Air Vents and Steam System Corrosion

I hope we’re at the point where we can agree that proper venting is going to save fuel and balance systems. Not only does air slow the steam down, it also acts as a great insulator. Did you know that a film of air just 1/25th of an inch wide offers the same resistance to heat as a 4-foot-thick wall of iron?

But there’s another important reason for efficient venting, and that’s condensate grooving. If you’ve worked with steam, you’ve seen the results of this destructive force. You just may not have fully understood what was causing it.

Condensate grooving is the main reason why so many underground returns leak. It’s a persistent, gnawing force that weakens joints and allows water hammer to tear pipes from fittings. It’s the primary cause of unit heater and radiator failure.

And it’s as natural to steam systems as boiling water.

The main cause of condensate grooving is carbon dioxide. It’s a simple process. When carbon dioxide mixes with condensate you get carbonic acid. And carbonic acid is bad news! If you were to take a pH reading on the condensate found in most steam systems, you’d get a reading well below 7.0. That’s on the “acid” side and, obviously, not good for the pipes.

The chemical reaction that takes place inside the system looks like this:


Carbon dioxide is a byproduct of boiling water. Every time a boiler steams, it produces the stuff that will eventually destroy the system. That’s ironic, isn’t it?

The boiler can’t help making this stuff because there are compounds called carbonates and bicarbonates present in feed water that break down when the water boils. That chemical reaction, the breakdown of the carbonates and bicarbonates, produces three things: water, hydroxide (in itself, also very corrosive), and carbon dioxide.

Carbon dioxide, which is, of course, a gas, moves out into the system with the steam. If it’s not quickly vented it will mix with and dissolve in the condensate that’s formed as the steam condenses. The acid that results from this chemical marriage goes right to work on the system components.

And to make matters worse, we also have oxygen in a steam system. It enters through the air vents every time the system pressure drops to zero.

Oxygen speeds up the corrosive attack of the condensate by throwing rust (ferric hydroxide) into the mix. (I’ll spare you the chemical formula for that whole process. If you’d like to see what it looks like, just open the drain valve on any steam system.)

So the importance of proper venting becomes even more apparent when you start thinking about rotting pipes and a bushel basket of sludge.

Want to limit system corrosion? It’s all in the venting!


(This is an excerpt from Dan Holohan's book The Lost Art of Steam Heating Revisited.)


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