GB2051239A - Centrifugal pumps and sealing arrangements therefor - Google Patents

Abstract

A sealing arrangement is described for a centrifugal pump of particular usefulness for pumping high temperature corrosive liquids. The pump housing 10 has an axial inlet 13 and a radial outlet 20, fluid being driven through the pump by a rotating impeller 11 mounted on a shaft 12 disposed axially away from the inlet 13. The shaft 12, in addition to having a static seal 19 is protected by a dynamic seal provided by parts 17 of the impeller blades 14 projecting behind a rearward face 16 of a generally conical defector 18, 15. The part 15 of the defector rotates with the impeller. The part 18, which is static and forms part of the casing, may be either omitted or replaced by an extension of the rotating part 15. The defector deflects the incoming fluid radially outwardly and the parts portion 17 of the blades provide sufficient back pressure to prevent leakage of fluid along the shaft (12). <IMAGE>

Description

SPECIFICATION Improvements relating to centrifugal pumps and sealing arrangements therefor This invention relates to a centrifugal pump and to a sealing arrangement therefor.

It may be of particular usefulness in the pumping of corrosive liquids, especially at high temperatures, but it is of general applicability.

It is known to provide what is termed a "hydrodynamic" seal in addition to static sealing arrangements in a centrifugal pump, to prevent the fluid being pumped from being forced along the central drive shaft of the pump impeller when the impeller is in motion.

In a previously proposed arrangement, there is provided a seal chamber at the rear of the main pump chamber (that is, at the face of the impeller remote from the fluid inlet). The seal chamber houses a seal disc mounted co-axially and keyed for rotation with the pump impeller, and having a plurality of vanes positioned on the face of the disc directed towards the shaft to be sealed. The disc rotates in unison with the pump impeller and the vanes on the rotating disc tend to force the fluid towards the periphery of the seal chamber by centrifugal force and to generate a region of reduced pressure leading to a clear space at the central shaft, preventing the fluid being pumped from seeping out along the shaft bearing.

Such an arrangement has certain disadvantages, which become particularly acute where corrosive liquids at elevated temperatures are being pumped, for example heated plating solutions, or pickling solutions.

The range af pumping pressures over which the conventional hydrodynamic seal can be used is limited because the size of the seal chamber and seal disc need to be increased in proportion to the size of the pump. This is to ensure that the back pressure developed by the seal disc vanes can resist the pressure developed by the pump. For a large, high suction pressure pump, the size of a hydrodynamic seal assembly therefore becomes impracticably large.

If a corrosive fluid is being pumped, the internal surfaces of the pump and seal which contact the fluid must be protected from corrosion, which may mean the use of expensive materials or additional assembly steps, for example to introduce a lining resistant to corrosion. The existence of a separate seal chamber and seal disc, distinct from the pump chamber and impeller, increases the cost of such measures compared with providing them for the pump alone.

Furthermore, the fluid in the seal chamber will tend to build up around its circumference during pumping and there will be little tendency for this portion of the fluid to circulate out of the seal chamber. If the fluid is hot, undesirable differential heating may occur, with the fast moving seal disc vanes imparting further energy in the form of heat to the "trapped" fluid at the periphery of the seal chamber, while a void exists adjacent the shaft.

The build up of heat may lead to damage to the seal chamber, and especially may cause reduction of any anti-corrosion properties it may have.

It is an object of the present invention to provide a centrifugal pump and a sealing arrangement therefor which overcome or reduce some or all of these disadvantages.

According to the invention there is provided a centrifugal pump comprising a housing, a rotary shaft extending into the housing, an inlet for fluid disposed generally axially of the housing and at the face of the impeller remote from the shaft, and an outlet for fluid, disposed at the periphery of the housing, the pump including a central fluid deflector means facing the inlet and adapted to deflect incoming fluid generally radially outwardly away from the rotational axis of the shaft, at least a portion of said deflector means being integral with the impeller and rotating therewith, and the impeller having a plurality of impeller blades having parts projecting rearwardly from said portion of the deflector means to generate a back pressure when the impeller is rotating, tending to resist passage of fluid towards the shaft.

Preferably, the fluid deflector means is generally conical in shape. It may include a generally flat radially extending rearward face, from which said parts of the impeller blades project in a direction towards the shaft.

The fluid deflector means may comprise a fixed portion integral with the housing of the pump and affording the apex of a generally conical body, the remainder of which is defined by said portion integral with the impeller.

