Centrifugal pump intermittent high amperage issue 1

[Marcus Gatenby]

Hoping somebody can steer me in a direction

Centrifugal pump controlled by VSD, control loop increases/decreases speed dependent on end of process line pressure (min 2.0 bar)
Higher than sustainable amperage only occurs when operating specific products (higher viscosity) although not every time
Process line operates at the same flow (21.5 m3/hour) at all times, amperage tends to increase as time (40 hour runs) increases
Specific products can be manipulated which eliminates amperage issue, a 1% increase in fat from run start eliminates issue
Impellor distance from volute "back plate" has been set to maximum (20.0mm), amperage issue slightly worse from this change
NPSHa is above pumps minimum requirement by approximately 0.1m, pump does have a 90 degree bend 4 pipe diameters away from entry
Pump operates to the far left of pump curve, pump flow at best BEP approximately 100 m3/hour, pump operates at 21.5 m3/hour
Even when pump operates with low amperage motor/pump housing has been seen shaking/vibrating although no cavitation can be heard
Pump operates with seal water, the mechanical seal/impellor do not appear to suffer infant syndrome as in same parts for 3 years
Thanks for taking the time to read & think this out, I appreciate it.

Feedback from replies:
JohnGP - Yes NPSHa is very low, pump probably is cavitating although I had not thought more about this due to lack of noise
QualityTime - Cannot throttle discharge, 4 - It could be possible that air is entrained during tank swap events, pipe sounds full
Littleinch - Agree NPSH not good, pump was selected for its head potential, operates at 80%, amperage exceeding maximum = unsustainable
DubMac - I will look into viscosity correction, the difference from good operating to poor in viscosity terms is approximately 15cP
Artisi - Measurement of heights above pump centreline, pipe line leading to measured and friction factors calculated, no gauges yet

 

[JohnGP]

I can make some general comment-
1. I think you realize that more power is required to pump a viscous product than would be for water under the same conditions. The increase in amps over time could be due to a slight increase in viscosity over time(?).
2. I would consider a margin of 0.1m of NPSHa to NPSHr as insufficient to avoid cavitation. I normally work with 1.0 to 1.5m on water type fluids, but some work with less. Published NPSHr data is usually recorded at the value corresponding to a 3% drop in head for a given flow, so cavitation is beginning to happen at that value.
3. Working so far to the left of BEP will mean pump efficiency is considerably lower, and adjusting for viscosity will lead to further reduction in efficiency. Presumably though the BEP is from the pump curve for water, so actual BEP would be lower for viscous material, but pump selection would have (should have) taken that into account.
4. Seal water system, so seals shouldn't be affected by the viscous fluid, although it's a wonder they have survived the reported vibration without damage. Pump could be cavitating without much noise.
5. Not sure what "infant syndrome" is in relation to pumps.

 

[QualityTime]

Agree with JohnGP.

[ol 1]
[li]Throttle the pump discharge. If he noises decrease then you actually have cavitation. If the noises do not decrease I don't think it is cavitation.[/li]
[li]Reproduce your pump curve at 100% speed and compare it to the maufacturers curve using water[/li]
[li]What is your shut in head and how does it compare to the manufactuerers curve using water?[/li]
[li]Is your fluid so viscous that you are getting column separation in the suction of the pump (i.e. are you pumping air and fluid?[/li]
[li]Is this a suction lift condition or a flooded condition?[/li]
[li]Put your ear against the pump suction piping. Does it sound full? Is the pump discharge flow metered? What is the reading? Is the value stable? Read my response to this particular thread

http://www.eng-tips.com/viewthread.cfm?qid=418631

[/li]
[/ol]

 

[LittleInch]

Quality Time,

1) In this instance throttling the pump discharge won't have any impact on the inlet conditions - that trick only works of the pump is on the RHS of the curve as flow reduces and NPSHR reduces.

The margin over NPSH is not enough by a long way. Cavitation often occurs 1-3 m above the official NPSHR curve.

The pump is basically far too big. You simply won't get any joy from this unit until you replace it with a more suitably sized unit.

At that flow you are probably below the minimum stable flow within the pump - I'm far from surprised you're having difficulty.

What sort of speed are you operating at compared to max pump speed?

What does this mean "Higher than sustainable amperage "?? What is sustainable amperage?

Is the pump trying to go so slow that it stalls?


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DubMac]

The first thing I would do is Google "viscosity correction for centrifugal pumps" and use the correction factor based upon your viscosity to replot the performance curve. You might be surprised at how much performance is affected by just a slight uptick in viscosity.

If viscosity is the culprit, looking at other factors first will most likely confuse the issue..

 

[Artisi]

how have you established NPSHa and NPSHr?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[QualityTime]

I agree with DubMac. Make sure all of your calculations are based on the viscosity of the liquid that you are pumping and do a proper system head curve. Only then can the proper conclusions be made.

