I just wanted to pass on a bit of knowledge on a pump problem we just experience this past week.

On my monthly route I noticed a dramatic increase on the casing of the 10hp motor which drives this caustic pump. From it's normal 105 degrees, it jumped up to over 200, mostly around the shaft and bearing area...Pump was running 15.2 amps and would kick off...Normal amp load for this motor is 6-8 loaded...

Upon examining the data, I noticed some rotor bar peaks with the 2-time line frequency side bands...The pump itself looked ok with the exception of a sharp increse in both 3 & 4 VVEL, HVEL okay...No evidence of bearing defect frequencies either, all looked normal, and no impacting!

The kicker. I replaced the motor, and we still had the same problem...Of course we all know that disconnecting the coupling and running the motor will show nothing, and it didn't...

We pulled the pump, and our millwrights broke it down and inspected it. Bearings looked okay we did have a hot area around the mechanical seal probably heat transferring through the motor shaft to it...There was about .020 wear on the impeller backing plate where the gap is adjusted.Some wear also on the impeller. It was one of those high tech style blades with 8 open vanes and 16 smaller ones between the large ones. Back plate was perforated and pitted. I recommended replacing that since we did everything else, but they did not want to spend the money...

Long story short, after wasting a week changing motors, and fiddling around with the starter & breaker. We ordered the backing plate (stainless steel) and installed that new impeller. Pump BTW is pumping a really hot charge of caustic, and runs 3550 rpm..Good for about 2 years pumping that stuff...We installed that pump and motor new on March 2004..

Moral of this story, when overhauling pumps don't skimp. Replace that worn impeller and backing plate, this caused the volume of fluid to build up in the rear housing and was not being pumped out, generating more work for the motor causing it to amp up and overheat and kick off...Problem solved, running smooth as silk at 100 degree on the motor at 9 amps..

BTW, our electricians even strung out some wire bypassing the pump breaker box just to make sure it wasn't a wiring problem. Megged out okay before they did that. jeeeez!! Also the pump outlet temp was the same (normal) as the other sister pump which is the same age, but it's pumping a cooler charge of caustic...

Rod

Good case study.

So, is it safe to assume the entire motor was probably hotter than normal due to winding overloading but it was only noticed on the bearings?

Or was the heat coming from excessive recirculating fluid within the pump... but heat not noticed at the pump?

The worn impeller and housing with the excess fluids was causing the motor to amp up generating the heat, then it kicked off. Once the pump problem was solved (root cause) the motor was back to normal operating temp. and amps.

Rod

Plese make sure I have this right.
The pump was operating at a lower
GPM but required more power.

I you could please post the motor speed
before and after work was done.

thanks

This message has been edited. Last edited by: Barry,

No Problem Barry...

The motor data plate has 3550 rpm on it...I strobed it and it was actually running 3505, 45 rpm lower than normal. The pump was taking in more fluid than it could pump out causing it to load up and over work the motor...Hope that helps...

Rod

Barry,

I forgot to mention the other day on that pump, that the RPM is running 3548 after the repair...Motor is rated at 3550, so thats pretty good! I just strobed it, both my caustic pumps running the same...

Rod

Rodney,

I am trying to understand your pump problem that has been resolved. How can a pump take in more fluid than it pumps out when the fluid is not compressible? What goes in has to come out, doesn't it?

Walt

Kind of like Walt on this; however, if you're bottom rated w/HP and it's a Wilfley Pump or similar design with pumping back-plate then you can have a fluid hang-up???????????????

It is interesting and does seem odd without more details. Thanks in advance.

What you say would stand to reason, laws of physics and all, but according to both our millwrights, which are in agreement, with that wear behind the impeller, for whatever reason, the fluids are not exiting the outlet at a fast enough rate, sort of like the phenomenon when a pump decavitates. The stuff just lays in there recirculating along with the suction still bringing more in. Perhaps view it as a backup. Tough to explain I know. At first we thought it may be a restriction somewhere in one of those lines which are braided stainless, with a lining inside. We also replaced the ball valves in the line from close port to "full" port...

All we know is , we changed out everything in the system, except for the impeller and housing. When we changed those items, it worked properly again.

A fluke?? Your guess is as good as mine...

Rod

Forgot to mention, that these are your basic up to date Goulds pumps, and the outlet is at the 12:00 position, not off to the side like some.

We purchased three of these at the same time and installed them. All on skids coupled with 10 HP motors...This is the only one with this problem, probably due to the hot mixtute of caustic that it's pumping..

Rod

quote:

Back plate was perforated and pitted.

quote:

Also the pump outlet temp was the same (normal) as the other sister pump which is the same age, but it's pumping a cooler charge of caustic...

To avoid repeat this problem I suggest you review NPSH datasheet of the pump to check if operating point is near to NPSH required = NPSH available wich is the begining of low pressure buble formation (cavitation).

Two elements help cavitation, high rate flow and high temperature. This pump has hotter soda than the syster so the coldest would have this problem infrequently than the hotter.

Very high area difference between the inlet and outlet between blades produce a low pressure front around blades boundaried for limit lineout, so this is the reason of this kind of high tech impeller ( 8 large blades with 16 litle blades between ) maybe not enough resource for this service and good solution for the sister wich is coldest. Speak with the builder of the pump and ask what would happen if the impeller for this hotter pump would be with a higher number of blades, for example 8+16=24 (complete, no partial length). Obviously that the loss pressure would be higher and the hydrauliceficiency woud be lowest but without incipient cavitation problems.