high amps

[mizkn]

Puzzled.
We have a durco centrifugal pump that keeps tripping on high amps (37 vs designed at 22)... (20 hp, 192 tdh, 3600 rpm, the pumped solvent fluid viscosity is sometimes that of water but today it was "syrupy" and cooled). There have historically been problems with NPSH--however, with the tank level at 70% not the case.

The vibration readings previously led us to believe we needed to "hot" align after we changed out the motor. The motor has sealed bearings and was changed out today, however, vibration is not the issue and NO change in the symptoms. When we throttle back the discharge valves the amperage goes down but the process needs more flow. The inline steam exchanger could not be used as the amps would hold for 30 min on throttled valves and then heat up without the hx in service.

the pump housing seal fluid hoses have air fins after noting a temp of 598F months ago. we need to check this pump housing temp again.

I have noted the thread407-59159 posting "during the motor inspection, it was found that the rotor was bowed and unbalanced (was not shown in the vibration spec) and grease was found on the windings. bearings were overgreased"... we will look into this possibility, however, since the motor when uncoupled did not have any increas in amps and have had two motors now tried and no success or change in symptoms...we are still puzzled.

any suggestions what to attempt next...

 

[d23]

mizkn

Was viscosity taken into account when selecting the pump and motor?

I would see if the system will pump water and compare that to your pump curve. First guess is you have a viscosity problem.

Good Luck!

 

[BobPE]

d23 hinted to one side of the coin, the other would be your pump is running to the right of its curve and is undersized since you indicated that it runs in amp range when you throttle the discharge. It's 50/50 with the viscosity and the run out, take your pick or give us some more information and maybe we can help more....Since you have a heat exchanger on the system I would venture a guess that someone missed the thermo and undersized the system.

BobPE

 

[pablo02]

Is the motor nameplate 22 amps? Typically a 20 hp motor FLA is closer to 27... apparently your overloads are taking the motor out... off the top of my head, I think your 22 amps is underdesigned and I also concur with the other posters -- recheck the operating requirements, you might need a bigger motor..

 

[mizkn]

Thanks for the ideas.

We went to the pump and found that the impeller was warn...very warn even two months ago. So, what did we learn? The reason it worked the last time we operated this portion of the process was that the solvent was new, viscosity like water; now, with viscosity much thicker and the HE not thining the fluid either and an older impeller which has warn more so that the tolerances are way off.

Still digging to learn the rest of the story. We will look into the motor sizing as we investigate the replacement impeller.

 

[BobPE]

The impeller would be worn from cavitation, replacing it will not correct the problem of the pump being incorrectly sized. You should relook at the system considering viscosity changes and known system needs. I think you will come up with a completely different pump and motor that what you currently have.

BobPE

 

[Assumptions]

Do the NPSHa calcs and the system hydraulic calcs indicate a problem for this viscosity/density fluid?