GB2559549A

GB2559549A - Pump assembly

Info

Publication number: GB2559549A
Application number: GB1701812.8A
Authority: GB
Inventors: K Yates Martin; N Keil Andrew
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2017-02-03
Filing date: 2017-02-03
Publication date: 2018-08-15
Anticipated expiration: 2037-02-03
Also published as: GB2559549B; GB201701812D0

Abstract

A pump assembly has a pump assembly casing 25 that includes gear pump(s) 31 which comprise a pair of meshing gears 31a, 31b. Each gear has a coaxial bearing shaft 41 which extends from each side of the gear and is journaled within a pair of bearing blocks 39a, 39b. The shaft emerges from the bearing blocks at ends thereof distal from the gear to form a pair of sealing lands 43 and a pair of carbon face seals 45 is mounted to the pump assembly casing to form respective dynamic seals with the sealing lands. Each carbon face seal has a carbon ring 47 which forms a sealing land for dynamically sealing to the corresponding sealing land of the bearing shaft and has an annular carrier 49 to which the carbon ring is mounted by an interference fit. The interference fit extends over an axial distance which is more than 66% of the total axial length of the carbon ring. A fuel supply system and gas turbine engine that comprise the pump assembly are also claimed.

Description

(54) Title of the Invention: Pump assembly

Abstract Title: Sealing arrangement of a pump assembly (57) A pump assembly has a pump assembly casing 25 that includes gear pump(s) 31 which comprise a pair of meshing gears 31a, 31b. Each gear has a coaxial bearing shaft 41 which extends from each side of the gear and is journaled within a pair of bearing blocks 39a, 39b. The shaft emerges from the bearing blocks at ends thereof distal from the gear to form a pair of sealing lands 43 and a pair of carbon face seals 45 is mounted to the pump assembly casing to form respective dynamic seals with the sealing lands. Each carbon face seal has a carbon ring 47 which forms a sealing land for dynamically sealing to the corresponding sealing land of the bearing shaft and has an annular carrier 49 to which the carbon ring is mounted by an interference fit. The interference fit extends over an axial distance which is more than 66% of the total axial length of the carbon ring. A fuel supply system and gas turbine engine that comprise the pump assembly are also claimed.

B

FIG. 2

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FIG. 1

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PUMP ASSEMBLY

Field of the Invention

The present invention relates to a pump assembly, and particularly, but not exclusively, to a pump assembly of a fuel supply system of a gas turbine engine.

Background

In a gas turbine engine fuel delivery system, pump assemblies, as shown for example in US 2005/0232784, are typically used for pumping the fuel. Where such assemblies include gear pumps, gear elements are commonly supported by bearing blocks which are adapted to journal respective bearing shafts of the gears through a bore of each bearing block. These bearing blocks also typically abut axially-directed faces of respective gears of the pumps.

The bearing blocks may be solid bearings, or pressure loaded bearings. A solid bearing can transfer load from journal to a (typically cast) casing of the pump assembly, and additionally can transfer axial load to the casing. Pressure loaded bearings also transfer load from journal to casing, and in addition can provide an axial force and a moment against the axially-directed face of the gear which the bearing block abuts.

In order to prevent both internal and overboard-drains leakage of fuel, a pump assembly may also contain carbon face seals which form dynamic seals between the journals of the rotating gears, and the pump assembly casing. Nonetheless, drains leaks are sometimes observed through carbon face seals at ambient (and cold) conditions following in-service running, typically at high temperature. Studies suggest that such leakage occurs due to uneven wear of the carbon face of the seal.

