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series centrifugal pump motor mystery

series centrifugal pump motor mystery

series centrifugal pump motor mystery

(OP)
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.

RE: series centrifugal pump motor mystery

Motor current can be constant even if shaft lod changes. It depends on what power factor you have. But it is not usual when running close to full load. You can usually see it at partial load.

Nonetheless. Take a P.F. reading just after start and another one when flow has decreased. If your other observations are correct, then your power factor probably is less the second time you read it.

Gunnar Englund
www.gke.org

RE: series centrifugal pump motor mystery

"Our increased runtime rules out flowmeter anomalies."

I don't understand this comment.

I would still suspect the flow monitoring.  I would add a pressure sensor temporarily too.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: series centrifugal pump motor mystery

(OP)
Thanks for the information.  We have the ability to feed the motors with A/B Soft Starters or A/B VFD's (we have dual capability for redundancy). We have the same occurance with either starter.  The load is constant, +/- 150 feet of hydraulic head, to the top of the force main.  I know the power factor will fluctuate with a change in voltage and load, both of those factors are static.  When we feed the motors with their VFD's the supply system does not see an inductive load due to the nature of the VFD's.  When operated with an SCR soft start, we see the same results.  I don't think power factor is the issue.  What do you think?

RE: series centrifugal pump motor mystery

You have a VFD? Then you should be able to read not only motor current but also power - perhaps even torque from the inverter. If P.F. frequency, voltage and current are constant and the load is changing then there must be something wrong with one (or more) measurement either electric or other.

Gunnar Englund
www.gke.org

RE: series centrifugal pump motor mystery

(OP)
With regards to the flowmeter question:  The flowmeter is of the magnetic flow meter type.  If the flowmeter were malfunctioning, we would expect to see normal pump times.  By comparing our flow chart recorder readings, we see that we have substanially longer pump run times that seem to indicate the validity of the flowmeter information.  We adjust this for daily flow changes and additional system loads and it seems the meter is working properly.  It cannot be calibrated in the field but we have the ability to verify the calibration and it appears okay.

Thanks for your suggestions.

RE: series centrifugal pump motor mystery

(OP)
Thanks, Skogsgurra.  If the load were decreasing, I would expect to see the current and power factor decreasing.  They appear to be remaining stable.  When we are testing this system to find out what is going on, the VFD is set to minimum and maximum speed based on 60 Hertz.  That way we can compare it with the Soft Starter. They both react the same.  I would expect that the torque shouldn't shift unless the voltage, load or frequency changed.

RE: series centrifugal pump motor mystery

Whats the piping system look like?  Are ou filling it up or otherwise "packing" it?  If so the water may not have anywhere to go and the flow drops off.  
Pumps can heat a lot of water under the right conditions, have you check the temperature?

RE: series centrifugal pump motor mystery

For balanced and constant Voltage and Frequency,the current change follows the load torque demanded on the motor shaft. The power factor is inherent to the motor parameters and it will not change unless the load, voltage or frequency change.

This means the motors have the mechanical load.

Each set has two pumps in series, so the discharge of the first pump is the suction of the second?
If so, could turbulence be the reason for the extra power demand?
I am not expert on hydraulics, have you any oher experience of performance with a similar pump array?

RE: series centrifugal pump motor mystery

(OP)
Thanks, BJC.  The water is discharged into a common header that charges a force main.  The main crests at an elevation of +150 feet above the pump.  From there it gravity feeds into a sewer main that is at the bottom of the grade a couple miles away.  If we stop pumping the checkvalve on our pump closes and the force main remains surcharged.  So basically, it is pumping into an open channel, although raised in elevation.

RE: series centrifugal pump motor mystery

(OP)
Thanks, Aolalde.  We utilize the two pumps in series to produce the desired head required to pump over the hill.  We have been monitoring both motors and pumps and everything looks fine.  The inlet to the first pump is submerged and has +/- 15 feet NPSH.  The discharge of the first pump is about 75 PSIG and travels through +/- 8 feet of pipe to the suction side of the second pump.  The discharge of the second (identical) pump is +/- 150PSIG. This equates to 346 feet of head which is adequate for the 310 feet of head in elevation change.  I mis-wrote earlier that the elevation was 150 feet.  It is about 310 feet, which accounts for the 150 PSIG discharge.  We utilize series pumps in a couple of locations and have never run into this problem.  Thats why I am scratching my head so much.

RE: series centrifugal pump motor mystery

Quote:

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
An increase in dynamic head.

Quote (BJC):

Whats the piping system look like?  Are ou filling it up or otherwise "packing" it?  If so the water may not have anywhere to go and the flow drops off.

Most likely there is a slight restriction in the line down to the sewer. As you are pumping the line is slowly backing up. When it finally fills up it increases the dynamic head. Put a pressure guage on the pump discharge this shopuld show an increase in dynamic head.
respectfully

RE: series centrifugal pump motor mystery

(OP)
Thanks Waross - The discharge shows +/- 150PSIG.  As the flow drops off there is a corresponding duplicate drop-off in the discharge pressure of both pumps.
Larry P.

