Steam Traps Trap Steam

steam trap test station

One of the things I loved about this business when I was first coming up was that the name of a component nearly always defined its function. All I had to do was reverse the word order to figure it out. Consider the title of this article, for instance. Steam traps trap steam. So simple!

Or think about condensate pumps. They pump condensate. It’s right there in the name. Isn’t that delightful?

How about air vents? Air vents, well, they vent air. Wonderfully simple!

Differential-pressure regulators? Why those little beauties regulate differential pressure, of course. All I had to figure out then was what differential pressure was.

And then there’s my all-time favorite: P-traps. Go ahead; figure it out on your own.

But I digress.

With the exception of some of the older vapor systems, thermostatic radiator traps belong on all two-pipe steam radiators. They go down low, on the side opposite the radiator’s supply valve. These wonderful automatic valves will respond to temperature, which is great because, within the radiator, steam is hotter than the condensate that forms when the steam turns back to water. That’s what opens and closes a thermostatic trap.

They’re normally open and air will pass through them ahead of the steam and vent from the system somewhere downstream. When steam arrives, the thermostatic element will expand because it’s partially filled with alcohol and under a vacuum. Alcohol boils at a temperature that’s lower than the temperature at which water boils. When the element expands, it pushes the pin into the seat and traps the steam inside the radiator.

Condensate builds up and flows into the trap. At this point, we need a drop in temperature of about 10-15° Fahrenheit before the trap will reopen. When it does reopen, the differential pressure across the trap will move the condensate from the radiator into the return so it can make its way back to the boiler.

Thermostatic radiator traps nearly always fail in the open position because this is the position that will cause you the most grief. But because they are sensitive to temperature, you can check a thermostatic radiator trap with a thermometer. Go ahead; take its temp on one side and then the other. You’re looking for that 10-15° difference, and you’ll need to keep in mind that this temperature is relative to the steam pressure. Steam at 1-psi pressure is about 216°, so on a low-pressure, space-heating system, the traps will open when the condensate is somewhere between 200° and 205° F. That’s nice.

However, if you’re a running, say, 10-psi pressure on that space-heating system because you happen to be a knucklehead, the steam temperature will be 240° F. and the trap will vomit condensate at about 225° into the return lines, where some of it will immediately flash back into steam, defeating the purpose of the trap altogether.

If you’d like to have more fun that a laser thermometer can provide, you can make yourself a trap-testing station out of a pot. Seriously. Just look at the photo up above. Isn't that delicious? It works like a charm and speaks well for American ingenuity.

Thermostatic radiator traps will open and close hundreds of thousands of times each heating season, and manufacturers give them about a 10-year lifespan. They get checked about every 50 years or so, and only because the building is coming apart from the water hammer.
But you and I can understand why this is so. The problem with checking traps is that we sometimes can’t get at them without breaking things. And the wealthier the tenants, the worse this problem seems to be. I’ve been in some Manhattan apartment buildings where the rich folks thought it was a fine idea to have their radiators encased in marble, leaving no access to the steam traps (or to the air). The marble looked marvelous, I have to admit, but since air couldn’t get to the radiators to warm the rich folks (which is why I was there), and because the radiator traps were entombed better than Ulysses S. Grant, all I could do was just look at the rich people. They looked back. I smiled.

Around 1975, when I was working for a manufacturers’ rep on the Isle of Long, we sold hundreds of thermostatic radiator valves to the people who ran a cooperative apartment building. We also sold them an equal amount of new thermostatic radiator traps. This was right after the first OPEC oil embargo and NYC was abuzz with talk of saving fuel. A combination of new radiator traps and TRVs seemed like just the ticket.

Now when do you suppose they decided to do this complicated heating work? Do you think they did it during the summer when the steam heat was off? Nah, that’s a silly time to work on heating. It’s too warm! Summer is the time to work on air conditioning, so that’s what they did. They were no different from anyone else.

So they were adding these TRVs and new steam traps to lines that were under steam pressure. They’d isolate one line at a time, of course, so as to not get burned, but they couldn’t get into all the apartments on that line on the same day, and they didn’t want the tenants to complain, so they turned the line off during the day, and opened it again at night.

Now think this through. That steam line rises though, say, the living room of all the stacked apartments. It goes from the first floor to the 20th floor in that Manhattan apartment building. We’re going to install new TRVs and radiator traps on the first three floors today. That’s about all we can get done because we’re using our own building superintendent to do the work and the tenants aren’t always answering the door when he knocks. He also works slowly.

Now the last time anyone worked on these traps, F.D.R. was sitting in the Oval Office. There’s been steam in the return lines, raising hell and the fuel bills since the Kennedy administration. The building super installs the new TRVs and traps in those three apartments and turns on the steam that night. The TRVs close on temperature, and so do the new traps. The steam condenses inside the newly refurbished radiators and traps and forms a vacuum because the radiator is now closed on the inlet side (thanks to the TRV) and on the outlet side (because of the closed trap).

With me so far? Great!

Now keep in mind that traps work on pressure differential and that high pressure goes to low pressure – always. When the new steam traps opened, the steam in the return line was waiting for it. Steam rushed backwards into the radiators because there was a bit of a vacuum in there. This created water hammer within the trap, which turned the trap elements into paperweights in a matter of moments. Oh, and the return-line steam also overheated the radiator, even though the TRV was closed.

Nature hates a vacuum.

I spent a lot of hours on that job with the factory guy from the TRV company. We were both young and we got a good education right there at the start of America’s first energy crisis. The tuition sure was expensive, though.

The main thing we learned that winter was that summer is the time to work on TRVs and traps.


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