Summary

In a first aspect, the present invention provides a pump assembly having a pump assembly casing containing one or more gear pumps;

wherein the or each gear pump comprises a pair of meshing gears, each gear of the pair having a coaxial bearing shaft which extends from each side of the gear and is journaled within a pair of bearing blocks sandwiching the gear, the shaft emerging from the bearing blocks at ends thereof distal from the gear to form a pair of sealing lands, and a pair of carbon face seals being mounted to the pump assembly casing to form respective dynamic seals with the sealing lands of the bearing shaft;

wherein each carbon face seal has a carbon ring which forms a sealing land for dynamically sealing to the corresponding sealing land of the bearing shaft, and further has an annular carrier to which the carbon ring is mounted by an interference fit between a radially inwardly facing surface of the carrier and a radially outwardly facing surface of the carbon ring; and wherein the interference fit extends over an axial distance which is more than 66% of the total axial length of the carbon ring (and preferably more than 80% or 90% of the total axial length of the carbon ring).

Differential thermal expansion of the carbon ring and its carrier can cause the sealing land of the carbon ring to become non-planar (typically concave) at high temperatures. This can lead to excessive wear, particularly at the outer diameter of the land, due to rubbing against the corresponding sealing land of the bearing shaft. As a consequence, when the face seal returns to ambient temperatures, the reverse differential thermal contraction does not restore the sealing land of the carbon ring to planarity. Rather it typically adopts a convex shape that can allow drains leaks. However, by increasing the axial distance over which the carbon ring is supported by the carrier, it is possible to reduce the distortion of the carbon ring caused by differential thermal expansion and contraction, and thus reduce uneven and excessive wear of the sealing land of the carbon ring. In turn this can reduce leaks through the face seal.

The carrier may extend axially to terminate in an end surface proximate to the corresponding sealing land of the bearing shaft, the radially inner edge of the carrier end surface being radially outwards of the radially outer edge of the corresponding sealing land of the bearing shaft. Such an arrangement advantageously improves the acceptance of the face seal to carbon wear, as even with such wear clashing between carrier and the bearing shaft can be avoided.

Indeed, in a second aspect, the present invention provides a pump assembly having a pump assembly casing containing one or more gear pumps;

wherein each carbon face seal has a carbon ring which forms a sealing land for dynamically sealing to the corresponding sealing land of the bearing shaft, and further has an annular carrier to which the carbon ring is mounted by an interference fit between a radially inwardly facing surface of the carrier and a radially outwardly facing surface of the carbon ring; and wherein the carrier extends axially to terminate in an end surface proximate the corresponding sealing land of the bearing shaft, the radially inner edge of the carrier end surface being radially outwards of the radially outer edge of the corresponding sealing land.

In a third aspect, the present invention provides a fuel supply system of a gas turbine engine having the pump assembly according to the first or second aspect for pumping fuel. For example, the fuel supply system may include a dual stage pump formed of a low pressure pump and a high pressure pump. The low pressure pump may be a centrifugal pump. The high pressure pump may be formed by the one or more gear pumps of the pump assembly of the third aspect.

In a fourth aspect, the present invention provides a gas turbine engine having the fuel supply system of the third aspect.

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

The annular carrier may be formed of a material having a coefficient of thermal expansion which is no more than three times greater (and preferably no more than 2.5 times greater) than the coefficient of thermal expansion of the material of the carbon ring.

The annular carrier can be formed of steel.

The radially outer edge of the sealing land of the carbon ring may be radially outwards of the radially outer edge of the corresponding sealing land of the bearing shaft. Additionally or alternatively, the radially inner edge of the sealing land of the carbon ring may be radially inwards of the radially inner edge of the corresponding sealing land of the bearing shaft.

Such an arrangement(s) also facilitates extension of the interference fit over a longer axial distance.

Conveniently, the interference fit can be formed by shrink fitting the annular carrier onto the carbon ring.

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) a longitudinal section view of the interior of a dual stage pump assembly of a fuel supply system of the engine of Figure 1, and (B) a close-up view of a dynamic seal region of the longitudinal section view;

Figure 3 illustrates schematically a longitudinal section view of a dynamic seal of Figure 2;

Figure 4(A) to (E) illustrates schematically a sequence of manufacturing and operational events of a carbon face seal of the dynamic seal;

Figure 5 shows (A) a longitudinal section view of an example of a dynamic seal having an increased length of interference fit, and (B) a close-up view of a selected region of the longitudinal section view;

Figure 6 shows a longitudinal section view of another example of a dynamic seal having an increased length of interference fit; and

Figure 7 shows a longitudinal section view of a further example of a dynamic seal having an increased length of interference fit.

