centrifugal impeller pump while the HP pumping stage may comprise a positive displacement gear pump having one or more pinion gear pairs.

The [P and HP stages are typically connected by a gear train to a common drive input, which facilitates a compact system and helps the LP and HP stages to operate synchronously. The inter-stage flow between LP and HP pumping stages may be used to cool engine lubrication oil in a fuel/oil heat exchanger.

The journal bearings and gear elements of an HP pumping stage gear pump are typically lubricated by the fluid (aviation engine fuel) being pumped, due to the impracticalities of providing appropriate sealing.

As the fluid being pumped is drawn into the gear elements it is possible to locally lower the fluid static pressure to the point that vapour bubbles form (e.g. dissolved gas bubbles or fluid vapour bubbles). This is called cavitation. As the bubbles collapse against the gear or bearing surface, shockwaves are generated which impact the local surfaces and lead to erosion and pitting of the material. This phenomenon can limit the operation and life of the pump.

In practice, it is often accepted that cavitation damage and mechanical wear will occur in service, and therefore the designed output of a pump may be increased to allow for a drop-off in performance at the end of the service life. This leads to an increase in weight and size of the pumping unit.

Typically, a gear pump is designed against several performance requirements at differing operating conditions. One such design point is the starting (or re-light) condition where the pump rotates slowly, delivery pressures and flows are low and any parasitic internal losses can form a significant percentage of the available output. Cavitation damage and erosion can significantly reduce pump output at this condition.

Modern gear pumps for aviation engine duty have two sets of gears within a single pump.

More particularly, two gear element pairs are arranged such that one pair (typically the smaller of the two) provides a continuous base load output from the pump (the base load may correspond to engine fuel demand at cruise conditions) and another pail (typically the larger of two) provides an additional output for periods of heavy demand (for example at take-offl. This arrangement allows the pump output to be matched with engine demand across the speed/load range. During periods of flight when the full pump output is not required (for example at cruise) the output of the larger pair is returned to the pump inlet and is not used by the engine. A special valve is used to return the surplus fuel with minimum obstruction while allowing the correct quantity to feed the engine demand.

Summary of the Invention

An aim of the present invention is to extend the service life and/or improve the performance of gear pumps.

Accordingly, in a first aspect, the present invention provides a gear pump (e.g. a fuel pump) having: a plurality of gear element pairs for pumping a fluid, each gear element pair having a respective input port for receiving the fluid and a respective output port for discharging the fluid pumped by the pair; and a connector device for connecting one or more of the output ports to a base load fluid line, and for connecting the other output port(s) to an additional load fluid line; wherein the connector device is operable to changeably select which output port(s) is connected to the base load fluid line and which output port(s) is connected to the additional load fluid line.

By changing which output port(s) is connected to the base load fluid line, the pump's service duty can be distributed more evenly around the gear element pairs. For example, each output port in turn can be connected to the base load fluid line, ensuring that over time each gear element pair wears at approximately the same rate. This is in contrast to a conventional gear pump where the identities of the gear element pairs providing the base load and additional load are fixed, such that one pair generally wears at a faster rate than another.

By spreading wear more evenly around the gear element pairs, service life can be extended, and, for a given service life, performance can be improved. Alternatively, the designed output of the pump can be decreased due to reduced performance drop-off, thereby allowing the weight and size of the pumping unit to be reduced.

In a second aspect, the present invention provides a fuel supply system for an aero-engine (such as a gas turbine engine), the system having a pumping unit including the gear pump of the first aspect, and a control arrangement downstream thereof which receives fuel from the base load fluid line and the additional load fluid line, the control arrangement forming from the received fuel a metered fuel supply to meet engine fuel demand, and further having a return line which returns to the pumping unit fuel received from the additional load fluid line in excess of the engine fuel demand.

In a third aspect, the present invention provides an aero-engine (such as a gas turbine engine) having the fuel supply system of the second aspect.

In a fourth aspect, the present invention provides use of the gear pump of the first aspect to pump fuel to an aero-engine (such as a gas turbine engine).

Optional features of the invention will now be set out. These are applicable singly or in any combination with any aspect of the invention.

Typically, the gear element pairs can be equally sized. In this way, they can provide equal outputs, such that changing which output pods are connected to the base and additional load fluid lines has no effect on the pump's characteristics.

The pump may typically have two, three, four or five of the gear element pairs.

The connector may connect just one of the gear element pairs to the base load fluid line.

Another option, in a pump having at least three gear element pairs, is for the connector to connect just two of the gear element pairs to the base load fluid line.

The gear element pairs may be angularly spaced, and preferably equally angularly spaced, around a central connector device. Conveniently, the connector device can be rotatably operable to changeably select the output ports.

The connector device may have a connector body with internal passages formed therein, each passage, in use, connecting a respective output port to, selectably, the base load or additional load fluid line. The body may be cylindrical. The passages may then extend along the axial direction of the body.

The gear pump may further have an actuator for operating the connector device.

