GB2266750A - Sheet-metal centrifugal pump casing. - Google Patents

Abstract

A centrifugal pump has a pump casting (6) in which a pump shaft (1) is mounted to be driven by an electric motor. The shaft (1) mounts an impeller (9) having arcuate radial passages (10) therein through which fluid entering an axial inlet (11) is caused to flow by rotation of the impeller. A collector ring (12) around the radially outer edge of the impeller (9) is formed by two trough-shaped pressings (12a, 12b) of stainless steel sheet or plate to form a torpid with a very smooth interior which provides minimal resistance to flow of liquid around the collector ring (12). <IMAGE>

Description

CENTRIFUGAL PUMP The invention relates to a centrifugal pump.

Centrifugal pumps are widely though not exclusively used in apparatus for dyeing yarn and other textile products by circulating dye liquor through the textile material in a dyeing vessel.

In order to increase the temperature of the dye liquor, the circulating system traditionally uses a heat exchanger comprising a bundle of tubes inside a cylindrical pressure vessel and the liquor is made to pass through these tubes at a high velocity to effect good transfer of heat from a heating medium.

In some cases a single pump has been made to serve more than one dyeing vessel, so that the total assembly may involve a series of separate units, and the pump has had to circulate the dye liquor through a throttle valve, the heat exchanger, a reverse valve and sometimes also through cut-off valves which allow separate vessels to be isolated to revert to single vessel operation for small loads. The dye liquor is passed out of the vessels and back to the reverse valve, finally returning to the pump via another piece of piping.

Each of the components in such a system offers a resistance to the flow of the liquor, as do the changes of sectioned area as the liquor passes into and out of components such as heat exchangers and dyeing vessels, and through throttle valves and other changes in direction of the piping system.

Because of the complexity of such a system with bolted flanges at every piping joint into and out of each component, designers tended to keep the cost to a minimum by using the smallest diameter of piping that could be made to do the job.

Both the resistance of the system to liquor flow and the power required to drive a given amount of liquor through a system of a given size vary inversely as the 4th power of the pipe diameter of the system. Such systems require the pumps to be designed to operate against high pressures and the power requirement is high in relation to the actual flow rate achieved. The magnitude of this increase of system resistance with reduced pipe size is seldom fully appreciated as the derivation is a little obscure, and may escape the notice, or be beyond the understanding, of the average draughtsman drawing up a circulating system.

It is known that A = K1 d2 Q = AV H = K2 V2 P = K3 Q H where A is the pipe cross sectional area d is the pipe diameter V is the flow velocity Q is the volumetric flow rate H is the resistance P is the power consumption KX, K2 and K3 are constants It can be shown that V = Q/(KX d2) H = K2 Q2/(K12 d4) P = K2 K3 Q3/(K12 d4) So that HO; l/d4 and P0; l/d4 This required the incorporation of a volute, in the design of a casing of the pump, the volute being shaped so as to provide a region of steadily increasing pipe section disposed around the periphery of the impeller. This enabled the pump to achieve its maximum efficiency when operating against a high resistance.

However, as the resistance to flow was the result of loss of pressure through the tortuous, small diameter piping system, the greater part of the energy supplied to drive the pump was dissipated into heat by friction in the piping system. In some cases very little difference in pump discharge pressure could be detected between an empty machine and a loaded machine as the resistance of the load was very small in comparison with the resistance of the circulating system.

Circulating the liquor against such a high system resistance calls for the use of a high pressure pump, incorporating a volute which is a trumpet shaped passageway of steadily expanding cross-section disposed around the periphery of its impeller. This enables the pump to achieve its maximum efficiency when operating against a high resistance.

However, the use of high pressure pumps in conjunction with high resistance systems in equipment which has to be taken to high temperature has a potentially dangerous side-effect. The total pressure resulting from adding the required static pressure to maintain the necessary NPSH at the pump suction to maintain the required circulation at high temperature can result in the relief valves discharging continuously on out-in flow. This can be most evident when processing tight loads such as sewing thread. It can result in loss of dye liquor and can cause shade control problems.

It also presents a safety hazard as the pressure in the vessel may exceed the designed maximum.

These problems are common to the chemical and paper processing industries and it has been normal practice for dyeing machine manufacturers to use volute type pumps originally developed for these industries and which are available in the appropriate grades of stainless steel.

Recent developments in the design of dyeing machinery have been directed towards the reduction of system resistance and piping system losses.

The tubular heat exchanger normally used has abrupt changes of section of the liquor flow path at the entry connection. There are additional entry losses as the flowing liquor is split into many small streams and made to pass through the heating tubes.

There are similar exit losses as the small streams leave the tube bundle and are recombined.

There are finally exit losses as the liquor is forced back into the small diameter circulating system.

These losses have been greatly reduced and very largely eliminated by discontinuing the use of an in-line tubular heat exchanger and replacing it by a more easily manufactured heating coil in which there are virtually no energy losses imposed on the circulating liquor. Such losses as do exist are confined to a steam system, and have no effect on the efficiency of the circulating system.

The reverse valve and a carrier seat have been redesigned to provide a smoother flow path for the flowing liquor both into and out of the load being treated.

The reverse valve is incorporated into the design of the. dyeing vessel itself so that there are only two short sections of pipe connecting the pump to the machine and has been designed to provide all of the facilities which previously required valves in the circulating system and which caused resistance to flow and reduction in performance.

