×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Pumping Problem - Any Ideas?

Pumping Problem - Any Ideas?

Pumping Problem - Any Ideas?

(OP)
Hello All,

I have a pumping system that has been giving a problem the past little while and we cannot seem to find the reason as to the why.

The System:
3 pumps (A/B/C)
Each pump rated for 6100 usgpm
Pump type - double split suction centrifugal
Primary Mover - Electric Motor - 3 phase 4160 VAC

Process Conditions:
Fluid - Water (80 F)
Common inlet header pressure - 80 psig
Common discharge header pressue - 450 psig
Flow - currently 12,000 usgpm, 14,000 before the problem started.
Pump A and C inlet pressure - 75 psig
Pump B inlet pressure - 35 psig
Pump A and C Motors - 200 amps
Pump B Motor - 150 amps

Additional Information:
1. There has been no obstruction found between B pump suction and the common inlet header.
2. Pump suction strainers have been cleaned.
3. Past problem with these pumps is that the seals failed between suction and discharge resulting in internal recycling.  A temporary seal replacement was done and the pumps worked fine for 7 months until this happened 2 months ago.
4. Minimum suction pressure required is 50 psig.
5. The 150 amps on B pump indicates that the pump is doing less flow than A and C (probably due to the reduced inlet pressure to this pump).
6. A D pump is currently being installed but I do not think it is involved problem as the system had this problem before the D pump installation was begun.
7. I do not think the suction/discharge pump seals have failed as the symptom to this was high pump current (250 amps) and low flow rates.
8. Picture located at:
http://i203.photobucket.com/albums/aa219/ptstacey/39s.jpg

This one has me stumped.  Any ideas are greatly appreciated.

Regards,

KL

RE: Pumping Problem - Any Ideas?

Your high amps and low flow rate symptom are confusing you.  What you probably had the first time was a complete or a fairly significant failure of the internal seal causing lots of bypass (the pump doesn't care what it is pumping, in the suction/out the discharge, or pumping around internally-the high amps indicates that the pump was pumping something unless it just had a significant rub associated with the seal failure) which resulted in a high (internal) flow rate, just that little of it was across the pump suction/outlet and most of it was inside the pump.

What it seems to me that you have here is another failure of the internal seal, whatever kind that is, but maybe not as extensive as the first one.

That pump's reduced head changes the operating point of the two good pumps on their curves resulting in reduced flow overall.

rmw

RE: Pumping Problem - Any Ideas?

(OP)
What you stated about the seal failure, high amps is nothing new to me as the pump had a significant internal recycle. I am well aware what was going on there.  Also with the flowrate and lower suction pressure how it correlates to the lower flow rate and lower amps....that is nothing new to me either....it pushes it to the left on the pump curve.

The real question is: Why does B pump NOT have the same suction pressure as A and C.

KL

RE: Pumping Problem - Any Ideas?

If there is a large pressure drop between the inlet manifold and the suction of Pump B there can be only two possible reasons. Either there is an extremely high flow or there is a restriction. There is no way the pump could pump sufficient water to cause this high pressure drop, so there must be a restriction.

Nothing else that is going on in the pump (seal failures etc) can cause this effect.

Once the logic has told us that there has to be a restriction between the manifold and the pump suction it is simply a question of finding it.  E.g. Has the B suction strainer screen been replaced recently with a finer screen?

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: Pumping Problem - Any Ideas?

There is another remote possibility.  What makes you trust the instrumentation that is giving the values that you posted?

I am an old gage tapper.  I never trust an instrument.  Maybe I am an old Reagenite-trust but verify.  While I rarely trust pressure gages, I do trust amps, however, assuming you are reading the amps with the same device.

rmw

RE: Pumping Problem - Any Ideas?

rmw, I too distrust pressure gauges but ChocLab's extremely well presented problem makes me believe that he shares our distrust of gauges and would have already swapped the gauges around to make sure of his readings.

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: Pumping Problem - Any Ideas?

Katmar,

I assumed the same, but I work on the theorum "never assume anything."  I would however, be surprised to find out that he/she hadn't.  

It is a puzzling scenario.

rmw

RE: Pumping Problem - Any Ideas?

I found this forum tonight and started membership right away. I like my first input as simple as practical as the following:

Based on the problem you stated and the sketch you provided, please consider troubleshooting the following probable causes:
1) The suction line of each pump will have an inlet isolating valve which can be butterfly valve, globe valve or gate valve. Your problem may be the due to the valve seat no longer in its proper place (in the case of globe or gate valve) or wrapped (in the case result of butterfly valve).
2) Use hydraulics calculations to determine if the current the set of pump operating parameters can cause "suction stealing" which is not uncommon to occur for 3 pumps operating in parallel.  You problem may be the result of your pump B suffering from suction stealing of pumps A and C.
3) Although identical pumps will likely show almost similar performances for some years after commissioning, come a time when one of these pumps will start to loss its performance  due to wear and tear ahead of the others, which can be as much as 10% (as I remember). I personally found this type of problem in a cooling seawater pumpstation in one refinery.  
   

doodscasay

RE: Pumping Problem - Any Ideas?