Detailed Description and Further Optional Features

Although a pump assembly of the present invention may be used in various applications, a significant intended use is in an aircraft fuel supply system, and the invention will be described hereafter in relation to such a system.

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 intermediatepressure 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.

Figure 2 shows (A) a longitudinal section view of the interior of a dual stage pump assembly of a fuel supply system of the engine 10, and (B) a close-up view of a dynamic seal region of the longitudinal section view. The dual stage pump assembly has: a casing 25; a centrifugal pump 27 forming a low pressure stage of the pump assembly; and two (primary and secondary 31) displacement gear pumps forming a high pressure stage of the assembly.

The smaller, primary gear pump (of which only the driver gear 29 is shown in the longitudinal section view of Fig. 2) is pressurised at all flight conditions, while the larger, secondary gear pump 31 (of which both a driver gear 31a and a driven gear 31b are shown in the longitudinal section view of Fig. 2) is pressurised for high power (above cruise) conditions, and for low speed starting.

A drive shaft 33, which accepts power from an engine accessory gearbox (not shown), has male spline couplings at each end. The drive shaft accommodates for misalignment and connects directly into the driver gear 31a of the secondary gear pump 31, and continues via a linking drive shaft 35 to an impeller and inducer of the centrifugal pump 27. A secondary drive shaft 37 transfers power from the secondary gear pump to the primary gear pump and also accommodates for misalignment. More particularly, one splined end of the secondary drive shaft is engaged internally with the driven gear 31b of the secondary displacement pump, whilst its opposite splined end is engaged internally with the driver gear 29 of the smaller, primary gear pump, which drives the driven gear (not shown) of the primary gear pump.

Each displacement pump gear 31a, 31b, 29 is sandwiched between a solid bearing block 39a and a pressure loaded bearing block 39b which are adapted to receive a bearing shaft 41 or journal which extends from each side of that gear. Each bearing shaft emerges from its bearing blocks at ends thereof distal from the gear to form a pair of sealing lands 43, as shown in the close-up view of Fig. 2(B). A pair of carbon face seals 45 are mounted to the pump assembly casing 25 to form respective dynamic seals with the sealing lands of the bearing shaft of the driver gear 31 a of the secondary gear pump 31. Figure 3 illustrates schematically a longitudinal section view of one of the dynamic seals. Each carbon face seal comprises a carbon ring 47 which forms a sealing land for dynamically sealing to the corresponding sealing land of the bearing shaft 41, and further comprises an annular carrier 49 (typically formed of steel) to which the carbon ring is mounted by an interference fit between a radially inwardly facing surface of the carrier and a radially outwardly facing surface of the carbon ring. The carbon face seals are used to prevent both internal (high pressure stage to low pressure stage) and overboard-drains leakage of fuel.

The pressure distribution between the sealing lands of a carbon face seal comprises:

• Hydrostatic pressure due to a difference in the sealed pressures across the sealing lands.

• Hydrodynamic pressure due to non-planarities in the sealing lands in the circumferential direction.

The hydrostatic pressure distribution depends strongly on the radial geometry of the parts, and in particular if there is any radial variation in the separation of the surfaces, such variation being known as “coning”. The hydrodynamic pressure distribution only exists when there is relative movement between the two parts of the seal.

Drains leaks have been observed through conventional carbon face seals at ambient (and cold) conditions following in-service running, typically at high temperature. Studies suggest that the leaks are caused by the sealing land of the carbon ring wearing to a convex shape. The force required to separate the sealing land of the carbon ring and the corresponding sealing land of the bearing shaft reduces as the magnitude of this convex shape increases.

