GB2039615A - Turbo pump - Google Patents

Abstract

Opposed faces of the pump housing 30 and impeller 33 define flow restrictors 43, 46 between the volute outlet 32 and a low pressure zone 48. Between the restrictors 43, 46 is an annular volume, the pressure in which is dependent on the relative axial positions of the impeller 33 and housing 30, changes in this pressure acting to oppose axial movement of the impeller. Fluid in the zone 48 is returned to the outlet by a passage 49 in the impeller 33. <IMAGE>

Description

SPECIFICATION Turbo pumps This invention relates to turbo pumps for fluids. The impellers of turbo pumps are subjected in use, to axial loads which are dependent on the fluid pressures associated with the pump. It has been found that it is not possible to predict with accuracy the magnitude and direction of these axial loads under varying conditions of pump operation. The loads on thrust bearings within the pump cannot therefore be forecast.

It is an object of the present invention to provide a turbo pump in which, in use, the impeller is axially positioned without coacting with thrust bearings.

According to the invention there is provided a turbo pump having a housing, an impeller rotatably mounted in said housing for limited axial movement relative thereto, said housing and said impeller having respective opposed cylindrical surfaces which define a first annular flow restriction, respective opposed axially-directed surfaces which define a second annular flow restriction radially inwardly of said first restriction, and further respective opposed axially-directed surfaces which define an annular volume between said first and second annular restrictions, said annular restrictions and said annularvolume providing a flow path between a radially outer high pressure zone of the pump and a radially inner low pressure zone of the pump.

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which Figure 1 is a longitudinal section through a known form of turbo pump, Figure2 is a corresponding section through a turbo pump according to the invention, Figure 3 shows, to a larger scale, a detail of the pump of Figure 2, Figure 4 shows a detail, corresponding to Figure 3 of an alternative form of pump, and Figures 5, 6 and 7 show details, also corresponding to Figure 3 of further alternative forms of pump.

The pump shown in Figure 1 has a housing 10 having an inlet 11 and a volute outlet 12. A bladed conical impeller 13 is rotatably mounted in the housing 10 for limited axial movement whose extent is defined by respective thrust bearings 14, 15. Fluid pressures within the pump act on the impeller surfaces. The magnitudes of these fluid pressures may vary in different uses of the pump and may also vary with changes in pump operating conditions in a given type of use.

It is therefore difficult to forecast the direction and magnitude of the resultant axial thrust on the impeller 13, and hence which ofthe bearings 14, 15 is required to support this thrust.

In order to reduce flow leakage from the volute 12 an annular labyrinth seal 18 is provided by projections on the housing 10 and impeller 13 respectively.

The required tolerances have the effect that leakage flow through the labyrinth 18 is considerable, and this leakage passes to a zone 19 of the pump. It is necessarvthatthezone 19 shall be maintained art a low pressure in order that the impeller 13 shall not be urged rightwardly by the pump outlet pressure.

The zone 19 communicates with an annular passage 20 by means of which leakage fluid can join that from the pump inlet 11. Due to tolerances on the labyrinth seal 18 the leakage flows may be a high proportion of the total amount pumped. Recirculation of the leaked fluid thereby results in a temperature rise, and when the pump fluid is a volatile liquid, as for example an aircraftfuel, reintroduction of heated liquid into the low pressure zone adjacent the pump inlet can result in vapourization and reduction in mass flow of the fuel. The pump to be described with reference to Figure 2 does not require leakage flow to be returned to the pump inlet.

Additionally, the labyrinth 18 results in shearing of fluid at the larger radius of the impeller 13, and this shearing action reduces the efficiency of the pump.

The pump of Figure 2 provides a means whereby leakage losses and the effects of shearing are substantially reduced.

The pump shown in Figure 2 has a housing 30 with an inlet 31 and a volute outlet passage 32. A helically bladed conical impeller 33 is rotatably mounted in a bearing portion 34 of the housing 33 and can urge fluid from the inlet 31 both axially and radially outwardly to the volute passage 32. The impeller 33 has limited axial movement relative to the bearing portion 34, and in a leftward limiting position of the impeller 33 (as seen in Figure 2) the impeller 33 engages a thrust bearing 35. A flange 36 of the impeller 33 is engaged by a spring loaded face seal 37 which is mounted in the housing 30 and which defines an annular chamber 38 which communicates by way of a passage 39 with the volute outlet passage 32. Pressure in the outlet passage 32 thus acts on the flange 36 to bias the impeller 33 leftward towards engagement with the thrust bearing 35.

Annular adjacent zones 40 of the housing 30 and impeller 33 have configurations which are shown more clearly in Figure 3. Opposed cylindrical surfaces 41,42 of the housing 30 and impeller 33 respectively define an annular flow restrictor 43 whose impedance to fluid flow is independent of the relative axial positions of the housing 30 and impeller 33. Annular axially-directed surface portions 44,45 of the housing 30 and impeller 33 respectively cooperate to define a second annular flow restrictor 46 whose resistance to flow is dependent on the relative axial positions of the housing and impeller. An annular volume 47 between the restrictors 43, 46 co-operates with these restrictors to define a flow path between the volute outlet passage 33 and an annular chamber 48 (Figure 1) between the bearing portion 34 of the housing and an inner surface of the impeller 33.Radial passages 49 in the impeller 33 extend between the chamber 48 and the passage 32.

