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Slurry pumping problems

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Slurry pumping problems

Slurry pumping problems

Dimtec

(Mechanical)

(OP)

6 Dec 05 11:07

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.

RE: Slurry pumping problems

checman

(Mechanical)

6 Dec 05 15:43

It does sound like you are slowly plugging a line. The 4” line looks to be more likely the problem looking at the velocity. After the fourth or fifth feed point (depending on where you go to 3”) the velocity must be getting quite low in the 4” line. Slugs of slurry may be migrating to a point that they can slowly choke back the flow. As far as the garden hose getting things going it makes sense, for practical purposes liquids are not compressible and your system is a loop, liquid in will mean liquid out. You are diluting and possibly cooling the slurry also. There are many variables but velocity is very suspect with the symptoms you describe. Another possibility is that the liquid is a dilatant or pseudo-plastic and you are whipping up a creamy slurry or maybe just air trapped at the impeller eye.

Regards checman

RE: Slurry pumping problems

ApC2Kp

(Mechanical)

6 Dec 05 16:18

Dimtec,
Could there be some pluggage of the pump suction line?  If the suction line is larger size 6" or more, then the slower pump inlet velocity may result in drop out of solids and plugging of the suction line.  The slower velocity and a longer length of suction line might explain why there is plugging, and adding 3/4" water hose has increase in flow so quickly.   Look for a pipe reducer at the pump suction, and see if it has flat on top (typical) or the flat on bottom (needed for slurry service).     

RE: Slurry pumping problems

Dimtec

(Mechanical)

(OP)

6 Dec 05 17:23

The suction pipe from the tank to the pump is 4", with an eccentric reduction at the entrance of the pump, flat at the bottom.
The pipe just changes from 4" to 3" after the last feeding point. We have a manometer just before the reduction from 4" to 3", and we can see that the pressure drop on the 4" doesnt indicate a problem on it. It is around 3 barg.

RE: Slurry pumping problems

stanier

(Mechanical)

6 Dec 05 18:20

I suggest that you get a copy of Slurry Transport Using Centrifugal Pumps by Wilson, Addie and Clift. Thios reference provides the best explanation and maths for such instances.,

When you add water as you suggest you are changing the density of the slurry held up in the pipeline. Thats why the Non Newtonian properties change relatively quickly.

With a slurry line there is a difference between the density of the slurry in and out of the line and the density of the slurry in the line itself. The hydraulic calculation needs to take into account this phenomenon.I suggest that you get a copy of Slurry Transport Using Centrifugal Pumps by Wilson, Addie and Clift. Thios reference provides the best explanation and maths for such instances.,When you add water as you suggest you are changing the density of the slurry held up in the pipeline. Thats why the Non Newtonian properties change relatively quickly.

RE: Slurry pumping problems

kavous

(Chemical)

6 Jan 06 14:48

by replacing 4" Flexible Hose with the existing suction line the problem should be solved. If you have 40m³/h lime slurry ( 4~12%) in a 3" line velocity would be around 4ft/sec that is low. recommended velocity for lime slurry flow is close to 4 to 7ft/sec( close to 7).
Thus I would say check the suction line first( more possible ) and then discharge line. I've assumed pump does work properly in mechanical and electrical poit  of view.

RE: Slurry pumping problems

bulkhandling

(Mechanical)

6 Jan 06 18:25

If the pump has a enclosed impeller, it is possible to have a air lock in the impeller. I assume the lime slurry have a high tension between the air-liquid surface. The flush water can dilute the lime and reduced the surface tension, so the liquid brought the air away gradually.

If there is a high point in discharge piping close to the pump, there may be a air pocket there. with reduced surface tension, it can be gradually brought away by the liquid. However, this possibility is low except there is something special in the pipe.

It does not look like any blockage at the suction pipe since you never noticed any cavitation. At 60C temp and major blockage at suction line, I would expect serious cavitation. But it is possible that air accumulated at the reducer and then entered the impeller eye and locked the impeller.

RE: Slurry pumping problems

LAsludge

(Chemical)

7 Jan 06 21:18

You have a vari drive on the pump.  Do you have a pressure gauge on the pump and does it indicate that you actually have 10 bar at 1500 rpm?  If you see a flow change back to the tank immediately after starting the water flow to the pump suction then I'd be inclined to look at the suction piping and monitor my suction side pressure.

RE: Slurry pumping problems

Dimtec

(Mechanical)

(OP)

9 Jan 06 03:26

kavous, 40m3/h in a 3" line, the speed would be around 2.3 m/s, aprox 7 feet/s. We have checked some spools on the line after 2 months on operation, and there is no presence of scale, neither 3" and 4" lines, so I assumed is not a problem of low velocity on the line.
We have a pressure transmitter on the impulsion, so we check continuously the impulsion pressure. With that data there is no evidence of suction blockage.