Finite element analysis of the axial deformation of a carbon face seal under the diametric interference fit between the carbon ring and the steel carrier over the typical operating temperature range experienced by the unit in service, suggests the sequence of events illustrated schematically in Figure 4(A)-(E). Figure 4(A) shows the carbon ring 47 immediately after interference fitting to the steel annular carrier 49. The interference fit operation causes the sealing land of the carbon ring to deform into a convex shape. After final machining/lapping of the carbon ring (Figure 4(B)), the sealing land becomes planar as more material is removed from the inner diameter (ID) than the outer diameter (OD) of the land. At elevated operating temperature, shown in Figure 4(C), differential thermal expansion of the steel and carbon causes axial deformation of the carbon, resulting in a concave shape of the sealing land. After a longer period at elevated operating temperature, however, the concave shape results in more wear at the OD than at the ID of the sealing land due to running against the corresponding sealing land of the bearing shaft 41, as shown in Figure 4(D). Finally, on return to ambient temperature (Figure 4(E), differential thermal contraction of the steel and carbon causes the axial deformation of the carbon to reverse, resulting in a convex shape of the sealing land.

In order to reduce the deformation of the carbon ring 47, one option is to increase the axial length of the interference fit provided by the interference fit between the steel carrier 49 and the carbon ring. In particular, studies suggest that if the axial distance of the interference fit is more than 66% of the total axial length of the carbon ring (and preferably more than 80% or 90% of the total axial length of the carbon ring), this can substantially reduce the axial distortion of the carbon ring due to differential thermal expansion and contraction effects, and thus reduce the convexity of the sealing land of the carbon ring at ambient temperature after elevated temperature operation. The reduced convexity in turn can decrease the leakage through the seal.

Figure 5 shows (A) a longitudinal section view of an example of a dynamic seal having an increased length of interference fit, and (B) a close-up view of a selected region of the longitudinal section view. In this example, the carbon ring 47 is supported over almost the entirety of its axial length by the annular carrier 49 such that interference pressure acts along the full length of the ring. To ensure that there is no clash between the carrier and the bearing shaft 41, tight control of the sealing land of the bearing shaft may be needed. Additionally or alternatively, a chamfer 51 may be provided on the exposed edge of this sealing land.

Preferably the carrier material also has a relatively low coefficient of thermal expansion.

This can help to reduce the tightness requirement of the interference fit and also reduce the impact of differential thermal expansion and contraction between the carrier 49 and the carbon ring 47. For example, a conventional steel may have a coefficient of thermal expansion which is about 3.3 times greater than that of the carbon of the ring. However, it is possible to use a “low expansion steel” to form the carrier, such a steel having a coefficient of thermal expansion which is no more than three or 2.5 times greater than that of the carbon of the ring. Indeed, a steel to carbon coefficient of thermal expansion ratio of about 2.3 is achievable.

Figure 6 shows a longitudinal section view of another example of a dynamic seal having an increased length of interference fit. In common with the previous example, the annular carrier 49 extends axially to terminate in an end surface 53 proximate the corresponding sealing land of the bearing shaft. However, in this example, the radially inner edge of the carrier end surface is radially outwards of the radially outer edge of the sealing land of the bearing shaft 41. This arrangement facilitates an increase in the axial length of the interference fit of the between the carbon ring 47 and the carrier without risk of a clash between the carrier and the bearing shaft following operational wear of the carbon. In particular, it allows the outer diameter of the carbon ring to be radially outwards of the radially outer edge of the sealing land of the bearing shaft. This concept can be extended so that the inner diameter of the carbon ring is radially inwards of the radially inner edge of the sealing land ofthe bearing shaft, as shown in Figure 7.

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 ofthe 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.

All references referred to above are hereby incorporated by reference.

