Pump design
Pump design
(OP)
On a slurry line I have a pump with the following design data :
Design @ duty : 10 000 gpm
BEP : 20 000 gpm
Actual flow max : 8 500 gpm
I'm experiencing frequent failures ( 150 mm shaft breakage)
My questions are :
- Should I blame the designer?
- Could I say that the failures result of a bad design?
- What should I do to return this pump near the BEP?
Design @ duty : 10 000 gpm
BEP : 20 000 gpm
Actual flow max : 8 500 gpm
I'm experiencing frequent failures ( 150 mm shaft breakage)
My questions are :
- Should I blame the designer?
- Could I say that the failures result of a bad design?
- What should I do to return this pump near the BEP?





RE: Pump design
Can you say failures are a result of bad engineering? Again, information is the key. I would say though that the failures you indicate can most likely be attributed to the pump running to the left of BEP.
What can you do to return the pump to BEP? An easy fix would be to install a high side by-pass return to the pump suction somwhere. The best fix, get a new pump as the one you have probably could not be refit to run at any sort of efficiency you could tolerate at those flows. The return on investment based on energy consumption would most likely make the new pump choice a good investment.
Did you have the system engineered by an engineer? If so, then asking those questions directly to that engineer should return you answers that you can work from.
BobPE
RE: Pump design
Meantime, check your system - do you have large solids (maybe broken screens) that jam the impeller against the casing? What about shut downs where the solids in the slurry can settle out in the pump shell causing extra load on the shaft at start up. Some slurries can pack just like cement if left long enough.
Where is the shaft breaking? If it's at the drive end, check your alignment or belt tension - cog belts tend to be misapplied in pumps and don't slip like v-belts. If it's breaking at the impeller end, is your suction plate/impeller gap correct or do you have parts rubbing?
Is the gland packing and flush water correct or is this contributing to excess load?
You mentioned Frequent Failures - that would indicate a problem that has existed for some time and should be brought to the attention of the pump maker.
Fix the PROBLEM, not the blame.
Keep the wheels on the ground
Bob
RE: Pump design
What I did today, is to determine a dead head and a running head. It comes out to be 25 ft lower than expected and the running head is quit similar to the dead head. I'm sure that the impeller size and the speed are as design (39", 590rpm). The pressure differentiel is 90 Psia.
On the other hand, looking at the suction plate, I found out that it is rubbed.
I will continue to get the system curve and then talk with GIW the pump maker.
Regards
RE: Pump design
Like BobPE has said - blame the engineer not the pump -
Naresuan University
Phitsanulok
Thailand
RE: Pump design
I can understand you seeing rubbing of the impeller on the wear plate probably a result of shaft deflection just before failure.
Naresuan University
Phitsanulok
Thailand
RE: Pump design
The shaft is broken at the impeller side.
What means, the dead head is far lower than the supplier curve?
RE: Pump design
The low NPSHR of the impeller may result in a high sss number which would tell you that you might need to run at 50% BEP as a minimum continuos flow to avoid low flow cavitation and its damaging vibration.
In many cases I have found that shaft breakage at the impeller is from sudden load changes such as a fast opening valve or slugs of product going through the pump. In slurries I have seen the discharge line partially plug then blow open causing a surge in the discharge line and the pump. I agree with sprintcar you need to determine why the shaft is failing.
Regards checman
RE: Pump design
Therefore:
1. Get a full hydraulic study undertaken on the application - what are you trying to achieve and what is actually being achieved.
2. Contact GIW with the problem together with the hydraulic study.
3. Get a qualified analysis carried out on the shaft failure - this will tell you the mode of failure - from here you maybe able to work backwards to establish what operating conditions can cause this type of failure.
Concentrate on the cause not the result.
Naresuan University
Phitsanulok
Thailand
RE: Pump design
I only wish it was that easy in the real world away from the desk or the text books.
Naresuan University
Phitsanulok
Thailand
RE: Pump design
Naresuan University
Phitsanulok
Thailand
RE: Pump design
Keep the wheels on the ground
Bob
RE: Pump design
We are talking with GIW at this moment.
On the other hand, we are not using the OEM pump shaft, but the part is not pirated as our supply is not in Thailand but a very well know, sitting in the USA.
RE: Pump design
Any 'non-oem' part is generally termed 'pirated' in most industries...'aftermarket' is the automotive term.
You need to get the actual specs on the material from your vendor so the pump maker knows what to look for.
Most highly stressed parts (such as pump shafts) will be designed with a specific material and an engineering factor of safety. Once you change material or process, you have deviated from the original design and the resulting failures are self inflicted...
....and you wanted to blame the Designer!!
Keep the wheels on the ground
Bob
RE: Pump design
How can I blame the part?
