heating coil material for use in Sodium Hypochlorite

[russiangoat]

Hi,
I'm trying to nail down a good, cost-effective material for heating a vat containing sodium hypochlorite, for the purpose of membrane cleaning. The temperature is only going to be up to 30 degrees Centigrade. I'm getting mixed signals on 316 steel, some information and companies highly recommend it, some say its not recommended. Any help would be appreciated.

 

[ApC2Kp]

The 30 deg C temperature would be no problem for 316SS, except you must consider the temperature of the heating media. Hot water temperatures could be marginal for 316SS, and any steam temperatures would rule out the use 316SS for the hypochlorite solution (unless very low concentration).
I would suggest using a steam jacketed (or heat traced) teflon lined pipe to heat the solution, if there is a circulation loop from a pump. You did not mention the vat's material. Could the vat be heat traced?

 

[russiangoat]

ApC2Kp,

Finally got some more information on the situation.

The vat is made of concrete, and is not a candidate for heat tracing. The hypochlorite solution is 12%, and the heating media is going to be water, not steam. Also, the heating coil will only be exposed to the solution for a week at a time, about four times a year.

So you don't think the 316 SS will have corrosion problems with the NaOCl at 30 degrees? I'm thinking that the limited exposure to the NaOCl will cause insignificant corrosion problems, but they don't want to have to replace the piping very often.

 

[moltenmetal]

If you have problems, they'll be at the liquid/air interface. And they'll depend entirely on your hot fluid temperature, since heat transfer conditions suggest that that the coil metal temperature will run closer to the heating fluid temperature than to the temperature of the hypochlorite solution in the vat. If your coil's kept at 30 C, I doubt you'll have any problems if it's kept completely immersed- but by definition, that won't heat your tank to 30 C. If your heating water is kept below 60 C, 316SS will probably last in your duty provided the coil exterior is cleaned after each week-long use. Beyond 60 C, consider titanium, or a large teflon-coated coil.

 

[glennpan]

Russiangoat,

Do NOT use SS with Hypochlorite. It is exceptionally bad for that service. Not only will it likely corrode in a week, but the metal contamination that it will cause in the Hypo will make the Hypo much more agressive as a stress cracking agent down the line in your piping, tanks, and pumps.

A PTFE coated coil is the best way to go and less expensive than using exotic alloys.

Glenn

 

[MJCronin]

This is a very interesting thread....

I am curious... Who has used and who makes these teflon coated coils and what are the operating limitations ???

Do these coils last very long and what is the primary market within the process chemical industry ???

Does anyone have any websites or weblinks that you can direct me toward ???

Thank You....

MJC

 

[glennpan]

MJC,

There are several manufacturers for these types of products. We have used the ones made by Process Technology.

http://www.process-technology.com

How long the coil lasts is application dependent. They are commonly used in metal finishing.

Glenn

 

[russiangoat]

First I want to thank everyone for responding.

Second, the plant used to just paint some SS pipe they ran through the hypochlorite vat. I want to know if anyone has opinions on whether painting the SS pipe would help prevent corrosion/metal contamination, or if that is just a band-aid solution. Also, would metal contamination affect/damage the membranes being cleaned?

Third, I was wondering if Glenn, or anyone else, could tell me whether PTFE is usable with citric acid. The plant would like to use a single pipe to heat both a vat of hypochlorite (for organic cleaning) and a vat of citric acid (for mineral cleaning), so if PTFE was going to be the only answer for the hypo, it has to work for citric acid too.

Thanks again for responses.

 

[glennpan]

Russiangoat,

As you stated, painting the SS would be a bandaid solution.

Without knowing anything about the membranes, it is impossible to say if they would be damaged. What type of pressure will they be subjected to? What material are they constructed of?

PTFE should work just fine with a Citric acid solution.

Glenn

 

[7604]

Hi there

You should not use 316 ss to work with sodium hypochloride. It will attack the steel even at low temps. Try to use higher grade alloys such as Inconel 600 or even Monel 400. You'll seee the difference.
Cheers