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Pump design
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(Chemical)
(OP)
4 Feb 05 08:26On 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?
Replies continue below
(Civil/Environmental)
4 Feb 05 08:41Should you blame the engineer? Hard to say, there is a lot more information you need to research as to what the original design conditions were, what is going on now that may be different that during orininal design, etc.
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
(Mechanical)
4 Feb 05 10:34The FIRST thing to do is contact the pump manufacturer - supply them with your ACTUAL duty conditions: head, flow, SG, etc. to make sure the pump is correct for your application. For example, did the motor power or RPM change after the initial pump installation? What is the tonnage change from the quoted specification?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
(Chemical)
(OP)
4 Feb 05 12:12Thanks to all for your good inputs.
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
(Mechanical)
6 Feb 05 06:15My first question would be - why is the duty point on such a large pump 50% of BEP - made worse by your actual duty of gpm.Like BobPE has said - blame the engineer not the pump -
Naresuan University
Phitsanulok
Thailand
(Mechanical)
6 Feb 05 20:30Where is the shaft broken?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
(Chemical)
(OP)
7 Feb 05 07:58This pump has a very low NPSH required, I think that is the reason they choose it, dispite the fact it's running far from the BEP.
The shaft is broken at the impeller side.
What means, the dead head is far lower than the supplier curve?
(Mechanical)
7 Feb 05 14:20boubacar, Is there a reason for not bypassing some flow back to the supply and put the pump operating condition on a better part of the curve? In some cases the agitation can help keep solids in suspension and be of some benefit to the process.
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
(Mechanical)
8 Feb 05 00:23For a problem of this nature you should be doing a number of things at the moment and not applying "shot-gun" therapy to the problem - forget the dead-heads, the speed, the NPSHr/a etc - this is all meaning-less at this point, what you need is a systematic study to get to the root cause. Most of the GIW pump range are built like battle-ships to account for all the upset conditions encountered with slurry pumping - your application being no different to hundreds of others out there in the real world.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
(Mechanical)
9 Feb 05 23:16For those fortunate enough to have fortune-telling skills and who can predict or all ready know that the shaft failure was either torsional failure, a bending failure or a manufacturing fault etc etc. without looking into it are certainly lucky. This allows them the benefit being able of pin-pointing the problem with certainty and offer immediate solutions.I only wish it was that easy in the real world away from the desk or the text books.
Naresuan University
Phitsanulok
Thailand
(Mechanical)
10 Feb 05 04:16are you using OEM pump shafts or pirated parts?
Naresuan University
Phitsanulok
Thailand
(Mechanical)
10 Feb 05 11:41boubacar - GIW has a new tech service line - 1-888-techGIW (832-) 9am - 4pm eastern time. Call them with your pump serial number and I'm sure they can help you
Keep the wheels on the ground
Bob
(Chemical)
(OP)
10 Feb 05 14:51Thanks at all again.
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.
(Mechanical)
11 Feb 05 15:40FYI -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
(Chemical)
(OP)
11 Feb 05 15:48We are using those parts since . 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?
(Chemical)
(OP)
11 Feb 05 15:52Please find below some facts listed after the shaft faillure.
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
7. The pump was stopped on the 26th Jan for 1 hour, between 9:18PM 10:12 PM. Approximately 54 minutes.
8. The pump then ran from the 26th Jan at 10:12 PM to failure at 4:09 AM on the 30th January .
9. The shaft currently used is provided by Quality Machine and is made of Steel. These shafts have been used since .
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.