RE: series centrifugal pump motor mystery

(OP)
Also, the line to the top of the grade is always surcharged (filled) so there is no "backing up".  The decrease in discharge pressures leads me to believe that we are pumping less.  My question is why?  We have eliminated air entrainment as a possible culprit.  The piping layout exactly follows the pump manufacturers layout.  They have no idea why things are operating the way they are.  We are busy collecting data and thermal image scans to send to them for help.

RE: series centrifugal pump motor mystery

Power curve of mixed flow impeller pumps is not a straight line and there is possibility that you can have same power consumption at two different flowrates. The peak load is some where in the middle of the curve.

When you say full load at 2250 gpm, is it the maximum power with in the operation range or limiting power of the motor?

What happens to the level in the well after every 40 minutes?

What is the suction pressure of second stage pump when the flow is 2250 gpm and also when it is 1500 gpm?

What are the discharge pressures of first stage pump for the two flow conditions?

RE: series centrifugal pump motor mystery

I predict based on your topography and the puzzling results that everyones suggestions are essentially occurring.

The downhill side of the line is longer than the uphill side.
With the pumps off the uphill side tries to drain back.
The check valve stops this.
The uphill side remains filled and you see the pressure sit basically static, pumps on or off, probably about 20PSI less than pumps on.
Meanwhile the slack side actually drains down the long distance to the main.
When the pumps start the water starts to tip over the top of the hill and then runs downhill rapidly in a partially empty pipe.
This leaves the pumps essentially pumping to the top of the hill and out of an open pipe.  You get full flow and expected HP requirements.
Meanwhile the downhill pipe fills from the bottom of the grade at the main back towards the hilltop.  This takes 40 minutes.  The downhill side never fully fills entirely leaving a large air space that cannot pass water. This leaves your downhill pipe essentially restricted at some fraction of its theoretical diameter. This does not occur immediately but takes the forty minutes.

Mess with syphons and clear hoses and you will soon see this phenomenon. I have seen it several times!  You will reach a steady-state static situation where the system just sits there acting like a pipe 1/2 or 1/3 the pipes diameter depending on the entrained air bubble.

This is not a packing but a dynamic choking state.

You could prove this by venting the line somewhere like 10 to 40 feet down the hill from the summit on the downhill side after the 40 minutes.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: series centrifugal pump motor mystery

Hi Keith
That was what I thought but you said it much better. The problem is that that would result in an increase in discharge pressure and larryp001 is reporting a drop in discharge pressure when the flow drops.
larryp001;
Does the suction pipe enter the pump in a straight line or is there a 90deg. bend at the pump? If the pipe runs straight in I'm wondering if you may be getting an internal vortex after awhile. With two pumps, the second pump may be pulling water past the vortex, but with unexpected operating conditions.
Re the temperature measurements; Can you get an instrument that will read very small differential temperatures? With the amount of water flowing and removing heat, differential measurements may be needed to locate internal heat generation.
respectfully.

RE: series centrifugal pump motor mystery

(OP)
It is great to get such good input from all of you.  The downhill side of the force main gravity feeds to a sewage lift station where the water is collected in a wetwell and then pumped to its final destination, a wastewater treatment plant.  The flows into the station are roughly constant on a day to day, or weekly basis but the hourly flows fluctuate with time of day. (morning showers, lunches, dinners evening showerws, etc.)  The station operates in a fill and draw mode rather than trying to maintain a specific level in the wetwell.  We keep the low level shut down setpoint high enough to avoid the creation of vortices that could be pulled into the suction line.  When inspecting the manhole at the top of the forcemain, it appears that water level never raises.  It just flows in the channel without surcharging the manhole.  Most importantly, as we close the discharge valve from the pump while it is operation we see the expected decrease in current (not moving water, not doing work, lower current).  Applying that observation to your suggestion, I would think that we should see a decrease in current as the water was prevented from flowing.  We keep seeing approximately the same current, but our flowmeter shows a gradual reduction in flow of up to 30% without a reduction in current.  That is what is really puzzling me.

RE: series centrifugal pump motor mystery

Is it possible that the density of fluid changes after initial flow? More sludge gets sucked and moved so more friction, work remain the same but less flow? Or on other words higher head and lower flow. HP=head x flow rate.

RE: series centrifugal pump motor mystery

Is there a way of monitoring the head in addition to flow?

RE: series centrifugal pump motor mystery

(OP)
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.

RE: series centrifugal pump motor mystery

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

RE: series centrifugal pump motor mystery

(OP)
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.

RE: series centrifugal pump motor mystery

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?



RE: series centrifugal pump motor mystery

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.

RE: series centrifugal pump motor mystery

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.

RE: series centrifugal pump motor mystery

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

RE: series centrifugal pump motor mystery

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

RE: series centrifugal pump motor mystery

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?

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RE: series centrifugal pump motor mystery

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.

RE: series centrifugal pump motor mystery

(OP)
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.

RE: series centrifugal pump motor mystery

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 ARE packing the line or pushing thru a bubble obstruction. Your motor IS really doing less work and IS drawing less real power.  Your amp reading is faking you out, as it were.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: series centrifugal pump motor mystery

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

RE: series centrifugal pump motor mystery

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

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

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