Can A Centrifugal Pump Run Backward?

Author: Sara Peters | August 24, 2017 | Category: Equipment Maintenance, Pumps

ps220.jpgAs we were setting up for our latest video product review with the PumpSmart 220, we realized we had a problem. The pump's impeller was turning backwards. I asked our Chris Holland, Sr. Fluid Sales Engineer, "what does that mean for our demo?" Honestly, I half expected the water to run in reverse through the pump, the suction becoming the discharge, and the discharge becoming the suction. Apparently, that's not the case.

Centrifugal_Pump.gifHow could this happen?

The chance for a pump to run backwards occurs most commonly when installing a new pump and motor, replacing a motor, changing a starter, or adding a VFD.

Before coupling the motor to the pump, run a "bump test" to ensure the shaft is turning the right way. A bump test is a quick power on/off so the rotation of the shaft can be seen. There are three wires on the lead that connect to the pump motor. Connect them to the motor and run the bump test.

Check the casing for indication of rotational direction. In most cases, when looking at the impeller from the coupling side, the shaft turns clockwise. If the bump test results in the opposite, switch two of the three wires, then run the test again.

Chris Holland demonstrated the test for me and it was clear, the shaft was turning backwards.

What's the big deal?

Why does impeller direction matter? As I mentioned before, flow doesn't reverse through the pump, so what's the problem?

Running the pump backwards definitely has adverse effects. Pumps that have threaded impellers can unthread inside the casing when run in reverse. With tight clearances inside the casing, the impeller and shaft can act like a car jack on the pump internals. The impeller can be galled, casing damaged, thrust bearing and mechanical seal destroyed. Be sure that even when bump testing the motor, the pump is not coupled!

Fortunately the pump we used for our demo didn't have a threaded impeller.

Though we didn't destroy our pump internals, we could see with a flow meter that the amount of flow was drastically reduced. The PumpSmart 220 was programmed to run the motor at a speed that should have generated 15 GPM. In reality, the pump was generating only 8-9 GPM. Chris said this is to be expected, as a pump running in reverse will generate approximately half the expected flow.

We were able to fix this problem quickly with a reference to the PumpSmart's operations manual. We found the setting that would allow us to change the direction of the impeller. The flow on the flow meter now matched the PumpSmart's expected output.

When installing a new motor be conscious of the shaft's rotation. Best case you get less flow than expected, worst case you get to buy a new pump! If you're unsure, contact a local distributor who is well versed in pump installation.

Problems with pump installation? Ask us about it! We gladly provide technical assistance to businesses and municipalities in Wisconsin and upper Michigan.

Problem pumps? Find out what's causing repeat failures and get it fixed now.

Sara Peters

Sara Peters

Sara leads Crane Engineering's blogging team, coming up with fresh stories and insights for our readers to apply to their every day work.

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