What do you use for disolved oxygen reduction in a pressurized system?

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Garth B

Member
Feb 25, 2010
42
New Brunswick
I wasn't sure if I needed to get rid of Oxygen in my pressurized system untill recently getting my wife to measure it with a DO probe from her work. It measured 0.8mg/L, american boiler manufacturer association suggests 0.04max and another source says less than 0.1mg/L for hot water systems.

My system has a few air vents at high points and a scoop, I was hoping that would be enough as I had read that some rusting and high temps would get the remaining Oxygen out.... apparently it dosn't get enough out.

Which would be a better investment; an air eliminator (spirovent ect.) or chemicals?
Which do people on here use with success?
 
It would be best to do both, install the air eliminator & continue to monitor & treat your water accordingly. Closed systems should need less treatment over time depending on how much make up water your system needs.
 
A common chemical used in OWB's to scavenge O2, and I know of no reason not to use it if needed in a pressurized system, is sodium sulfite. My notes show that it is pH9, also in a good range for boiler water. I have on hand a bottle, nearly full, which I no longer need. PM me if you want to talk about that.
 
How long has your system been up and running? And how long were you able to keep the high temps up for? Is high close to 200? If you are still getting the system up and running, you might just need to keep it running hot for a bit until the air comes out of the water. Also, are you making anything up to the system? Any leaks? Im just wondering if its staying high because of newly introduced water with oxygen in it.
 
The storage was installed and filled in sept. I have had it up to 170F a few times and 150f a few times, maybe it does just need a bit more time? Do I need to heat it up to a higher temp?

I could only find one place localy that had chemical treatment, it was Masters brand and 40$/gal.... sales guy said my system volume would require 16gal, he only had six but that is way too much to spend I think.
 
I honestly am no expert on boiler water treatment, so keep that in mind. Others here know much better than I.

If it were my system, I would wait a little bit until I could keep the system up at a higher temp for an extended period of time before I would spend the money on chemicals. It may depend on your system, which could be designed to operate on low temps, but I would want everything up to at least 180 for a while before I would go chemical. Running things good and hot with zones calling I would think can only help.

With the colder temps coming soon, getting your system hot shouldnt be too much of a problem.
 
jebatty said:
A common chemical used in OWB's to scavenge O2, and I know of no reason not to use it if needed in a pressurized system, is sodium sulfite. My notes show that it is pH9, also in a good range for boiler water. I have on hand a bottle, nearly full, which I no longer need. PM me if you want to talk about that.

IMHO, sulfite chemicals work but my working experience on various commercial closed-loop hydronic systems is that the sulfite seemed to always be depleting, and we had to add more. On the other hand, I presently work with molybdate-based inhibitors that are more tolerant of dissolved air / oxygen as they coat the inside of the pipes, and generally provide cleaning and aggregating capabilities depending on the balance of the chemicals in what you get. The following URL is just an example of what I am referring to. To not add chemicals with the concept that they are too expensive is potentially a very bad choice. This is a lot like never changing the oil in your car. It may work for a while, and save money, but then all of a sudden ...

http://www.heatlink.com/en/product/heatlink-heating/01205/molybdate-based-corrosion-inhibitor

On the first application if this type of inhibitor, the moly levels will go down as they plate out on all the interior surfaces of the piping. After some operational time and the presence of sufficient levels of the moly in solution, the measured levels will become stable and the system will remain protected for a potentially long time. If you decide to flush in the future, especially if you use a boiler flushing chemical solution, you may need to add the moly again. So, it is best to ensure the entire system is as clean as you can get it before adding the expensive chemicals.

Also, I have seen many (professionally engineered) systems where there where excellent air-vent and scoop arrangements that were either incorrectly placed in the piping loop or the flow was too fast to be operationally effective. Good luck.

As always, the John Siegenthaler's book on hydronics can help anyone learn lots more on the finer points of hydronics and heating in general.

http://www.hydronicpros.com/publications/

I am not affiliated with any of the above URL links!
 
