GB2539514A

GB2539514A - Impellers for centrifugal pumps

Info

Publication number: GB2539514A
Application number: GB1510920.0A
Authority: GB
Inventors: James Beer William; Charles Nicholas Randall Jack; Peter Walker Samuel
Original assignee: Gilbert Gilkes & Gordon Ltd
Current assignee: Gilbert Gilkes & Gordon Ltd
Priority date: 2015-06-20
Filing date: 2015-06-20
Publication date: 2016-12-21
Also published as: GB201510920D0

Abstract

An impeller for a centrifugal pump comprising a plurality of first vanes 2 extending from a hub to the circumference of the impeller, and a plurality of second vanes 2B which are shorter and extend only part way to the circumference of the impeller. The impeller also features a shroud 15B, which has been trimmed so as to have a scalloped circumference. The shortened blades and cutaways on the shroud may reduce the influence of boundary layer friction on the impeller and increase the output pressure (head).

Description

IMPELLERS FOR CENTRIFUGAL PUMPS

This specification relates to the shrouded impellers used for centrifugal pumps.

Pumps have thousands of uses in domestic and industrial applications and designers are striving ceaselessly to improve the efficiency of the units, i.e. to improve the volumetric throughput and / or head generated per unit of power input. The impeller is a key element in any centrifugal pump and designers have tried a multitude of variations to improve its performance There are three basic aspects of the design where improvements are possible; these are:-i) the condition of the fluid and its flow into the impeller volute; ii) the profile, arrangement and number of the blades on the impeller and the size of the impeller; and iii) the condition of the fluid discharged from the impeller into the casing and out of the pump. This specification is concerned with the third of these aspects.

According to the invention, there is provided an impeller for a centrifugal pump capable of pumping a fluid comprising:-i) a centrifugal pump having a body, a fluid inlet (eye), a rotatable impeller located within the body, a means to drive the impeller and a fluid outlet; ii) the rotatable impeller having:-a) a fluid inlet (eye); b) a hub having an axis of rotation; c) a radial shroud fast with the hub and having parts of its circumferential edge and adjoining areas removed; d) a plurality of impeller blades fast with the radial member and extending outwardly from the hub towards the circumference of the radial shroud the plurality of blades having some extending fully out to the circumference of the radial shroud and others extending only part of the way out towards said circumference; and e) a means to discharge the fluid from the impeller into the pump outlet; characterised in that the rotation of the impeller causes the fluid essentially to fill the volumetric spaces between adjacent pairs of blades, develops a pressure in the fluid and discharges the fluid in the spaces to the pump outlet and beyond and further characterised in that because certain of the blades do not extend fully to the circumference of the radial shroud and also because parts of the circumference and adjacent areas of the radial shroud are removed, the total impeller surface area in contact with the fluid being discharged to the outlet is reduced thus minimising boundary layer friction and improving throughput and / or head generated.

According to a first variation of the apparatus of the invention, the eye is essentially coaxial with its axis of rotation.

According to a second variation of the apparatus of the invention, the plurality of blades 10 alternates with one blade extending fully to the shroud circumference and the next extending only part of the way out towards said circumference According to a third variation of the apparatus of the invention, the parts of the circumference of the radial shroud which are removed are those circumferential lengths 15 between those where the alternating blades extend fully to the circumference.

According to a fourth variation of the apparatus of the invention, parts of the radial shroud extending inwardly from the circumferential lengths are also removed.

According to a fifth variation of the apparatus of the invention, the alterations to the impeller act to maximise the head (pressure) generated by the pump in the pump outlet.

In a preferred application of the apparatus of the invention every other blade on the impeller is shortened so that it extends outwardly from the hub but does not extend to the circumference of the radial strengthening shroud. Additionally, parts of the radial shroud's circumference between the remaining blades which do extend to the circumference are removed and adjacent inboard areas are also removed. The cut-away portions are hydrodynamically faired to give a scalloped form to the circumference of the adapted impeller.

The purpose of these improvements is to reduce the boundary layer friction acting on the fluid contained between adjacent pairs of blades and so improve the discharge of this fluid into the pump outlet. Tests have shown that both changes act in combination to produce a significant improvement in head (pressure) in the pump outlet For a clearer understanding of the invention and to show how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawing in which:-Figure 1 is a diagrammatic sectional representation of a typical centrifugal pump

impeller in a housing (Prior Art);

Figure 2 is a diagrammatic representation of the forces applying to the fluid bring pumped at a point A in Fig. 1; Figure 3 is a diagrammatic sectional representation of an intermediate stage of the impeller development in a housing; Figure 4 is a diagrammatic sectional representation of the impeller design according to the invention in a housing; Figure 5 is a perspective view of an impeller according to the invention; and Figure 6 is a part cut-away, perspective view of the impeller shown in Fig. 5.

In the following description, the same reference numeral is used for the same component in different Figures and / or for different components fulfilling identical functions.

