How NOT to Size Steam Traps

ft steam trap

I was in the basement of this six-story apartment building in a suburb just north of New York City. This goes back some years when I was working for a manufacturers' representative. We sold stuff that had to do with hot-water systems and steam-heating heating. I was the Contractor Boy, which meant I spent my days in the field, trying my best to help installers. The thought was that if I made them feel good about our company, they would ask for our products when they went to their wholesalers. This was somewhat of a thankless task because, if I were successful, the contractor would often ask his wholesaler for something that the wholesaler didn’t have in stock (my company’s stuff). And when the wholesaler told the contractor that he would have to wait, the contractor would invariably tell the wholesaler that he would take whatever was on the shelf. I’d lose.

And then the wholesaler would call me and tell me to stop trying to change his contractors’ minds about products, and instead, bring him his competitors’ contractors.

And that's how it goes.

Anyway, I was in this basement, looking at a steam trap with an engineer, which was my first problem because I am not an engineer and I was quite young at the time. I was a good reader, though, and I had lots of old books. I was also a bit of a steam whisperer. I had a sixth sense for this stuff. I’d imagine myself as the steam, or the water, or the air, and then I’d ask myself what I would do, and where would I go, if I were inside the pipes. It was all very Zen-like, and a little spooky.

Anyway, on this job, the engineer had specified these float-and-thermostatic steam traps that the company we represented made. The traps had failed and the engineer wanted to know why. They were all passing steam into the return lines, and when that happens, some of the tenants aren’t going to have any heat because steam in a return line blocks the air that’s trying to leave the system. And where there is air, steam will not go. It also causes water hammer that can lift the building off its foundation. So there we were.

Now this seemingly guilty trap was at the base of a two-inch steam riser that went from the basement up to the sixth floor, feeding six radiators as it went. The radiators also had steam traps, of the thermostatic variety, and any condensate that formed inside the radiators when the steam gave up its latent heat energy would drain through those traps and back into a dry return line. The dry return worked its way back to a boiler-feed pump. It was standard steam stuff; the supply-and-return risers were like the two sides of a ladder, and the radiators on each floor were like the rungs on that ladder.

The trap that the engineer and I were looking at had just one job – it had to drain the condensate that formed in the two-inch riser when the pipe first got hot. It didn’t have to handle any of the condensate that formed in the radiators; the radiator traps took care of that. So we were asking this trap to deal with just the water that would form when about 50 feet of two-inch diameter, insulated pipe went from ambient temperature to 215-degrees Fahrenheit. Can’t be much water, right? And once the pipe gets hot, it stops making condensate. I knew that the trap was too big, and that’s why it, and all the others, had failed.

“Why did you spec a two-inch F&T trap for this line?” I asked.

“Because it’s a two-inch riser,” the engineer said.

“But, this riser is only going up about 50 feet.”

“I know that,” the engineer said. “I can see that. Duh!”

“Well, how about this?” I said. “If you were going up 300 feet, you’d use a three-quarter-inch F&T. That would be the right size. A two-pipe riser that’s this short doesn’t make that much condensate. The trap is oversized, and that’s why it’s failing.”

He rolled his eyes. “But it’s a two-inch line. If I specified a three-quarter-inch trap, it would look stupid.”

I looked at the trap and I had to admit it did look snazzy – very uniform. Two-inch pipe. Two-inch trap. It was like matching the jacket and the trousers. “Trap looks good,” I said.

He snapped his head up and down. So there.

“Trouble is, when an F&T trap is oversized,” I said, “it barely has to open to release the little bit of condensate it collects. And when that happens, the pin stays too close to the seat inside the trap. The condensate moves so quickly across that tight space that it erodes the surfaces. That’s why this trap is passing steam. It’s way too big, and your client paid way too much. If you rebuild the trap, or replace it with another of the same size, the same thing is going to happen again. If you keep doing what you’re doing, you’ll keep getting what you’ve got. In fact, a three-quarter-inch trap would be oversized for this duty. Sure, it would be better than what you’ve got here, but it would still be too big.”
He shook his head. “Three-quarter-inch would look stupid,” he said. The man knew his mind.

I looked again at the trap. “I have to admit it does look good,” I said. He nodded. “But now you look stupid,” I added.

Just couldn’t resist.

So here’s the thing: Within the realm of space heating (and please don’t apply this to commercial systems), if the trap is the same size as the line that it’s on, it’s the wrong size trap. This will be true every single time. Line size is the wrong size. That trap will always be oversized, which means that someone paid more than he or she should have paid for a trap that’s going to fail long before its time.

But it will look good.

The right way to size a steam trap is to figure out how much load the trap is going to see. You do this my considering what the trap is serving. Is it at the end of a steam main? Is it on a unit heater? Is it at the base of a riser? If it’s a heater of some sort, the manufacturer will be able to tell you the load. If it’s serving a main or a riser, you’ll have to figure out the length of the pipe and take it through its temperature rise to figure the pounds per hour of condensate that you’ll get when the pipe first heats. There are good books available that will give you that that information.

Next, you have to know the differential pressure across the trap. In space-heating work, that differential pressure is usually very low because most buildings heat on 2-psi pressure or less. If a line gets back-pressured (because a trap somewhere is passing steam into a return), it will affect the performance of other traps, making them look bad when they’re really not. You have to look at big pictures when you’re sizing traps.

Every job is different.

And if you’re not comfortable with that, get with a willing and enthusiastic manufacturers’ rep for a trap company. Have that eager person go out to the job with your contractor customer and size all the traps, just as I used to do. The rep will trip over himself to help you. He’ll happily spend hours crawling around in horrible places, thinking of all the business he’ll soon have from you. He’ll do all the math and he’ll come up with a list of perfect products for the job, and he’ll show it to you. You’ll add your margin and give the price to your contractor customer. He’ll tell you that your price is too high, and ask if there is something you can do, like maybe get a second price.

So you’ll call the other rep and give him the first reps’ list of materials. The second rep will, of course, always be cheaper because he didn’t have to go to the job, crawl through the nasty places, do all the math, and size all the traps. So he’ll get the business.

It usually worked that way for me.

Made me feel stupid.


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