GB2266749A - Centrifugal pumps - Google Patents

Abstract

A centrifugal pump comprises a pump casing 15 and a pump backplate 9 which together define a pump chamber in which an open impeller 14 is mounted for rotation. Means 25, 26, 27, 28 are provided to effect axial adjustment between the pump casing 15 and the pump backplate 9, whereby the axial clearance between the pump casing 15 and the impeller 14 can be accurately set, and sealing means 12, 13, 24 are provided to effect a seal between the pump backplate 9 and the pump casing 15 at all relative positions of adjustment thereof. <IMAGE>

Description

CENTRIFUGAL PUMPS The invention relates to a centrifugal pump.

In a centrifugal pump a disc like impeller may be provided, usually for rotation about its centre by a motor and provided with passages which extend from its centre to its periphery such that liquid supplied generally axially by a pump suction pipe to the centre of the impeller is accelerated outwardly by the rotation of the impeller. It is collected in a peripheral chamber surrounding the impeller and expelled through an outlet port.

In the manufacture of small centrifugal pumps, the production of impellers has always presented quality control problems. It can be demonstrated that the best performance is obtained when the total cross-sectional area of the flow paths in the impeller does not greatly exceed the cross-sectional area of a pump suction pipe. Attempts to cast "closed" impellers, that is to say impellers which in themselves define all the walls of the passages extending from the centre to the periphery of the impeller, have resulted in lack of uniformity in shape and area of the narrow passageways and loss of balance in both mass of metal and hydraulic flow area. Such difficulties have led to the passages being made much larger than is hydraulically correct in order to provide strong enough cores to withstand the flow of molten metal as it is poured into a mould to cast the impeller.

Open impellers have been proposed, that is to say impellers where the passages are formed as grooves in the face of the impeller disc and are effectively closed by maintaining a close clearance between that face of the rotating impeller and a stationary inside face of a pump casing. The faces may be flat or slightly conical according to the design requirements.

In order to prevent leakage of pressured liquid at the impeller periphery down between the faces to the impeller suction connection at the centre of the disc, the clearance between the faces must be accurately maintained at a very small dimension.

Wear of the faces can be caused by corrosion or erosion by sand, salt or other particulate matter in the liquid being pumped and will change the clearance between the impeller and the pump casing.

To obtain the required close clearance, both in manufacture and during the life of the machine, different methods have been used.

In a first method, the components are machined to ' close tolerances so that, when assembled, the correct clearance will be obtained. This is expensive since it requires all parts to be very accurately machined and in the absence of any means for adjustment in the field, parts have to be accurately set up and re-machined by maintenance personnel to compensate for wear.

Another approach is to effect adjustment by the provision of shims in the form of large gaskets to be inserted between the pump casing and a pump backplate. Such gaskets require clearance holes for the eight to twelve small bolts customarily used to secure the casing to the backplate. A serviceman is required to place the casing in position, determine the thickness of the shims needed to provide the required clearance, select the appropriate shims and complete the installation. This requires skill and patience and provision of the necessary replacement parts.

In an attempt to simplify the adjustment process, shims have been used cut to suit the spaces between the bolts but this can result in damage to the pump casing or the backplate if bolts without shims underneath them are overtightened. Shims in the shape of plain washers under each bolt are difficult to work with as are U-shaped shims to be trapped behind the bolts. It is not easy to cut such shims from shimstock with tin-snips.

The result of the difficulties is that there is a tendency for pumps to be returned to service after maintenance incorrectly adjusted and for performance therefor to be below standard even after adjustment has been attempted.

According to the invention a centrifugal pump has a pump casing, a pump backplate, a pump chamber defined by the pump casing and the pump backplate, an open impeller mounted in the pump chamber, means to effect relative axial movement between the pump casing and the pump backplate thereby to vary the clearance between a face of the impeller and a cooperating face of the pump casing and means to effect a seal between the pump casing and the pump backplate at all relative positions of adjustment therebetween.

Preferably, the sealing means comprises generally cylindrical cooperating surfaces of the pump casing and the pump backplate with one of such cylindrical surfaces having a groove therein to receive an 0ring to effect a seal with the other of the surfaces.

The means to effect adjustment of the relative positions of the pump casing and the pump backplate may comprise elongate threaded members secured to one of the pump casing and a member secured to the pump backplate and extending through the other of the pump casing and said member and nuts threadedly engaged on the threaded members and to bear against the other of the pump casing and said member.

Thus, the nuts act as adjustment and locking nuts to control the axial position of the pump casing with respect to the pump backplate and thus the relative axial position of an inner face of the pump casing and a face of the impeller.

The impeller may have passages therein. through which liquid flows from a suction connection at the centre thereof to a discharge at the periphery of the impeller, formed as grooves accurately produced by machining the face of the impeller to provide the required crosssectional area for the passages. It may be preferable to cast the open passages into the impeller but since they are open they are then accessible for quality control inspection and easy correction of casting defects.

The invention is diagrammatically illustrated by way of example in the accompanying drawing in which: Figure 1 is an end view of a centrifugal pump according to the invention mounted on an electric drive motor; Figure 2 is a sectional view taken on line II-II of Figure 1; and Figure 3 is a perspective view showing an alternative impeller for the pump of Figure 1.

Referring to the drawing, an electric motor 1 has a housing 2 incorporating a mounting plate 3 and an output shaft 4 which extends through and is perpendicular to a reference face 5 of the mounting plate 3. A centrifugal pump 6 comprises a backplate 7 located at its inner periphery by means of a spigot 8 on the mounting plate 3 and having a pump backplate 9 secured thereto by four bolts 10. A cylindrical outer surface 11 of the pump backplate 9 has a groove 12 therein and an O-ring 13 engaged in the groove. The pump 6 further comprises an open impeller 14 secured on the shaft 4 of the electric motor 1 and a pump casing 15.

