Water Treatment for Closed Boiler System 1

[moltenmetal]

We're considering a thermosiphon reboiler, carbon steel construction with an electric element, providing saturated steam at 260-300 C to a coil in a pilot plant process vessel.

The water/steam system is totally closed, and will only be opened if leakage has depleted the water sufficiently. There are very few joints in the system so it shouldn't be opened often.

Any recommendations on a passivation procedure prior to protracted operation? Any recommendations for water preparation? Don't direct us to Nalco or the like, because they're not anxious to bother answering questions like this if there's no ongoing need for chemicals.

 

[unclesyd]

Do you have enough heat load to justify a vaporized oil system, such as Therminol or Dowtherm. This way your could just use the latent heat and and keep a low pressure system.
Our plant operates in this range on a very large scale. Our pilot plant has numerous small systems operating in this range.

 

[Metalguy]

You can use tri-sodium phosphate (TSP) to raise the pH tp ~10.5, and kill the oxygen with a little sodium sulfite.

These chemicals are cheap and have worked well for many years.

 

[moltenmetal]

Metalguy: the TSP and sulphite was exactly what I was looking for. Any worries about the sulphate from the decomp of the sulphite? And is there any benefit to pre-boiling the thing with phosphate and soap, prior to filling with better-quality water?

Unclesyd: Good suggestion. We've considered it, but there's too little space inside the pressure vessel to fit a big enough coil to so we could get away with using hot oil vapour instead of steam- and too much of a problem if (when) the process vessel coil fails if we go that route.

 

[Metalguy]

I'm not aware of any sulfite-sulfate failures in boilers, but I haven't performed a lit. search. The amount of sulfite used is very small. I don't think you need a pretreatment unless the system is dirty/oily, etc.

What can be important is to sterilize the system by heating it up very soon after filling it. Some of the worst bacteria involved with MIC are the sulfate-reducers (desulfovibrio desulfuricans--try saying THAT fast!), and the don't need oxygen to set up their drill presses on your system. In fact, oxygen makes they go dormant.

They'll drill thru 304/316 faster than a union worker on group-piecework!

 

[WitchDoctor01]

Suggest that you consider sodium nitrite and borate mixtures.