Fluoride in Pipelines

[agileengineering]

Hello all!

I'm trying to find out if there is any information available on the best material to use for fluoride solution (sodium fluorosilicate) in a pipeline.

We are experiencing an issue with the build of fluoride solution building up/scaling on the inside of the pipe. This pipe is steel pipe but the run of pipe from the dosing room is UPVC pipe (DN100), with build up unknown. I'm trying to find out if PVC was used for the full run, whether that would reduce scale up? Or any other material that may be better for fluoride solution?

 

[agileengineering]

Example of build up/scaling attached.

 

[StoneCold]

agileengineering
When I look at the picture, it looks to me like you are operating the system below the solubility of the fluoride solution. You need either a lower concentration or a higher temperature to dissolve the fluoride compounds. You might benefit from a higher velocity in the piping as well, try to get up to the 10 ft/sec range. I don't think it's piping material related because I don't see the fluoride corroding the walls of the pipe and making a rough surface.

Regards
StoneCold

 

[bimr]

It looks like your problem is calcium fluoride scale caused by either too little dilution water and/or too much hardness in the water.

Sodium fluorosilicate is a crystalline powder with limited solubility in water. It takes 60 gallons of water to dissolve 1 lb. In areas with high hardness, a water softener is often needed on the make-up water feed to minimize the potential for calcium fluoride scale formation. To avoid scale formation in hard water, either a water softener is needed, or other fluoride chemical should be used to minimize scale formation. 10-State standards specifies water softening if hardness exceeds 75 mg/L as calcium carbonate.

Unlike sodium fluoride, which has a constant solubility, sodium fluorosilicate has a solubility that varies with temperature, from 0.44 g per 100 mL (which is 1/10th that of sodium fluoride) at 32°F to 2.45 g per 100 mL at 212°F. When the water temperature is 77°F, the solubility is approximately 0.762 g per 100 mL, or 1/5th the saturated solution strength of sodium fluoride at the same temperature. Consequently, it requires a lot more water to form a solution.

PVC pipe is relatively resistant to scaling. A velocity of 3-4 ft/sec will also not allow any solids to settle in the pipeline.

Carbon steel piping should be avoided as it will be attacked. PVC, CPVC or Polyethylene piping are commonly used for fluoride solutions, but polypropylene can also be used. PVC has been more commonly used, but its disadvantages are that it is a rigid connection with many joints for potential leaking where polyethylene is a flexible piping.

Polyethylene and Polyvinylchloride are the two pipe materials best suited for fluoride solution piping. They are both formed from ethylene gas, which as 2 carbon atoms, but are polymerized into long-strand carbon chains. PVC has one alternate hydrogen replaced by a chlorine atom. Their chemical resistance is approximately the equivalent, but their physical properties are quite different. PVC is historically the pipe material used for most facilities, and it is a rigid thermoset which needs to be “glued” using a chemical solvent. It will have many joints which are prone to leaking and as a rigid pipe, is subject to fracture. PE is a flexible pipe and as thermoplastic, can be joined by melting the edges to a continuous run without any leaks or potential leaking. However, this requires specialized equipment and experience by the fabricator.

http://www.jpands.org/vol10no2/kauffman.pdf

 

[agileengineering]

Thanks Bimr - is there any literature or sources of information to back this [pipe materials best suited for fluoride solution piping] up so I can put a bit of a report and recommendations together?

 

[bimr]

Force Flow has some information:

http://www.forceflow.com/Fluoride/Equipment_notes.pdf