high motor load

[upm]

gents,
i am going through a buzzle here. two identical 3 kw pumps, motor for one of them is pulling 95% FLC and cannot even throttle the discharrge block valve? we inspected motor, pump bearing, impeller and even seal. do not know what else can i do?motor no load test is 30% of FLC and vibration levels are ok. i am only questioning the volute geomatry for that pump which will effect the hydrolics? if they are pumping water and no change in the process.
pls. advice

 

[torbjorn88]

Tricky,

have you checked the rpm? Perhaps a simplistic approach, but since the rule of thumb is that a decrease or increase in pump speed change the motor load to the power of three. Pehaps it could be a start.

 

[Scipio]

What's the motor load on the second pump, and what's your anticipated load at normal operation? Also what did you check for on the impeller & bearings? My first guess, assuming it's just one of the two pumps that's showing significant load, would be wear ring clearance (not sure if that's applicable on your impeller, it's a pretty small pump) or a coupling misalignment - might cause some radial forces on your bearings that's causing extra motor load. Depending on the bearings, the extra load could still be well within what they can handle, so not be apparent as damage. Just a guess at this stage in the game though.

 

[lilliput1]

You sure one pump is not turning backwards? Check the elect connection for correct rotation.

 

[electricpete]

Sorry but I just don't understand what you are telling us.

"motor for one of them is pulling 95% FLC and cannot even throttle the discharrge block valve?"

What does it mean? Why can't you throttle the discharge valve? What is the position of the valve? What is the flow and dp? How does it compare to the pump curve? What is the current, flow, dp of the other pump? What is it that you consider abnormal?

 

[upm]

Gents,
Thank you all for your reply and sorry i may not explained the problem well.
electricpete:
I ment the motor is alreday pulling 95 % load while pumping against blocked disharge valve. Therfore, once we line up the pump (throttle the discharge block valve) the pump will operats on the curve to develop the diff. head and flow which will requires more power. therfore, the motor load will increase as a result so there is no more room to develope the head/flow.
I checked the rotation and it is as per the design even as the other pump.
the motor load on the good pump is 74 % at the operated point, and at SOH it is 60 %
We did inspect the pump internals and no bearing damage/excessive clearances or any clear mechanical damage.The motor is also inspected and no problems, even we cheked the impeller dimentions against the good pump impeller.
The RPM is correct.
Gents, it is a buzzle and am trying to work it out and need your help as if one of these pumps failed and theother tripped on overload, the plant will be down
Regards

 

[Kawartha]

upm,

From the information that you have provided, we can not determine if the problem is the pump or the motor. If you remove the high amp motor (from pump 1) and couple it to the other pump (pump 2), if it continues to draw high amps at shut-off, you will know the motor (from pump 1) is not correct. And the corollary, if the high amp motor operates properly on the second pump - the problem is pump number 1.

I would secondly suggest that you contact the pump supplier for their comments and recommendations. We assume that these are new pumps. Warranty should cover such problems.

 

[upm]

Kawartha,
Thanks for the advice, however i thout of your suggestion but there is no way to stop both pumps as the level in the drum increases and might reach the trip setting for the main compressor.
Pump/motor vendors are puzzled as well and have nothing to add. i am trying to prove that the new pump hydrolics is some hoe has different behaviours that the other and then i will force the vendor to buy a bigger motor as the train is strail under warnty
upm

 

[d23]

upm:

Looking at all the post you have considered about all there is. I liked Kawartha's idea about changing motors. I would like to add the following:

1) Did you no-load test the motor on a shop bench or on the pump/motor frame? Using the actual power/control system may tell you there is a starter/power problem with this one unit.

2) Assuming you have checked both the pump and motor could there be an alignment problem? This could include a mounting frame problem that will twist or piping problem that will allow mis-alignment only during operation.

3) Considering how critical these pumps are for your operation can you justify an extra motor or pump for inventory? Do you have a motor rebuild shop that you could borrow or rent a motor from?

Good Luck

 

[upm]

well,
I did not check the pipe strains yet , however there is no missalignment frequency appear in teh spectrum. we have no solo run data at the work shop.
any way today we will install the motr monitoring software to watch the electric parameters and see how it goes.
Do you think the seal tension can impact the load? it will but how to justefy changing the seal?
upm

 

[electricpete]

A few scattered ideas:

#1 - If it is a pure axial flow pump (often used in hi-flow, low dp applications), then maximum shaft horsepower is delivered to the pump in the valve-closed position. Then what you have reported may be normal... opening the valve would decrease motor current. Do you perhaps have a reading of current with the other pump in similar configuration?

#2 - It could be a severe air-gap problem on the motor which allows it to draw excessive magnetizing current. Measuring the power factor angle of the input power will help you determine whether the current drawn by the motor is reactive (magnetizing) or real (associated with some real power dissipation).

