Buildup of Hard Scale in Conveying Line

Posted in: , on 14. Dec. 2007 - 10:21

Hydrated Lime - buildup of hard scale in conveying line

We have a positive pressure pneumatic conveying system for Hydrated (slaked) Lime installed at a waste incinerator. Dosing to approx. 200kg/hr is done via a dosing screw through a rotary feeder into a feeding tee.

Air flow is generated by means of a lateral channel blower and the conveying pipeline is a 50NB steel pipe. Bend radii are 1000mm. We are experiencing a rapid buildup of a very hard scale (it has to be chisseled out) inside the feeding tee as well as in the first 2m of the conveying pipeline.

Our immediate check was for water ingress and moisture content of the Lime but those all checked negative. Our pickup velocity is in the region of 27m/s at an material to air ratio of 0.8. We did have a larger blower installed for a while to increase the dispersion velocity of the Lime at the exit of the pipeline and the pickup velocity was around 37m/s at that stage. That seemed to have worsened the problem. During one test it took less that 3 hours for a 5-6mm scale buildup in the pipe.

A similar type of problem is being experienced at a Lime conveying plant with 80NB pipelines.

We are currently looking at the possibility of the system running through the dew point and moisture codensation taking place in the first section of the pipeline.

Has anyone had a similar problem and been able to identify the cause of the buildup. Do we alleviate that by using stainless steel pipes or even going to plastic piping?

Karsten Wodrich

Re: Buildup Of Hard Scale In Conveying Line

Posted on 14. Dec. 2007 - 09:57

This is a very common problem with lime injections plants if they are not designed properly. 37 m/s is more then double the conveying velocity required for these size pipes and I wont be surprised if this gets impact related build up in the bends. But generally for hydrated lime hard deposits formed are probably of calcium carbonate and they always form in initial few meters of the conveying lines. Reaction with H2O / CO2 is very fast. There are a number of cures for it and since this is free advice I will recommend rubber hose in the initial few meters preferably first bend also. It will not cure the problem but it will shake the build up off and the system will perform better and don’t forget to reduce the conveying velocity.

(not verified)

Pipe Build Up

Posted on 15. Dec. 2007 - 04:27

Do you have a heat exchanger and dryer installed between the blower and the material inlet point?

When pneumatically transporting moisture and heat sensitive and material, I always reccomend that this equipment be used.

Try Xchanger

turbo molch d_en-

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Bryair for a regenerative dryer

turbo molch d_en-

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You can also try pigging the line. Information attached.


turbo molch d_en- (PDF)

Ca(Oh)2 Build Up

Posted on 25. Dec. 2007 - 01:04

The problem you are faced with is (as Mantoo said) wellknown since about 25 years when we discovered (during my employeement at Steinmueller) that high velocities inside pipes combined with low material/air ratios cause buildups by CaCO3.

The nature of this phenomen is, that CO2 of air reacts with Ca(OH)2.

We made same trials and have analysed carefully the composition of buildups which contains more than 65% of CaCO3.

The behaviour of hydrated lime at low velocities and high material/air ratios is completly different. There is no build up if you unload a truck, for example.

To avoid buildup:

1.) decrease air velocity to 14 - 16 m/s.

2.) keep material/air ratio as high as possible

3.) use rubber hose for the entire piping

4.) use a minimum diameter of 80NB.

There is no need for airdriers in this case.

Regards from Cologne

Klaus Schneider


Hydrated Lime Conveying

Posted on 3. May. 2010 - 11:35

Use of hydrated lime to mitigate SO3 in coal-fired electric power plants is becoming very common now in the U.S. I found out about this Forum accidentally while doing a Google search.

We are having similar problems with hardening of lime within 6 to 10 meters from the rotary valve discharge. We are not treating the air (i.e., no drying to remove moisture). I have read quite a bit about the formation of CaCO3 based on reaction with CO2. Howeer, I also understand that in order for the CO2 uptake to happen, the lime particle needs to absorb moisture first. CO2 is soluble in water, and will readily be absorbed by wet lime. I read this in the following paper:

Biaohua Chen, Mary L. Laucks, and E. James Davis, Carbon Dioxide Uptake by Hydrated Lime Aerosol Particles, Aerosol Science and Technology, 38:588-597, 2004

I have an electronic copy of this paper as a "pdf" file but I am not able to attach it because the instructions for file upload gives a list of the file types and "pdf" is NOT one of them. I will be happy to send it separately as an email if anyone is interested.

Based on the discussion presented in the above publication, I am inclined to conclude that drying the air will help with the prevention of CO2 uptake. Does anyone have any comments about my conclusion?

All our ducts are metal, I see that one of the recommendations is to use rubber hose. What is the basis of this recommendation? Does it have anything to do with static electricity and accumulation caused by attraction of charged particles to one another?

Finally, one of the recommendations is to use minimum 80 mm nominal bore. I would like to understand the basis of this recommendation.

I would deeply appreciate your reactions to the above questions.conclusions.

Thank you.

Ramesh Kalagnanam

Progres Energy

Raleigh, NC, USA

Hydrated Lime

Posted on 3. May. 2010 - 11:59

Namaskar Ramesh Kalagnanam

A pipeline pigger with scrapers is your answer and easy to implement.

As for an air dryer you would be money ahead if you simply diverted some of the exhaust exiting the stack into the air inlet for the blower to solve the moisture issue as slaked hydrated lime loves moisture and then clogging which is why its so well used in waste water treatment plants for creating sludges in anerobic digestion sewage treatment plants.