Centrifugal pumps are regularly used for handling fluids, i.e. liquids and gases. The following description is written with reference to liquids, almost all of which are essentially incompressible, e.g. water. The principles described apply equally well to gases but allowance must be made for their compressibilities and lower viscosities.

Fig. 1 shows a sectional representation of a typical centrifugal pump impeller (Prior Art) in its casing 1, consisting of the impeller 14, rotating about an axis 5, with blades 2 arranged spirally outwards from the hub to the circumference. Shrouds 15 are provided on each side of blades 2 to improve the hydraulic efficiency of the flow through impeller 14 and reduce re-circulation. They also add to the structural integrity and strength of the impeller.

Impeller 14 rotates 6 drawing water in from the pump inlet (not shown) via the impeller eye 24 (Figs. 5 and 6) as the blades 2 pass zone P. The water is compressed as blades 2 pass zone Q and any voidage, e.g. caused by incomplete filling at P, is made up with more water from the eye. Due to the small clearance 16 between the blade tips and casing 3, the water (hereinafter referred to as 'contained water') between each adjacent pair of blades 2 rotates essentially as a solid body until that pair reaches zone R, where most of this water is discharged tangentially 17 into outlet 4.

Clearly, to maximise throughput, the contained water volume between each adjacent pair of blades 2 should be maximised and the part of this volume discharged in zone R should also be maximised. However, pumps have to generate the required output head (pressure) and high pressures require many blades 2 with tight volutes. These two criteria (high throughput and high head) tend to be mutually exclusive. For example, closely spaced, impeller blades will generate high heads but the contained water volumes will be relatively small and the boundary layer friction (between the metal surfaces and the contained water) will reduce the proportion of this water discharged 17 in zone R. This inevitably leads to design compromises according to whether the requirement is for a high throughput or a 10 high head.

Fig. 2 shows the dynamic forces exerted on the contained water by a blade 2 at a point A in Fig. 1. Blade 2 exerts a force 8 on the water normally to the tangent 7 to the curvature of blade 2 at point A. Force 8 may be resolved into a radial component 10 and a circumferential component 9. Radial component 10 compresses water 12 locally against the internal circumference of casing 3 and boundary layer friction 11 acts to oppose motion 6. Flow conditions in the annulus 16 between blade tips 2B and casing 3 will be highly turbulent but boundary layer friction 11 will be enhanced by the pressure effect on water 12 due to radial force 10. Water passing into angle of nip 13 and through the clearance 16 will be choking flow and this will add to the magnitude of frictional force 11.

(Forces 8, 9, 10 and 11 are all vectors but (in Fig. 2) the length of each arrow is not intended to indicate the relative magnitude of the actual force. (In particular, tangential arrow 11 is extended so that the barbs are clear of line 3, for purposes of clarity.) Though the basic principles can be described, exactly what is happening inside the volutes between adjacent pairs of blades 2 is not clear and a series of experiments was undertaken to test a variety of full-size impeller designs and, in particular, to assess the role of boundary layer friction, e.g. 11, on the discharge of contained water into outlet 4. As the impeller designs would all have the same nominal output, the target was to increase and maximise head (pressure) generated in the pump outlet.

Clearly, there is considerable flow resistance in the volumes designated 12 and 13 as this water tries to squeeze through blade tip gaps 16 and so having fewer blade tips will reduce the cumulative resistance and improve discharge of contained water in zone R. However, reducing the blade number will normally lower the head that can be generated. A compromise (Fig. 3) is to retain the same number of blades on impeller 14A but cut, say, every other one 2A short so that their tips 2B' are not so close to the casing. Radial strengthening shroud 15 consists of an inner portion 15 and an outer annulus 15A. Radial strengthening 15 of volute blades 2 is essential.

The impeller design 14A shown in Fig. 3 is an intermediate stage in the development, referred to above, between the Fig. 1 starting point (Prior Art) and the final stage (Fig. 4).

The Fig. 4 design shows the impeller of the invention 14B with every other blade 2A cut short, as per Fig. 3. Radial shrouds 15A (Fig. 3) have also been trimmed 15B; this provides support to the distal part of each blade 2. They are also hydrodynamically profiled and faired 15C into the leading edge of the following blade 2. The purpose of part shrouds 15B is to support blades 2 adequately yet minimise the surface area in contact with the contained water (i.e. to minimise boundary layer fiction) while providing hydrodynamic fairing 15C. The portions of the part shrouds 15A (Fig. 3) at and near the circumference and between the outer parts of adjacent blades 2 provide adequate strengthening but do also contribute to additional boundary layer friction. As shown in Fig. 4, impeller 14B has a scalloped' circumferential form which is characteristic of this invention (Figs. 5 and 6).

