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Posted by Tony Conner on July 11, 2002 at 11:14:16:
In Reply to: return line chem treatment posted by mike on July 10, 2002 at 18:22:46:
The first thing to do is to make sure that you're getting all of the condensate back to the boiler that you can. That mechanical act will solve about 75% of your water treatment/chemical problems before they start. Alkalinity in make-up water breaks down into carbon dioxide in the boiler, and gets carried out with the steam. The CO2 dissolves in the condensate, and forms carbonic acid. This is what causes condensate lines to "groove" along the bottom half. No make-up, no CO2. No CO2, no carbonic acid. The other return line corrosion you can get is oxygen pitting. This will be from air drawn in through vacuum breakers. Many chemical guys tell the plant people to "get rid of those things". Chemically, this is correct. However, mechanically, this is often bad news. You can have things like outside air intake coils freeze and split, various and exciting water hammer events, etc. Corrosion from air drawn in from vacuum breakers is something that needs to be addressed chemically, or by using stainless steel return lines. DO NOT USE COPPER PIPING FOR CONDENSATE SERVICE. Copper in and of itself won't necessarily be a problem. Some plants have had copper return lines for years. However, if something else happens, like a chemical upset in the boiler resuling in lots of carryover, etc you can have copper being pulled off the piping, and carried back to the boiler. The copper can concentrate in the boiler, and plate-out, forming a galvanic cell. Very nasty.
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