While you are at it you might as well measure the three phase currents (compare for balance).

#3 - You absolutely must measure the shaft-speed with a hand-held tach. It is a very easy thing to do. Then confirm that speed against speed assumed in pump curve. Also use it as a way to evalute motor.

#4 - Any abnormal vibrations seen? What frequencies?

 

[electricpete]

Since it is a 3kw pump, is it perhaps a single-phase motor? If it is a capacitor-start motor, there may be a centrifugal switch which is supposed to cut out that cap once it comes up to speed. Failure of that switch to open might cause high current.

Also might be worthwhile to check your connections and voltages.

 

[upm]

electricpete,
it is a centrifugal pump 3 phase motor. we did measure the speed by hand hels tack and was normal, diecrtion of rotation was correct . we did measure the current usng a clamp on ameter at each faces, check the motor manager settings as well.
\how to measure the power factor angle?

 

[d23]

upm:

About the seal: Yes the seal can cause your problem.

About electricpete: Pete is "very smart" about power systems and pumps. Very smart! For what it's worth in this case I will make a few assumptions. If all three pumps are in the same location I would expect they are feed from the same power supply. Being on the same power supply if one has a problem due to phase angle or power factor then all three of them would have the same problem. You can use an oscilloscope to check them, but I doubt this will solve your problem. In most cases if you suspect a seal you should contact the seal manufacture. We use John Crain seal on out pumps and they are more than willing to come to a location and validate seal alignment/operation for us. Your manufacture will most likely do the same for you.

(Pete please take it easy on me, I'm still trying to learn)

D23

 

[me3]

For a cetrifugal pump, the load should be minimum when the discharge valve is closed. But, in your case it is not. My first impression would be that there is something wrong with the motor itself, and not a mechanical fault. specially if the pump was not showing any upnormal noise, vibrations of heat.

You might try to replace this motor with a spare one and see the results. this will confirm to you if the problem is mechanical(pump), or electrical(motor).

 

[electricpete]

d23 - thanks for the kind words... not entirely deserved. There are plenty of knowledgeable people here including yourself.

upm - In the terminology of the Karassik's pump handbook, an axial flow pump is a "centrifugal" pump. Axial flow pumps draw max power under valve-closed conditions.

The only axial flow pumps I have seen are large power plant circ pumps. I don't know if there are 3kw applications for axial flow pumps, but I think it is worthwhile to verify that either:
1 - this is not an axial flow pump
or
2 - other pump/motor in similar condition drew less current.

 

[dadfap]

upm,

seing as how you can't shut down both units at once, I'd get the manufacturer to come out with a 3rd unit and replace each in turn.

Start with the suspect pump, change it out with the new pump. Measure the performance of the "new" pump and if it is OK, tell the manufacturer to take his pump away and fix it... problem solved.

If the "new" pump displays the same problems, replace the second pump with the pump you removed from the system.

If the pump then behaves as per catalogue, you have a system problem, not a pump problem, either way you have made progress.

If it is a syatem problem I'd look at the following
1. Voltage drop on the "problem" pump under load
2. Discharge pressure on the problem pump

Just some thoughts.

dadfap

 

[CB2]

Check to see if the impeller rotation direction matches the volute rotation direction ( the volute cutwater and impeller tips should be pointing in the same direction). The manufacturer could have installed the incorrect rotation impeller.

 

[d23]

upm:

The impeller design can be part of this. Is your pump a radial, mixed, axial etc design?

What happens with your "good" pump when you close the valve on it?

A low specific speed pump where you have high head, low flow and low pump eff. could have a very flat BHP curve. It is possible for a pump to require more HP at shut-in that at open flow. (dadfap check out one of my DN-4000 pumps.) The probability of this increases with smaller OD impellers.

You can look at this web site for more info on specific speed:

http://www.mcnallyinstitute.com/CDweb/s-html/s072.htm

D23

 

[upm]

Gents,
Thanka lot for all your advices and support, sorry for not replying but i was busy with this pump amd some other as well. To answer some of your concerns, these are radial flow pumps and not axial, the impeller is open. The good pump is drawing 60 %FLC at SOH
CB2
what do you mean by "Check to see if the impeller rotation direction matches the volute rotation direction ( the volute cutwater and impeller tips should be pointing in the same direction" we measured the cutwater angle but did npot know how to check the direction of volut? pls. help.
new info:
1.wehen i ternded the motor load i noticed the cyclic waves at constand process demand?? thus the motor load is varying from 88 to 100% FLC at constant discharge control valve opening? who can we explain it? we measure the mps by clamps on ameter and was matching the motor manager? what is this???? have no idea
2. There is a R.O installed on the GOOD pump discharge nozzle to control the curve shape and not in the new pump? does it make any difference since i am throttling the discharge valve?
pls. help
upm