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Centrifugal pump intermittent high amperage issue

Centrifugal pump intermittent high amperage issue

(OP)
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



RE: Centrifugal pump intermittent high amperage issue

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.

RE: Centrifugal pump intermittent high amperage issue

Agree with JohnGP.

  1. 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.
  2. Reproduce your pump curve at 100% speed and compare it to the maufacturers curve using water
  3. What is your shut in head and how does it compare to the manufactuerers curve using water?
  4. 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?
  5. Is this a suction lift condition or a flooded condition?
  6. 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

RE: Centrifugal pump intermittent high amperage issue

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.

RE: Centrifugal pump intermittent high amperage issue

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..

RE: Centrifugal pump intermittent high amperage issue

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.)

RE: Centrifugal pump intermittent high amperage issue

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.

RE: Centrifugal pump intermittent high amperage issue

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.)

RE: Centrifugal pump intermittent high amperage issue

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

RE: Centrifugal pump intermittent high amperage issue

Some of my thoughts:

  1. 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?
  2. My previous question was never answered. Is the pump under a flooded suction or suction lift condition?
  3. Does the pump flow meter or discharge pressure bounce all over the place?

RE: Centrifugal pump intermittent high amperage issue

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.)

RE: Centrifugal pump intermittent high amperage issue

(OP)
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





RE: Centrifugal pump intermittent high amperage issue

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.)

RE: Centrifugal pump intermittent high amperage issue

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

RE: Centrifugal pump intermittent high amperage issue

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.

RE: Centrifugal pump intermittent high amperage issue

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

RE: Centrifugal pump intermittent high amperage issue

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.)

RE: Centrifugal pump intermittent high amperage issue

(OP)
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

RE: Centrifugal pump intermittent high amperage issue

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.

RE: Centrifugal pump intermittent high amperage issue

At 21.5m3/hr, what is the pump discharge pressure and the pump suction pressure measured to the centerline of the pump. Send a photo of your installation and a sketch showing elevations of yur system from start to finish with approzimate pipe length and diameters and valves

Just looking at the pump curve you are way way oversized

RE: Centrifugal pump intermittent high amperage issue

(OP)
LittleInch:
Head pressure is consumed through the process line, the pump in question is controlled by a pressure at the end of line to 2.0 bar
Process feed pressure from pump is 8.0 bar at start, 10.0 bar at end of run, fouling/friction consumes the pressure
Start of run output 65% @ 30.0 amps, end of run output 85% @ 45.0 amps - Running a 30 KW motor

QualityTime:
21.5 m3/hour discharge pressure 8.0 bar, rising to 10.0 bar at end of run
I have been advised by pump manufacturer not to infer too much from pump curve due to this pump being so heavily modified
however as I have mentioned to them this pump curve clearly shows this pump is not ideally suited to the process.
They are reluctant to support this philosophy due to so much water under the bridge and the fact that they have said
repeatedly it is the correct pump, at the end of the day they do not want to accept this issue.

They do have another pump that is more ideally suited but refuse to even entertain the option which surprises me

RE: Centrifugal pump intermittent high amperage issue

This is weird. No use looking at anything unless you have a proper pump curveto look at. Look at this link on how to apply the correction factors. http://pumpresource.us/files/5514/2400/9276/5E_Eff...

What is their reason(s) for not going to another pump?

Look at this paper on when to use a PD pump versus a centrifugal pump. Look at the part on shaft deflection. Shaft deflection will cause the pump issues that you have observed. Talk to a PD pump supplier

http://www.pumpschool.com/intro/pd%20vs%20centrif....

RE: Centrifugal pump intermittent high amperage issue

(OP)
QualityTime:
I have yet to put my complete case to them, the years have gone by with very little analytical assessment of the situation
I am trying to paint a picture to them so they can help us to get past this, sometimes people don't like reversing their prospective
It is a very strange position to take when we are more than likely going to purchase a more suitable pump from them
There is only one other pump within their range which could do the job, I shall post that curve when received from supplier

RE: Centrifugal pump intermittent high amperage issue

OP and others:
Lets go back to the original post, where you made the following statement: "NPSHa is above pumps minimum requirement by approximately 0.1m,...."
From day 1 NPSHa 0.1m was nonsense, but without data it could only be assumed as correct (although very suspect to my thinking and the reason I was asking questions about inlet side conditions) - we now only understand that the inlet is flooded - this means the NPSHa is not a problem unless of course the inlet line losses total 8 or 9 metres.

For flooded (positive) inlet, NPSHa = Hs - Hvpa + Hst -Hs.

What is the 0.1M you are reporting, can it be assumed it is the tank low level above the pump centerline less the losses you have assumed.

If this is the case, the problem is more than likely the pump drawing in air and going off-prime and hunting on / off.
As for the problem about pump size etc commented on by others - I haven't addressed this as there is probably a major installation problem on the inlet side of the pump which needs to be addressed prior to worrying about pump sizes.-

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.)

RE: Centrifugal pump intermittent high amperage issue

(OP)
Artisi:
Pump centre situated slightly lower (0.45m) than tank outlet, pressure drop through valves/bends/piping calculated to be 0.35m
I don't understand where you get 8-9 metres of line losses from, if I have steered you wrong I am sorry, certainly not intent
Please consider that not everybody uses the same exact terminology or has the same definition for words across the world

We are going to organise a couple trials like running feed tanks at a minimum 2.0 metres above current low level &
Raising the low level cut-off of tanks.