RE: Slurry pumping problems

kavous

(Chemical)

9 Jan 06 18:51

Dimtec,
You’re right, I wanted to say 4". Because I thought after the first feeding point Q will be reduced and you don't have 60 m³/h any more. That is getting worse close to the last feeding point.
option one- as I mentioned the problem is in the suction line ( more possible). By having Flexible Transparency Hose ( 4" ) as a suction line ( without any branch, bend, low point and so on. the problem will be solved. I saw the same problem many time in suction lines.
option two -Some companies use a flushing water connection with a solenoid valve. The valve gets open timely and flushes the line periodically.
Option three- if you have ant pressure/flow control on the recycle line such as back pressure control valve or flow orifice (FO is not a right application for this service)adjust them for lower presure and increase pump flow rate. by this you can increase the slurry velocity in the loop.

there is no scale issue. it's solid build up issue and may happen at low points, bends, branch connections, vertical points ....
Hope it does work.
Kavous

RE: Slurry pumping problems

LAsludge

(Chemical)

9 Jan 06 19:16

Questions:

When the pump goes from 1400rpm to 1500rpm you can observe a pressure increase from 120 psi to 150 psi?

When the flow back to the holding tank drops to near zero what is the pressure reading at your feeding ports?

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Additional resources:
Slurry Pump vs. Water Pump | Why & Where Slurry Matters

RE: Slurry pumping problems

bulkhandling

(Mechanical)

10 Jan 06 20:13

To check the discharge pressure is necessory and may give you answer to your questions.

There is unlikely solid settling in the suction pipe. 7fps velocity is pretty high for a 4" suction pipe. Even if some settling in the pipe, It's pretty easy to be brought away when the local velocity increased. I would prefer the reducer at the suction line top-flat instead of bottom flat because the bottom flat will make air bubbles accumulated at the top of the reducer.

To find out if there is air lock in the impeller or not, you can check the pressure gauge at the discharge pipe when the pump start to speed up. When the pump speed-up, the discharge pressure shall keep the same and when the pump reached max. speed, the discharge pressure starts to drop, so the recirculated lime reduced.

RE: Slurry pumping problems

Dimtec
Slurry Pump Bearing Assembly

(Mechanical)

(OP)

11 Jan 06 02:39

The pressure transmitter we have in the impulsion goes from 6.5 bars at 1300 rpm, to nearly 10 bars at 1500 rpm. This doesn’t indicate any blockage or air bubbles trapped on the suction side. The pressure at the suction could be around 0.6 bar, the pump is feed by a tank, liquid level around 5 meters, constant.
If there were air bubbles on the suction side, it would notice a pressure drop, right?
At 1300 rpm, 6.5 bar at the impulsion, we have around 4 bars at the feeding ports. At 1500 rpm and 10 bars at the impulsion, we have around 6 bars at the feeding ports.

RE: Slurry pumping problems

Dimtec

(Mechanical)

(OP)

11 Jan 06 03:00 http://es.pg.photos.yahoo.com/ph/dimtec1972/my_photos

This is a graph of the systems parameters.
Brown line: pump speed
Red line: Discharge pressure
Pink line: Returning flow
Yellow line: Density

There is a density drop (around 5 gpl), is when the hose is added at the suction, and the speed drops from 1500 rpm to 1300 rpm in around 1 hour.

This is a graph of the systems parameters.Brown line: pump speedRed line: Discharge pressurePink line: Returning flowYellow line: DensityThere is a density drop (around 5 gpl), is when the hose is added at the suction, and the speed drops from 1500 rpm to 1300 rpm in around 1 hour.

RE: Slurry pumping problems

LAsludge

(Chemical)

11 Jan 06 19:34

Interesting data; look at what’s happening.

When you’re operating in “clean” mode you have a discharge pressure of 6.5 bars.  When you get to your feeding ports the pressure has dropped to 4 bars; a drop of 2.5 bars.  That number is consistent with the expected pressure drop of water flowing through 800 meters of schedule 40 steel pipe.  Since you’re pumping a lime slurry I would expect it to be a little higher but maybe your pipe is schedule 10 but that’s immaterial.  When you operate at 1500 rpm your discharge pressure is 10 bars and your feeding port pressure is 6 bars.  That’s a 4 bar pressure drop vs. a 2.5 bar pressure drop in the “clean” mode.   Assuming you have a constant flowrate then you must ask why is the pressure drop from the pump discharge to the feeding ports 60% higher?  

