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Pump keeps kicking out
3

Pump keeps kicking out

Pump keeps kicking out

(OP)
Hi
I work in pulp and paper and am troubleshooting a pump. It is a centrifugal pump 9" impeller (goulds 3196mtx); motor 3HP, 1760RPM, 3phase constant speed induction motor, 575V, 3.2A. The flow rate we are putting out is 23gpm. The motor is the right size for application (meets the pump curves)

The problem is the motor keeps kicking out. It is pumping polysulfides orange liquor (pulp and paper jargon). We think it is scale build up in the pump that is causing the motor to trip.We do an acid wash once a year right now. It is a time intensive process and increasing the frequency is a bit difficult. Putting in a bigger motor seems unreasonable as we don't know if the amps drawn by the motor ever level out.
Any pumps out there in the market which decrease the rate of scale build up.
Or maybe there is some other reason for the kick outs.
Thanks
Naqshe Iqbal E.I.T

RE: Pump keeps kicking out

Have you ever taken the time to open the pump and to actually check if the "build" of liquor is the prime cause?
Has the liquor SG been factored into the power calculation?
What is the liquor temperature?

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: Pump keeps kicking out

(OP)
Have you ever taken the time to open the pump and to actually check if the "build" of liquor is the prime cause?
Has the liquor SG been factored into the power calculation?
What is the liquor temperature?

Hi,
At the flow rate we have and with the S.G factored in (viscocity was very similar to water)the pump shouldnt require more than 2.5HP.

We cant open the pump for sometime. It is important for production.

RE: Pump keeps kicking out

The motor overload relay measures current and trips when the current exceeds the setting. Have the plant electricians checked the actual current being drawn by the motor and compared it to the relay setting? Is the overload relay in a very warm environment? Most overload relays are compensated up to about 55°C ambient, where 'ambient' means the air within the starter compartment while the drive is running.

Is the voltage at the motor terminals low? Induction motors, over a limited range, behave as relatively constant power devices, meaning that a slightly low voltage will result in more current being drawn.

RE: Pump keeps kicking out

(OP)
The motor overload relay measures current and trips when the current exceeds the setting. Have the plant electricians checked the actual current being drawn by the motor and compared it to the relay setting? Is the overload relay in a very warm environment? Most overload relays are compensated up to about 55°C ambient, where 'ambient' means the air within the starter compartment while the drive is running.

Is the voltage at the motor terminals low? Induction motors, over a limited range, behave as relatively constant power devices, meaning that a slightly low voltage will result in more current being drawn.

The actual current varies.The current drawn is constantly increasing from the time of a kick out to the next kick out. The overloads are currently set to 110% of 3.72A. They are in a relatively cool room; the room has mcc for other motors with no isses. Voltage is never below 575V.
We checked the RPM of the motor once and found when the motor was drawing more current the RPM were lower than full load RPM of the motor (more slip) showing that there was more load on the motor.

What is interesting is that right after a kick out the amps drawn are actually lower than what was being drawn before kick-out and then they gradually rise till kick out (constant flow rate).

RE: Pump keeps kicking out

Just brainstorming:

Something to do with a kind of rheological time dependent behavior of the fluid. The longer the fluid undergoes shearing force, the higher its viscosity. So the longer the fluid is flowing in the pump and the pipes (shearing effect) the viscosity keep on increasing.
When the motor kicks out, the fluid property are reset, thus the viscosity.
On restart, the power required on the motor returns to its origin value.

It the fluid pumped in a closed loop ?

RE: Pump keeps kicking out

Have you confirmed the flow and head independently?

RE: Pump keeps kicking out

So far you've mentioned flow rate (do you know what it actually is over time to any degree of accuracy?), but not differential head / pressure or density. Do these change over the time taken for your pump to trip out? Can you / do you actually measure them? Do you have an actual pump test curve or are you relying on the standard one / one supplied in the bid. For such a small pump you might not have had it tested.

It's clear your pump is demanding more power form the motor. This can only really come about due to increased density compared to the deisgn over time, increased flow?, increased head differential or decreased efficieny (your scaling).

However if it "resets" between trips it's difficult to see how scaling can do this.