The invention also provides a sealing arrangement for a centrifugal pump of the type comprising a housing, a rotary shaft extending into the housing, an impeller mounted on the shaft for rotation therewith, an inlet for fluid disposed generally axially of the housing and at the face of the impeller remote from the shaft, and an outlet for fluid, disposed at the periphery of the housing, said sealing arrangement comprising, in combination, a central deflector means disposed axially and facing the inlet of the pump, at least a portion of said deflector means being integral with the impeller and being adapted to deflect incoming fluid generally radially outwardly away from the rotational axis of the shaft, and means for generating a back pressure tending to resist passage of fluid towards the shaft and comprising parts of said impeller blades projecting rearwardly from said portion of the deflector means.

The invention will now be described in more detail by way of example only with reference to the accompanying drawing which is a radial section of one half of a pump embodying the invention.

Referring to the drawing, a pump comprises a housing 10 within which is mounted an impeller 11 keyed for rotation with a shaft 1 2. The shaft 12 is mounted in suitable bearings extending into the housing from one face thereof, referred to as the "rear" face for purposes of description.

An inlet 13 is provided in communication with a source of fluid to be pumped. The inlet is disposed at the face of the impeller 11 which is remote from the shaft 12, which will be referred to as the "front" face of the impeller. The inlet is axially in line with the shaft 12.

The impeller 11 comprises a plurality of blades 14 which are of generally known construction and need not be described in detail. However, the central portion 1 5 of the impeller is formed as part of a conical solid body, having a flat rear surface 1 6 extending radially outwardly with respect to the shaft, and so disposed that a small part 17 of each impeller blade 14 extends slightly rearwardly of the surface 16.

A fixed conical body 18 is provided in the central axial region of the pump inlet 1 3, this body 1 8 being so positioned that fluid entering through the inlet is diverted somewhat radially outwardly away from the axis of the shaft 12.

The conical body 18 and the central portion 1 5 of the impeller form, in combination, a generally conical deflector means, one part of which (the portion 15) rotates with the impeller and the other part of which (the conical body) is stationary throughout operation of the pump.

Suitable static sealing means are provided and are generally indicated at 1 9 in the drawing.

In operation, the impeller 11 of the pump is rotatably driven by the shaft 12, tending to force fluid radially outwardly from the axis of the shaft towards an outlet 20 at the periphery of the housing, preferably disposed tangentially. Fluid is drawn into the housing through the inlet 13 and is defiected somewhat radially outwardly as it enters by the deflector means formed by the conical body 18 and portion 1 5 of the impeller.

Thus, the bulk of the fluid is driven more or less directly from the inlet 13 to the outlet 20 by the impeller. However, some of the fluid will tend to pass over the portion 1 5 of the impeller and, in the absence of any means to prevent this, would be forced through the static seal 1 9 and along the shaft bearings, resulting in a loss of fluid pressure and, if the fluid being pumped is corrosive, possibly serious damage to the shaft and bearings.

The parts 1 7 of the impeller blades 14 which project behind the rear surface 1 6 of the generally part conical portion 1 5 tend, however, to act on any fluid passing over the portion 1 5 towards the rear side and force such fluid outwardly by the centrifugal force. The pressure generated by these parts 17 of the blades is equal to the pumping pressure generated in the main body of the pump housing and hence the liquid is prevented from passing past the impeller to the rear and into the region of the shaft.

It will be seen that the parts 17 of the blades have a fairly substantial length, owing to the taper of the central portion 1 5 of the impeller. Any impeller can be scaled so as to generate an adequate back pressure to form a hydrodynamic seal. In fact, it may be necessary to provide additional vapour sealing means on the shaft to prevent air from being drawn in along the shaft since a low pressure condition develops in the region immediately surrounding the shaft at the rear of the impeller. This can be compared to the lowering of pressure which causes the fluid to enter at the inlet 13.

It will be seen, therefore, that an efficient hydrodynamic seal can be provided without the use of a separate seal chamber or seal disc. Thus, considerable savings in manufacturing costs can be achieved because of the reduced amount of machining necessary to construct the pump and, where it is made of or lined with a corrosion resistant material which may be expensive, considerable cost savings can be made in relation to materials and/or the assembly of a lining.

