Booster pump station breakdown 2

[TotemCruise]

Hello!
Before 300 m3/hr RO system 6 pump booster pump station used to work round the clock for three months. At the end of this period the customer informed us about periodical alarm disconnections of pump station. The message informed about the frequency converter fault and pump #2 short circuit. The service center engineers disassembled pumps and discovered that the impellers of all pumps except one that was switched off all the time are almost fully destroyed. Undamaged parts of the pump #2 check valve were found there in the output collector of the station.
In their conclusion they wrote that the reason of destruction was operation of the pumps in wrong duty point. According the project the station has to operate under the input pressure 2 bar and produce the changeable at any time output pressure from 8 to 9 bar. The customer is not ready to use the system in full power till now so they used to operate it most of the time in conditions as follows:

Inlet Pressure 2 bar
Duty point 6 bar.

At that the lowest point of the curve is at 5.8 bar.

They said there was an overload that lead to damaging of the impellers. They absolutely didn't take in account the check valve. Were there any overload that could be so dangerous for motors and pumps there. The input pressure was high enough to prevent the suction cavitation. The view of debris confirms this. The surface of the damaged impellers looks new. If somebody wants to have a look at them I can send some pictures.

We have our own version of this destruction. We think exactly the broken of check valve initiated this destruction. A high speed water flow run back through the broken check valve and pump #2 when it was switched off according to schedule off the PLC. Setting this pump
in motion lead to superhigh impeller loading that caused the damage of the impeller. The starting current because of the rotaition of the pump in opposite direction was too high and was interpreted by the frequency converter like short sircuit.
What do you think about this situation. Could this be a
reason. Has somebody ever faced with problem like this.

 

[DVQF]

Appears to be "classic" upthrust. A temporary loss of inlet pressure could cause this. Normally a failed check valve on the discharge would result in a broken pump shaft.

 

[BigInch]

Poppeye, I like your explanation. Must admit I didn't think about that. Certainly has a ring to it. True or not, (but I think it just might be), its worth a star.

It only depends on if he ran out of NPSH at those flows on the right side of the curve or not. Certainly possible.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[TotemCruise]

Thank you for discussion.
The extreme right point of NPSH curve is only 7m. I would like to remind that as I wrote the pump station input pressure has never been less then 2,5-3 bar. Do you think it is an appropriate to cavitation condition.

 

[BigInch]

That's a lot more than the 7m needed, so if that is truely the case... no.



BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[TotemCruise]

Poppeye, Have you seen the photo's of the impellers at

http://new.photos.yahoo.com/totemcruise/

I would appreciate if you as very experienced service and repair manager look at them and share your opinion. Do these damages similar to that that you wrote about.
I do not see erosion there on the vanes. Some vanes of different stages look like new.

 

[Artisi]

Just to satisfy my interest, I re-looked at your photos, I still only see impact damage, nothing to do with running off the curve, insufficient flow, thermal expansion, running backwards or any signs of cavitation.
I too have spent a great deal of time of my pump life diagnosing failures and I can assure you that impact damage is clear and obvious.
You need to investigate what / how and why you have this damage.

Phitsanulok
Thailand

 

[poppeye]

TotemCruise,
this is pretty much classic failure of a canned pump. Either Grundfos, Flowserve or who ever makes these style of pumps. Normally caused by insufficient lubricating fluid passing through the stages. These pumps can build up a lot of heat during operation. Should there be insufficient NPSH or NPSH close to the vapor point. Usually the second, third or forth stage will start letting go as the fluid flashes in the eye. Once this starts happening, you will have impact damage on subsequent stages. Then comes the imbalance of the remaining stack pieces. This usually takes out the entire stack of impellers, alignment bushings and shaft. The repair is fairly easy by replacing the stack. You didn't say the speed of the unit but at either 1700 or 3600 rpm. It does not take long once the damage starts to take out the entire stack assembly.

 

[BigInch]

It appears that, if there was suction pressure available, there was no flow causing subsequent heating and vaporization. At this point, I'd question the reliability of the operating data at the time of the event.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[poppeye]

Running to the left of BEP is just as bad for this type of pump. When sizing these units you MUST ensure that operation below Minimum Allowable Contiuous Flow is avoided. If not you end up with a lot of scrap metal.