As with many pump "problems", in reality it's the system that is the problem, so watch the critical things over time (differential preessure, flow rate, density) to see if they change. Even a relatively small change or the fact that your pump is actually putting out a greater head or lower efficiency than you think can easily eat into your relatively samll margin of pump shaft power vs motor output power.

As ever, a flow diagram or sketch of the system, the pump curve, the system curve etc all helps us to understand your system and point out potential issues much easier than words, though figures and data always help...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Pump keeps kicking out

How long does that pump take to start? Sure HP from the curve might check out, but for small motor sizing you need to consider the usual starting characteristics (moment of inertia, WR^2) of the rotating element (impeller, shaft, coupling.) And also the not-so-usual characteristics for such a small motor: the power absorbed by the mechanical seal, or packing, and normal friction from bearings and wear rings could easily exceed your 0.5hp margin. These things are negligible for larger pieces of equipment (although the WR^2 still needs to be given to the motor vendor to ensure proper starting.)


I'd guess that once there is a drop in speed for whatever reason (change in conditions that cause a brief increase in flow, therefore power?) the motor can't quite manage to accelerate back up to "full speed" so instead it trips. Almost like the motor is repeatedly experiencing the tail end of a warm start, until it gives up and trips.

RE: Pump keeps kicking out

(OP)
So far you've mentioned flow rate (do you know what it actually is over time to any degree of accuracy?), but not differential head / pressure or density. Do these change over the time taken for your pump to trip out? Can you / do you actually measure them? Do you have an actual pump test curve or are you relying on the standard one / one supplied in the bid. For such a small pump you might not have had it tested.

The flow rate is 23gpm. It varies very little, there is a throttling valve controlling the flow rate from the pump to maintain certain level in the tank. The maximum I have ever seen it increase to is 26gpm but that was just once. When the motor trips the flow rate is around 23gpm. So it is not an increase in flow rate causing that.
S.G of the fluid is 1.17. Viscocity is very close to water.
It is pumping out 96ft of head, incoming head is not even half a foot.
Pump curve.
http://i43.tinypic.com/mj1tw6.png


It's clear your pump is demanding more power form the motor. This can only really come about due to increased density compared to the deisgn over time, increased flow?, increased head differential or decreased efficieny (your scaling).

I am guessing that your loses go up as pipes have buildup in them. Also the elevation head is only 32ft but it takes a long path.

From the pump curve motor shouldn't be demanding more than 3HP at CURRENT settings.

However if it "resets" between trips it's difficult to see how scaling can do this.
As for reseting i was thinking that maybe the flow of liquid through pump kind of dislodges it.

Right now I am open to any reasonable suggestion.



RE: Pump keeps kicking out

wait a minute.

First, Your point is at 23 gpm and a BEP at around 70 gpm at constant curve from the graph.
This is the first abnormal observation.

Second, you have poor reserve margin with a 3HP motor since the iso-power line is very close and it limit completely your map. It does not even allow you to move to the right of the curve. In case of flow fluctuations (system curve with slightly less resistance) you trip the motor.

If the motor is really rated 3HP than it is undersized and the furthermore (which may be at the root of poor motor sizing) the operation is not in the safe area of the map. In fact this is the problem not the Motor itself.

RE: Pump keeps kicking out

A couple of things.

Your generic pump curve will be based on water. For shaft power you need to work on multiplying the power by 1.17, so gets you much closer to 3HP. An earlier post said you hasd allowed for that, but this curve says it hasn't. It is also a generic curve, not a specific as tested one so you could easily be + 10 to 15% more power requirment as most suppliers tend to allow for a bit of wear and know that people tend to complain when they don't get enough flow / pressure, but are usually quite happy with a bit more. Also you're working so far on the LHS of the pump curve the efficiency lines are actually pretty vague and all sorts of odd things start happening inside the pump at reduced flow compared to it's BEP to increase power requirments.

I can only assume you are working in US GPM. If either of your flows is in imperial it will add another factor.

Basically your pump is underpowered and as the motor warms up seems to loose the sysnchrous speed which is just going to drag the power up and lead to overload.

If you can stick in a 5 HP motor all your problems may go away or better still swap the pump for one which has a duty point (BEP) much closer to your 23gpm.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Pump keeps kicking out

(OP)
This is how I calculated theoretical power requriements.
Pump power required = (Flow rate (GPM)* Head (ft)* Specific Gravity )/(3960*Efficiency) =(23 (GPM)* 96 (ft)* 1.17 )/(3960*0.26) = 2.5HP

First, Your point is at 23 gpm and a BEP at around 70 gpm at constant curve from the graph.
This is the first abnormal observation.

