series centrifugal pump motor mystery 1

[larryp001]

We have a pump station with four sets of paired centrifugal pumps operated in series to produce the necessary head for the force main application. Any set of pumps will run for a period of time and then system flow will begin to drop off from 2250GPM to 1500GPM. The weird thing is the motor current remains the same. We have monitored voltage, current, frequency, harmonics, and performed thermal scans and don't find anything outside specified parameters. If we stop the motor and wait for three or four minutes the pump will restart pumping 2250GPM and then drop after about 40 minutes of runtime. We have recently cleaned the wetwell to eliminate any grit and we have raised our level setpoint to avoid the ingestion of any vortices into the pump. Our increased runtime rules out flowmeter anomolies. My question is: What conditions will allow a motor to pump less fluid and maintain the same full load current when the amount pumped was as much as 30% less than full load? We have used recently calibrated Fluke instrumets for all of our measurements. These are 200 HP dry-pit submersible motors connected to the pumps. Any ideas or suggestions would be greatly appreciated. Thanks.

 

[larryp001]

Thanks rbulsara. We cleaned the wetwell to eliminate any effects from the ingestion of grit. We verified that there were no scum blanket getting sucked into the system. It appears to be just your typical run-of-the-mill residential wastewater,+/- 97% water, 3% oganic solids,that doesn't vary in density. Also, less work should show up as reduction in current level at motor. Motor shows same current even though flow rate is indicated as dropping. We have opened the manhole at the crest and the air/vacuum breaker immediately before the manhole is never pulling blowing even with three sets of pumps running. That seems to indicate there is no back up of water on the down hill side.

 

[waross]

I have had problems with a single sewage pump cavitating after it was in operation for a short time. The single pump would stop pumping and the current would fall as expected.
I suspect internal cavitation and or vortex to be the problem. Have you considered installing a straightening vane in the inlet of one of the pumps to see if this helps?
respectfully

 

[larryp001]

Thanks waross. No cavitation noises are being heard and no cavitation erosion of the impellers has been noted. The inlet pipe is a typical bell end pointed down. We raised the wetwell setpoint to eliminate the possibility of vortices being ingested into the suction line. Again, with ingestion of air, less room for water meaning less work with less motor current. Our motor current is staying the same while flow reduces. There I go scratching my head again.

 

[rbulsara]

Well some more information may help..(this electrical engineer playing mechanical), you shoud post this in mechanical/hydraulic/fluid dynamic forum as well.

1. Discharge pressure and flow alone are not sufficient. What is the "differential pressure" across each pump? and across the two pumps in series? You may have lower discharge pressure but even lower inlet pressrure..at least check that.

2. What is the HP and voltage rating of the pump. What is the current reading? Any kW readings?

 

[rbulsara]

more questions:

What happens when only one set of pumps is running?

What happens when more than one set of pumps run?

Are sets of pumps in parallel fighting each other?

Is there a bypass loop formed? Post a flow diagram..

I am not saying I will have the answers, it may enable others to have more insight.

 

[BJC]

somebody else check me. At low flow your pumping about 12,000 lbs per minute. The horse power to raise the temperature of that mass just one degree F is close to 300, right?
I am not near my books so what does someone else think.
I have seen seen pumps used to heat water.

Can you post a link to the pump types. The pumps I have worked with in similar application were "trash" pumps designed to pass fairly large solid objects. The curves were not like a typical water pump.

 

[waross]

What may be the problem is a submerged vortex. If a vorex sucks air the problem is obvious. I am wondering if the configuration of the piping is encouraging the liquid to "Spin" in the pipe. This would absorb energy to overcome the inertia and impart the angular velocity. This may be the explanation for the 30% of the power that seems to be disappearing. I have no idea if it would be in the primary suction or the intermediate suction. Maybe you can figure out how to check this out.
respectfully

 

[itsmoked]

larryp001; You say you have two pumps in series and that "the" pump stays at the same power level but the flow diminishes.

Are you saying "neither" pump's power changes?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Another opinion from a non-mechanical engineer (for what it''s worth).

It really sounds like a gas or air compression problem to me. I've done a lot of controls on force mains in the past and have seen this once before, although it was found to be a long vortex (formed around a level sensor tube) as waross suggested. Since you've ruled that out, any chance that you maybe have a lot of suspended air bubbles in the intake well that take a while to coalesce in the pipe under pressure and become a gas pocket somewhere? This is a problem I first was made aware of in the Trans-Alaska Pipeline, because there are a lot of microscopic natural gas bubbles suspended in the crude and over the long distances it becomes a problem. They have to vent it out along the way, otherwise they lose too much pumping capacity. The "reset" you describe where it functions normally after a 3-4 minute rest is puzzling to be sure, but maybe the gas is being reabsorbed when the pressure from the pumps goes away?

In a properly designed force main system any gases would rise to the top of the force main and be expelled with the flow of course. However, even if there are no trap points in the original design (humps in the piping profile), there is always a chance that the installing contractor made an error, or something has shifted on you. That was the case in the one I saw. There was a slight land slippage problem that didn't break the pipe, but caused a hump that allowed the air to stay trapped and compress. It took a long time for enough air to get trapped in the pocket to make a difference, but the result was similar to what you are seeing: decreased flow with constant power. If so, I think that flow would not continue to decrease indefinitely because eventually the gas would be at maximum compression and flow would continue on around it. So you would see a drop off and then a flattening of your flow profile. Have you tried running longer to see if it flattens?

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[BJC]

Can the downhill side of the force main be filling up with floating solids? If the velocity is not enough to force them out they could accumulate in the upper part of the downhill run.
This is interesting a drawing would help.

 

[larryp001]

I just got back to my office. Thank you all for all of your excellent suggestions. I will do some extensive testing this weekend to try and deal with some of your questions. I will post my findings on Monday or Tuesday, workload depending.

Thanks again.

 

[itsmoked]

One thing to add, skogs suggested it early on.

How are you measuring the current?

Let me give you an example. I have a 5Hp motor that runs a 6 cylinder single stage compressor. If I pull the belt off and run the motor, a clamp on current reading will show a reading that is about 2 amps less than FLA.

Now I put on the belt and run the compressor. When it initially starts and runs into an empty tank the current reading drops to something like 60% of FLA now that the motor has a load as apposed to no load 90% FLA. As the compressor starts working against a higher head pressure the current goes up. Finally reaching 100% FLA when the head pressure reaches 180psi.

This is a power factor thing. As the motor becomes unloaded its power factor drops raising the reactive component of the current. The clamp on doesn't know this! It just read current! As the motor loads up its power factor improves lowering its current or possibly keeping it the same but because the power factor is actually changing the actual work being done is actually increasing.

In your case the pump load is diminishing but as it does the PF shifts and so your amp reading stays the same. I believe this is what you are seeing. I believe either you [green]ARE[/green] packing the line or pushing thru a bubble obstruction. Your motor [green]IS[/green] really doing less work and [green]IS[/green] drawing less real power. Your amp reading is faking you out, as it were.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Hi Keith;
Check larryp001's post of 18 May 06 10:31
I had similar thoughts re: power factor, but when the pump is shut-in, the current does drop.
respectfully

 

[itsmoked]

Ah the valve test you mean? Okay, well the mystery continues then.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com