(Mechanical)
14 Feb 05 00:29The latest information is what we have been looking for -" 11.The current draw increased over a period of 9 minutes from 235A 393A12. 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
(Mechanical)
14 Feb 05 14:48boubacar, I find it strange that the pump is running and drawing more power the last 8 minutes but not pumping. Even if the impeller is deflecting into the suction plate it should still pump something if it is turning close to rated RMP. Is the wear on the suction plate mostly towards the top or is it some what even? If some what even it may suggest the pump is running with no flow either due to a plug or vortex (trapped air) at the suction or a plugged discharge. Causing the fluid trapped in the casing to heat up in the pump which heats up the pump shaft and impeller until they expand into the suction plate. There are many ways to wreck a pump not knowing all the specific information about your system one can only make guesses at where the problem with your system is. Is it the pump ? Is the problem a severely unbalanced impeller? Is it something in the system? Is it your operating condition? It maybe any one or any combination of the aforementioned or maybe something that has not yet come to light.
Regards Checman
(Mechanical)
16 Feb 05 13:27boubacarIs 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
(Mechanical)
21 Feb 05 19:27what is the L^3/D^4 ratio?for metric use <2.0what is the suction specific speed?check these industry accepted parameters to see where your pump is operating.
saludos.
a.
(Mechanical)
22 Feb 05 10:42another thought - is it possible that at some point during one of the outages that the pump was started in reverse? This could have backed the impeller off until it hit against the suction plate, and possibly initiated some cracking damage to the shaft.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
(Mechanical)
Link to SUNBO PUMP
3 Mar 05 10:11Hey guys.. did we scare him off or something???
Keep the wheels on the ground
Bob
(Mechanical)
3 Mar 05 23:18My thoughts exactly - seems a shame after all the input.
Naresuan University
Phitsanulok
Thailand
(Chemical)
(OP)
4 Mar 05 08:19Hey guy,
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.
(Mechanical)
4 Mar 05 09:42I think you might want to rephrase that 'poorly designed' comment.... as YOU stated: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
(Mechanical)
6 Mar 05 22:42I second strintcar comments - don't blame the pump - maybe you really mean to say that the system requirements that exist now are not compatable with the installed equipment.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
(Chemical)
(OP)
7 Mar 05 08:07Guys,
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.
(Mechanical)
7 Mar 05 22:18I was just reading, learning a little, have been very busy so have not participated much.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
(Mechanical)
23 Mar 05 14:43Well - it's been 2 weeks - any resolution?
Racing and bullfighting are the only real sports...everything else is just a game.
Bob
(Industrial)
29 Mar 05 12:36greetings, I have a project in which
I must evaluate the cost of a pump high pressure
for salt water for one makes of desalination, the parameters that must satisfy the pump are a production of m3/h and pressure 70 bars.(Mechanical)
29 Mar 05 22:27JotunbaniSaludos, you should start a new thread for this question, that way you will get answers from all the forum.
Naresuan University
Phitsanulok
Thailand
(Chemical)
30 Mar 05 09:28jotunbani:
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.
(Industrial)
30 Mar 05 13:19Hello I excuse myself for my former question which is not clear, I can't evaluate the price of a pump high pressure having the characteristics : Flow: m3/hr Press: 70 bars The graphs which I have are all according to H in meter (total head)
i need your help for to evaluate the cost of this pump
(Chemical)
30 Mar 05 23:05jotunbani:
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.
(Mechanical)
13 Apr 05 07:26I heard from a business contact that apparently this was simply due to insufficient NSPHA - the sump level was well below what the pump required and it ran in constant cavitation, with the failures becoming an issue when they switched to the 'pirate shafts'.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
(Chemical)
(OP)
13 Apr 05 11:48Don't worry Sprintcar, I'm still here and my problem still pending. As you said, there is a low NPSHA and the pump doesn't suit the application. But, as it is more than likely that we cannot change the process to fit the pump, we are still looking for a pump wich can do the duty in good conditions.
Do you pratice Bullfighting when racing ?
(Mechanical)
21 Apr 05 07:43Sure do - except I've got 23 other "Bulls" to deal with instead of just one....all of them lbs and 700 horsepower at 130mph on a dirt track....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
(Mechanical)
24 Apr 05 16:43boubacar,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
(Mechanical)
26 Apr 05 07:00rmw----- What you are describing is no different than what is available from many of the major slurry pump manufacturers.
Naresuan University
Phitsanulok
Thailand
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Product Description
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