I agree, we are operating in a very inefficient area of the curve.

Second, you have poor reserve margin with a 3HP motor since the iso-power line is very close and it limit completely your map. It does not even allow you to move to the right of the curve. In case of flow fluctuations (system curve with slightly less resistance) you trip the motor.

If the motor is really rated 3HP than it is undersized and the furthermore (which may be at the root of poor motor sizing) the operation is not in the safe area of the map. In fact this is the problem not the Motor itself.

I don't think it is fluctuations of the flow that are causing the motor to trip. What we see is even if the flow rate is relatively contant the amps drawn rise steadily. We have taken ammeter readings over a period of 4-5 days. The motor tripped three times during that period and after a trip the amps drawn will decrease a little bit and hten steadily rise till the motor trips again.

RE: Pump keeps kicking out

So the problem could be heat related. What are the ambient conditions? Are there other constants when it trips? You should get a bigger motor (maybe pump too) but for now, point a box fan at it and see if it still trips...

RE: Pump keeps kicking out

(OP)
Basically your pump is underpowered and as the motor warms up seems to loose the sysnchrous speed which is just going to drag the power up and lead to overload.

If you can stick in a 5 HP motor all your problems may go away or better still swap the pump for one which has a duty point (BEP) much closer to your 23gpm.

We were thinking of the same thing. But we don't know what are the max amps drawn by the motor at nominal flow rate. 5 hp might kick out later than 3hp but we cant guarantee it wont.

RE: Pump keeps kicking out

iqbalnad,


This is bad design because as it cannot be an unexpected off design or even an oversizing on purpose since the motor has been rated on this point i.e. 23 gpm ; apart if the motor was intended to be replaced on purpose as part of a future revamp.

You say you agree your pumps operates on a poor efficiency area but that is not quite the point. The point is you will end up with mechanical issues with your pump. No need to discuss the flow fluctuations story. The point is completely in the left corner of the map. Unless you run with some funny pump and impeller (you should tell us if it is so) there is NO justification for this design for any conventional single or multiple stage pump. Anyway the iso efficiencies speak for themselves.

Furthermore I disagree on your point related on the stability of the flow. All it takes is a small excursion or upset in your process to start running the 3HP motor on what should be (at least) a power reserve. You are looking at second order issues to explain a situation which is primarily improper design.

The motor rating you mention in the Original post is 3HP, it should be mentioned anyway on the Nameplate.

RE: Pump keeps kicking out

1gibson,

quote
... You should get a bigger motor (maybe pump too)...
unquote

not quite correct:
pump is too big.
motor is too small.

 

RE: Pump keeps kicking out

(OP)
So the problem could be heat related. What are the ambient conditions? Are there other constants when it trips? You should get a bigger motor (maybe pump too) but for now, point a box fan at it and see if it still trips...

We have overloads at 4.10amps for a motor with fla of 3.2Amps.

It trips becasue it is drawings more amps than it should. According to ammeter reading that can be from 4.1 to 4.2 amps

RE: Pump keeps kicking out

Your calc is correct as it stands, however if you look at the curve and do the same right on the 3hp line for say 38gpm by 94ft at 37%, you get 2.5hp. Therefore for the other losses referred to above (seals bearings etc) the manufacturer has added 0.5hp. Therefore add this to your calc and you get over 3hp. I think your motor is overheating as it is right on the limit or over and experiencing extra slip.

There could be some fouling issue as the flow is so low compared to what it is designed for so I would go for a properly sized pump as your main resolution issue. Short term, not much I can think of, but clean the motor fins, make sure the motor fan is clean and not deformed, or maybe remove the motor fan and use an external bigger fan to give the motor output that little bit more power, check the stuffing box and lubricate the bearings. Every little thing will help.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Pump keeps kicking out

Is the pump actually fitted with a 9" impeller, has it been physically checked?????
Has the flowrate, total head, and SG been measured?????? - or is it a guess or your expectations.
looking at your pump curve the pump should have been fitted with a 5hp motor to make it non-overloading across the full pump curve.
The selection of 26% for your efficiency may well be wishful thinking, operating where you are on the curve so far left of BEP the real efficiency could be anything other than 26% probably a lot less, unless you have ideal inlet conditions to the impeller which is unlikely. Bear in mind this curve was produced in an accurate test facility, something you can't reproduce on site without accurate and correctly placed certified gauges and flow measuring equipment.

