Boiler Engineering Question

R Dougan

I’ve got an answer to a head-scratcher question which, I’m hoping can be confirmed or rejected by the experts-in-residence herein. It starts as follows:

If I replace all the water in my system during a complete flush-out, there will be a certain period in which my return piping is occupied by oxygenated tap water that will possess corrosive properties that will have an increased deteriorative effect on that piping as compared to boiler condensate. It will then take a certain period of operation for this volume to be replaced by condensate of a deoxygenated quality that will be less debilitative to this return pipe. (If this corrosion is negligible or there is no appreciable difference between the properties of fresh water and system condensate feel free to correct me, …but, if there is a difference, ...let’s take a trip.)

My boiler is rated at 358 EDR, dividing by 4 gives me 89.5 lbs./hr. of condensate (see pg. 45 of LAOSH by Holohan.)

Next, dividing my 89.5 lbs./hr. by 60 min./hr. to get 1.4917lbs./min

Then, divide the 1.4917 lbs./min. by waters’ density of 8.34 lbs./gal. to yield .1789 gal./min. of condensate produced by my boiler.

I can double check this with the .5 gal/min per 1000edr constant I found in some book somewhere (sorry, I can’t remember the source.)

(358edr/1000edr) x.5gal/min =.1790 gal/min. …rounding my previous result to the nearest thousandth, these are equal. Good.

Now, if I have a wet return of 98.5 ft. of 1 ¼ inch pipe, as measured from my Hartford loop connection to the water line of every wet return drip and given that 1 ¼ inch pipe has a volume of .011 cu.ft./ lineal ft., this would yield a total volume of 1.0835 cu.ft. of water in my return piping.

Converting this as follows, 1.0835 cu.ft. x 7.48 gal./cu ft. (for water) = 8.105 gal of water in the return piping.

Okay, hopefully everyone is still with me, if we take the amount of water in the return piping and divide by the rate at which the boiler can displace that water with condensate, this should result in the period it takes to completely change over the volume of water in the return line.

8.105 gal in the return piping divided by .1790 gal./min of boiler condensate =45.28 min of runtime.

Okay Experts, have I got this right? And if so, would this then mean, after I give my equipment a good thorough wand-style flush-out, that I can then be satisfied that after about 46 min. of runtime that I have deoxygenated my system to limit corrosion during an offseason period.

Hope to hear anything besides crickets, thanks.

Gsmith

the calculations appear to be correct so steaming for 45 minutes to an hour should provide one turnover of the boiler return piping. You probably need to add the volume of water in the boiler so that you completely de-aerate the entire water volume.

JUGHNE

Bearing in mind that if you only drained the boiler that would not drain the wet return.....but you said complete change of water?

R Dougan

@JUGHNE
>>Bearing in mind that if you only drained the boiler that would not drain the wet return.....but you said complete change of water? <<

...understood, I should have been more specific. I was including the return in my complete change of water. If piping work was performed, oils and dope will find their way into the return and ultimately into the boiler. I know this wasn't part of the initial posit, but it would be an example of when you would want to flush both the boiler and the return, ...don't you agree?

Having repiped my entire system last summer, I'm still getting contaminants in the boiler, ...small amounts at this point but they do accumulate over time and need to be removed. By draining the return I see a lot of blackish/grey water being flushed out. I know some may still stick to the walls of the pipe as it drains but quite a bit is removed this way. Eventually it will be completely cleaned out or get to the point where skimming will be the best remedy. With the amount of blackwater in the return I don't think I'm there yet, even this far into heating season and after an incredibly cold Dec/Jan.

So, ... is my mathematical solution valid or am I making a wrong assumption.

Jamie Hall

Looks like your basic arithmetic is correct. And as to the amount of corrosion that you can expect from a day or two even of new water... unless that water is really horribly aggressive for some other reason you'll not see any difference.

JUGHNE

Plus if you only boiled the water in the boiler without enough time for all of it to make the round trip you would have a certain amount of dilution in the cond water.