(OP)
I've been looking around on the web and in my books for information about suction starvation of pumps in parallel, but cannot find anything.  Does anybody have any information on this or know where I can find something on it?

Thank you,

KL

RE: Pumping Problem - Any Ideas?

I'd agree with doodscasay.  If the gauges are accurate and have changed, then you are losing 40 psig on the suction.  In broad brush terms, the average flow per pump was 4700 gpm.  After the problem, you lost 2000 gpm.  If the flow on pump B is 2700 gpm (it will actually be less), then you are the energy being lost in between the header and gauge is 2700 gpm with a differential of 40 psig.  If I've done the sums right, this is about 45 kW.  It it highly likely this power dissipation is going to create some noise.  My first action would be take a screw driver and MK 1 ear and try to detect some variation of noise around the pumps.  Something is restricting the flow.  If is not a blocked strainer, its either a valve gate or butterfly not connected to the shaft as it should be or you have a hard hat, a site over coat or something else in the pipe.  If its a butterfly relatively close to the header as "designers" often put them, then the turblence could easily destroy a blade or gear box.  On a site where I was commissioning once, the gear box on a butterfly valve located near the header gear box started rattling withing 2 hours.  By the time we stopped it, the blade was pretty well doing its own thing. (I'd doubt it would be a butterfly problem because you would have most likely heard it).
Best of luck.  Keep us posted.

RE: Pumping Problem - Any Ideas?

Hi KL
My first guess is suction starvation.Usually the middle pump,I am guessing you have a suction specific speed close to 3400 or so.The pumps you are describing pump such a high volume that it is a common problem if for example you shut down any one of the three then restart it.It can go unnoticed for long periods if you have automatic on/off controls.During any inspection of the pump you will most likely see signs of cavitaion.If you have the particulars feel free to contact me at jmills@ieua.org  

RE: Pumping Problem - Any Ideas?

This my be irrelevant to this problem but I witnessed 2 similar problem with sets of three pumps over the years. I can't recall the particulars but the head was there but low flow. Though not directly involved I was close enough to hear the cussing.
The last occurrence started after a seal failure on the poorly performing pump in a set of three. The seal was replaced and the problem was still there. After the second seal was installed on the pump still had the problem. There was the normal routine of checking and swapping instruments and assurances from everybody that everthing was in order.
It was eventually found out that the motor on the problem pump was turning backwards.   

RE: Pumping Problem - Any Ideas?

If the pumps used to perform according to design, but for the last two months they have not, then there is nothing wrong with the piping design or layout and you can rule out "suction starvation". Unless pipe modes were done two months ago?

The B Pump suction pressure is low and its motor amps are low. This means the pump is pumping below par and that there is a restriction in the inlet piping. Its that simple and no other theory advanced above can explain the symptoms and timing.

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: Pumping Problem - Any Ideas?

With a centrifugal pump, if you lower the suction pressure with a fixed discharge pressure, you will "back up its curve".  The pump will pump less volume at a higher head and use less energy. Thats what its doing.  There has to be something in the system between where they measured the 75 psig to one pump and 35 to the other.

RE: Pumping Problem - Any Ideas?

Did you checked if this pump, by any way had a problem in the impeller or casing (corrosion, loss of material, etc.).
Open 2 pumps (the one with problems and one of the others)
and check clearances between impeller and casing.

Good luck.

RE: Pumping Problem - Any Ideas?

ref. to katmar, the piping configuration is permanent, however the common inlet pressure is a variable that may create suction stealing. That is why, choclab should compare the common inlet parameters before and after the problem date, and also should investigate if there was any significant variation done in the supply tank (examples: switchover to a different supply tank, or opening of a tie-in line upstream of the pumpstation to supply other users, or another user connected to the common feed header increased its demand)... A significant hydraulic change may result in the reduction of feedrate/pressure to pumps A/B/C, and if this change coincided to the start of pump B problem, then most probably you have suction stealing....  Suction stealing is caused by inadequate available pressure to satisfy the combined suction requirements of pump A/B/C. The pump with the least ability to pull suction due to its piping configuration is always the middle pump (B), and it will have the lowest suction pressure.     
(I run out of writing space).     

doodscasay

RE: Pumping Problem - Any Ideas?

ref. to dcasto: what you said was correct, and the reverse is is also correct, which is, you lower the energy input (power supply) to the pump driver and the result is lower pumprate.

