troubleshooting a vertical multi-stage pump

[Andy22]

We have a 4 stage vertical pump which suddenly has a lower output pressure and flow. The motor amps has also dropped by 10% or 20%. Marking the pressure and flow on the pump curve, it would appear that the pump curve has dropped by about 1/4. Is it possible that 1 impeller lost its key so only 3 of the 4 stages is rotating?

 

[MikeCool]

It's either that, or your motor has slowed.

Mike Cool
Mechanical Engineer
American Stainless Pumps, Inc.
Los Angeles California USA

http://www.aspumps.com

 

[Artisi]

Mike
If it's a synchronous motor (more than likely) how can it slow?

OP
Yes you could have an impeller problem.

But, you could have a restricted inlet - less flow, head and power. Look further before pulling the pump.

 

[MikeCool]

Bad bearings, lower voltage, high temperatures (not likely these days) furry animals crawling inside. VFD...

Mike Cool
Mechanical Engineer
American Stainless Pumps, Inc.
Los Angeles California USA

http://www.aspumps.com

 

[Artisi]

Motor speed is a function of the frequency being feed to it, anything else causing a slowdown of the motor will cause o/load and increased power consumption.

 

[Artisi]

excluding the VFD - of course

 

[MikeCool]

You're right. He's dropped an impeller.

Mike Cool
Mechanical Engineer
American Stainless Pumps, Inc.
Los Angeles California USA

http://www.aspumps.com

 

[Artisi]

Maybe.

 

[ione]

Below some bothers which could affect your pump performance (capacity and pressure drops):

Speed of the motor
Impeller loose
Impeller plugged
Wear rings worn
Strainer clogged
Suction valve throttled

 

[JJPellin]

How are the impellers mounted? If it uses tapered collets, you could have broken one impeller loose and it is not spinning. If the impellers are key driven (suggested by your question) with split rings or snap rings to locate, I would not expect there to be room to drop far enough to come completely clear of the key. Is there any change in the vibration to indicate a rub or cavitation. I would expect a failed wear ring or restricted suction before a dropped impeller. But either is possible. If it is in an open sump, I would pull it up and check for 2x4's and bucket lids. If it is in a can, I would check for suction strainers or dropped valve gates.

Johnny Pellin