Slurry pumping problems

[Dimtec]

We are having some problems with an installation we just put in operation a month ago.
We are having some flow restrictions and we don’t know the cause.
We are pumping lime slurry (density 1080 gpl, temp about 60ºC).
The installation consists on a feeding tank, a centrifugal pump, a pipe ring, and 5 feed points with a control valve each one.
The tank is about 10 m3 capacity with an agitator.
The pump is centrifugal, with variable speed drive, 1500 rpm max, and 90 m maximum head. The vsd is controlled by the flow returning to the tank
The pipe ring is about 800 m – 4” to the feeding points, and 3” on the returning to the same feeding tank. We are pumping about 60 m3/h on the 4” line, and returning about 40 m3/h on 3”.
The feeding points have their own control valves and flow meter each one.
We have a pigging system to clean the line. The bends on the line are 10D radius, and the valves are ball valves full pass.
In normal operation the pump is working around 8barG on the impulsion, about 1400 rpm. But sometimes, the speed and the pressure starts to increase slowly, (let says, in 5 hours), and we it reaches the maximum speed (1500rpm, 10 barg) the flow returning to the tank starts to drop, and in a matter of 2-3 hours falls to nearly 0.
We haves some manometers on the feeding points, and with that and the % open of the valves, and the flow meters, we can see that there is no blockage on the 4” inches line.
So we are guessing that there is a blockage on the 3” line.
The thing that annoys me is that we have achieved a partial solution that consists on connecting a ½” hose to the pump suction and feed it with water.
As soon as we connect the hose, the flow stars to increase again, and in a matter of 20 minutes it reaches its normal condition and works properly.
His tends to happen by 2-3 times per week.
If is a matter of blockage on the 3” line, and the water helps to clean the line, why as soon we connect the hose the pump starts to increase the flow? The water would need about 20 minutes to run the 4” line and another 15” to return to the tank, so I suppose the should expected to have to wait about 30 minutes or more to see any improvement on the flow.
I am really lost with this, and any help would be appreciated.

 

[LAsludge]

What about the return slurry temperature? Do you monitor that too?

 

[Dimtec]

No, we don’t control the temperature of the return.
But about that, the temperature of the process has been increased from 40 to 75 ºC. The problem hasn’t been affected for the temperature change; we have exactly the same problem now and before. I have checked also the relationship with the ambient temperature, and I can not see any measurable effect.

 

[LAsludge]

Ok so temperature is not an issue. Let’s review what we know.

From the data you’ve provided I draw the following conclusions:
1) The pump is operating normally
2) There is no blockage in the suction line
3) The pressure required to pump 60m3/h through your pipe loop is variable and increases to the point that it cannot fulfill the pumping requirements.

What is causing the increased resistance to flow in the piping loop? It would seem that it must be either piping restrictions or fluid dynamics. Time for more data.
I’d like to see a more complete pressure/temperature profile of the pipe loop. If possible I would gather a new dataset at the pump discharge, at a point halfway between the pump and feeding port 1, before feeding port 1 and after each feeding port. Measurements should also be taken at the 4 to 3 inch pipe transition and also halfway back on the 3” return line. This dataset should be done at constant flowrate and at both “clean” operating conditions and during problem conditions. With this data we will be able to see if the problem occurs throughout the entire loop or if it’s just isolated to a pipe loop section.

Shear thickening has been suggested. I don’t know anything about this subject but I would expect it to be a continuous issue not an intermittent one. Never the less it would seem worthwhile to test the viscosity at various points during good and bad operations.

 

[kavous]

Dimtec,
Please give us a feedback if you had any action, resolution.

 

[Dimtec]

I have sent some samples of product to laboratory for viscosity test. I would inform you when I have them.
Also it seems that there is some relationship between the problem and fast variations of the speed of the screw conveyor that feeds the mixing tank. For the moment not sure that it could be the cause or and effect.
I would post further info

 

[Dimtec]

Finally, we have received the viscosity test. Our datasheets for the hydrated lime indicated a viscosity of 10 cps. The viscosity seems to follow a Power law.
The parameters for the viscosity goes from k=2000 cps; n=0.13 at 1030 kg/m3 density, to K=4500 cps; n=0.25 at 1080, that is the density we would like to work. Temperature in the range from 30 to 80 ºC seems to have very little effect on the viscosity.
SO, a small density increase seems to have a very large impact on the viscosity.
To solve this issue, we are thinking on connecting existing pumps to the suction of the lime pumps to boost the pressure by 5 bars.
We hope with this modification we could work very close to the objective density

Regards

 

[Alanator]

You mentioned using ball valves for the control valves. I understand that ordinary ball valves are great for on-off service, but lousy for modulating service. I think I remember seeing ball valves that are designed for modulation.

 

[bulkhandling]

Dimtec,

I quite suspect that your modification will resolve your problem.

From your test result, I cannot find the answers to our questions although I actually do not quite understand the exact meaning of the k's and n's. For my understanding, the test result only tells that when density increases, the viscosity increases, no matter power law or not.

Lime addition to the process is pretty common in mining and chemical industries with a density of 1080 to 1100 kg/m3. Viscosity at this point is normally not a major concern. Like your data 10 cps, or 6 cps for another test (last year I asked a lab do a test for a density of 1090kg/m3??). With the pump TDH of about 50 ft and similar recirculation loop for a few applications I or my coleagues designed, we never heard any problem like you met.

I understand your piping loop is excessively long (800m+?? for 3") and your max pump head 90m (tip speed 8000fpm to 9000 fpm??)is very high to me. Although your normal operating speed may be lower, but may still create excessively high agitating load or shear rate , which may change some normal properties of the lime slurry. Viscosity may be among those changes.

