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

 

[Artisi]

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.)

 

[iqbalnaq]

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.

 

[ScottyUK]

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.

 

[iqbalnaq]

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).

 

[rotw]

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 ?

 

[77JQX]

Have you confirmed the flow and head independently?

 

[LittleInch]

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

 

[1gibson]

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.

 

[iqbalnaq]

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.

 

[rotw]

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.

 

[LittleInch]

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

 

[iqbalnaq]

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.

 

[1gibson]

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...

 

[iqbalnaq]

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.

 

[rotw]

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.

 

[rotw]

1gibson,

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

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

 

[iqbalnaq]

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

 

[LittleInch]

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

 

[Artisi]

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.)