Conveniently, the actuator can operate the connector to sequentially select the output port(s) connected to the base load fluid line. This can ensure that, at least on average, each output port in turn is connected to the base load fluid line for the same amount of time, helping to spread wear more evenly around the gear element pairs. For example, the actuator may be an indexing device which operates the connector after each period of operation of the pump.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a longitudinal cross-section through a ducted fan gas turbine engine; Figure 2 shows a perspective view of a gear arrangement of an HP stage of a fuel pumping unit for the engine of Figure 1; Figure 3 shows the gear arrangement of Figure 2 with its bearing blocks removed; Figure 4 shows a cross-section through gear element pairs of the gear arrangement of Figure 2; Figure 5 shows (a) a longitudinal section and (b) a transverse section through a connector device of the HP stage; Figure 6 shows schematically three possible arrangements (a) to (c) for internal passages of the connector device; and Figure 7 shows schematically three possible arrangements (a) to (c) of gear element pairs of the HP stage.

Detailed Description and Further Optional Features of the Invention With reference to Figure 1, a ducted fan gas turbine engine incorporating the invention is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.

During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.

The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.

The engine 10 has a fuel pumping unit including an HP stage and an [P stage.

The [P stage operates to draw fuel from a fuel tank, and may be in the form of a centrifugal pump, driven by rotation of an input drive shaft. This in turn may be driven by a drive output pad of an accessory gearbox of the engine.

The HP stage comprises a positive displacement pump in the form of a gear pump. Figure 2 shows a perspective view of a possible gear arrangement of the pump, which is in the form of three identical gear element pairs 30. Each pair includes a first toothed gear 30a, and a second toothed gear 30b, the teeth of which are meshed with one another, the gears being sandwiched between respective bearing blocks 32. Rotation of each gear element pair (also by the input drive shaft) positively displaces fuel from an inlet port (not shown) of the pair to an output port 34, pressurising the fuel at the output side.

Figure 3 shows the gear arrangement of Figure 2 with the bearing blocks 32 removed, and Figure 4 shows a cross-section through the gear element pairs 30 of the geai arrangement of Figure 2.

The gear element pairs 30 are equally angularly spaced around a central cylindrical connector device 36 having a first internal passage 38a which connects one of the outlet ports 34 to a base load fuel supply line (not shown) and two second internal passages 38b which connect the other two outlet pods 34 to an additional load fuel supply line (not shown).

The base and additional load fuel supply lines then deliver the fuel at high pressure to a hydra-mechanical unit (HMU), which controls the delivery of fuel to the combustion equipment of the engine. More particularly, the HMU contains a metering valve which receives the high pressure fuel from the base and additional load supply lines. As is conventional in metering valves, a spool of the valve is moved within the housing of the valve to control the degree of opening of the metering orifice of the valve and thus the metering of fuel flow through the valve. A delivery line from the metering valve conducts metered fuel at a reduced pressure through a pressure raising and shut-off valve (PRSOV) of the HMU. The FRSOV serves, in use, to maintain a minimum fuel pump pressure rise, so as to ensure that internal HMU valves and any fuel-pressure operated auxiliary devices (such as variable stator vane actuators, variable inlet guide vane actuators and bleed valve actuators) arranged to receive fuel under pressure can operate correctly. An output line from the PRSOV exits the HMU to pass the metered fuel to the engine burner manifold(s).

Not all the fuel from the additional load fuel supply line may be burnt in the engine. The part that is to be burnt enters the metering valve via a non-return valve, but a substantial proportion may be recirculated back to the fuel pumping unit via a combining spill valve and a spill return of the HMU. When the engine is operating at altitude where the engine burns little fuel, all the fuel from the additional load fuel supply line may be recirculated in this way.

Figure 5 shows (a) a longitudinal section through the connector device 36 and (b) a transverse section at the level of the outlet ports 34 through the connector device. The first internal passage 38a extends along the axis of the device and sealingly joins to the base load fuel supply line at a central discharge zone 40a in an end face of the device. The second internal passages 38b extend parallel to, but at a radial distance from, the axis of the device and sealingly join to the additional load fuel supply line at an annular discharge zone 40b in the side face of the device. Figure 6 shows schematically: (c) this arrangement for the internal passages of the connector device, (a) an arrangement in which the second internal passages 38b extend to the additional load fuel supply line at an annular discharge zone 40b in the end face of the device, and (b) another possible arrangement in which the first internal passage 38a extends to the base load fuel supply line at an annular discharge zone 40a in the side face of the device. 0-rings 42 seal the discharge zones to the supply lines.

These example configurations of the discharge interface between the connector device 36 and the fuel supply lines all allow the separate streams of pumped fluid to be directed to the relevant supply line independent of the angular position of the connector device itself More particularly, the pump also has an actuator (not shown) which rotates the connector device by 1200 after each period of pump operation. The actuator can have, for example, an indexing device of known type, such as a ratchet and pawl. The ratchet can be driven forward by a small piston to rotate the connector device, with the outlet fuel pressure providing motive force. At the end of pump duty, e.g. when the engine shuts down, the pressure decays to zero and a spring can retuin the piston and pawl to their start positions, with the pawl indexing onto the next tooth of the ratchet. In this way the connector device is indexed each time the engine starts and the indexing mechanism is reset at the end of every flight. However, alternative indexing mechanisms are known.

