PUMP IMPELLER AND METHOD

Abstract: A pump impeller (10) which is modified so as to achieve selected operating performance parameters. The impeller (10) includes a front shroud (12) and a rear shroud (14), the shrouds (12, 14) being spaced apart so as to form a plurality of passageways therebetween which are separated by a plurality of impeller blades (15), each having an outer edge (19) extending between the front and rear shrouds (12, 14), the impeller (10) having an outer diameter D (15). The method of modification includes the steps of trimming the outer edge (19) of the blades (15) so that the outer diameter D1 of the front shroud (12) is less than the outer diameter D2 of the rear shroud (14).

Description: PUMP IMPELLER AND METHOD This invention relates generally to pumps and more particular, but not exclusively to high specific speed pumps. The invention is particularly applicable to high specific speed pumps typically used for flue gas desulphurisation (FGD) applications in power stations. Such pumps are utilised in wet limestone scrubbing process to circulate the slurry to remove the sulphur from the flue gases before the sulphur can enter the atmosphere. Reference to this particular application is not to be taken as a limitation to the scope of the invention. It will be readily apparent to those persons skilled in the art that the invention is also applicable to other applications. Pumps required for FGD applications typically must deliver high flowrate at moderate to low head. To achieve this they can, for example, be direct coupled which increases the speed at which they operate above say a lower speed gearbox driven pump. These higher speed pumps for FGD applications can also be referred to as a mixed flow pump, as opposed to say a normal slower running radial style of slurry pump. The flow in a radial pump is predominantly radial, whereas a mixed flow pump, the flow is both radial and axial. The duty specifications for FGD pumps are normally very stringent and users require high efficiency. For FGD plants to operate correctly and efficiently, the FGD pumps must pump a precise set volume of limestone slurry through the FGD system. As the volume flowrate needs to be set precisely, so does the head (or pressure) that the pump supplies. Normally, the pump specification does not allow any negative tolerances eg. , if the specified pump head is 25m, then on test, the pump must produce 25m or more. How much more is also normally spelt out in the Pump Test Standard Acceptance Criteria that are given in the contract. This can be say +5 % more head. Design of a pump for FGD must take account of the duty requirements, particularly in regard to head as it is the head, which ultimately controls the volume flow of slurry in the FGD system. With a direct driven mixed flow pump, the impeller diameter can only be changed marginally to meet the duty requirements. Hence there will be occasions that the pump would generate more head than the specified and in some cases even more than the allowable upper limit specified in the contract. When the head is higher than the allowable tolerance, it must be reduced so the final tested head is within the tolerance band to meet the acceptance criteria. To reduce the head to within the allowable tolerance (say -0% to +5 %), the impeller can be modified by causing it to be trimmed i.e. a small reduction is made to the impeller diameter. Trimming to reduce the head also changes the power absorbed by the pump and this impacts on the pump efficiency. The currently known technique as detailed in technical papers and texts is to make an angled cut on the drive side or rear shroud side of the impeller. This is shown in Figure 1. Depending on the amount of trim (diameter reduction), the head and power are affected at different rates as shown in Figure 2. Hence while the prediction of the necessary trim is complicated, the main problem relates to the fact that the head and power reductions do not follow the same pattern. Hence, trimming by this known method generally results in a reduction in head, a lesser reduction in power and a consequential decrease in efficiency. Hence it may occur that the head and flow produced by the pump are correct, but that the power absorbed is higher than the allowable tolerance. In such a case, the pump would be unacceptable. It is an object according to one aspect of the present invention to provide a method of modifying a pump impeller so that it alleviates the problems discussed above. It is a further object according to another aspect of the present invention to provide an improved pump impeller. According to one aspect of the present invention there is provided a method of modifying a pump impeller so as to achieve selected operating performance parameters, the impeller including a front shroud and a rear shroud, the shrouds being spaced apart so as to form a plurality of passageways therebetween which are separated by a plurality of impeller blades, each having an outer edge extending between the front and rear shrouds, the impeller having an outer diameter D, the method including the steps of trimming the outer edge of the impeller blades so that the outer diameter D, of the front shroud is less than the outer diameter D 2 of the rear shroud. Preferably, the outer peripheral edge is trimmed so as to taper inwardly from the outer diameter D 2 of the rear shroud to the outer diameter D, of the front shroud. According to another aspect of the present invention there is provided an impeller for a pump, the impeller including a front shroud having an outer diameter O, and a rear shroud having an outer diameter D 2 , the shrouds being spaced apart so as to form a plurality of passageways therebetween which are separated by a plurality of impeller blades each having an outer edge extending between the front and rear shrouds characterised in that the outer diameter D, of the front shroud is less than the outer diameter D 2 of the rear shroud. Preferably, the ratio of D,/D 2 ranges from between 1.0 to and including 0.85. It has been surprisingly discovered that the reduction in head and power follows a more predictable pattern and more importantly, the reduction in head and power achieved for any trim are more equal. The effect is that the pump efficiency is far less effected by the trim as compared to the known method. Hence it is more likely using the new method of impeller trimming, that the head and flow can be achieved within tolerance and at the same time that the efficiency is likewise acceptable. Preferred embodiments of the invention will hereinafter be described with reference to the accompanying drawings and in those drawings : Figure 1 is a schematic partial view of an impeller which has been trimmed using a known trimming technique; Figure 2 is a graph illustrating the general performance characteristics of the impeller shown in Figure 1 ; Figure 3 is a schematic partial view of an impeller which has been trimmed according to the present invention; and Figure 4 is a graph illustrating the general performance characteristics of the impeller shown in Figure 3 where percent trim is determined by 100 - ( — ) x 100 Referring to Figures 1 and 3 there is shown two impellers each generally indicated at 10, each impeller including a front shroud 12 and a rear shroud 14 with a series of blades 15 extending therebetween separating the interior of the impeller into a series of passageways. The impeller further includes a impeller inlet 17 and a series of outlets between the blades at the peripheral edge 19 of the impeller. The diameter of the front shroud is indicated by D, and the diameter of the rear shroud is indicated by D 2 . In the prior art arrangement shown in Figure 1, the trimming is effected by removal of a portion of the outer peripheral edge so that the diameter of the inlet shroud D, is greater than the diameter of the outlet shroud D 2 . As mentioned earlier, depending on the amount of trim (diameter reduction), the head and power effected at different rates as shown in Figure 2. According to the present invention, the impeller is trimmed by removing material so that the diameter of the front shroud is less than the diameter of the rear shroud. The effect of this trimming is shown in Figure 4. Thus, according to the present invention, the pump efficiency is far less effected by the trimming operation as compared to the prior art method. As such it is more likely that under the method according to the present invention, the head and flow can be achieved within tolerance at the same time that the efficiency is held within acceptable limits. Finally, it is to be understood that various alterations, modifications and/or additions may be incorporated into the various constructions and arrangements of parts without departing from the spirit or ambit of the invention.