The results of these changes are a reduction in resistance to flow from elimination of the in-line heat exchanger; a reduction in the complexity, length of pipe and numbers of components in the piping system so that a larger pipe size can be used, resulting in further reduction in resistance to flow; and an improvement in design of the reverse valve and use of a larger valve still further to reduce the total flow resistance.

The effects of these design changes when compounded result in a significantly lower resistance to flow in the final machine. This means that the previous need for the pump to force liquid against a high resistance no longer exists.

High pressure volute type pumps as previously used when operated under conditions of lower head were found to be operating at low efficiency so that the full benefits of the improvements in the design of the system cannot be obtained.

According to the invention there is provided a centrifugal pump having an impeller mounted in a pump body, an axial inlet connection, a radial outlet connection and a collector ring surrounding the periphery of the impeller, the collector ring being of toroidal shape, formed from pressings of sheet metal secured together and leading to the outlet connection.

Such a centrifugal pump can provide the performance characteristic required for new dye liquor circulating systems, and can be fabricated from commercially available materials without the need for highly sophisticated stainless steel foundry facilities.

In a preferred embodiment the collector ring is fabricated from two pressings made from stainless steel sheet or plate. These are welded together to form a torus of circular section which collects liquor as it is delivered from the periphery of the impeller.

The collector ring has the smooth internal surface characteristic of stainless steel sheet instead of the rough and usually irregular surface of the inside of a hollow stainless steel casing.

It can provide an exceptionally smooth transition for the liquor leaving the impeller and allows for unrestricted rotation of this liquid in the pump body. Thus the restriction to flow which is offered by the narrow region in a volute housing is removed and the maximum flow rate provided by the new design is greatly increased.

The invention is diagrammatically illustrated by way of example in the accompanying drawings in which: Figure 1 is a sectional view through a centrifugal pump according to the invention taken on line I-I of Figure 2; Figure 2 is a view taken in the direction of arrow II of Figure 1; Figure 3 shows a plot of head against flow rate with the characteristic for a conventional pump shown by a dotted line and the characteristic for a pump according to the invention shown by a solid line; and Figure 4 shows a plot of power against flow rate with the characteristic for a conventional pump shown by a dotted line and the characteristic for a pump according to the invention shown by a solid line.

Referring to the drawings, a pump shaft 1 has a keyway 2, whereby it can be coupled to be driven by drive means such as an electric motor, and extends through a bearing stand 3 in which bearings 4 and 5 are provided. It further extends through a simple pump casting 6 which is secured by bolts 7 to the bearing stand 3 and which mounts shaft seals 8.

An impeller 9 is keyed to the other end of the shaft to that having the keyway 2 therein and has arcuate radially extending passages 10 shown in Figure 2 into which, in operation of the pump, fluid flows from an axial fluid inlet 11, the fluid being thrown radially outwardly of the impeller by the high speed rotation thereof.

A collector ring 12 is provided around the radially outer edge of the impeller 9. The collector ring 12 is provided as two annular trough-shaped members of pressed stainless steel sheet, that is to say a rear member 12a and a forward member 12h welded together by a peripheral weld. The rear member 12a is welded to a simple cast ring 13 secured by bolts 14 to the pump casting 6. The forward member 12b is secured by a weld 15 to the axial inlet 11. Reinforcing webs 16 extend around the members 12a and 12h and at the underside, as shown in Figure 2, mount support feet 17. Figure 2 also shows a radial outlet connection from the collector ring 12.

Since the collector ring 12 is formed of stainless steel, it can have a very smooth interior which provides minimal resistance to flow of liquid around the collector 12 from the positions at which the liquid leaves the radially outer edge of the impeller 9 until it passes out through the radial outlet 18.

The performance comparisons shown by Figures 3 and 4 clearly illustrate the advantages gained by a pump according to the invention, in both tests the same pump bearing stand, test equipment, impeller and motor were used, only the impeller pump casings were changed.

In dyeing machines of the latest design which offer minimum resistance to rate of flow the same amount of applied horsepower may be able to circulate 50;1 more dye liquor with the new design of pump casing.

In addition to the performance improvement, there are considerable manufacturing advantages. Stainless steel sheet and plate can be produced to very high standards of composition and can be formed by pressing methods with a degree of uniformity unobtainable with castings.

The many problems such as porosity and holes caused by contraction when cooling, and irregular thickness caused by malalignment or shifting of sand cores that make the cast bodies so expensive and their supply to unreliable can be completely eliminated.

Some castings, ie the castings 6, 11 and 13, may be used in the construction of the pump body but these can be of simple form requiring straight-forward patterns without expensive core boxes and which are readily available in acceptable quality.

Claims (5)

1. A centrifugal pump comprising an impeller mounted in a pump body, an axial inlet connection, a radial outlet connection and a collector ring surrounding the periphery of the impeller, the collector ring being of toroidal shape, formed from pressings of sheet metal secured together and leading to the outlet connection.

2. A centrifugal pump according to claim 1, in which the collector ring is fabricated from two pressings made from stainless steel sheet or plate.

3. A centrifugal pump according to claim 2, in which the two pressings are welded together to form a torus of circular section which collects liquor as it is delivered from the periphery of the impeller.

4. A centrifugal pump according to any one of claims 1 to 3, in which the collector ring has such a smooth internal surface characteristic that it provides an exceptionally smooth transition for the liquor leaving the impeller and allows for unrestricted rotation of this liquid in the pump body.

5. A centrifugal pump substantially as hereinbefore described and illustrated with reference to the accompanying drawings.