Now, I think choclab should consider troubleshooting using a voltage meter which will take only few minutes. Take the actual voltage being feed to the pump, is it still 3PH/4160V? OR take the RPM of the pump/motor shaft and compare result to its design spec... Once, I have a pumpstation problem almost identical to yours and the problem was traced to low voltage to the problem pump only (the other pumps have correct voltages) and the electrician rectified the problem.
Good luck.

doodscasay

RE: Pumping Problem - Any Ideas?

ref to MedicineEng suggestion, due to the big sizes of the pumps and the big maintenance job it will incur, it is more advisable to troubleshoot for any pump internal deterioration by using first the pump curve. If it is possible, pump B should operate alone, make flow variations by throttling the  discharge valve at different positions and taking suction pressure, discharge pressure, and power. The pressure gauges must be installed as near as possible to the pump suction and discharge flanges.  If the pump is almost following its performance curve, there is no pump internal deterioration that affects its performance, and further troubleshooting should be done outside the pump casing.  For example, how about the check valve at the pump discharge. If it is a flapper-type, the flapper may have fallen off and it is restricting discharge flow, consequence of which is lower suction pressure and lower energy consumption.


     

doodscasay

RE: Pumping Problem - Any Ideas?

ref to Nvaporman, I totally agree that cavitation is common to pumps with big volumetric flowrates. now, choclab can easily find out if his problem pump is losing flow/suction due to cavitation. Go to each pump and point one end of your pen to the reducer spool piece (this is the spool connected immediately to the pump discharge flange), and listen intently to the other end of your pen. If pump B is cavitating, it will emit a hizzing sound different from that of pump A and C.  If it does, the water inside pump B reducer spool bubbles and collapses creating the cavitation. Cavitation is caused by inadequate suction pressure OR very low (discharge) backpressure. Considering that pump A/C are providing backpressure, we can safely say that if pump B is cavitating, its cavitation is caused by low suction pressure and should be eliminated by providing pump B the required suction pressure.          

doodscasay

RE: Pumping Problem - Any Ideas?

dodoyrog,
You cannot lower the energy suppy to a pump, the pump creates the demand for energy and will get it from the motor.  If the motor can not supply the energy it will fail (or breakers trip).  On an AC motor, if the voltage drops, the amps go up, the breaker trips.

On a DC motor the the voltage drops and the PUMP slows down and then you could see the reverse.  So unless this is a DC motor....

RE: Pumping Problem - Any Ideas?

It is a very reasonable to assume that choclab water pumpset has 3 identical pumps (as shown in his sketch) with identical DC motors.  An AC motor, purpose of which is to deliver a variable flowrate as required by on-off users, is only installed in systems foreseen with high demand fluctuations to prevent pressure surges and consequent piping failures, for example, in firewater system with 2 DC motors and 1 AC motor. The cost of AC-driven pump is very high compared to a DC-driven pump, so I am sure that all choclab pumps have DC motors.

Also, choclab should be aware that a 3-pump system is designed for 2 pumps duty/1 pump standby, and never 3 pumps duty as it is operating now. Meaning, that the common suction header of pump A/B/C was originally sized only basing the flowrates for 2 pumps. But with 3 pumps now in operation there is a higher-than-design flowrate (PLUS) as the suction line gets older, its internal roughness increases and its C factor changes (EQUALS) higher flow resistance, lesser available flowrates, and suction starvation to least-efficient pump/s.  To determine if the suction line is adequate for 3 pumps, choclab should figure out the current water velocity in the suction line and compare it with the acceptable pump suction velocity range as given in many references. If choclab calculation is significantly higher that the reference, you have hydraulic overloading in your suction line and that is the cause of your problem.



       

doodscasay

RE: Pumping Problem - Any Ideas?

Hi Choclab

I did a simulation of these pumps and found them OK.
Acc. to me,B pump suction pressure gauge is faulty and B pump is OK.
Such big pumps got very low curve slope and you can reduce flow and you did not notice  any discharge pressure drop.

B pump got some restrictions on discharge pipe and that/s why  the flow through it is  smaller , it can be discharge valve problem or the  discharge line is fouled.
Another advice is that,
I suppose that impeller diameter inside is  rated one rand there is  possibility to replace it with  max. diameter impeller to increase capacity  all the pumps instead of to  add a new D pump. Check the pumps data sheet and the impeller diameter or ask about it your maintenance gay.
You can ask your Vendor too.

Regards
kemot

RE: Pumping Problem - Any Ideas?

kemot, I'm curious as to how you managed to do a simulation, not knowing any pipe diameters and lengths at the suction and discharge header, no downstream system curve details, no valve info, or any pump details other than flow, inlet and outlet pressure and amps?

http://virtualpipeline.spaces.msn.com

RE: Pumping Problem - Any Ideas?