The pump casing 15 is generally circular, has an axial central fluid suction inlet 16, a circular radially extending chamber 17 defined by a face 18, a peripheral chamber 19 and a fluid outlet 20.

At three positions around the pump casing 15 apertured lugs 21 are provided in line with apertured lugs 22 on the backplate 7. The pump casing 15 has an inward annular projection 23 which defines the inner face of the peripheral chamber 19 and terminates in a cylindrical surface 24 which extends parallel to and closely adjacent to the cylindrical outer surface 11 of the pump backplate 9, the surface 24 cooperating with the O-ring 13 to form a seal between the pump casing 15 and the pump backplate 9.

Bolts 25 are engaged in the apertured lugs 22 in the backplate 7, two nuts 26, 27 are engaged on each of the bolts 25 between the backplate 7 and the lugs 21 of the pump casing 15 and a further nut 28 is engaged on each bolt 25 on the free end thereof. By means of the bolt 25 and the nuts 26, 27, 28 the lugs 21 and thus the pump casing 15 can be moved axially with respect to the pump backplate 7 and thus with respect to the impeller 14 mounted on the shaft 4.

In the embodiment shown in Figure 3, the impeller 14 comprises a back disc 14a with blades 14b mounted thereon in the chamber 17, the outer axial face formed by the blades 14b being slightly conical and the blades 14h defining impeller passages therebetween, which passages are closed by the face 18 of the pump casing 15.

Figure 3 shows that the impeller 14 can be a flat disc with opposite parallel faces with three curved grooves 29 extending between an axial bore 30 to receive the shaft 4 and the periphery of the disc 14, the grooves 29 being of generally rectangular section and increasing in width and depth between the inner central aperture and the periphery.

The use of three bolts 25 and four bolts 10 allows the pump discharge 20 to be mounted vertically, horizontally or at 30C intervals therebetween.

The procedure to set the clearance at the impeller 14 is as follows. First remove a rear motor cover 3 so that a cooling fan is exposed and can be used to rotate the shaft 4 with the fingers.

Assemble the pump casing 15 over the O-ring 13 and, with the rear adjustment nuts 26, 27 run backwards to their fullest extent so that they are clear of the lugs 21 and do not create an obstruction, the casing 15 should be pushed backwards until it squarely rests on the impeller 14. Assemble the front nuts 28 and gently run up with the fingers to make contact with the lugs 21. Check that all the nuts 28 are finger tight against the lugs 21 and that the shaft 4 cannot be rotated with the fingers. Run the back nuts 27 up to the rear of the lugs 21. Ease the front nuts 28 off a fraction of a turn and take up the clearance with the back nuts 26 until the shaft 4 can be rotated, checking each pair of nuts 28, 27 in turn to make sure that the pump casing 15 is only just clear of the impeller.

Each front nut 28 should be backed off one sixth of a turn, and the rear nut 27 tightened against it. The use of standard hexagon nuts and wrenches enables this to be done accurately. Suction and discharge connections may be reconnected and the rear motor cover replaced after a final check that the shaft 4 spins freely, and the unit can be returned to service.

If there is any doubt about the accuracy of the setting at any time, the clearance can be checked and reset in this way without removing the pump from service or disturbing the pipework.

With suitable machining fixtures the impeller casting can be held accurately centred for machining by locating it on the passageways themselves and thereby ensuring their concentricity in use.

The pump casing can be designed to have sufficient freedom of lateral movement to be able to use in the same pump, impellers of different width, each of which may be best suited for a different purpose, so that a single basic pump can be used in a number of different applications.

Claims (8)

1. A centrifugal pump comprising a pump casing, a pump backplate, a pump chamber defined by the pump casing and the pump backplate, an open impeller mounted in the pump chamber, means to effect relative axial movement between the pump casing and the pump backplate thereby to vary the clearance between a face of the impeller and a cooperating face of the pump casing and means to effect a seal between the pump casing and the pump backplate at all relative positions of adjustment therebetween.

2. A centrifugal pump according to claim 1, in which the means to effect a seal comprises generally cylindrical cooperating surfaces of the pump casing and the pump backplate with one of such cylindrical surfaces having a groove therein to receive an O-ring to effect a seal with the other of the surfaces.

3. A centrifugal pump according to claim 1 or claim 2, in which the means to effect relative axial movement between the pump casing and the pump backplate comprises elongate threaded members secured to one of the pump casing and a member secured to the pump backplate and extending through the other of the pump casing and said member and nuts threadedly engaged on the threaded members and to bear against the other of the pump casing and said member.

4. A centrifugal pump according to claim 3, in which the nuts act as adjustment and locking nuts to control the axial position of the pump casing with respect to the pump backplate and thus the relative axial position of an inner face of the pump casing and a face of the impeller.

5. A centrifugal pump according to any one of claims 1 to 4, in which the impeller has passages therein, through which passages liquid flows from a suction connection at the centre of the impeller to a discharge at the periphery of the impeller, formed as grooves accurately produced to provide the required cross-sectional area for the passages.

6. A centrifugal pump according to claim 5, in which the grooves are produced by machining.

7. A centrifugal pump according to claim 5, in which the grooves are to cast as open passages in the impeller.

8. A centrifugal pump substantially as hereinbefore described and illustrated with reference to the accompanying drawing.