Water treatment for iron boiler?

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Ray Wangler
Posts: 53
Joined: Wed Dec 10, 2008 5:55 pm
Location: Bismarck, ND

Re: Water treatment for iron boiler?

Post by Ray Wangler »

I work in an oil refinery and asked our water expert to give a short treatise on traction engine boilers (obviously steel) that would run at our show in Rollag MN, 4 days a year. Here's his answer and it's all scalable to model boilers for both size and amount of time operating. I found it interesting, hope you do as well.

Water Treatment Philosophies for Steam Traction Boilers (assuming surface water source).

Surface water in central ND has approximately 250 ppm of hardness, mostly in the form of Ca and Mg. It’s reasonable to guess that MN lake water is in that range. Add a bunch of hardness to the water (normally found in well water) and the discussion changes. Given your steam boilers nominally run at 150 psi of saturated steam, the temperature is nominally 360F (don’t split hairs, they aren’t “high pressure boilers” in the real world). Assuming the boiler volume to be 300 gallons and you use 500 gallons/day X 4 days, without blowing down, that’s nearly 7 cycles. Meaning 1750 ppm of hardness on the last day. That’s a lot. No treatment is going to “fix” that. So I think the question really is what measures can you reasonably take to preserve your boilers.
Without question, the first answer is treat the water with an RO, fixed bed ion exchange unit, zeolite softener………..anything! Understanding that this application makes that choice nearly impossible, what else can you do?
Daily blowdowns of 50 gallons, (assuming a ¾” diameter hole open to atmosphere at 150 psi for 15 seconds) is about a 10% reduction in hardness (a little more as the # of days goes up, but that’s close enough.) The calculation is linear so it looks like this:
800 gallons * 250 ppm = 200,000 grains of hardness
250 gallons * 667 ppm + 50 gallons *250 ppm = 179,000 grains of hardness (this is effect of getting rid of 50 gallons at 625 parts hardness and adding 50 gallons at 250 parts hardness).
179000/200000 = 0.896 or about 90% of the grains that would be there if you didn’t blow down.
So far, the discussion above has been hardness in water and scale. If the PH of the water is between 4.2 and 8.2 (very likely without treatment), you’ll be in the carbonate (Phenolphthalein) alkalinity range, meaning precipitating out Calcium Carbonate (worst of the scale). If the PH drops below 4.2 it’s free ion carbonic acid which is extremely corrosive to carbon steel, meaning your losing a lot of boiler. The best case is holding the PH between 8.2 and 10.2 where you have hydroxyl alkalinity and the formation of magnesium hydroxide (a soft, fluffy, washable scale). The calcium hydroxide is pretty soluble and stays in solution.
In conclusion, blowdown like you are. More is better, maybe not required, but better. Use chemical treatment. Good boiler treatment will contain an oxygen scavenger (bisulfite), a polymer that coats the steel and resists scaling, and has PH buffering, maybe borate, to keep the PH above that magic 8.2.
A little sodium or potassium hydroxide in the water wouldn’t hurt as well, but be sparing with that so as not to overdo it. An ounce or two of 50% in 100 gallons is enough.
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