In use, high pressure fluid from the outlet passage 32 passes through the restrictor 43, volume 47 and restrictor 46 to the chamber 48. Fluid within chamber 48 is urged radially outwards through the passages 49, by centrifugal action. The pressure in the chamber 48 is thus substantially below that in the outlet passage 32, and remains substantially constant for given values of pressure in the inlet 31 and the pump speed. The pressure in the volume 47 is dependent on flow through the restrictor46, and hence on the axial position of the impeller 33 relative to the housing 30.

In a particular use the pump outlet is supplied with fluid at a relatively low pressure by a pressure raising device within a fluid reservoir. This inlet pressure and the pressure in chamber 38 combine to urge the impeller 33 to the left. The impeller 33 is urged rightwardly by the pressure in volume 47, rightward movement increasing the flow through restrictor 46 and reducing the pressure in volume 147. The impeller 33 will thus move to an equilibrium position in which the pressure forces thereon are in balance. It can readily be ensured that the aforesaid equilibrium position is such that the impeller 33 is not coacting with the thrust bearing 35. It will be apparent that leftward movement of the impeller 33 causes an increase in pressure in the volume 47 which opposes that movement.

Return of leakage fluid to the relatively high pressure in the outlet passage 32 has the effect that the tendency of recirculated fluid to vapourize is substantially reduced. Since the restriction 43 at the larger radius of the impeller 33 has only two opposed surfaces, the shearing area on the leakage fluid is substantially reduced.

The cross-section of the annular volume 47 is large in relation to the flow area of the restrictors 43, 46.

Axial movements of the impeller 33 thus result in relatively large changes in the volume 47, and fluid displaced as a result of these changes must flow through the restrictor 43 or restrictor 46. The speed of axial movement of the impeller 33 is thus limited by the restrictors 43, 46 and these movements are thereby damped.

In the modification shown in Figure 4 the restriction 43 of Figure 3 is replaced by two restrictions 50, 51 which define between them a volume 52 the pressure in which is dependent on the amount of leakage flow, and hence the axial position of the impeller 33. This arrangement provides for increased damping of axial movement of the impeller 33, and also, by providing two restrictions 50, 51 in series, enables the dimensional tolerances of the parts defining the restrictions 50, 51 to be wider than the corresponding tolerances defining the restrictor 43 in Figure 3.

The further alternative shown in Figure 5 has a restriction 55 corresponding to the restrictor 43 of Figure 3. The impeller 33 has a T-section projection 56 which cooperates with a V-section groove 57 in the housing 30. The projection 56 and groove 57 define two flow restrictions 58, 59 in series, and a volume 60 exists between the restrictions 55 and 58.

By virtue of the sloping walls of the groove 57, rightward movement of the impeller 33 does not increase flow through the restrictions 58, 59 as much as a corresponding movement of the impeller 53 would increase flow through the restriction 46. The rate of change of pressure in the volume 60 is thus less than that of the two preceding embodiments.

Since the total range of axial movement of the imnPllor IRIR will hp rmnil the nhnnne in volume flow through the restrictors 43,46 or their equivalents, will also be small. The change of pressure in the chamber 48 will also be small, and the pressure in chamber 48 may thus be regarded as substantially constant.

It is desirable to ensure that the centrifugal pressure difference along the passages 49 is not such that, when flow through restrictors 43, 46 is at or near zero, the pressure in chamber 48 is not reduced to a level at which cavitation can occur. In some embodiments, therefore, the radially inner ends of the passages 49 are not aligned with the radially inner ends of the restrictors 46. Such an embodiment is shown in Figure 6. It will be understood that the inner ends of passages 49 may be located as required, either radially inwardly or radially outwardly of the restrictor 46.

It will also be understood that the restrictors 43, 46 and volume 47 may be defined between the impeller 33 and housing 30 in the alternative manner indicated in Figure 7, and further that the impeller and housing may be transposed in the examples of Figures 4 and 5, without departing from the invention.

Claims (7)

1. Aturbo pump having a housing, an impeller rotatably mounted in said housing for limited axial movement relative thereto, said housing and said impeller having respective opposed cylindrical surfaces which define a first annular flow restriction, respective opposed axially-directed surfaces which define a second annular flow restriction radially inwardly of said first restriction, and further respective opposed axially-directed surfaces which define an annular volume between said first and second annular restrictions and said annular volume providing a flow path between a radially outer high pressure zone of the pump and a radially inner low pressure zone of the pump.

2. A turbo pump as claimed in claim 1 in which said impeller includes means for discharging fluid from said low pressure zone to said high pressure zone.

3. Aturbo pump as claimed in claim 1 or claim 2 which includes means for applying the pressure in said high pressure zone to said impeller, to urge the latter in a direction opposite to that in which said impeller is urged by the pressure in said annular volume.

4. A turbo pump as claimed in any preceding claim in which said first flow restriction comprises opposed first cylindrical surfaces on said impeller and said housing, second opposed cylindrical surfaces on said impeller and said housing, and a further annular volume between said opposed first and said opposed second surfaces.

5. A turbo pump as claimed in any of claim 1 to 3 in which at least one of said opposed axially directed surfaces defining said second flow restriction is inclined to the direction of axial movement of said impeller.

6. A turbo pump as claimed in claim 5 in which said one axial directed surface comprises two walls which are mutually inclined and each of which is inclined to said direction of axial movement, the other opposed axially directed surface comprising a projection coacting with both of said walls to define two restricted flow paths having a further annular volume therebetween.

7. A turbo pump substantially as hereinbefore described with reference to Figures 2 and 3, and as modified by any of Figures 4 and 7 of the accompanying drawings.