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
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
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
RE: series centrifugal pump motor mystery
Gunnar Englund
www.gke.org
RE: series centrifugal pump motor mystery
Thanks for your suggestions.
RE: series centrifugal pump motor mystery
RE: series centrifugal pump motor mystery
Pumps can heat a lot of water under the right conditions, have you check the temperature?
RE: series centrifugal pump motor mystery
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
RE: series centrifugal pump motor mystery
RE: series centrifugal pump motor mystery
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
Larry P.
RE: series centrifugal pump motor mystery
RE: series centrifugal pump motor mystery
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
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
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
RE: series centrifugal pump motor mystery
RE: series centrifugal pump motor mystery
RE: series centrifugal pump motor mystery
RE: series centrifugal pump motor mystery
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
RE: series centrifugal pump motor mystery
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
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
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
respectfully
RE: series centrifugal pump motor mystery
Are you saying "neither" pump's power changes?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: series centrifugal pump motor mystery
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
RE: series centrifugal pump motor mystery
This is interesting a drawing would help.
RE: series centrifugal pump motor mystery
Thanks again.
RE: series centrifugal pump motor mystery
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
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
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com