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS : 1. A method of modifying a pump impeller so as to achieve selected operating performance parameters, the impeller including a front shroud and a rear shroud, the shrouds being spaced apart so as to form a plurality of passageways therebetween which are separated by a plurality of impeller blades, each having an outer edge extending between the front and rear shrouds, the impeller having an outer diameter D, the method including the steps of trimming the outer edge of the impeller blades so that the outer diameter D, of the front shroud is less than the outer diameter D 2 of the rear shroud. 2. A method according to claim 1 wherein the outer peripheral edge is trimmed so as to taper inwardly from the outer diameter D 2 of the rear shroud to the outer diameter D, of the front shroud. 3. A method according to claim 1 or claim 2 wherein the ration of D,/D 2 ranges from between 1.0 to and including 0.85. 4. A method according to any preceding claim wherein the pump impeller is for a pump use for fluid gas desulphurisation. 5. An impeller for a pump, the impeller including a front shroud having an outer diameter D, and a rear shroud having an outer diameter D 2 , the shrouds being spaced apart so as to form a plurality of passageways therebetween which are separated by a plurality of impeller blades each having an outer edge extending between the front and rear shrouds characterised in that the outer diameter D, of the front shroud is less than the outer diameter D 2 of the rear shroud. 6. An impeller according to claim 5 wherein the ratio of Dj/D 2 ranges from between 1.0 to and including 0.85. 7. An impeller according to claim 5 or claim 6 wherein outer edge is trimmed so as to taper inwardly from the outer diameter D 2 of the rear shroud to the outer diameter D, of the front shroud. 8. An impeller according to any one of claims 5 to 7 wherein the impeller is used as a pump for fluid gas desulphurisation.

Inventors: BURGESS KEVIN EDWARD, BURGESS, KEVIN, EDWARD
Applicants: BURGESS KEVIN EDWARD, WARMAN INT LTD, BURGESS, KEVIN, EDWARD, WARMAN INTERNATIONAL LIMITED
Application year: 1998
Main classification: F04D29
IPC codes: F04D29