Some additional information about shaft deflection 15-11

Whenever you troubleshoot a centrifugal pump, there are only two things visible to the observer:

This knowledge makes the troubleshooting task a bit easier. About the only other thing we ever see, is that the product has attached its self to the impeller or volute. Until this buildup throws the rotating assembly out of dynamic balance that can cause rubbing and damage problems, or interferes with the hydraulic flow in the pump, product attachment is not of much consequence.

Shaft deflection occurs as a result of pipe strain, misalignment, lack of dynamic balance, thermal growth and the one we want to talk about now, operating off of the pump's BEP

In an other paper we discussed this type of shaft deflection in detail. It can cause rubbing and damage, especially to mechanical seal and precision bearings. To be effective troubleshooters we must be able to confirm the problem and then come up with a sensible fix to prevent it from happening again.

In that other paper I mentioned that Francis Vane impellers deflect from the shaft centerline towards approximately 60 and 240 degrees depending upon the percentage of flow through the pump. This direction is always measured from the cutwater, and in the direction of shaft rotation. In this paper I want to get a little more specific.

Please take a look at the following chart. It will show you the direction of deflection, as a function of capacity, for four different specific speed impellers.

Impeller specific speed number
Capacity, as a % of the BEP
Deflection, from the shaft centerline towards:
2370
0%
35 degrees
2370
55%
40 degrees
2370
75%
60 degrees
2370
80%
85 degrees
2370
110%
140 degrees
2370
130%
250 degrees
1735
0%
65 degrees
1735
60%
89 degrees
1735
140%
300 degrees
785
0%
110 degrees
785
50%
140 degrees
785
135%
250 degrees
530
0%
135 degrees
530
90%
145 degrees
530
78%
180 degrees
530
40%
265 degrees
530
170%
320 degrees

If you are experiencing this problem, you will observe a continuous rub mark all around some part of the rotating assembly that is running very close to a stationary piece, and a partial rub mark on the stationary part. The bottom of the stuffing box is a good example of this type of rubbing. You will frequently observe that the bottom of the stuffing box has become egg shaped as a result of contact with the rotating shaft, although a dial indicator verifies that the shaft is straight. Remember the shaft is deflecting or bending, it is not bent! You will see these same rub patterns on the wear rings used with closed impeller designs

Whenever you are pumping close to the pump's best efficiency point, the radial thrust is insignificant. It never really hits zero, but it gets close.

There are a lot of reasons why a pump operates off its BEP and some of them are difficult to correct:

You have a couple of choices if you want to lessen the shaft deflection problems. Pick the one that makes the most sense in your application:

 

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