I also think that saying the pump is definitely pumping 23 GPM at 96ft at a constant 1.17 SG at 26% efficiency is also wishful thinking and not actual operating conditions.

For instance, if the flow is 25GPM head 97, SG 1.2, efficiency 24 then power requirement is above 3hp.

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: Pump keeps kicking out

(OP)

The impeller is 9" (physically checked), S.G also checked.
Flow rate is 23gpm (nominal doesn't fluctuate much).

We just opened the pump and found very little scale build-up. The impeller was not rubbing against scale in the housing.

I also think that saying the pump is definitely pumping 23 GPM at 96ft at a constant 1.17 SG at 26% efficiency is also wishful thinking and not actual operating conditions.

I agree the pump needs more power. But why are the amps drawn constantly rising. We see amps increasing and not a steady draw of amps.

RE: Pump keeps kicking out

Suggestion : because of the slip factor.
The motor has so poor margin that the load impact directly the slip.
The speed reduces and so the speed of the pump and the head, therefore the flow decreases further and the efficiency drops. The efficiency drops imply again more load on the motor and slip increase again.
and so forth till you trip the motor.

does this make some sense ?

RE: Pump keeps kicking out

You should give us more info to possibly help you :

- you mention throttling valve. Is it automatically controlled ? If so give us the percentage of opening versus time from one trip to the other.

- Do you have some monitoring of the speed of the pump ? if so give us the speed evolution with the time from one trip to the next one.

- Please provide the percentage of increase in current from min to max

- How much time it takes from one trip to the next. Is it in the range of magnitude of the time needed for the motor to cool down? You mentioned constraint to keep the production so how long you keep up continuous operation until the trip.

- What type of intervention on the motor do you perform after trip. Let the motor cool down and restart, make replacement of thermal switches or whatsoever ?

- Do you monitor the suction, discharge pressure and flow somewhere in the process ? if so please give us the trend vs. time

- Can you arrange and monitor the temperature in the motor winding or somewhere to see the trend ? is the room temperature stable what are the % expected variations ranges ?

These are a lot of info. but the more we get, the better.

RE: Pump keeps kicking out

(OP)
You should give us more info to possibly help you :

- you mention throttling valve. Is it automatically controlled ? If so give us the percentage of opening versus time from one trip to the other.

It responds to the level of the tank pump is sucking from. pneumatic control valve.

- Do you have some monitoring of the speed of the pump ? if so give us the speed evolution with the time from one trip to the next one.

That will be a bit hard to do. But I can confirm that when the motor os drawing more amps the motor rpm is lower than full load rpm

- Please provide the percentage of increase in current from min to max

2.7- 4.3 (trips)

- How much time it takes from one trip to the next. Is it in the range of magnitude of the time needed for the motor to cool down? You mentioned constraint to keep the production so how long you keep up continuous operation until the trip.

Time varies between trips. Sometime it wouldn't trip for days.Sometimes it will trip 2-3 times in a day.

- What type of intervention on the motor do you perform after trip. Let the motor cool down and restart, make replacement of thermal switches or whatsoever ?
Just reset overloads.

What I was wondering if heating of motor can result in larger amps drawn?


Also we opened the pump up and found barely any scale on it. There was some scale on the control valve. Now that we have put everything together the amps drawn have dropped to 2.7amps from 3.9-4amps. So I was thinking that maybe something to do with control valve as the pump was clean.

RE: Pump keeps kicking out

As the problem is so random, I would say it's an application problem, ie, SG changes or total head reduces or you have a power supply problem - voltage drop, any high demand equipment on the same supply line?

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: Pump keeps kicking out

(OP)
As the problem is so random, I would say it's an application problem, ie, SG changes or total head reduces or you have a power supply problem - voltage drop, any high demand equipment on the same supply line?

total head reduces? Can you please elaborate on that.


