Pump keeps kicking out 3

[iqbalnaq]

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

 

[iqbalnaq]

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.

 

[rotw]

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 ?

 

[rotw]

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.

 

[iqbalnaq]

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.

 

[Artisi]

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.)

 

[iqbalnaq]

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

 

[rotw]

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.

 

[ccfowler]

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.

 

[rotw]

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%).

 

[QualityTime]

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.

 

[ccfowler]

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.

 

[rotw]

ccfowler,

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

 

[Artisi]

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.)

 

[ccfowler]

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.

 

[rotw]

I also agree Artisi

 

[rotw]

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.

 

[786392]

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)

 

[iqbalnaq]

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.

 

[LittleInch]

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

 

[iqbalnaq]

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.