EBEBRATT-Ed

Think your way over thinking this. Run the boiler until the condensate returning into the boiler is warm. Run it 20 min longer and call it a day. Some new make up water isn't going to harm anything. It's continuously adding make up water that's the culprit

R Dougan

EBEBRATT-Ed said:

Think your way over thinking this. Run the boiler until the condensate returning into the boiler is warm. Run it 20 min longer and call it a day. Some new make up water isn't going to harm anything. It's continuously adding make up water that's the culprit

You're right, I probably am over-thinking this. Mainly because I suffered a waterline boiler failure a couple years ago due to too much make-up water over time. This was before I understood the problem inherent in having a small leak which required excessive filling to maintain the normal water level. (For anyone interested, the attached photo shows how a small pinhole leak will manifest itself with the staining on a floor joist.) This experience has probably contributed to me being a bit paranoid about corrosion in both the boiler as well as the piping. I assume that flushing a boiler completely has a different effect than adding make-up water in that there is not an increase in dissolved chemicals. (I also get that they are two completely different procedures for two completely different functions, but they do both result in new water replacing initial water that has exited the system.) If I understand the excessive make-up water danger correctly, its due to an increase in the concentration of the dissolved chemicals present in that water. This increase in concentration creates a greater propensity for the system fluid to corrode, ...mainly inside the boiler. Have I got that right? Flushing, eliminates this fluid of greater concentration (particularly chlorides?) and returns the system to the base level composition of the feed-water. Isn't this also correct? Sorry if this all seems very basic, but I'm trying to wrap my head around what is the greatest source of deterioration to a steam heating system. It also might be good info for folks that are new to steam systems and my save a premature failure. Thanks all for your comments.

Jamie Hall

Corrosion is an incredibly complex set of processes. The safest thing to say is that like taxes and a few other things it's going to happen. From the chemical standpoint, what you need to look at, though, is that you need to have a metal -- and you need to have something in solution which can accept electrons from the metal atoms to make them into ions, which may or may not be soluble.

The worst culprit for corrosion -- by far -- is oxygen. Any of the halogens (chlorine, iodine, etc.) will do, too, but not their ionized forms (such as chloride). But oxygen is pretty much everywhere.

From the standpoint of preventing corrosion, a completely closed system, such as many hot water systems, would be in good shape from that standpoint (they have other problems which are made worse by things like chloride, though) as their is a limited amount of oxygen present at the initial fill, and once it's gone it's gone, and corrosion will stop. One might think, then, that steam systems would go quickly, but they don't -- except in wet returns or parts of the boiler which are wet or damp. The reason for that is that water accelerates corrosion by removing the corrosion products (that's a bit simplified, but never mind). However, one can reduce the corrosion in steam systems by reducing the oxygen -- and by far the best way to do that is to minimize makeup water, which contains the stuff (the air breathed in by vents would be a problem if it weren't that most of the system is either dry or filled with steam most of the time).

Total dissolved solids aren't usually as much of a problem for oxidation corrosion, although they may cause problems in other ways.

So fill the boiler. The initial boil will get rid of most of the oxygen and the corrosion will be much reduced. Flush out from time to time just enough water -- but no more -- to get rid of gunk or, if there is a foaming problem from total dissolved solids, to control that. But no more. It's better not to drain and refill, as that just starts the cycle all over again.

R Dougan

Thx Jamie.

brandonf

Just me thinking outside of the box but does anyone think it's a good idea to rig a large reducing coupler to act as a funnel on the feed water line to pour in distilled water? Logically it might help with water quality inside the boiler. May not be too much of a hassle on systems that only need new water every few weeks. Just my "Average Joe" thoughts ......

BobC

You can buy boiler water at some wholesalers and it's worth doing if your area has known water problems. Unless you have a water quality problem some flushing and a few skims should suffice. Some of us then add a Steamaster tablet to raise the PH and help reduce corrosion.

Bob