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.

 

[checman]

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

 

[ApC2Kp]

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).

 

[Dimtec]

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.

 

[stanier]

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.

 

[kavous]

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.

 

[bulkhandling]

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.

 

[LAsludge]

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.

 

[Dimtec]

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.

 

[kavous]

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

 

[LAsludge]

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?

 

[bulkhandling]

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.

 

[Dimtec]

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.

 

[Dimtec]

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.

 

[LAsludge]

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.

 

[bulkhandling]

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.

 

[LAsludge]

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

 

[sprintcar]

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

 

[Dimtec]

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.

 

[bulkhandling]

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.