1000HP Deep Well Water Pump 2

[gokulkrish2]

Hi All,

We have a deep well water pump in our site and we have had more than 5 failures on it now. I am given the task of finding the root cause for the failures and give recommendations of what can be done to prevent future failures.

Here is the Pump Information:

1000 Horse Power Centrilift 725 HMI UT Motor
29 Stage HC3500 Centrilift Pump
4 Bag Sealing Section 6.75 OD

4160v Switch Gear with 469 Multilin Power Management Relay

Load information

Avg Discharge Flows of 1350 GPM
Avg Water Temperature of 95 Degrees Fahrenheit

Background:

I talked with the Electrical Supervisor and he thinks that the pump is being started lot many times ( 8 to 9 starts per day ) and that is the reason it fails. He argues that the pump fills the store water tank quick and so the level sensor shuts the pump. He recommends we do a lake and pump water into that and store so that we have less starts on that pump.

Other argument is that the inrush on the pump would be too high. Right now the pump is started with a DOL starter. Some recommend that we go with a VFD or a Soft starter to start and hence reduce the inrush and increase the number of starts. I think this is good than the lake option.

Also, do anyone have a recommendation of how to measure the sub transient and transient inrush current values of the pump during the start? I want to measure and see how much is the current rise. Wouldnt that help in the appropriation for VFD or soft start?

I know a little about pumps and hence whatever you guys give would be a valuable peice of information for me.

Thanks.

gokul

 

[jcarrr]

LionelHutz is bringing up the big question when he asks to rotor/impeller lift. If the water level above the pump is being alowed to fall back to the static level between starts, the motor and pump may be called upon to deliver far more that rated power for more than a few moments. What does the manufacturer say about essentially open discharge operation?

We run several 4160 deep well pump motors on drives with excellent results. They are set to match delivery exactly to demand. The only time they shut down is if the power goes off or maintenance is in progress. Power consumption has been reduced, demand is controller and load factor increased.

After a few more motor replacements you will have the drive and transformer completely paid for.

 

[LionelHutz]

Power consumption has been reduced

Have you calculated the gallons pumped per kW consumed before and after the VFD install?

So much of the "energy savings" in water pumps is focused on how lowering the speed a little lowers the HP or kW usage a lot. This is completely the wrong approach. You have to focus on increasing the gallons pumped per kW of power used if you truely want to save energy.

After posting that, I will now post that a VFD is an excellent tool for process control. You can lower the pump speed and maintain a constant water flow and steady pump operation which will easier on the pump system. In this application, A VFD may well be a very good solution.

However, I still believe that if it the failures are due to mechanical forces in the windings then a soft-starter will reduce those forces which should give the motor a long life.

 

[MikeHalloran]

I'm wondering if blowing the water out of the riser would allow the pump to start more easily...

Yeah, I know it takes ~900 psi, so a cheap compressor won't do it.



Mike Halloran
Pembroke Pines, FL, USA

 

[waross]

Hi Mike;
I think that there are enough deep well pumps working well that starting isn't the issue. We have to find what's damaging the windings. Vacuum contactors have been known to generate High frequency transients that may give rise to high voltages that often attack the end turn windings, but I have no reply to my query re vacuum contactors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

I agree with the suggestion about vacuum contactors and an open mind to all possibilities beyond the effects of starting.

I don't see the logic for ruling out frequent starting as a contributor when it has been stated the manufacturer recommends 1-2 starts a day. Is 8-10x per day cycling of 1000hp motors supposed to be a common application? (Yikes).

Another clue may be the location of the failures. If failures are not confined to first few line end coils. we expect probably it is mechanical stresses as was suggested by Lionel. There are also many clues of winding movement (at least in air-cooled motors). Cracking of resin around ties in the endwinding areas. Dusting visible at the coil exits and perhaps in the vent ducts.

Was a lot of copper missing at the failures (usually suggests turn to turn fault to me, which also leans toward mechanical causes).

I may have missed it but do we know which protection took out the motor... phase overcurrent (time, instantanios) or ground fault or something else?


Another question I was curious about... how long roughly from installation to failure. Or if not that, then 5 failures over what time period?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[LionelHutz]

I agree with these guys. If the failures are in the first few windings at the motor leads then that points to a transient over voltage or high dV/dt event causing the failure.

 

[gokulkrish2]

Thank you all for the valuable feedbacks. First of all I need to correct one information i gave you guys before. It is not one single 1000HP motor. It is 2x500 HP motor running in tandam. I got it from the manufacturer when i talked with him today.

Here is more what i have found out.


waross : yes we are using a vaccuum contactor.

Electricpete: Manufacturer tell we need to start the motor by 2 secs. yes there is a ramp up time constraint here. The motor has to start by 2 secs. Sooner the better.

Also rough duration between installation and failure is around 6 months and the motor is a 2 pole motor.