Question:
I have not completed yet but vibration analysis on this pump for several runs under normal & trial conditions
I am also considering doing temperature monitoring pre/post pump for several runs under normal & trial conditions
Would you or anybody else suggest any other form of monitoring that may help prove cavitation is the issue, sound monitoring?

RE: Centrifugal pump intermittent high amperage issue

You are not accounting for atmospheric pressure which at sea level is approx. 9.8 m.

This is the term Hs in the NPSHa calculation.

so NPSHa = 9.8 minus vapor pressure plus liquor height above pump minus suction line losses.

Calculating

Hvpa 0 (as we don't know vapor pressure so for argument assume zero)
Hst 0.45
Hs 0.35

Calculated NPSHa is 9.8 - 0 + 0.45 - 0.35 = 9.9m.

Hope this clears NPSHa for you.
NPSHr is a pump hydraulic function.

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.)

RE: Centrifugal pump intermittent high amperage issue

(OP)
Artisi:
I understand your point of view, I had not accounted for atmospheric pressure, thanks for educating me
I shall endeavour to re-consider some aspects as soon as possible
Cheers

RE: Centrifugal pump intermittent high amperage issue

NPSHa/r will not sort your problem - my feeling is with such low water level to pump inlet you will be entraining air possibly in sufficient amounts to put the pump off prime.

Edit: added not - to read "not sort"
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.)

RE: Centrifugal pump intermittent high amperage issue

(OP)
Artisi:
Entrained air a very real possibility, I will endeavour to mash together some trials where we keep well away from
tank low levels currently experienced.

Thankyou & all others for chipping in, must say this has be a humbling experience. Kind regards

RE: Centrifugal pump intermittent high amperage issue

Ok, time to re-group a bit here.

You have a pump which is operated on variable speed to maintain a fixed pressure some distance from the pump, hence the system curve is mainly friction based. The pump curve ( I assume the impellor diameter is 230mm). From the data provided, this pump will be operating at a higher speed than shown on the pump curve
question - please provide initial pump speed / motor frequency on the pump curve and what the pump speed / frequency is when you're operating at 8 to 10 bar output pressure).

The (unknown density??) liquid is fairly viscous (65 cP), but the pump seems to work fine with a different product of viscosity 50 cP.
question - Does this increased viscosity affect the required outlet pressure?, i.e. what is the outlet pressure at 50cP and what is it at 65cp? for the same flow rate?

The NPSH, on the basis that the tank is atmospheric, would seem to be more than adequate (NPSH is always measured in head using absolute pressure converted to m).

The flow from the tank seems to be a potential concern, though 2m or more height should be ok. What is the inlet pipe size / nozzle diameter? Is there visible vortexing or air entrainment?

The key issue for me remains the fact that the pump, despite what the pump vendor is saying, is clearly the wrong size for the duty. I suspect the vendor is saying that simply because he thinks he is going to be liable for the change out.

when you operate that far left on a pump curve, strange things start to happen around the impellor which can disappear when the pump is stopped, hence your inspections find nothing. However if the action you state
"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"

indicates to me that this cold water flush is either
a) clearing out some build up of material around the impellor
b) somehow clearing out a build-up of viscous material in the impellor
c) somehow clearing out a build up of material somewhere downstream and hence reducing output pressure.
d) somehow clearing out an obstruction building up in the inlet pipework

Item c should be fairly easy to check? - Do you have a history function built into your control system you can check process parameters before and after this flush?

Items a and b, will, I believe based on information available to date, only able to be addressed by installing a smaller pump where the flow is much closer to the BEP and also the pump speed at normal operation is closer to the notional pump curve. When you operate pumps faster than their "normal" speed, you can also get some strange effects which don't occur at lower speeds

Item d will need some more data on the fluid - viscosity is quite high, but what temperature is this at? Does it have a pour point close to operating or does material start to coagulate or coat the inside of the pipes at lower temps? Remember we know nothing about this liquid other than what you tell us.....

please keep us informed as to the tests / decisions made / things you've discovered.

LI

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

RE: Centrifugal pump intermittent high amperage issue

If re-grouping, the solution is get a pump suitable for the duty, operating at 21.5m3/h with this pump is not just silly but absolutely ridiculous.
The pump is more than likely suffering discharge recirculation, the flow onto the impeller blade is anything other than ideal and probably suffering inlet recirculation, this is not even considering the likelihood of air being entrained from the minimal amount inlet head on the pump, no wonder you have surging and completely screwed up performance. Further, the amount of power being wasted per year operating (or trying to operate) running at such low efficiency would pay for a new pump probably a couple of times over.

My view for what it's worth is: for such a poor pump selection and installation there is no cure to the problem, you and everyone else are spinning wheels and wasting good time.

I suggest you employ a competent pump engineer to review the whole system and sort-out a correct pump and engineered 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.)

RE: Centrifugal pump intermittent high amperage issue

I agree - you're operating this unit at an efficiency of 30% instead of around 65 to 70.

Hence in simple terms your electricity bill for this pump will reduce by ~55-60% if you get a pump more suited to the duty.

Doesn't take long to do a quick calc of annual running hours x 30kW x $/kWH x 0.6 to get you a cost saving for this pump.

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

RE: Centrifugal pump intermittent high amperage issue

Probably a lot less than 30% in real terms factoring in all the hassles and upsets.

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.)

RE: Centrifugal pump intermittent high amperage issue

Marcus - Any feedback / update??

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

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