Thrust bearing for multistage centrifugal pumps

Details Thrust bearing for multistage centrifugal pumps Thrust bearing for multistage centrifugal pumps

1998-02-26

2001-10-30

Look, Edward K. (Department: 3745)

Rotary kinetic fluid motors or pumps

With shaft connected fluid force subjected thrust balancing...

C415S131000, C415S229000, C415S058400

Reexamination Certificate

active

06309174

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to thrust bearings for pumps and turbines. More specifically, the present invention relates to an improved thrust bearing for accommodating axial thrust acting on the rotors of pumps.
Pumps are rotating machines that increase the pressure of fluids traveling through the pump casing. An impeller or series of impellers are mounted on a shaft within the pump casing. The rotating impellers increase the pressure on the fluid as the fluid travels through the casing. A motor also having a shaft is coupled to the pump shaft to provide the rotating movement.
Barrel pumps are a type of centrifugal pump that use many impeller stages mounted on a common shaft. The impellers face the same direction and are separated by a diffuser, cross over channel and return channel. The shaft with the impellers is mounted into a casing. Low pressure fluid enters the casing at one end of the impeller array and passes through each impeller. The fluid pressure is successively increased by each impeller. At the opposite end of the impeller array the fluid exits the casing from the discharge outlet at a high pressure.
Various forces act on the impeller shaft during operation of the pump. For long shafts with a number of impellers, the shaft must be supported at intermediate points along its length to prevent excessive sagging or curvature. Commonly, bushings that closely fit around the shaft are mounted between the impellers. The bushings act as bearings to counteract the radial forces and to maintain the desired radial position of the shaft.
Axial thrust also acts on the impeller during operation. The axial thrust is additive for each impeller. Since the impellers are attached to the shaft, very strong axial forces may develop along the shaft.
Another force acting on the shaft is an axial force generated by the difference between the low pressure at the inlet of the pump and the high pressure at the outlet of the pump. This axial force, depending on the pump configuration, is typically in the same direction as the axial thrust generated by each impeller. The axial thrust tends to bend the shaft if the thrust bearing is at the inlet end of the shaft.
In pump design, it is desirable to provide a shaft having the smallest diameter possible. By providing a small diameter shaft the amount of flow permitted in the impeller inlet region, which is sometimes called the eye, is maximized. By reducing the shaft diameter, however, the tendency for the shaft to bend is increased.
One way in which axial thrust is accommodated is to allow the bearings of the motor driving the pump to absorb any axial thrust. Because of the high thrust developed by some pumps, special motors may be required to accommodate the axial thrust. By providing a special motor, the typical life of the motor is increased. The cost of the motor is also increased.
Another way in which axial thrust is accommodated in pumps is that a bearing assembly is located between the shaft of the pump and the shaft of the driving motor. Typically, the bearings are rolling contact type such as ball or roller bearings that are lubricated by oil or grease. The bearing housing is independently supported to transmit the thrust force from the pump shaft to the base plate or other structure supporting the bearing housing. One problem associated with a separate bearing assembly is that the components are extremely difficult to manufacture, they require a separate lubrication system and they also require shaft seals. All of these drawbacks undesirably increase the cost of the pump system.
FIG. 1
illustrates a typical barrel pump
10
coupled to a bearing assembly
12
. Bearing assembly
12
is coupled to a motor
14
. Bearing assembly
12
is used to absorb axial thrust from pump
10
and prevent the axial thrust from reaching motor
14
.
Pump
10
has a casing
16
. Casing
16
has an inlet port
18
and a discharge port
20
. A pump shaft
22
runs longitudinally within casing
16
. Radial shaft bearings
25
and
26
support shaft
22
in a rotatable fashion within casing
16
. Shaft
22
supports a plurality of impellers
28
. Impellers
28
, spaced axially apart by shaft sleeves
30
, are affixed to shaft
22
. Each impeller
28
increases the pressure from the previous stage. Seal
24
seals the end of casing
16
from leaking around shaft
22
.
Each impeller stage has an inlet
32
which transmits water toward impeller outlet
34
. Impeller inlet
32
is located adjacent to shaft
22
. Impeller
28
changes the direction of flow from inlet
32
in an axial direction to a radial direction when fluid is discharged from impeller outlet
34
. A diffuser
36
redirects fluid from impeller outlet
34
into a crossover channel
38
. Crossover channel
38
transmits fluid into a return channel
40
. Return channel
40
redirects water in a radial direction toward shaft
22
. Return channel
40
ends at the input to the next impeller stage or at the discharge port
20
in the final stage. An interstage piece
42
separates the various impeller stages.
Each impeller
28
has an impeller hub
44
and impeller ring
48
. To reduce leakage between impeller stages, a close clearance is established between each impeller ring
48
and wear ring
46
. A close clearance is also established by interstage piece
42
and impeller hub
44
to prevent leakage.
A strut
50
is used to support shaft bearing
26
. Strut
50
extends across casing
12
and allows fluid to reach discharge port
20
. Strut
50
only minimally effects the flow of fluid exiting pump
10
.
Bearing assembly
12
has a shaft
52
and a bearing housing
54
. Within bearing housing
54
are a plurality of bearings
56
supporting bearing shaft
52
. Seals
58
enclose shaft
52
within housing
54
to prevent leakage of fluid from within bearing housing
54
. A bearing housing strut
60
supports bearing housing
54
. Bearing housing strut
60
is preferably not coupled to a support for pump
10
or motor
14
so that any forces may be transmitted through bearing housing to strut
60
.
Motor
14
has a motor shaft
62
. Bearing shaft
52
is coupled to motor shaft
62
and pump shaft
22
by couplings
64
.
As is common in prior known barrel pumps, a force is developed in an axial direction as shown by arrow
66
. Bearing assembly
12
absorbs axial thrust to prevent the axial thrust from being transmitted to motor
14
.
SUMMARY OF THE INVENTION
It is therefore one object of the invention to provide a barrel pump and driver assembly that does not require special couplings, separate bearing housing or motor bearings. By eliminating expensive couplings and bearings the overall cost of manufacture and operation of the pump is significantly reduced.
In one embodiment of the invention, a pump has a casing with an inlet port and a discharge port. A shaft is positioned within the pump. A thrust bearing is coupled to the shaft. A pressure alterable bearing cavity is located within the casing. The pressure alterable cavity allows the axial thrust on the shaft from the impellers to be counteracted. The thrust bearing has an annular seal coupled with respect to the casing. A disc is coupled to the shaft and positioned adjacent the seal. The seal and the disc have a gap therebetween. A feedback pipe couples the bearing cavity to the inlet port. As the axial thrust acting on the shaft changes, the gap between the seal and the disc also changes. The changing of the gap changes the pressure within the bearing cavity. The disc and thus the shaft are repositioned in response to the change in the bearing cavity pressure change.
One advantage of the invention is that axial thrust acting on the driving motor is reduced.
Another advantage of the invention is that expensive lubricated bearings are not required in the operation of the pump.
Yet another advantage of the invention is that a thrust bearing has been integrated into the pump housing and is lubricated and cooled by the pumpage. The thrust bearing ensures that the shaft is under tension from axial thrust which h

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