Cavitation erosion and wear are cumulative processes. A result of the indexed rotation is that the gear element pair 30 that provides the base load output of the pump is rotated to the next gear element pair, in sequence, thus spreading the duty load around the pairs, and reducing the cavitation damage on the loaded gears. In this way, individual pairs can retain their designed output performance for a longer period in service. This can be used to extend the service intervals and increase the time that the pump can spend on-wing, providing a reduction in maintenance burden for the engine and lower ownership costs. However, the gear element pairs are the same size and of equal output so that the output characteristic of the pump is not affected by the indexed rotation.

The indexing function can be combined with a pressure relief valve of the pump to promote integration and reduce part count. In such an arrangement, the piston can be pressurised by the fluid pressure on pump/engine start, and moved to a defined position to index the connector device. If the outlet fluid pressure continues to rise, the additional pressure causes further travel of the piston and this over-travel opens the pressure relief valve, allowing excess fluid pressure to escape to a lower pressure part of the circuit (e.g. the fuel tank, or the [P stage inlet).

In the above example, the three gear element pairs 30 of the HP stage are equally angularly spaced around the central cylindrical connector device 36. However, many other configurations are possible with e.g. two, three, four or five gear element pairs. Thus Figure 7 shows schematically possible arrangements of gear element pairs (a) two, (b) three and (c) four gear element pairs. Conventional HP stage gear pumps tend to be arranged so that the output of the gear set providing the additional load is approximately twice the output of the gear set providing the base load. A configuration with three equally sized gear element pairs (as in the above example and Figure 7(b)) can emulate this characteristic of a conventional HP stage gear pump. However, depending on the ratio of fuel demand at cruise and maximum take-off conditions, it may be desirable to include four (as in Figure 7(c)) or five gear element pairs. Further, it is possible for two gear element pairs to provide the base load.

The gear pump of the present invention can provide other advantages. For example, the reduction in cavitation damage enables the end-of-life capacity allowance of the pump to be reduced. The gears and their bearings can thus be reduced in size and weight. As another example, the gears and their bearings are typically high tolerance components with tightly controlled surface finishes. They are therefore expensive components, and may in fact be the chief material cost elements of a gear pump. By spreading cavitation damage more equally across all the gears, their replacement frequency can be reduced. Additionally, having all the gears and bearing blocks the same size can assist with spare part provisioning and can reduce inventories even though the number of parts in the pump may be increased relative to a conventional pump. Thus cost benefits can be achieved through economies of scale. Finally, by using a number of smaller gear element pairs rather than a single large gear element pair to provide the additional load, the gears can be located closer to the adjacent housing wall of the pump. As this wall is also typically the mounting face of the pump to the engine, the overhung mass of the pump and its applied bending moment to the mount can be reduced. The pump can thus be made more compact and lighter.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

Claims

CLAIMS1. A gear pump having: a plurality of gear element pairs (30) for pumping a fluid, each gear element pair having a respective input port for receiving the fluid and a respective output port (34) for discharging the fluid pumped by the pair; and a connector device (36) for connecting one or more of the output pods to a base load fluid line, and for connecting the other output port(s) to an additional load fluid line; wherein the connector device is operable to changeably select which output pod(s) is connected to the base load fluid line and which output port(s) is connected to the additional load fluid line.
2. A gear pump according to claim 1, wherein the gear element pairs are equally sized.
3. A gear pump according to claim 1 or 2, having two, three, four or five gear element pairs.
4. A gear pump according to any one of the previous claims, wherein the connector connects just one of the gear element pairs to the base load fluid line.
5. A gear pump according to any one of claim 1 to 3, having at least three gear element pairs, and wherein the connector connects just two of the gear element pairs to the base load fluid line.
6. A gear pump according to any one of the previous claims, wherein the gear element pairs are angularly spaced around a central connector device.
7. A gear pump according to claim 6, wherein the connector device is rotatably operable to changeably select the output pods.
8. A gear pump according to any one of the previous claims, wherein the connector device has a connector body with internal passages (38a, 38b) formed therein, each passage, in use, connecting a respective output port to, selectably, the base load or additional load fluid line.
9. A gear pump according to any one of the previous claims, further having an actuator for operating the connector device.
10. A gear pump according to claim 9, wherein the actuator operates the connector to sequentially select the output port(s) connected to the base load fluid line.
11. A gear pump according to claim 10, wherein the actuator is an indexing device which operates the connector after each period of operation of the pump.
12. A gear pump according to any one of the previous claims which is a fuel pump.
13. A fuel supply system for an aero-engine, the system having a pumping unit including the gear pump of claim 12, and a control arrangement downstream thereof which receives fuel from the base load fluid line and the additional load fluid line, the control arrangement forming from the received fuel a metered fuel supply to meet engine fuel demand, and further having a return line which returns to the pumping unit fuel received from the additional load fluid line in excess of the engine fuel demand.
14. An aero-engine having the fuel supply system of claim 13.
15. Use of the gear pump of any one of claims 1 to 12 to pump fuel to an aero-engine.
16. A gear pump as herein described with reference to and as shown in Figures 2 to 7.