When I switched to pressurized storage, I first did my own pH adjustments, alkaline adjustments, and O2 treatment. Then I used Mike at Precision Chemical to recommend a pre-treatment and then a boiler water treatment chemical mix. Cost was a few $100 but I thought it worth the long term investment.
 
A good air elimination device is a must. Be sure the system is leak free first, clean and flush, fill with good quality water. Hydronic treatment is always a good idea. A tight system without any additional fill water should be fine also.

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And just to clarify, I wasnt trying to propose not adding any chemical treatment to your system. Its probably just a good idea to let it run for a while at the higher temps it may see during the peak of the system to drive any air out before you go the chemical route. That way you can get a more accurate reading of what you may need.
 
Thanks for the advise! Think I will end up following the same route as jebatty, I didn't do a very good cleaning or flush on the tanks before i started running it, so I would like to do that this summer, no point in adding chem just to dump it out. I have used naoh for ph allready, and will add some sodium sulfate if the o2 level dosn't go down for this season, if I can find boiler chem treatment for under a few hundred I will go that route. I am all for preventitive maintenance, but 650$ for chemical treatment is pretty steep if you can achieve the same protection for 50$ using naoh and sodium sulfate.

I am curious to know what thermal plants use for chem treatment, I know they often have large complex water treatment rooms with sandfilters, softeners, clarifiers ect. but what chemicals get added? Are there any power engineers on here? even though they are making steam they still have lots of warm water flowing through steel condensate lines... hopefully they dont spend 0.75$ to treat every gallon of water they put in.
 
Garth B said:
I am curious to know what thermal plants use for chem treatment, I know they often have large complex water treatment rooms with sandfilters, softeners, clarifiers ect. but what chemicals get added? Are there any power engineers on here? even though they are making steam they still have lots of warm water flowing through steel condensate lines... hopefully they dont spend 0.75$ to treat every gallon of water they put in.

/Snip; I am currently working as a power engineer in a medium size thermal plant with natural gas boilers in a regular power plant and a separate Bio-mass power plant. The bio-plant is a Nexterra design, if anyone is interested. Where we have steam, the systems are all for humidification or sterilization uses and so the steam is non-recovered. We therefore use water softeners and manually blow down the mud drums of these boilers daily. The main power plants are hot-water plants (240F at 70 psi) and are configured as closed loop systems. Except, of course, for the leaking heat exchangers in several mechanical rooms. However, we use a molybdate based chemical that treats several hundred thousand gallons of water, both on the thermal heating side and the closed-loop cooling water from the chiller heat exchangers. This system has always been molybdate based, for the last 17 years and no problems related to that have been experienced.; except, of course, for those times when the water chemicals were not maintained well, but that is another story. For the record, I am also a licensed electrician, which in this job seems to get me more work than my coworkers, but at least I am employed. Cheers.
 
I work on a large marine steam plant which is basically the same as a large shore based one except for the propulsion aspect. A steam cycle is a bit different in that make up water is continually added. In my case maybe 20 tons/day. Even though we use distilled water in the boilers we still have to "blowdown" when the "Total Dissolved Solids" level reading is up. We maintain the ph with caustic soda. the sludging out with trisodium phosphate and the O2 with hydrazine. The O2 is particularly corrosive at high temps in this situation and it get into the system through condensers under vacuum and the make up water. I brought up this subject with a friend who ran a nuclear powered ship. How do you test radioactive water? You don't. The water is ultra purified (and I presume deoxygenated) before it is introduced into the reactor. My point? I don't know.. LOL No I guess I would say if you use a good quality water filling your system that has proper air vents/scoops over time, a closed pressurized system should last a long time (as long as you don't have leaks which necessitates make up water all the time). My mother's hot water boiler has been in her house 40 years and doesn't have any problems.
 
lol well that answers my question on what is used in large boilers for o2 control!

I don't think i will be running out to buy hydrazine though, sounds like very unhealthy stuff to be handled by a novice. I will test my O2 again mid heating season to see where its at, I may have introduced some air into the sample while getting it out of the system as well, kind of hard not to, i took the water out through a submerged tube in the sample container, but there was still air in the container befor sampling.
 
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