I had expected to see test result for the viscosities for 1080 kg/m3 only at 1. fresh or relatively fresh hydrated lime (recirculated); 2. close to but before water addition.

For your long piping, a little bit increase in viscosity, total piping friction loss can be considerably higher and speeds up your pump.

So if my assumption is correcte, I would suggest to reduce your pump speed, instead of increase it - reducing the recirc rate can do it. Of course, you have to pay attention and take measurements to ensure the dosing rate to the five dosing points.

Good luck.

 

[LAsludge]

Dimtec,

Good to see you've solved your mystery.

Small changes in density create large changes in viscosity and the changes in viscosity explain all of your operational problems.

More sensitivity on your density monitoring and increasing your pumping capacity sounds like a winning combo.

Best of luck.




Alanator,

Ball valves are used for control. The "hole" in the ball can be shaped for either linear or equal percentage flow.

 

[bulkhandling]

The point is that density has not been changed all the time before water addition, but friction loss increased so that you have to increase the speed and finally adding water to the solution. Question: friction increased due to viscosity change? How was viscosity changed? Due to density? Increased shear rate? So as I said, the test result actually did not answere the questions.

Your questions answered? How can you make a good solution without answers on your questions?

 

[Dimtec]

Density is controlled by controlling the speed of the screw conveyor feeder (vsd), and the water flow (by a control valve). Screw feed rate is only estimated, since we don’t have any kind of weight controller or similar.
I am not very sure also about the density measurement. It is done by difference of pressure with 2 pressure transmitters, on the bottom and on the top of the feeder tank, respectively. Since the tank only has about 3 meter height, and we are talking about 50 gpl difference of density, I think the system is not very accurate. For example, changing the amount of gland water of the bottom pressure transmitter, (we are taking from 2 to 5 lpm) produces density variation readings of 30 gpl).
Bulkhandling, the test were done with fresh lime form each feeding tank, also from the suction of the pump, and from the discharge point at the returning loop, and the test don’t show big differences on viscosity, only on the results on the tank with higher density. The hose was disconnected from the pump suction before the test.
The viscosity tests were done at a range of temperature from 30 to 80ºC, and a range of shear rates, and all show that the viscosity of the lime reduces at higher shear rate, so at first glance increasing the pump speed should reduce the viscosity, should it?
Anyway we have reduced the recycle rate to 30 m3/, and the pump normal speed at 1030 density is lower that 1100 rpm, and we are having exactly the same problem. We have reduce the flow in order to disconnected the hose, and have more time of reaction and connect the hose only when is strictly necessity and the speed of the pump increase over a certain point.
I would post the result of the modification.

 

[bulkhandling]

I do see density increase as the pump speeds up. What is the cause? Higher pump speed with higher discharge pressure reduced the amount of gland seal water? What caused the original density increase? Does this happen periodically with a fixed cycle?

 

[LAsludge]

I’m sorry, I guess I got ahead of things. I thought you had your problem solved.

I thought it was obvious from your data that the operational problems were the result of high viscosity. It still seems like that’s your problem. How do you get high viscosity? How about too much lime in the mix?

Before you go looking for any esoteric solutions you need to verify that your density control system is working properly. When your system goes bad take a sample of the slurry and check the density in the lab. See how that compares to a sample running normally. You need to independently verify that your densitometer is working properly. Maybe your sensor is fouling up and maybe when you flush it with water you clean it up.

Maybe the next time you have this high pump speed situation you should try lowering the lime feed to see if that solves your problem. Maybe you should control viscosity instead of density. You could adjust your controls to moderate the VFD to the screw conveyer when the pump reaches a certain speed threshold.

 

[LAsludge]

Here's another note on lime slurries from another website:

Viscosity of slurry

The viscosity of hydroxide slurry can vary greatly from lime to lime as well as different process conditions. Certain changes in the hydration conditions or impurities in the lime will increase the viscosity of the slurry, thus affecting settling time. Often times the viscosity increases at slaking temperatures of 180°F and above. The relationship of viscosity size, particle size, specific surface, and settling rate has not been completely researched. In general, it is presumed that higher viscosity means smaller particle size of hydrate and greater specific surface and settling rate. Variations of viscosity of hydrated lime slurry have been reported between a range of 45 to 700 centipoises.

 

[bulkhandling]

Dimtec,

"I would post the result of the modification."???

 

[mizzoueng]

I know I am a newbie, but from what I have read you have had problems with the slurry right at the pump. After adding a water hose near the suction side of the pump everything works right. That makes me think the problem is on the pump impeller housing itself. We are getting ready to install a slurry pump, the manufacturer added a flushing connection to the impeller housing to combat blockage in the housing during operation and cleaning when the pump goes down.

Also, I know its a dumb question, but the startup screens aren't still in there are they?

 

[Artisi]

There has been considerable and constructive input into this so far- but the problem exists.
From you description of the problem and the simple cure of injecting water into the inlet seems to indicate a problem on the inlet side of the system.

So a few thoughts for you to think about / look at.

Does the returning slurry "feel-fall" into the supply tank.

One of the major pump inlet problems encountered in pump applications is the return product free-falling into the supply tank - this has the effect of entraining air into the pumped product which puts the pump off performance and can cause cause pumping to stop all together.It can be a slow process with a gradual build-up of air in the pump case - as you have described over a number of hours.

What type of agitator is installed, these are also known to "beat" air into the product if the tank level is low and air in entrained from the free surface of the tank.

Repositioning the agitator in relation to the pump inlet can sometimes overcome this problem. And at worst you could have a combination of both problems.


Naresuan University
Phitsanulok
Thailand