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Calculating volume of water that can be lost in the form of steam

NYCDave
NYCDave Member Posts: 72
Hi All,

We recently discovered a hole, above water, in our oil-fired steam-generating boiler after noticing that the boiler is taking in a lot of water from the automatic feed meter. It is currently using 30-40 gallons a day of new water. This made me curious as to how much water the boiler could actually lose in the form of steam, and I wanted to see if people thought my math and reasoning were valid.

The boiler is a Turnham V86, and according to the specifications it generates around 200 MBH, or 200,000 BTUs/hour. This corresponds to about 6 BHP (boiler horse power) or around 200 pounds of water-into-steam/hour. 2 pounds is about a kg or one liter of water, and there are about four liters in a gallon, so 200 pounds/hour is about 25 gallons/hour.

So, it seems if our boiler was running flat out, it could turn 25 gallons of water into steam every hour.

Now of course, even with a hole in the boiler, not all the steam is getting out (our radiators still get warm, the house is still warm) and of course, the boiler is not running all out. In fact, I'm not sure how often it runs... if I had to guess, it could be as little 10 minutes per hour. But even if say just 6 minutes/hour (10% duty cycle) that means 2.5 gallons/hour of steam, or 2.5x24=60 gallons/day of steam. If we were losing half of that to our hole, that could account for the 30 gallons/day of added water.

So the final question is, how much of the steam that the boiler generates could be escaping through the hole...

Any opinions out there on that question, or on the rest of the logic?

I figured this discussion would be relevant to anyone else who has to deal with a boiler that's taking a lot of water. We've looked as hard as we can for a leak elsewhere (in the returns, etc.) but haven't found anything else. We'll eventually replace the boiler, and if the water intake does not decrease then, we'll know we have another leak somewhere, but until then I'm thinking the hole can explain even this much water loss?

Thanks!!!

Comments

  • Fred
    Fred Member Posts: 8,518
    edited January 2017
    @NYCDave , the assumption I would question, in your analysis it the Run time per hour for the boiler. Assuming it takes about five minutes for steam to get to the end of the mains and maybe another 3 to five minutes for it to push air out of all the radiator run-outs/risers, it probably takes two to five minutes for it to start condensing in the radiators (maybe a full five minutes or more to heat all the way across, depending on the size of the radiators). I would say the average (typical) heat cycle on most boilers is between 20 to 30 minutes per hour and possible as much as 40 to 45 minutes per hour as you near design days. I believe you water loss is at the hole(s) in the boiler but it is always good to walk the basement and inspect all the piping periodically.
    Missing assumptions probably include the size of the hole(s) and the amount of steam that could escape through that hole, relative to the amount that actually moves into the mains/radiators and condenses during each heating cycle. A 1" hole can vent 11 CFM of air @ 3 Ounces of pressure
  • NYCDave
    NYCDave Member Posts: 72
    Thanks Fred, you comments are insightful, as always!! I agree the boiler is probably on more of the time, I was being conservative on purpose. Great to have some numbers on venting rate!! so a cubic foot is 7.5 gallons, and the volume of steam is 1700 times that of liquid water... so 11 CFM of steam is about 0.05, or 1/20th, gallons/minute of water (11*7.5/1700). So 20 minutes would vent 1 gallons worth of water through this 1" hole. If the boiler was on 20 minutes/hour (still conservative) this could account for 24 gallons per day...

    It's hard to say how big our hole really is... and there are probably two... I took some photos with my cell phone (attached, these are two adjacent sections, first one just shows corrosion, the 2nd you can just see the edge of the hole), but they mostly show the corrosion, hard to see the actual hole...

    I'm guessing you are right and that the hole in the boiler can explain the water loss. Am kicking myself for not keeping closer tabs on the water meter for longer (wish these things had a memory!!! Would be a good product feature if someone wants to design a new one). Guess I understand better now why some people on here are not very positive on the automatic feeders.

    In any case, happy to assume for now it's the hole. Would hate to install a new boiler only to find out we're still using 40 gallons per day, which would presumably rust it out in short order...

    That reminds me of another question!!! If I fill my boiler to the top of the sight glass, and turn it off for 24 hours... if there was a leak in the return lines, would that lead to a drop in the water level inside the boiler? I'm not exactly sure how the hydrostatic pressure works in these systems...

    Thanks as always!

  • Fred
    Fred Member Posts: 8,518
    Keep in mind that the water won't drop below the level of the Hartford loop (That's why that loop is there) but If you mark the current water line on the sight glass,( above the Hartford loop) or fill it up to near the top, and the water drops down, it probably indicates a leak in a wet return or it could mean another leak in the boiler block. If it's in the boiler block but below those holes that you have identified in the top of the block, you should see water around the boiler, on the floor.
  • NYCDave
    NYCDave Member Posts: 72
    Ok, this made me think of another probably silly question. Are the wet returns essentially full of water all the time? So that only excess water returning from steam can get over the Hartford looping back into the boiler? I guess this must be the case or the water would never get over the loop… Does this mean when you first start a new system up that you have to fill up The whole volume of the wet return lines before water starts coming back to the boiler? We have probably a good hundred feet of returnpipe running around the perimeter of the house, I imagine it's a sizable volume of water to run through the boiler before those fill up?
  • Fred
    Fred Member Posts: 8,518
    Yes, the wet return is always filled with water, up to the Hartford loop. The vertical drops are full to a level equal to the boiler water line. Typically your Hartford loop is a couple inches below your boiler water line so when you fill the boiler, it will also fill the wet return. In any case, before a new boiler is commissioned, it should be skimmed and the over-filling of the boiler for that process will also fill the wet returns.