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What makes a steam boiler fail?

7/21/2016 | 778 Views | 0 Comments

I started my career in the heating industry in 1970, when I was 20 years old. I was fresh out of college and my number in the Draft Lottery that year was 355. My father was in the business, so it seemed like the right thing to do at the time and I have had no regrets.

There weren't many young people in the heating business back then. I came to realize that this was because of Viet Nam. The young people had either gone to war, or gone to college, and not many who came to the end of either of those paths chose the heating industry as their next career.

The old-timers who knew steam heating boilers and steam heating systems had learned their business during the '40s and '50s, and those guys were wonderful professors. The ones I met when I was very young loved to talk about what they knew, and I loved to listen. These men had few students, though, and as the '70s rolled around, most of the professors had retired or died. The next generation was left to figure out the steam on their own. 

In 1973, the first oil embargo hit America. The price of fuel soared and many people looked to get rid of their old coal-converted steam boilers. They wanted something more efficient and the boiler manufacturers met the public's need by introducing smaller steam boilers with higher efficiencies. They fired these little boilers hard, and the steam left them much faster than it used to leave the older, larger boilers. And it left through much smaller holes, holes that the Dead Men never would have considered wise. I know this because they wrote about it in their books.

Many of these boilers that arrived in the '70s were actually hot water boilers, trimmed for steam. I don't think the people making the decisions at the time fully understood what made for a good steam boiler. It's understandable, though. Steam heating had been a profitable, non-growth business for decades, and many of the engineers who worked for the boiler companies during the '70s had never designed a steam boiler. They had been selling the designs of the Dead Men, and everything had been fine, but now they had to come up with something small and efficient in a hurry, and that's when the troubles began.

I remember one oil-fired steam boiler from the late '70s that had very narrow sections. The water inside those tight sections would fill with steam and rise to a level higher than the level in the gauge glass. When the boiler reached high-limit on the presssuretrol's setting, the burner would stop, which caused the steam that was rising through the boiler water to collapse. This drove the waterline inside the boiler to a point lower than the waterline inside the gauge glass. And as the different water levels tried to equalize, the automatic water feeder would kick in. Within a day or so, the boiler would be flooded.

From the outside, it looked like the water was backing out of the boiler, so the installers started adding check valves to their wet returns, but this did nothing. I visited many of these jobs. It was a problem created by a birth defect in the boiler. The contractor and the homeowner were left holding the bag.

The manufacturer stopped making this boiler, and that was a good thing. But before that happened, many of the installers ordered larger-than-needed boilers and then down-fired them. This lessened the amount of steam in those narrow boiler sections and seemed to solve the problem, but it often left the homeowner with a less-efficient boiler. So what's the point?

Years went by, and better steam boilers arrived – ones with wider sections, larger (and more) openings that allowed the steam to leave at a lower velocity, and bigger steam chests. But then, even some of these began to fail within a few years, and many from holes at the waterline.

I'd hear about the holes often on the Wall at Contractors and homeowners would say that the boiler was only a few years old, but now it had a hole in it, and right at the waterline. Some homeowners talked about more than one boiler suffering the same fate.

I went back to the old books and confirmed that a leading cause of this is often too much fresh water and the oxygen it brings along with it. Every steam system is an open system, and excessive oxygen will always cause iron to rust and eventually fail. If the piping system is leaking (and many old steam systems have buried returns), the automatic water feeder will do its job regularly, and that leads to holes at the waterline.

Hydrolevel introduced their VXT feeder, which has a water-meter function, to help with this problem, and it has helped, but some of those jobs I heard about had no leaks that anyone could find, and that had me wondering.

I was at a party not long ago when a rep from a boiler company sat next to me at the bar and told me that they had figured out what was causing the holes in the boilers on those systems that weren't taking on a lot of fresh water. This company had hired an independent lab to analyze the metal of boilers that had developed holes at the waterline. The problems were clustered in a geographical area, which made the company and the laboratory suspect that there was something in the water in that area that was causing the problem. It turned out to be high levels of chloride.

Now this is not chlorine, which some water companies use for treatment; this is chloride, or to put it more simply, salt. Chloride can come from the runoff after deicing trucks treat the roads during the winter, or it can come from natural sources. Once it gets into a boiler, though, it can cause problems, particularly if it comes in contact with abnormally hot surfaces..

Most of the boilers we use for steam heating nowadays have cast-iron sections with pins for efficient heat transfer from the flame to the water. These pins are also in the upper part of the boiler, up there where the steam disengages from the water. These boilers are also fired hard to get the most steam from a relatively small appliance. And that's where the problems start.

The combination of the chlorides in the water, and the very hot pins in the steam section of these modern boilers (it's too bad they have to be so hot), creates a condition known as "graphitic corrosion." You can Google that and learn more, but if you're not up for that, just know that graphitic corrosion eats cast iron. You wind up with holes at the waterline through which you could toss a tomcat.

The high temperature is the trigger that starts the corrosion process and those pins are as hot as they are because modern, vertical-flue pinned-section steam boilers are more efficient than the ones they replace. But if you look at the Dead Men boilers you'll notice that none of them had pins. They had smooth, ribbed surfaces and a lot more metal, and because of this, they weren't as sensitive to chlorides in the water.

Some manufacturers tried to solve this corrosion problem by using more sections for the same Btuh rating. This lowered the temperature of the pins in the steam section, but it also made the manufacturers who tried this less competitive. And as it turned out, it only prolonged the graphitic corrosion problems.

The chlorides are the problem, and the excessively high temperature on the pins is the trigger. The problems are widespread, occurring in the western and southern suburbs of Boston, the northern and northwestern suburbs of Providence, in New York State, on the eastern side of the Hudson River, from Albany to Peekskill, and in many areas of Long Island – all places with lots of steam heat.

So if you're using steam boilers of this type and you're having problems with corrosion, look to the water. Are you taking on a lot of feed water? The only way to know for sure is to meter it. Is the pH of the water between 7 and 9? The only way to tell is to test it and to monitor it.

Once you've checked those things, have the water tested for excessive amounts of chlorides. That could be the reason why the boilers you're installing aren't lasting. And this sort of internal corrosion isn't covered by most boiler manufacturers' warrantees, and neither is the labor involved in replacing these boilers once they fail.

Steam heating is a lost art, and we continue to learn as we go. Keep asking questions.

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