It will be seen that the fluid being acted on by the parts 1 7 of the blades 14 is driven centrifugally into the main bulk of the fluid in the pump housing. This contrasts with the arrangement previously proposed and using a separate seal disc and seal housing, in which such fluid could effectively be "trapped" because it was not directly in communication with the fluid in the main housing of the pump. Where a heated fluid was being pumped, it will be appreciated that this could cause a serious over-heating, since the heat being brought into the seal chamber by the fluid was unable to escape adequately and the work done by the vanes of the seal disc tended, in any case, to increase the temperature of this fluid.In the pump described in relation to the drawing, the whole bulk of the fluid being pumped effectively forms a heat sink for the heat generated by the impeller blades and in particular by the parts 17.

Thus, localised over-heating is most unlikely to occur.

It will be appreciated that the absence of any seal chamber and seal disc reduces the overall size of the pump assembly compared with the previously proposed pump having a hydrodynamic seal. The back pressure generated to provide the seal is related closely to the pressure generated by the impeller so that, for a given pump, the back pressure generation will increase with increasing rate of flow of fluid through the pump, thereby providing what might be considered a selfregulating seal pressure.For a large capacity pump, the size of the parts 17 of the blades which project behind the rear surface 1 6 of the central portion 15 of the impeller is adjusted to ensure adequate back pressure during operation of the pump and it is thought that an efficient hydrodynamic seal can be provided by this means in larger capacity pumps than has hitherto been possible using a separate seal disc and chamber, because of the problem of excessive size and bulk.

The function of the deflector is to deflect the bulk of the fluid more directly towards the outlet of the pump and away from the axis of the shaft so as to minimise the back pressure which needs to be generated by the parts 17 of the blades in order to prevent the fluid being forced along the shaft It is thought that the presence of a static conical body such as that shown at 1 8 may not be necessary but is desirable in order to avoid turbulence in the fluid being pumped which might reduce pumping efficiency and produce excessive heating. The central portion 1 5 of the impeller could, if desired, be extended forwardly to a generally conical shape with the rigid conical body 18 being omitted.

Claims (10)

1. A sealing arrangement for a centrifugal pump of the type comprising a housing, a rotary shaft extending into the housing, an impeller mounted on the shaft for rotation therewith, an inlet for fluid disposed generally axially of the housing and at the face of the impeller remote from the shaft, and an outlet for fluid disposed at the periphery of the housing; said sealing arrangement comprising, in combination, a central deflector means disposed axially and facing the inlet of the pump, at least a portion of the deflector means being integral with the impeller and being adapted to deflect incoming fluid generally radially outwardly away from the rotational axis of the shaft, and means for generating a back pressure tending to resist passage of fluid towards the shaft and comprising parts of the impeller blades projecting rearwardly from said portion of the deflector means.

2. A sealing arrangement according to Claim 1 wherein the fluid deflector means is generally conical in shape.

3. A sealing arrangement according to Claim 1 or Claim 2 wherein the fluid deflector means includes a generally flat radially extending rearward face, from which said parts of the impeller blades project in a direction towards the shaft.

4. A sealing arrangement according to Claim 2 or Claim 3 when appendant to Claim 2 wherein the fluid deflector means comprises a fixed portion integral with the housing of the pump and affording the apex of a generally conical body, the remainder of which is defined by said portion integral with the impeller.

5. A sealing arrangement substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

6. A centrifugal pump comprising a housing, a rotary shaft extending into the housing, an impeller mounted on the shaft for rotation therewith, an inlet for fluid disposed generally axially of the housing and at the face of the impeller remote from the shaft, and an outlet for fluid disposed at the periphery of the housing: the pump including a central fluid deflector means facing the inlet and adapted to deflect incoming means facing the inlet and adapted to deflect incoming fluid generally radially outwardly away from the rotational axis of the shaft, at least a portion of the deflector means being integral with the impeller and rotating therewith, and the impeller having a plurality of impeller blades having parts projecting rearwardly from said portion of the deflector means to generate a back pressure when the impeller is rotating, tending to resist passage of fluid towards the shaft.

7. A pump according to Claim 6 wherein the fluid deflector means is generally conical in shape.

8. A pump according to Claim 6 or Claim 7 wherein the fluid deflector means includes a generally flat radially extending rearward face, from which said parts of the impeller blades project in a direction towards the shaft.

9. A pump according to Claim 7 or to Claim 8 when appendant to Claim 7 wherein the fluid deflector means comprises a fixed portion integral with the housing of the pump and affording the apex of a generally conical body, the remainder of which is defined by said portion integral with the impeller.

10. A pump substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.