RE: Pump design
1. Radial (Impeller end) cover broken
2. Bearings found to be in satisfactory condition
3. Stuffing box bolts worked themselves loose. Some of the bolts were found loose.
4. Shaft broken outside of RA.
5. Broken impeller plate.
6. The pump ran continuously between the 19th and 26th January 2005
7. The pump was stopped on the 26th Jan 2005 for 1 hour, between 9:18PM – 10:12 PM. Approximately 54 minutes.
8. The pump then ran from the 26th Jan 2005 at 10:12 PM to failure at 4:09 AM on the 30th January 2005.
9. The shaft currently used is provided by Quality Machine and is made of 4140 Steel. These shafts have been used since 2001.
10. The BOT level gradually increased from 32% - 89% up to the point of failure at or around 4:09AM. This occurred over a period of 8 minutes.
11. The current draw increased over a period of 9 minutes from 235A – 393A
12. Suction plate scored.
13. Impeller face badly scored.
14. The pump operates to the far left of the BEP point, which contributes to excessive shaft deflection, and may affect the alignment of the impeller in the volute chamber.
RE: Pump design
" 11.The current draw increased over a period of 9 minutes from 235A – 393A
12. Suction plate scored.
13. Impeller face badly scored.
14. The pump operates to the far left of the BEP point, which contributes to excessive shaft deflection, and may affect the alignment of the impeller in the volute chamber. "
It certainly looks like the shaft has failed from excessive radial loading by operating so far left of BEP.
I would suggest that what you consider to be "normal" operation is marginal and the shaft is being fatigued from excessive deflection during operation, the 9 minutes of excessive current draw is the final stages of the shaft failure as the impeller rubs on the casing.
A close metalurgical examination should tell you exactly what has happened but I wouldn't be suprised to see a typical fatigue failure starting at one point and increasing over time with a final torsional break as power exceed the remaining shaft strength.
Next step is to establish why the pump is being forced to operate under such duress and how to rectify the problem.
Naresuan University
Phitsanulok
Thailand
RE: Pump design
Regards Checman
RE: Pump design
Is it possible that since you are running 2 pumps in parallel from the same suction point that you are reducing the NPSH available (for the subject pump) below what is required, thus causing severe cavitation and excess shaft loads? If subject pump is being supplied thru an elbow off the pipe to the other pump, this too can restrict suction.
If you have 4 similar pumps running and only one has recently become a problem, this is not a design flaw but rather something that has changed in your flow system or physical mounting of the pump itself. When was the last alignment check? Since the shaft has been replaced recently, it is possible that a different bearing housing (barrel) was installed and it has a different center height or alignment feature that was not realigned? If the housings were not tightened down properly, could the impeller have pulled it forward and caused the suction plate contact?
What material is the OEM shaft made from compared to what you have now? Where is the failure point - at the plug thread (small diameter) or next to the bearing on the large diameter?
Inquiring minds want to know...
Keep the wheels on the ground
Bob
RE: Pump design
http://www.mcnallyinstitute.com/11-html/11-06.html
for metric use <2.0
what is the suction specific speed?
http://www.mcnallyinstitute.com/09-html/09-12.html
check these industry accepted parameters to see where your pump is operating.
saludos.
a.
RE: Pump design
Since you are using pirate shafts, perhaps the shop rebuilding the bearing units is not following the OEM procedures or tolerances for the housing and bearings.
Keep the wheels on the ground
Bob
RE: Pump design
Keep the wheels on the ground
Bob
RE: Pump design
Naresuan University
Phitsanulok
Thailand
RE: Pump design
I'm sorry for this long silence, I had too many thing to do that put this shaft failure in second importance. Nevertheless, our mechanical engineer has send the shaft to a specialised company for analyses and after many exchangers with GIW, it seems to us that they do not understand why we have those pump at that service, they are really poorly desgned. So, they are going to design new pumps wich mach the process as we can not change this later.
Thank to all for your inputs, I' not abble to say that is the real cause of the failures ( pump design, pump assemby, part ...) but we will move forward from now.
RE: Pump design
boubacar (Chemical) 11 Feb 05 15:48
We are using those parts since 2001. Furthermore we have 4 pumps using them, but we have faillure only on one of them ( 2 shafts broken in 3 weeks). This later has commun suction and cummun discharge with another one, which experienced no problem.
How can I blame the part?
It appears 3 pumps are running fine. Perhaps your System, not the pump, is the 'poorly designed' culprit.
Any pump (or other equipment) is selected based on customer supplied parameters (head, flow, npsha, etc.) which may change over time due to production requirements, etc. It's incumbent on the the system manager or process engineer (like You) to understand both the original criteria and the current conditions so that either the pumps or the system can be updated to accomodate the new values.
I think just blaming GIW for this seems a bit short-sighted on your part. I looked at their website ( www.giwindustries.com ) and they've been doing this for over 100 years. Perhaps when this pump was originally installed, it met the system requirements. Or perhaps your previous systems engineer supplied the wrong information.
Remember, you're making these comments to an International Forum of folks who work in design and engineering...
