Centrifugal pumps, shaft deflection

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

• Evidence of parts rubbing against something.
• Evidence of damage to a pump component.

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:

• This is a standby pump that starts and stops a lot. At each start and shut down, the pump is operating on either side of its BEP.
• The process is always changing, due to fluid demand. The operator frequently opens and closes a control valve to satisfy the changing needs of production.
• The pump is being used as an accumulator to keep a head on a system. The pump is started each time the level in the head tank falls to a predetermined causing the discharge head to gradually increase as the level in the tank rises
• We should be using a positive displacement pump in this application but PD pumps do not have enough capacity for our needs. The head is constant in this application because we are pumping into a pressurized container. The capacity is varying with demand.

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:

• Go to a double volute design.
• Purchase a pump with a lower L3/D4 ratio shaft.
• Install a bypass line. When the control valve begins to throttle the discharge, the bypass will open and recirculate the unwanted capacity to a storage tank. You should not bypass to the pump suction because it can heat the incoming fluid.
• If the head is mainly system or friction head, you could use a variable speed driver.
• Use multiple pumps in parallel, and run them as needed to meet the changing capacity needs.

Posted

• On February 18, 2018