 

[TotemCruise]

Poppeye,
You are writing about an insufficient flow again. I do understand this case.
Let us take a concrete example. Grundfos CR 90-4 50 Hz is working in conditions as follows(hope you have diagrams or can find them easily):
1. inlet pressure is 2 bar
2. discharge pressure is 5 bar
If to approximate the curves the flow at this point should be about 130 m3/hr, NPSH about 9m. The inlet pressure is more then double of NPSH. I can agree that the flow is too high, but not insufficient, certainly it would better if the inlet pressure was threefold or even fivefold of NPSH but it is enough to prevent vaporisation and cavitation, is not it.

 

[poppeye]

I looked up the information from the grundfos web site unfortunately the NPSHR is not shown on the curve. Your station is running this pump at 40% below the BEP, or to the right of the curve. This is normally way beyond the recommended limit. 15% allowable on either side of the BEP for a pump is what is required by Flowserve to maintain factory warranty. Operating the pump at anyother point will void this warranty. Opening up a bypass valve because the customer does not want to fully operate the system is a classic mistake. You may not be delivering the full amount of water because you are recircing 60% but you are operating the pump at a point that you are in the critical zone of mechanical reliability. To save the pump while you are not wanting the full amount of water, install a discharge valve before the recirc valve. This way you can instill a false head on the pump maintaining the hydraulic duty point. The recirc can be opened 100% if need be to satisfy pre-operating 100% conditions.

 

[TotemCruise]

I do understand the pumps have been working out of zone and it is clear that the manufacturer has a ground to void the warranty. It is not the question.

The question is if it could cause the damages like these.
The probability of this is unknown and this can not be proved while the probability of damaging of the pump by water hammer because of the check valve fault is 1.

Nobody has opened a bypass valve. There is not it there. They simply set up the discharge pressure in the controller. They are going to install the orifice then.

By the way, the specialists of service center that put the station in operation set the minimal discharge pressure parameter limit to 6 bar. It was a mistake because the input pressure was only 2 bar.

 

[poppeye]

How is the check valve causing water hammer? Simply by closing? When one pump is started it back pressures another thus closing the check valve? You say they simply set up the discharge pressure in the controller? This would either imply a VFD lowering the pressure or a three way control valve controlled by the PLC. To lower the pressure you would have to either slow the pump down or divert the unused water back to the source. The latter of the two will take the pump to the far right of the curve and to the greatest NPSHR of the pump. and thus the damage.

 

[TotemCruise]

This is another kind of water hammer. This is a wellknown centrifugal pump backflow prevention problem. Please read my this thread messages and references, particularly the Lawrence Pumps Inc. newsletter "Preventing reverse rotation".

http://www.lawrencepumps.com/newsletter/news_v03_i3_mar06.html

As I wrote we have our own version of this destruction. We think exactly the broken of check valve initiated this destruction. A high speed water flow run back through the broken check valve and pump #2 when it was switched off according to schedule of the PLC. Setting this pump
in motion lead to superhigh impeller loading that caused the damage of the impeller. The starting current because of the rotaition of the pump in opposite direction was too high and was interpreted by the frequency converter like short sircuit.

 

[poppeye]

Back flow is not really water hammer. While starting a pump in operation that is pinwheeling in reverse due to reversed flow can cause damage. I have seen Hazleton pumps that sheared 4" shafting just below the coupling. What caused the damage to the check valve that would set up this scenerio? Low bidder installing inferior materials? I have replaced and repaired more canned pumps than I would really like to have seen, although good for business. In each catastrophic failure, which I am seeing in you pictures, the cause was either the pump running to far to the right or left of BEP. And in most cases the cause was either improper control valves that would let the pump operate in this condition, or operator error. Should you start the pump up in reverse rotation it would have casued severe damage at the impeller hubs to the shaft from the start up torque or twisting and damage at the coupling hub to the motor.

 

[TotemCruise]

Unfortunately I have not took a part in disassembling and have not seen neither impellers nor check valve peculiarly, only photo's. Moreover the customer harried to repair the check valve in presence of service center representative. I do not know for what. They even did not make a photo.
I do not think this #2 pump impeller could be damaged by the the backflow at start up at once. The vanes could got rickety in joint welds bit by bit.