Fig. 1 shows a current state-of-the-art (prior art) impeller, Fig. 3 is an intermediate stage of the development process and Figs. 4-6 the final inventive stage. For illustrative purposes the contained water 18 and 19 between the two adjacent blades 2 discharging (zone R) into outlet 4 is shown hatched. The contained water in the annulus outside the tips 2B' of shortened blades 2B is shown doubly hatched 18, 18' and 18" (in Figs. 1, 3 and 4 respectively), while that inside it is singly hatched 19, 19' and 19" respectively. Reference numerals (21, 21' and 21") and (22, 22' and 22") refer to the surface areas of radial shroud 15 in the annulus area 15A and inside it 15 respectively in contact with the contained water.

Referring to Fig. 1, as each pair of adjacent blades reach zone R, annular contained water 18 is flung outwards by centrifugal forces 8 and 10 and tangential force 9 into outlet 4 against the frictional resistance from surfaces 3, 15, 15A, 20 and 21. The outward movement of water 18, exerts a suctional force on inner water 19, which is constrained by boundary layer friction. In spite of the high rotational speed 6, not all the relatively small volume of water 19 will be discharged into outlet 4 in the time available before the trailing blade has passed cutwater 23.

Referring to Figs. 3 and 4, the larger (and more massive) volume of annular contained water 18' and 18" (compared to 18 in Fig. 1) will have a greater suctional force on inner water 19' and 19° than in the Fig. 1 example. Comparing the ratios of the contained water volumes (hatched areas) in Figs 1, 3 and 4 to their respective surface areas (frictional resistances), i.e:- 18, 19 18', 19' 18", 19" . 15, 15A, 20, 21, 22 15, 15A, 20, 21', 22' 15, 15B, 20, 21", 22" shows a strongly increasing trend and this is matched by the increase in head generated, as shown in the test results Table below.

Design Output Pressure Torque (Ex. mass) Efficiency (%) (Pa) % Gain (Nm) % Increase % % Loss Figure 1 (Prior art & starting datum) 319 017 46.8 77.79 Figure 3 325 307 1.97 47.74 2.01 77.76 0.03 Figure 4 344 902 8.11 55.11 17.8 71.90 5.89 The gains in output pressure of 1.97% and 9.11% from the Fig. 3 and Fig. 4 designs respectively demonstrate the degree to which boundary friction acts to oppose water flow. While this gain in output pressure increases the torque needed to drive the pump and consequently reduces the overall efficiency. the percentage pressure gains are considerably greater than the percentage efficiency losses, proving the value of full scale trials with possible new designs of impeller (Figs. 3 and 4-6).

At a time when designers are working hard on new designs of blade profile to gain an extra 20 tenth of a percentage improvement, increases of the order of 1.97-8+% are spectacular. The skilled person will appreciate the teaching herein and its application to making further improvements, all falling within the scope of the invention.

Claims

Claims:- 1. An impeller for a centrifugal pump capable of pumping a fluid comprising:-i) a centrifugal pump having a body, a fluid inlet (eye), a rotatable impeller located within the body, a means to drive the impeller and a fluid outlet; ii) the rotatable impeller having:-a) a fluid inlet (eye); b) a hub having an axis of rotation; c) a radial shroud fast with the hub and having parts of its circumferential edge and adjoining areas removed, d) a plurality of impeller blades fast with the radial member and extending outwardly from the hub towards the circumference of the radial shroud the plurality of blades having some extending fully out to the circumference of the radial shroud and others extending only part of the way out towards said circumference; and e) a means to discharge the fluid from the impeller into the pump outlet; characterised in that the rotation of the impeller causes the fluid essentially to fill the volumetric spaces between adjacent pairs of blades, develops a pressure in the fluid and discharges the fluid in the spaces to the pump outlet and beyond and further characterised in that because certain of the blades do not extend fully to the circumference of the radial shroud and also because parts of the circumference and adjacent areas of the radial shroud are removed, the total impeller surface area in contact with the fluid being discharged to the outlet is reduced thus minimising boundary layer friction and improving throughput and / or head generated.
2. An impeller for a centrifugal pump as claimed in claim 1, wherein the eye is essentially coaxial with its axis of rotation.
3. An impeller for a centrifugal pump as claimed in claims 1 and 2, wherein the plurality of blades alternates with one blade extending fully to the shroud circumference 30 and the next extending only part of the way out towards said circumference
4. An impeller for a centrifugal pump as claimed in any preceding claim, wherein the parts of the circumference of the radial shroud which are removed are those circumferential lengths between those where the alternating blades extend fully to the circumference.
5. An impeller for a centrifugal pump as claimed in claim 4, wherein parts of the radial shroud extending inwardly from the circumferential lengths are also removed.
6. An impeller for a centrifugal pump as claimed in any preceding claim, wherein the alterations to the impeller act to maximise the head (pressure) generated by the pump in the pump outlet.
7. An impeller for a centrifugal pump as described in and by the above statement with reference to Figs. 2-6 of the accompanying drawings.