 

[Artisi]

NPSHa is not established by measurements above pump centreline, need more info. on NPSHa available.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[EdStainless]

First, look into buying a lower flow pump. This one will not last forever, and the way that you are running it will shorten its life. You can probably justify replacing it simply on the basis of power savings.
Would it hurt your product to recirculate it? Allowing the pump to run at higher flow, and using a control valve to spill back a lot of that flow to the tank would help the pump a lot.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[QualityTime]

Some of my thoughts:

[ol 1]
[li]If the material is that viscous a centrifugal pump may not be the proper pump to use. The feed stock may not be moving fast enough in the suction line to fill up the pump volute. A progressing cavity pump running at 300-350 rpm should be looked at. What is the viscosity of the feedstock?[/li]
[li]My previous question was never answered. Is the pump under a flooded suction or suction lift condition?[/li]
[li]Does the pump flow meter or discharge pressure bounce all over the place?[/li]
[/ol]

 

[Artisi]

Seems once again everyone is spinning wheels, OP posted 10/1, there has been a lot of time spent by members with good info forwarded but bugger-all back from the OP.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Marcus Gatenby]

More feedback:
QualityTime:
Product viscosity measured at 65 cP, other products processed without issues 50 cP
Pump situated slightly lower (450mm) than tank outlet, pressure drop through valves/piping approximately 0.35m
Pump flow very stable, pressure not stable at pump outlet "oscillation", pump output (VSD %) stable

Edstainless:
Product recirculation, absolutely would like to but cannot, very likely shear issue would occur, plus wouldn't NPSHr increase?

Artisi:
Trying to answer questions as best I can, sometimes a struggle to break away for keyboard time, great comments and ideas from all contributors
Liquid height above pump impellor centre measured throughout all aspects of tank level movement
All pipe lengths/bends/valves taken into account for pressure drop feeding pump

Recent changes:
Low level cut-off for feed tank/s raised slightly as concern is air could be pulled into pump on tank changes
Planning trial to only operate feed tank/s with approximately 2.0m liquid level (min 40%) level for entire run (40 hours)

A little more history:
I did not spec pump or system design, walked into an issue which has been present at this particular plant for 12 years
Pump has been modified 3 times, 10-20 KW motor, Install VSD, 20-30 KW motor, modification of pump to allow impellor spacing out from volute
Before I started looking into the issue the business was considering installation of 50 KW motor, I suggested we re-think
An initial thought process was that impellor was in contact with the back/front housings due to fouling build-up
I think I have eliminated this as a causative by inspections which have not revealed any substantial fouling within pump
Question:
Pump has been operating for 20 hours, amperage is nearing maximum (45 amps), a cold water flush is applied for 3 minutes
Pump resumes normal processing but amperage has dropped to 40, takes 30-60 minutes to reach 45 amps again
The above effect can be repeated many times if fact that is how this plant has worked around this, water time is not desirable though

 

[Artisi]

OP: you are missing step 1 of the bare basic data required;
is the pump under suction lift or flooded inlet, either supply drawing with dimensions or fully describe the application.
Information on motor changes, impeller back clearance, your mother in laws maiden name etc. are all irrelevant without the basic hydraulic data of the installation.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Marcus Gatenby]

Artisi:
Sorry thought I included, flooded inlet.
I will post pump curve tomorrow

 

[LittleInch]

Also please let us know what outlet head the pump actually works at - your OP just says 2 bar min - what is "normal" or what is normal range.

Having read all this again I strongly suspect your pump and motor are operating too far left in a very flat part of the curve where very small changes in outlet head are causing large fluctuations in flow.

GET A PUMP SIZED CORRECTLY FOR THE DUTY.

Sorry to shout, but everything else is simply wasting everyones time - IMHO

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[QualityTime]

Please provide everyone with the catalogue pump curve with HP and NPSRr and impellor diameter and HP draw on it

 

[Artisi]

Flooded by how much, is it open to atmosphere or under vacuum?
LittleInch probably has a good solution, but still interesting to know what is happening hydraulically.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Marcus Gatenby]

Littleinch:
Pump outlet pressure operating at start of run (8.0 bar) progressing to (10.0 bar) at end of run due to fouling
The minimum operating pressure (2.0 bar) I mentioned is at the very end of a lengthy processing line
No flow does not fluctuate as mentioned several times, flow is fixed at all times to 21.5 m3/hour

Artisi:
Feed tank/s are open to atmosphere, at tank low points (tank swaps) head pressure available from feed tank very low
This process system operates like a batch process, tank swaps every 4 hours for 40 hours

 

[Marcus Gatenby]

Pump curve attached:

 

[LittleInch]

My point is that in the OP you said "control loop increases/decreases speed dependent on end of process line pressure (min 2.0 bar)"

Now you're saying the flow is fixed at 21.5m3/hr?? Exactly??

So why is the speed controlled by end of line pressure and not flow rate?

"motor/pump housing has been seen shaking/vibrating although no cavitation can be heard" Now that sounds like possible flow surging to me.

The question will be what does the system curve look like for this compared to your pump curve? Is most of the head required for static pressure or most of it used in friction down the line? You've got to match pump to system.

What power do you think is being consumed versus motor size.

A quick BOE for 10 bar at SG 1.5 with efficiency of 50% gives 17.5 kW. If efficiency drops off due to low flow, you could be looking at 25kW +. Your motor is what exactly?


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.