As I stated earlier - Fix the PROBLEM, not the blame.
Keep us posted on how the issue is resolved, ok?
Keep the wheels on the ground
Bob
RE: Pump design
Slurry pumps like GIW - don't break for no apparent reason.
Find the reason and make us all happy and you will also learn something.
Naresuan University
Phitsanulok
Thailand
RE: Pump design
I second Artisi too, something is wrong. None wants me to talk about the pump althouhg the supplier itself agrees the pump is not convenable for this appication, good. But that I can tell you since that pump has designed, the process flow has increased by 20% we are still at 77% of design flow and at 42 % point of BEP ( desgin point was @ 55 % BEP). The NPSHa remains the same as at design.
As I wrote in my last post, we are going to design a new pump wich match the current process parameters.
Thanks again I will post you the final solution.
RE: Pump design
Artisi is good theory of radial deflection.
That theory is even more likely if ...
You mentioned very low NPSHr, which of necessity drives the Nss high. If Nss reaches into the "high suction energy" zone, then any radial deflection occuring from Off-BEP operation may be dramatically exacerbated by the well known behavior of high suction energy pumps - THEY HATE TO RUN OFF-BEP, VERY STRONG REACTIONS AGAINST LOW FLOW RATES.
HI publishes a chart showing the high suction energy zone.
Scrounge around the HI site to find it, I don't know where it is, but that is where I got mine.
PUMPDESIGNER
RE: Pump design
Racing and bullfighting are the only real sports...everything else is just a game.
Bob
RE: Pump design
thanks for your help
spanish version :
saludos , tengo un proyecto en el cual debo evaluar el coste de una bomba de alta pression de agua salada para una fabrica de desalacion, los parametros que debe satisfacer la bomba es una produccion de 1042 m3/h y una pression de 70 bars
gracias por vuestra ayuda
RE: Pump design
Saludos, you should start a new thread for this question, that way you will get answers from all the forum.
Naresuan University
Phitsanulok
Thailand
RE: Pump design
Además de lo que te sugiere Artisi, toma en cuenta que no has indicado cual es tu problema o inquietud. Es decir, ¿que es lo que necesitas o requieres con respeto a esta bomba? Lo que nos has indicado es una descripción de tu proyecto. Supongo que eres capaz de llevar adelante la tarea que se te ha asignado.
Besides heeding Artisi's advice, you should state what your problem is regarding the pump. All you've said so far is what you have to do. I have to assume you're capable of doing what you've been assigned.
RE: Pump design
i need your help for to evaluate the cost of this pump
Thanks sirs for your help
Para MONTEMAYOR :
Saludos señor, me alegro que haya alguien aqui que sepa español,ya que soy un inutil en ingles :( asi que espero que usted me ayude en mi problema.
mi problema es el siguiente tengo que evaluar el coste de una bomba de agua salada que pueda satisfacer los parametros que ya dije arriba , los graficos que tengo solo me dan el coste en funccion de la altura manometrica y puesto que mi instalacion lo importante es la pression y el flujo, pues no llego a encontrar nada que me ayude en ello, espero recibir en breve su respuesta. o por lo menos que traduzca adecuadamente a los señores mi pregunta ya que como he dicho soy un patan en ingles :)
saludos jotunbani
RE: Pump design
I reiterate Artisi's recommendation to start a NEW thread on your specific question. Multiple subjects or queries in one thread cannot be addressed efficiently nor accurately.
Nuevamente te recomiendo que presentes tu(s) pregunta(s) en un nuevo "thread". Es practicamente imposible el poder responder a varios temas en un mismo "thread" y esto causa confusion, resultando en un enredo.
RE: Pump design
It was refreshing to see how all the engineering folks immediately 'defended' the original design. I guess we all understand that any product can be mis-applied beyond the designer's intent. This is a great 'family' to be a member of!
Hopefully boubacar has left with some education in system evaluation and problem resolution.
Racing and bullfighting are the only real sports...everything else is just a game.
Bob
RE: Pump design
Do you pratice Bullfighting when racing ?
RE: Pump design
Glad you're still here - let us know how you finally solve this!
Racing and bullfighting are the only real sports...everything else is just a game.
Bob
RE: Pump design
Check out www.naglepumps.com
They make pumps for pumping slurries as well as fluids with solids, like coal boiler bottom ash, and steel mill scale to name a couple. I think I remember that they have the capabilities up to 14" dia. solids. Now that is an entrained particle.
Because of this, some of their models that I have seen in the field have some pretty hefty shaft diameters.
Their designs have generous clearances so as to allow lots of recycle. They are favored for people who have to pump explosives like TNT and DNT.
They may have the pump you are looking for.
But, they do use threaded impellers, so bump checking the motor in the wrong direction with the pump coupled can cause impeller to suction plate and/or shaft damage described earlier in this thread.
rmw
RE: Pump design
Naresuan University
Phitsanulok
Thailand