LionelHutz : I thing I answered your question too. Yes, there is a ramp up time constraint. It has to ramp up by 2 secs

I actually spoke to support engineer from the manufacturer and he recommends VFD. He says his VFD has some kind of sensor which actually communicates with the motor and smoothes the flow of water.

I have to provide the appropriation to the management on whether to spend that cost for VFD.

What do you guys recommend. From what all we discussed i am starting to confirm that its a starting current issue. So should I go with a VFD or save some bucks with the Soft Starter.

Thanks once again for all your feedbacks.

gokul

 

[MikeHalloran]

This is not the time to save some bucks; this is the time to fix the problem and be done with it.


Mike Halloran
Pembroke Pines, FL, USA

 

[Marke]

It certainly sounds like transient torque issues fatiguing the end windings rather than thermal issues.
Start problems usually result in rotor damage rather than stator damage and if there is no rotor damage, then it does not sound like a thermal problem resulting form excessive starts or starts per hour.
The use of either a soft starter or primary resistance starter would reduce the start transient and should also reduce the end winding damage resulting from initial start transient torques.

Another source of very high transients, is "auto reclose" which is essentially the situation where the supply to the motor is lost for one or more cycles. On reclosure, there is a very high transient which can do considerable damage including breaking shafts. If there is an issue on the supply, or in the contactor control that allows the contactor to bounce, it could also be the cause of the problem.
I have seen a number of level control systems where the contactor is able to bounce at the top level.

A VFD with a fast ramp could also be used, but in this case, I would caution against any expectation of energy savings because the flow is not being mechanically restricted. If the flow is being reduced by some means causing the pump to "slip", then reducing the flow by reducing the speed will result in energy savings. If the flow is not restricted, then a soft starter will have a lower running cost than a VFD.

Best regards,
Mark.

Mark Empson
L M Photonics Ltd

 

[LionelHutz]

There are still 2 possible causes of damage.

1. Mechanical damage due to the magnetic forces during starting or due to the contactor bounce/chatter as brought up by Mark.

2. dV/dt damage due to the vacuum contactor.

What is causing the damage still needs to be determined. The motor uses a vacuum contactor. Are there surge arrestors and/or surge caps on the motor side?

I know the manufacturer says 2 secs to full speed but if you use a VFD that's not possible because you will run below lower speed continuously.

As for the energy savings - I would expect this system to use at least 10% more energy with a VFD.

 

[burnt2x]

Just to be clear on this problem:
1. Submersible pump - pump assembly and motor is at the bottom of the wellhead, motor is water-cooled (distilled water inside motor cavity), check valve at the surface.
2. Deepwell pump - the motor is at the top while pump stages are at the bottom, motor is air-cooled provided by ventillating fans, pump column equipped with foot valves.

Since you confirmed it is a submersible pump, the problem could be (1)a check valve passing that allows water in the pump column to drop and spin back the shaft; (2)transients due to vacuum breaker's high dv/dt or (3)over-capacity resulting to many starts/day. To check if the check valve is passing; use an ultrasonic flowmeter(strap-on) and see if there is backflow when the pump is off. On the transients problem, you need to install surge arresters at the motors. If you can determine that your pump is too big, try taking out one unit(if it is possible- you mentioned two arranged in tandem, I presume they're in arranged in-line with the discharge of the first stages routed to the second set of pump stages) or throttle your discharge valve a bit. A softstarter could also be your solution if all those mentioned are in order.
Glad to be of help.

 

[alehman]

Check with the the mfg. on any special requirements for the VFD.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[edison123]

End winding damages are mostly due to switching surges in vacuum circuit breakers. Frequent starts & stops will only lower the MTBF.

Solution - use a surge suppressor across the motor supply terminals.

Muthu

http://www.edison.co.in

 

[gokulkrish2]

I want to know if this will work.

Our engineer has suggested that we remove some number of impellers from the pump such that the water pumping flow is reduced and so we smooth the operation of the motor. He says it can configure it such that the pump will run for 24 hour operation and pump the same amount of water.

Any comment about this??

gokul

 

[waross]

Personnaly I would go for a calculated 18 hrs per day and go for one start. Or you could go for 18 hrs per day and throttle your output to 24 hrs a day. Run this past someone who is familiar with the pumping curve of your pump for a final say. If your demand increases a little bit or the water table falls you may want a little extra capacity available without pulling the pump.
BUT first I would investigate the possibility that the vacuum contactor is causing the problems. Fix the cause, not the symptoms.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I agree.

Yes, you will want some extra capacity so you don't run out of water.

Yes, you will still need to address the cause of the motor failures.

 

[oftenlost]

i think that ElectricPete nailed it Two second start? Two pole? Failing in the endturns after 6 months? Sounds like the long endturns are under braced. I would make sure that the endturns are tied, anchored, tied top to bottom in more than one location, looped connections and the windings are VPI treated.

 

[LionelHutz]

I thought I was the first that suggested mechanical damage due to the magnetic forces...