Claims

1. A pump assembly having a pump assembly casing (25) containing one or more gear pumps (31);

wherein the or each gear pump comprises a pair of meshing gears (31a, 31b), each gear of the pair having a coaxial bearing shaft (41) which extends from each side of the gear and is journaled within a pair of bearing blocks (39a, b) sandwiching the gear, the shaft emerging from the bearing blocks at ends thereof distal from the gear to form a pair of sealing lands (43), and a pair of carbon face seals (45) being mounted to the pump assembly casing to form respective dynamic seals with the sealing lands of the bearing shaft;

wherein each carbon face seal has a carbon ring (47) which forms a sealing land for dynamically sealing to the corresponding sealing land of the bearing shaft, and further has an annular carrier (49) to which the carbon ring is mounted by an interference fit between a radially inwardly facing surface of the carrier and a radially outwardly facing surface of the carbon ring; and wherein the interference fit extends over an axial distance which is more than 66% of the total axial length of the carbon ring.

2. A pump assembly according to claim 1, wherein the carrier extends axially to terminate in an end surface (53) proximate to the corresponding sealing land of the bearing shaft, the radially inner edge of the carrier end surface being radially outwards of the radially outer edge of the corresponding sealing land of the bearing shaft.

3. A pump assembly having a pump assembly casing (25) containing one or more gear pumps (31);

wherein each carbon face seal has a carbon ring (47) which forms a sealing land for dynamically sealing to the corresponding sealing land of the bearing shaft, and further has an annular carrier (49) to which the carbon ring is mounted by a interference fit between a radially inwardly facing surface of the carrier and a radially outwardly facing surface of the carbon ring; and wherein the carrier extends axially to terminate in an end surface proximate the corresponding sealing land of the bearing shaft, the radially inner edge of the carrier end

5 surface being radially outwards of the radially outer edge of the corresponding sealing land.

4. A pump assembly according to any one of the previous claims, wherein the annular carrier is formed of a material having a coefficient of thermal expansion which is no more than three times greater than the coefficient of thermal expansion of the material of the carbon ring.

10 5. A pump assembly according to any one of the previous claims, wherein the radially outer edge of the sealing land of the carbon ring is radially outwards of the radially outer edge of the corresponding sealing land of the bearing shaft.

6. A pump assembly according to any one of the previous claims, wherein the radially inner edge of the sealing land of the carbon ring is radially inwards of the radially inner edge

15 of the corresponding sealing land of the bearing shaft.

7. A fuel supply system of a gas turbine engine having the pump assembly according to any one of the previous claims for pumping fuel.

8.

A gas turbine engine having the fuel supply system of claim 7.

Amendments to the claims have been made as follows:

20 10 17,

wherein the or each gear pump comprises a pair of meshing gears (31a, 31b), each gear of the pair having a coaxial bearing shaft (41) which extends from each side of the gear

5 and is journaled within a pair of bearing blocks (39a, b) sandwiching the gear, the shaft emerging from the bearing blocks at ends thereof distal from the gear to form a pair of sealing lands (43), and a pair of carbon face seals (45) being mounted to the pump assembly casing to form respective dynamic seals with the sealing lands of the bearing shaft;

10 wherein each carbon face seal has a carbon ring (47) which forms a sealing land for dynamically sealing to the corresponding sealing land of the bearing shaft, and further has an annular carrier (49) to which the carbon ring is mounted by an interference fit between a radially inwardly facing surface of the carrier and a radially outwardly facing surface of the carbon ring; and wherein the interference fit extends over an axial distance which is more than 66% of the total axial length of the carbon ring.

3. A pump assembly according to claim 1 or 2, wherein the annular carrier is formed of a material having a coefficient of thermal expansion which is no more than three times greater than the coefficient of thermal expansion of the material of the carbon ring.

4. A pump assembly according to any one of the previous claims, wherein the radially

25 outer edge of the sealing land of the carbon ring is radially outwards of the radially outer edge of the corresponding sealing land of the bearing shaft.

5. A pump assembly according to any one of the previous claims, wherein the radially inner edge of the sealing land of the carbon ring is radially inwards of the radially inner edge of the corresponding sealing land of the bearing shaft.

30

6. A fuel supply system of a gas turbine engine having the pump assembly according to any one of the previous claims for pumping fuel.

7. A gas turbine engine having the fuel supply system of claim 6.

20 10 17

Intellectual

Property

Office

Application No: GB1701812.8 Examiner: Mr Mat Smith