Installing a pipeline pig launcher and reciever for any pipe is easily done and pipeline pigs of all sizes are used world wide. in your case a cable pig is probably going to be the fastest way to clean your delivery piping.

for that matter a pig pushed and pulled with rods or a larger length sewer cleaning tape would work also or a very small 12 volt winch,

type pipeline pigs in your browser nd you will find a wealth of information and many firms that do the work.


Re: Buildup Of Hard Scale In Conveying Line

Posted on 4. May. 2010 - 09:55

Dear Ramesh Kalagnanam,

From your thread, I understand that you are operating a pneumatic conveying installation for (quick)lime (CaO) and that you have scaling of Calcium Carbonate (CaCO3) in the first section of the pipeline.

The first section of a pneumatic conveying installation is the zone where water condensation from the air can be expected, as the pressure is there the highest and the air is cooled down approx. to the lime temperature.

A pneumatic conveying calculation of the installation can show this possibility.

The original humidity of the atmosphere air and the Solid Loading Ratio as well as the conveying pressure are the key parameters to perform the chemical calculations whether CaCO3 can be formed.

Replacing the first section by rubber hose does not prevent the formation of scaling but by the constant movement of the rubber hose, the scaling is broken and removed into the system.

As I understand, Raleigh has a humid subtropical climate (Wikipedia).

Is there a pattern in the formation of scaling with the seasons? (dry versus wet)

If you like, I can calculate your system and the amount of possible water condensation.

Therefore, I need the installation data.(pipe routing, compressor, conveying pressure, conveying rate, particle size)

If you prefer, you can use the private messaging function of this forum.

Have a nice day



Hydrated Lime

Posted on 6. May. 2010 - 03:45

To: lzaharis - Namaskar to you also. Thank you for yoru reply. Thank you and I will do a search on pipeline pigs.

To: Teus Tuinenburg - Thank you for your reply also. We are not using quick lime (CaO), rather, we are using hydrated lime, Ca(OH)2. This system was placed in service, Feb 14, 2010. Therefore, we do not have much information about seasonal variations. I will put together the information you indicated would be necessary todo the calculation and send it to you very soon. Thanks for your generous offer.

All the best to both of you.

Ramesh Kalagnanam

Re: Buildup Of Hard Scale In Conveying Line

Posted on 6. May. 2010 - 04:35

Dear Ramesh,

The chemical reaction involved is for hydrated lime:

Ca(OH)2 + CO2 --> CaCO3 + H2O

Resulting in:

74 kg Ca(OH)2 + 44 kg CO2 --> 100 kg CaCO3 + 18 kg H2O

The atmospheric air contains approx. 582 ppm nowadays

For 44 kg of CO2 we need 44/582*10^6 = 75601 kg of air.


74 kg Ca(OH)2 + 75601 kg air --> 100 kg CaCO3 + 18 kg H2O

From the pneumatic conveying data, it is possible to calculate the mixture of Ca(OH)2 with the air (In fact the SLR) and how much CaCO3 can be formed in that mixture per unit of time.

Assuming that this is the only way that CaCO3 can be formed and that this takes place in the first few meters of the pneumatic conveying system and within the available time.

I will wait for the pneumatic conveying installation data.

Have a nice day



Re: Buildup Of Hard Scale In Conveying Line

Posted on 7. May. 2010 - 02:55

Dear Ramesh,

Extending the use of the formula to a pneumatic conveying system:

74 kg Ca(OH)2 + 75601 kg air --> 100 kg CaCO3 + 18 kg H2O

With the applied conveying air mass (airmass) in kg/hr gives:

74/75601*airmass kg Ca(OH)2 + 75601/75601*airmass ---> 100/75601*airmass kg CaCO3 + 18/75601*airmass kg H2O

With airmass = Capacity/SLR this becomes:

74/75601* Capacity/SLR kg Ca(OH)2 + 75601/75601* Capacity/SLR ---> 100/75601* Capacity/SLR kg CaCO3 + 18/75601* Capacity/SLR kg H2O


Capacity = 10 tons/hr

SLR = 20

Air mass flow = 10000/20 = 500 kg/hr

Filled in:

74/75601*500 kg Ca(OH)2 + 75601/75601*500 kg air ---> 100/75601*500 kg CaCO3 + 18/75601*500 kg H2O


0.489 kg Ca(OH)2 + 500 kg air ---> 0.6614 kg CaCO3 + 0.119 kg H2O

In such a system, 0.489 kg Ca(OH)2 can form 0.6614 kg CaCO3 per hour, when all CO2 in the conveying air is used.

The air at the outlet of the pneumatic conveying system should then contain no CO2 anymore.

This can be measured.

For the formation of scales it is necessary that the randomly formed CaCO3 settles to solid crystals in a small area. Otherwise, the formed CaCO3 crystals are removed by the pneumatic conveying system itself.

It seems that condensation of water vapor is more likely.

Is there a chemical analysis of the scales?

Take care



Hydrated Lime

Posted on 7. May. 2010 - 04:35

Dear Teus:

I thank you very much for the active interest you are taking in trying to help me with a solution.

We have done a chemical analysis of the hardened deposit and here is what we found:

Gypsum - 6.74%

CaCO3 - 30.37%

Miscellaneous - 16.57%

We believe the rest is Ca(OH)2.

I am still trying to assemble the information that would be used to do the calculations you suggested.

Again, I thank you for your active interest in this problem.

Best regards.

Ramesh Kalagnanam