I'd like the OP to post an actual piping configuration in some form.  Often the real piping is much different than a simple sketch like we have seen.

I think there might be some nuggets of information there for those of us who are following this thread and scratching our heads.

I'm thinking there might be venturi effects.

rmw

RE: Pumping Problem - Any Ideas?

If the pump curves are flat, it may just be some divergence from the ideal condition where the system happens to reach a stable operating point.  Difficult to simulate those effects without precise info (even with it).

http://virtualpipeline.spaces.msn.com

RE: Pumping Problem - Any Ideas?

Hi
I did a simplified simulation. It can be done in Excel .
Suction line DP can be  expressed as k*V^2, whereas  k-coefficient and V-as a flow.
If we know that suction lines DP for A,C pumps are negligible so the "k" is equal to almost 0. The same  estimation we can set for discharge lines and header.
If we have a substantial DP for  B pump so this "k" should not be  neglected.
A pump equation we can set A -B*V^2 where A is  equal to a diff. head with flow =0 and B is slope coefficient.
Now you can made one test.
Having three pumps running , it is possible to stop pump B for a couple of  minutes and recording suction pressure.
I stopped  sea water intake pump(1200m3/h capacity) several times while 2 other pumps were running on a previous contract so is to be a safe experiment.

If then B suction pressure increases to a value about 80 (comparable with A&C suction pressure) psig that would mean that we have substantial obstruction inside B suction line
in spite of  a suction strainer is reported to have been cleaned earlier and suction valve is OK.
If  we got the same B suction pressure(35-40psig), pressure gauge is faulty.
To coming back to a simulation problem, you can write simple equations for suction and discharge lines as described above.

The pump curve one point we already  have (6100 uspgm with 450 psig)
The second point we can estimate i.e. 500psig with 0 uspgm flow . Having these two points we can calculate A&B coefficients of pump curve.
That's seems to be enough to solve these equations and understand the problem.
If we find there is  no obstruction inside suction line found then we can start investigation of discharge line.
From my experience I met several times that discharge check valve happened to be obstructed, generally check valves  should be checked periodically.

B pump 150 Amps current means this pump is underloaded, and might be about at min. flow  limit and  check it with the Vendor please.
I suppose to clarify my point.
Regards

RE: Pumping Problem - Any Ideas?


 BigIinch, such calculations and drawing pump curves will show you that there is pump internal recycle or  anything else is going on wrong  if  your  operating points don't match the curve.
Acc. to rated flow 6000 uspgm, estimated suction line  nominal  diameter  should be about 30" with 2ft/sec  velocity. In such cases , butterfly valves is the most likely to be used. I had a case that internal rubber lining of such valve has gone and pieces of worn rubber increased noticeably  DP in suction line.
Another possibility  to solve the problem is to request to have gamma scaning done on the suction and discharge lines of B pump by a suitable service.
Such service is now easy  available and they will find where is a obstruction causing this problem.
Regards

RE: Pumping Problem - Any Ideas?

I don't really consider checking an assumed pump operating point a simulation, but my point was that with all your curve kv^2 assumptions, you're still no better off than what you already know, pump B is flowing less (lower amps) and the lower suction pressure indicates suction line problem.  The lower psig says that and lower amp use verifies it.  Anything further, such as 30" diameter, 2 ft/sec, worn rubber, partially closed valve, broken valve, venturi flow or unbalanced header flow from poor piping configuration, etc. is 100% speculation, none of which can be deduced or confirmed from your limited simulation.  Might be a crushed pipe, hardhat or dead animal too.

http://virtualpipeline.spaces.msn.com

RE: Pumping Problem - Any Ideas?

BigInch, sorry but you are just tempting me to answer you that  I would  feel  much better if I could know  the pump curves.
As you know every pump undergoes a water performance test at Vendor  site to prove its capacity and efficiency.
So this is the similar situation, if I knew performance curves of these pumps I could exclude them from further investigation and concentrate to identify a problem on both suction and discharge pipeworks.
There are many causes for pumps to decline their performance like already mentioned internal circulation, also impeller could be fouled or damaged by debris, etc..
If I have problems with pumps , compressors and so on I try to involve mechanical engineers immediately to assist me to solve it  due to their duties.
As far I can see, most of us are chemical engineers and we are not enough  qualified to sort out all mechanical problems.
Regards

RE: Pumping Problem - Any Ideas?

Hmm, too bad it looks like this thread dried up.  It definitely sounded like an interesting problem.

I too would like to see a picture of the piping configuration.  The OP never stated that the problem started after the pumps had been successfully running for some time.  He also never stated that the system was not designed for three pumps normally running.  It is a bad assumption at this point that this is supposed to be a two pump + spare that is now working in three pump service.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources