Pumping Problem - Any Ideas?

[ChocLab]

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

 

[rmw]

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

 

[ChocLab]

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

 

[katmar]

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

 

[rmw]

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

 

[katmar]

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

 

[rmw]

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

 

[dodoyrog]

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

 

[ChocLab]

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

 

[4Pipes]

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.

 

[NVAPORMAN]

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

 

[BigInch]

Let's see a picture of the suction piping.

http://virtualpipeline.spaces.msn.com

 

[unclesyd]

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.

 

[katmar]

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

 

[dcasto]

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.

 

[MedicineEng]

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.

 

[dodoyrog]

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

 

[dodoyrog]

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

 

[dodoyrog]

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

 

[dodoyrog]

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