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

 

[QualityTime]

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

 

[Marcus Gatenby]

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

 

[QualityTime]

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_Effect.pdf

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 vs centrif.pdf

 

[Marcus Gatenby]

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

 

[Artisi]

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.)

 

[Marcus Gatenby]

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?

 

[Artisi]

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.)

 

[Marcus Gatenby]

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

 

[Artisi]

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.)

 

[Marcus Gatenby]

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

 

[LittleInch]

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.

 

[Artisi]

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.)

 

[LittleInch]

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.

 

[Artisi]

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.)

 

[LittleInch]

Marcus - Any feedback / update??

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