The pump seems to be running fine.  If cavitation or suction line blockage were a problem then you wouldn’t be able to maintain pressure let alone increase it.  The problem does not seem to be the 3” return line either.  You’re seeing a loss of pressure in the 4 “ line also.  Whatever is causing the pressure build up in the 4” line is the issue.  You say you’ve inspected the pipe and see no evidence of buildup on the pipe walls so the idea of a large dropout on a pipe joint seems pretty remote.

Your chart shows a fairly constant density until you start adding the water.  Also the pump pressure continues to rise even after the pump speed is maxed out and while the return flow decreases.  Something is acting to restrict the flow.

What about fluid viscosity?  You are running this slurry around in a loop at about a 200% recycle rate (Why so high?).  It wouldn’t take much of an increase in viscosity to explain everything.  Your water injection would effect viscosity immediately and it would certainly account the increased pressure drop in the 4” line.  I’m at a loss to explain why pumping lime slurry in a loop would increase it’s viscosity but maybe there’s another ingredient you’re not telling us about.

RE: Slurry pumping problems

bulkhandling

(Mechanical)

11 Jan 06 20:24

I'm thinking the possibility of shear-thickening.

Shear-thickening happens on some fine slurries especially with higher solid concentration. When shear rate increases, liquid viscosity increases. The thickened fluid will increase the friction loss starting from the pump casing to all along the piping length. When the pump speeds-up (increased shear rate), it aggravated the shear-thickening and further increased the viscosity and friction loss in the pipe.

To check if the fluid viscosity is higher at the first dosing point should give some indication if the above assumption is true or not.

RE: Slurry pumping problems

LAsludge

(Chemical)

11 Jan 06 20:29

The temperature will also effect the viscosity.  Are you monitoring the slurry temp?

RE: Slurry pumping problems

sprintcar

(Mechanical)

11 Jan 06 23:59

Have you consulted the pump manufacturer? They generally have solutions based on experience and testing.

"If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut."
-- by Albert Einstein

RE: Slurry pumping problems

Dimtec

(Mechanical)

(OP)

12 Jan 06 02:52

The temp of the lime is monitored and controlled by mixing hot condensate and water. It stays constant around 75ºC with variations around 2ºC. There is no evidence of relation of temperature on this problem.
The recycle rate is round 200% for this phase. In the future it would be around 100% (if we solve the problem with the flow)
I have no data about viscosity, only it’s assumed to be 2cp from previous projects that worked with the lime.

RE: Slurry pumping problems

bulkhandling

(Mechanical)

12 Jan 06 11:32

It is worth of your checking of the viscosity since shear-thickening can answer all your questions if it is true.

Since the friction loss is more than doubled when pump has the maximum speed, the change of viscosity shall be visible. Just get a sample from the mixing tank for fresh lime and another sample from the first dosing point. you should be able to tell the difference.

Diluting the slurry when needed is obviously a solution. If your process allows, to lower the mixing density at the beginning can keep your dosing density constant. When the lime slurry concentration is lower than certain percentage (for example 4 to 6%), the change on viscosity will be negligible.

RE: Slurry pumping problems

LAsludge

(Chemical)

12 Jan 06 12:02

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

RE: Slurry pumping problems

Dimtec

(Mechanical)

(OP)

12 Jan 06 12:19

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.

RE: Slurry pumping problems

LAsludge

(Chemical)

12 Jan 06 15:24

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.  

RE: Slurry pumping problems

kavous

(Chemical)

26 Jan 06 18:22

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

RE: Slurry pumping problems

Dimtec

(Mechanical)

(OP)

27 Jan 06 02:46

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

RE: Slurry pumping problems

Dimtec

(Mechanical)

(OP)

31 Mar 06 02:09

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

RE: Slurry pumping problems

Alanator

(Electrical)

31 Mar 06 10:27

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.

RE: Slurry pumping problems

bulkhandling

(Mechanical)

2 Apr 06 04:22

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.

RE: Slurry pumping problems

LAsludge

(Chemical)

2 Apr 06 16:33

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.
 

RE: Slurry pumping problems

bulkhandling

(Mechanical)

3 Apr 06 02:04

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?

RE: Slurry pumping problems

Dimtec

(Mechanical)

(OP)

3 Apr 06 02:49

 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.

RE: Slurry pumping problems

bulkhandling

(Mechanical)

3 Apr 06 14:35

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?

RE: Slurry pumping problems

LAsludge

(Chemical)

3 Apr 06 19:05

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.

RE: Slurry pumping problems

LAsludge

(Chemical)

4 Apr 06 14:07

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.

RE: Slurry pumping problems

bulkhandling

(Mechanical)

23 Jul 06 14:16

Dimtec,

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

RE: Slurry pumping problems

mizzoueng

(Mechanical)

23 Jul 06 17:35

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?

RE: Slurry pumping problems

Artisi

(Mechanical)

24 Jul 06 22:31
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.  
 

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

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