Also I am trying to understand what happens to the fluid around control valve and how it affects the fluid in the pump. Just in a normal application. For example, if the pressure drop across the control valve increases how does it affect the fluid before the pump.
I know that pressure after the control valve is lower than before and the flow rate is same.

Thanks

RE: Pump keeps kicking out

The motor is 3HP, cost nothing - why not replacing it with a 5 HP or so and close the matter ?
I see that it trips regularly and you have to reset and create outage to your process.

That will troubleshoot the problem. Moreover there is a consensus here that you motor is undersized.
That means your system is not robust to fluctuations in the whole system. Would n't this a better approach then inspecting components.
There might be a chance that the fluctuations are inherent to the big system and not necessarly a malfunction somewhere, in such case you would have wasted time and money possibly.

RE: Pump keeps kicking out

From what I've seen in the information provided, my only surprise is that your problems aren't more severe. The pump is very poorly selected for the duty, and in this size range variations of 10% in the actual characteristics from the generic published curves is not unusual. Since the operation is so far away from the BEP, the actual power required can be very much greater because of the poor efficiency range in which the pump is being forced to operate.

Since there was already an apparent choice to waste significant power by sizing the pump to operate inefficiently, it appears that operating energy costs are not a matter of much concern. It seems likely that a 5 HP motor may be enough to fix the problem, but I suspect that there may system operating characteristics that may present themselves after a change to a 5 HP motor. Because of the unfavorable pump selection, I would not be surprised that you may find operating conditions may develop where the 5 HP motor could be overloaded as the pump duty shifts into even less favorable efficiency ranges. I would seriously consider installing an even larger motor to avoid the need to make yet another replacement when other troubles show up. The difference in operating costs will be relatively trivial.

If operating costs were to be considered realistically, including the costs of troublesome interruptions, you may find that replacement of this pump with a properly chosen one may have a surprisingly short payback time.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

RE: Pump keeps kicking out

ccfowler,
With regard to efficiencies it does not seem to be an issue. Because if you look to the map w, the isopower lines decrease in power value with the lower efficiencies. in reverse, if the flow moves to higher efficiency (toward the BEP), the Isopower lines increase in power which could become an issue for the motor. This 5HP shoud cover the end of curve in anticipation of system curve change.
So if the fluid properties are constant, as it is said, that should be fine BUT we have to keep in mind that if the duty remains more or less around the specified flow, the motor will be partialy loaded (around 50-60%).

RE: Pump keeps kicking out

Does the temperatrue of the fluid change? My handbook talks about this. An excerpt is as follows?

Thermal Factors

The increase in temperature of the liquid within the pump is directly
related to the pump’s efficiency. The energy that is available to heat the flowing liquid and the pump casing is basically the difference between the power input to the pump (brake horsepower) and the useful work done by the pump (liquid horsepower). At low flow conditions, centrifugal pumps are very inefficient and a significant amount of input energy is lost and heats the liquid and the pump assembly. Refer to Subsection 2.3.1 and Chapter 12 for more discussion on thermal effects.

RE: Pump keeps kicking out

QualityTime,

You are correct. At low flow rates, pumps become very expensive "mechanical water heaters" to the point at zero or near zero flow they can even become even more expensive " mechanical steam generators."

rotaryworld,

My expectations about the 5 HP motor are that it will probably work, but I suspect that there may be unknown process fluctuations that may involve changes in the pumped fluid where density or viscosity may change to shift power demands beyond the 5 HP motor. The cost differences between a 5 HP or a larger motor are probably trivial compared to having operations disrupted yet again for the lack of enough power to get through an "excursion." As is so commonly the real situation, the claimed pump problems are actually some combination of system or pump selection problems.

I've known of "successful" cases where much larger and more powerful pumps have spent their entire operating lives at well below their BEP, but various compelling reasons made this the least costly alternative even when their seemingly excessive repair and energy costs were taken into consideration. In these cases the pumped fluid was just water, so no goofy business of varying composition, viscosity, or density introduced compounding complications.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

RE: Pump keeps kicking out

ccfowler,

I fully adhere to your approach.
With regard to running the motor at part load, what is your feeling about it ?

RE: Pump keeps kicking out

Come on guy's, we are talking about a "toy" pump here, some home swimming pools run bigger pumps that this and a 3 - 5 HP pump is inefficient to start with plus the amount of time water is in the pump will probably have zero effect on temp. rise anyway.

