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.photo bucket.com /albums/aa 219/ptstac ey/39s.jpg
This one has me stumped. Any ideas are greatly appreciated.
Regards,
KL
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://
This one has me stumped. Any ideas are greatly appreciated.
Regards,
KL





RE: Pumping Problem - Any Ideas?
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?
The real question is: Why does B pump NOT have the same suction pressure as A and C.
KL
RE: Pumping Problem - Any Ideas?
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?
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?
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Pumping Problem - Any Ideas?
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?
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?
Thank you,
KL
RE: Pumping Problem - Any Ideas?
Best of luck. Keep us posted.
RE: Pumping Problem - Any Ideas?
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?
http://virtualpipeline.spaces.msn.com
RE: Pumping Problem - Any Ideas?
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?
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?
RE: Pumping Problem - Any Ideas?
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?
(I run out of writing space).
doodscasay
RE: Pumping Problem - Any Ideas?
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?
doodscasay
RE: Pumping Problem - Any Ideas?
doodscasay
RE: Pumping Problem - Any Ideas?
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?
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?
"Mover - Electric Motor - 3 phase 4160 VAC"
http://virtualpipeline.spaces.msn.com
RE: Pumping Problem - Any Ideas?
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?
http://virtualpipeline.spaces.msn.com
RE: Pumping Problem - Any Ideas?
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?
http://virtualpipeline.spaces.msn.com
RE: Pumping Problem - Any Ideas?
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?
http://virtualpipeline.spaces.msn.com
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?
http://virtualpipeline.spaces.msn.com
RE: Pumping Problem - Any Ideas?
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?
http://virtualpipeline.spaces.msn.com
RE: Pumping Problem - Any Ideas?
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.