High ampere and low flow problem 4

[terrorist]

We have one newly installed pump. Its a between bearings, axially split casing, single stage centrifugal pump. Suction is from a flooded tank. Pump is tripping at overload even at a flow lesser than 50% of the rated discharge. Saw its power curve. Input power vs flow curve slopes downwards. Is it possible for any pump that on increasing its dis flow, it starts taking lesser current? If its possible, kindly advise how to start such a pump.

 

[ashtree]

Usually the power increases with flow and maximum power consumption is at the max flow rate.

So how extensive have the electrical checks on the system been?
Things to check would be:
Are the overloads set correctly.
Is the motor and overloads the coreect size for the pump.
Is the voltage correct for the motor.
Are the supply cables the right size.
Are all three phases available.
Are all the terminals tight.

How long does the pump operate for before it trips on overload? I suspect by what you are saying that its tripping very quickly.
Has the pump been checked to make sure there is not some sort of mechanical defect?

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[LittleInch]

Can you post pump curve and details of the motor and density of the fluid you're pumping.

This will save a lot of time in useless speculation.

first off please make sure yourself, by physically seeing the shaft that the pump is going the right way around. Very easy to get a three phase motor going the wrong way.

Might need the coupling guard to be temporarily removed.

Is it tripping on start? Do you have a motor starter which allows for excess current on start?
Are you starting against a closed valve?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JJPellin]

It is not unusual for very high flow, very low head pumps to experience a reduction in horsepower as flow increases. We see this commonly on large vertical turbine pumps in cooling water applications. The formula for horsepower includes flow (gpm) and head(feet) in the numerator. For a pump with a very steep curve, the product of head and flow can be dropping as flow increases. This trend usually reverses as you approach shut-off. You end up with a horsepower curve with a hump in the middle and dropping off at both very high and very low flow. The driver should have been sized to allow operation across this hump. If not, you might need to start-up the pump with valves open to get to the high side of the curve before the motor overloads.

Johnny Pellin

 

[Artisi]

I will just repeat what has already been asked to reinforce the comment/s, post a curve and YOU check direction of rotation.
Suction and discharge pressure wouldn't go astray at this point.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Compositepro]

Axial flow (propeller) type pumps have such a curve.

 

[terrorist]

Thank you everyone,
Most of the things that you have suggested have already been checked. Like
* Direction of rotation
* Alignment and freeness of the pump
* Motor is of adequate rating. Voltage also checked.
* There seems no mechanical rubbing
* Suction strainer also checked.
* Electrical team has checked from their end (connection, cable etc)
* overload setting are right
* Pumping fluid is clean water only
* This is happening with all three pumps installed in parallel. We tried operating them in parallel as well as separately.

Littleinch Sir,
It trips as soon as flow reaches 50% of the rated flow.

Mr. JJPellin sir,
Yes, Its a low head pump. We will try to start the pump with discharge valve opened (Earlier i also thought to do so)as you suggested. But as per start-up procedure given in the O&M manual, pump has to started with discharge valve closed.
One important point i had forgot to mention is that pump was running normal for 3..4 days since its commissioning.
Also please tell how come power decreases on increasing the flow.

I have attached data sheet, sectional drawing and performance curve of the pump.

 

[terrorist]

As seen in the curve max power drawn by pump is around 68 KW (in shut off condition). Motor rating is sufficiently large i.e. 75 KW.

 

[LittleInch]

Ok. Nothing jumps or so you'll need to do a bit more analysis and data gathering. You need to measure inlet and outlet pressure, flow rate and electrical power during the start up.

You ask how come power goes down as flow goes up? Your pump has quite a large drop in head as flow goes up coupled with a massive change in efficiency as flow increases. These two functions outweigh the flow increase to the extent that power should be going down.

Things to think about. You say the suction is flooded but the npsh is quite high at 7m so worth a check as with an inlet filter, even if empty and a long suction line you could be cavitating the pump. Maybe you've already wrecked the impellor? ? Or something has happened to decrease the efficiency.




Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JJPellin]

Based on the additional information, I would add one item to check. Check the voltage supply to the motor. If the voltage is lower than design, the amps will increase to try and maintain the same power. For a three phase motor, I would check the voltage and amperage on all three legs of the power supply to the motor.

Johnny Pellin

 

[LittleInch]

BTW where is the motor rating of 75KW mentioned I couldn't see it in the data sheet

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ashtree]

If all three pumps in paralell do the same thing then it has to be something shared in common , like , suctions , discharge or perhaps electrical supply.
I think someone else has asked, but maybe the head is still high which means that the flow is less than required and the power is still high. The head needs to be checked. If it worked for a couple of days , has anything (anything at all including electrical settings) changed in that time? What are you pumping into?

Are the overloads set at the motor rating?
Ask the electrical team what they have actually checked?
What sort of motor starter is fitted?

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[terrorist]

Littleinch sir,
Thank you for explaining how power can go down when flow go up.
Tank level is more than 5 meters above suction line and suction line is hardly 5 meters long. So NPSH doesn't seem to be problem.
Motor KW rating is written on motor name plate. Its 75 KW only.
Also impeller MOC is SG Ni resist, so little chances of impeller getting wrecked in a week.
We will take another trial and record all the parameters as suggested by you. I will post it soon.

JJPellin Sir,
Voltage has already been checked by electrical engineer. Its ok.
Will ask electrical guy to check ampere and voltage on all three legs during next trail.

Ashtree Sir,
Exactly, there has to be something common in all there machines.
No work was done on pump or motor since its commissioning.

OEM is also supposed to visit the site in 2/3 days.

Thanks

 

[georgeverghese]

Also check if there is a motor winding high temp trip protection loop and any other motor side trip loops.

 

[Artisi]

If the pumps ran for 3 / 4 days without any problems what has changed, it seems unlikely it could be hydraulic, mechanical, or application problem. Has the power supply lost one phase, has any electrical work been undertaken on the installation or near by that could have inter-changed phase connections to the installation?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[georgeverghese]

If this trip occurs when differential pressure goes down, maybe its got something to do with inadequate shaft seal flushing at lower dp? Seal flush lines blocked up with dirt ? You havent said anything about the seal plan.

 

[itsmoked]

I have seen almost exactly this same thing before. The supply voltage was precisely correct but the motor was actually wired for the higher voltage. In this case the motor was tapped for 460V but they were supplying it with 230V. They'd been limping along throttling the pump for two years before I was called. At about 50% load it would trip off.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LittleInch]

From everything we know about at the moment, I would say the issue more than likely lies in the electrical / motor side rather than the pump side, but we don't have enough data to make sure.

It smoked - This ones a little odd as the power requirement at 50% is more than that at full flow, so starting current and initial flow uses more amps.

How and why the system apparently ran for 3-4 days then not seems to be a key issue.

Did the flow reduce over time? If so the motors would have been operating at a higher power.

Are these motors definitely tripping on excess amps or temperature? you need to measure running voltage and amps, not just volts at rest.

How do you know what the flow is through this single pump when it apparently a new pump in a three pump parallel operation?

Also do you have the test results for this actual pump ( and not a "typical" or computer generated curve?)

This is a curious one - even if you work it out and fix it, please respect us and let us know what you found. Thanks.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

You may have an inadequate power supply. What is the length of the wiring to the pump. What is the capacity of the electrical wiring.

If the wire size is too small, the amps will increase.

The pump motor may also not be large enough so that the motor is being overloaded. An overloaded motor will also cause the amps to increase.

 

[Strong]

"Input power vs flow curve slopes downwards. Is it possible for any pump that on increasing its dis flow, it starts taking lesser current?"
The drawing indicates a typical double inlet centrifugal pump, so I would expect to see a rising power versus flow curve. Could there be an error in technical data for the pump you have?

Walt