On the other hand, if we were discussing a 2000kW re-circ pump then we could have reasonable discussion on hydraulic efficiency or lack there-of so, lets get back to reality, it's the wrong pump for the application as well as being under powered for the situation it is in and if the OP can't see or doesn't want to see or accept this fact, then we are all spinning wheels discussing ideal applications, pump selection, hydraulics and theory as none of it will solve his problem.

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: Pump keeps kicking out

Oh, Artisi! As usual, you are so very correct! I fully agree with your last comment!

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

RE: Pump keeps kicking out

I also agree Artisi

RE: Pump keeps kicking out

However with a small bemol...to "this pump is a toy"...
We never know the economic rational of this plant, neither the financials behind and less the minset of the operator/owner.
It might be that owner is private (investing his own money to the last cent) who must operate with low cost imperatives.
Who knows that for him replacing a motor would appear or turnout to be something non negligible.
I personally doubt, but you never know.

RE: Pump keeps kicking out

Moreover "Just a wild thought"

We should check for and

eradicate any possibility of physical obstructing material

as if being sucked-in at random intervals

thus affecting pump out operations with increased E/power demand.

Best Regards
Qalander(Chem)

RE: Pump keeps kicking out

(OP)
I have looked into changing the pump. But the problem is that system requires a head of 96ft with only 23 gpm flow rate. Options are very limited. I am open to suggestions.

I have nothing against putting a bigger motor. I am only worried that it will start kicking out too as we dont know the max horsepower needed.

RE: Pump keeps kicking out

Why are options "limited"? Have you actually prodcuced a data sheet and put in the leg work with the major vendors to see if they have a more suitable pump? Technical sales engineers are very knowlegable about the pumps they produce and you can search for suitable vendors very easily.

You know the max shaft HP of the pump - the issue is the max shaft HP of the motor is far too close or under this maximum demand. Double or triple the motor size and you are very unlikely to have this occur again. Get the right sized pump and the max power will decrease so you won't need to change cbale sizes or motro starter etc.

I would be very interested to see what the winding tmeperature rise is during these outages either from internal thermocouples or an external casing temperature. may give you a better explaaination...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Pump keeps kicking out

(OP)
Okay 5hp motor was installed and it is still kicking out. We set the breakers such that they wont kick out even at 10 amps just to see what is the max. The motor still kicked out, so even a 10 HP motor will start kicking out.
Is there anything that can be done to reduce scale build up. We were thinking of a coating but are worried it will chip and enter our pulp stream. Can we use special impeller type that scrapes? Or any other solution.
Problem is not not enough power.

RE: Pump keeps kicking out

On 6th August you said "We just opened the pump and found very little scale build-up. The impeller was not rubbing against scale in the housing."

So what is happening now, why have you now asked how to reduce scale build up?

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: Pump keeps kicking out

One possible option is a rubber lined pump, this is sometimes a solution but cannot be guaranteed. Also sourcing a small pump with high temp. elastomers maynot be possible.
You will need to talk with a slurry pump manufacturer and ask their advice.

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: Pump keeps kicking out

Reading this again, it struck me that one thing not considered is whether to increase the flow through the pump by say double your current flow by means of a return line either to pump inlet line or back to original tank. You would waste a bit of energy, but the additional velocity and operating more in the centre of your pump curve would be helpful and the increased efficiency means that at double your flow, you would only increase power by 50%.

Given you have a control valve on level control, the loss of a few feet of head shouldn't make any real difference.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Pump keeps kicking out

(OP)
I think when we opened the pump the flush affect knocked some of the scale inside the pump. That is why we didn't see any scale rubbing against the impeller. That will also explain when the pump kicks out the amps drawn by motor go down.

Scale is the only possibility I can think of. After acid wash amps go down, after flushing the pump with water amps go down.
As for rubber pump, or a coating we are worried it will chip off and enter our pulp stream.

I was thinking of an impeller with a scraper vane on it. That might prevent buildup.

Increasing the flow will increase the efficiency, but might not help with scaling. Or maybe more fluid will prevent scale build up. Will try that.
Thanks

RE: Pump keeps kicking out

It's not guaranteed, but at low flows you get all sorts if strange Eddy flows within the pump which are much reduced at higher flow rates.

Let us know if it helps.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

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