Tom Székely, P.E., LEED AP

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May 5, 1993

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Twas the Season

When I started writing this issue in December, this segment was called " 'Tis the Season" because it was that time of year when some of us got to experience the clanging of steam pipes as harmony to the dulcet tones of holiday bells. Though the season has passed, the issue remains. Why do steam pipes clang? Is it a manifestation of The Immutable Law of Steam Heating Systems? You know, all steam systems knock. Yeah, and steam heat is dry heat, right? (Why does that sound familiar?)


Well, I remember some conventional wisdom ascribing it to pipe joints undergoing stresses as the system heated up (wrong), and, of course, that it is an immutable law of steam systems (also wrong). Did you ever wonder why it sounds like someone banging on the pipes with a hammer? The answer to that puzzlement is that it is, like water hammer in water piping, a case where the pipe is actually being hammered by a slug of water inside it. What?! Water is soft and squishy. How could it hammer at anything?


If you bounced an ice cube off a frying pan, it would probably ring rather nicely. Ice is water. Understand that water, like all liquids, is incompressible. That is, though you can compress air (a mixture of gases) into so small a volume that a SCUBA tank might provide you with as much air as would fill a building under normal atmospheric conditions, you can't squeeze water down at all. That's why hydraulic lifts work.


OK, so we have liquid ice (more commonly known as water) banging off the inside of steam pipes. Great. Where did it come from, what the blazes is banging it into the pipes, and why is it banging into pipes anyway, instead of just flowing in them?


You sure ask a lot of questions.


Well, we already know that ice is water, and I guess you know that steam starts life as water. Stands to reason that it ends life the same way. We technical types call this phenomenon change of state, with the three states of matter (normal states - there is something wacky called the plasma state which is beyond the scope of this discussion) being, solid (ice) liquid (water), and gas (steam or water vapor).

We make steam in a boiler by pumping so much heat energy into the water that the molecules don't like each other very much any more and they go flying off into all directions. That is, in a solid (low heat content) adjacent molecules are perfectly content to remain so, and stuff has a definite shape. In a liquid, the increase in heat content which melts ice or pig iron has made the molecules energetic enough that they slide over and around each other but still are interconnected so the substance has no definite shape, though it still has a fixed volume.

You try to pour a gallon of water into a coffee cup and what you get is a mess. (There is a some­what more crude analogy in the popular construct known as a blivet which discusses trying to fit ten pounds of waste in a five pound bag). When you've pumped enough heat into a system to turn a liquid into a gas, the gas expands to fill all the available space of the container it's in. When, in doing so, it mixes with the gas already occupying that space, the relative concentration of the two gases (as reflected by a fancy term like partial pressure) determines how diluted the newly created gas is by the already extant gas. This is important in early detection of blivets.

Steam heat works because of the tremendous amount of heat that has to be pumped into some water to turn it into steam. When the steam gets to the radiator to give up its heat, it does so by condensing back into water.

Why doesn't it turn back into water in the pipes which transmit it to the radiators? Because the pipes don't have enough surface area to suck enough heat out of the steam to turn any appreciable amount of it back into water. Some, however, does turn into water on the way, normally carried along by the steam, in diffuse form to the radiator. If the piping has undergone some local settlement since it was installed, the resulting low spot holds a puddle of condensed water that would have drained out of the system on boiler shutdown when the system was new and all piping was pitched so as to promote drainage.

All right, so we have this puddle in a low spot. Why doesn't the steam simply pass over the puddle when the boiler is restarted? Because, being a gas, it fills all the space in the pipe, and further, being under some pressure, it pushes the puddle out ahead of it as it expands into the supply pipe from the boiler. If there were no elbows or tees in the pipe (tough to do when the system has to return condensate to the boiler which made the steam), the steam would simply push the puddle ahead of it out the end of the pipe. As it happens, when the puddle reaches the first elbow or tee, it's plowing merrily along in accordance with Newton's first law of motion. That is, it won't change direction of its own accord, so it slams into the wall of the elbow or tee (and makes the pipe clang in accordance with Newton's second law).

OK, OK, so the steam flings the slug of water at the pipe, but clanging and shaking? You bet your bippy. It has to do with how fast the steam is moving in the pipes. Huh? What's pushing it along that it should have any velocity to speak of? Nothing. What's sucking it along (the vacuum created in the radiators when the steam condenses to water), is another matter entirely. In low pressure residential steam systems, this pressure differential can move the steam along in the pipes at about 29 feet per second or 20 miles per hour.

You get the same effect by dropping a pipe elbow from a height of 15 feet.

Should clang and bounce rather well.  In process plant steam systems, the steam can be moving along at between 150 and 200 miles per hour. Fling a slug of water at this velocity and you turn pipe elbows into scrap iron and steam boilers into geysers.

You've gotta be careful when you fool with Mother Nature. If that weren't the case, who would need Engineers?

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Fig. 1 - Water droplet getting bell rung