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What You Ought To Know About F&T Traps


Dan Holohan for Plumbing & Mechanical
August 1, 1990
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The stuff I'm about to tell you, I had to learn the hard way, but I didn't have to learn them more than once.

I hope you won't either.

1) The Only Time You Need F&T Traps Is When You Have A Condensate Pump Or Dry Return Lines.

F&T traps keep steam from passing by. They are normally closed automatic valves that work like ballcocks. When they're working, they stop steam dead in its tracks.

You need them if you have a condensate pump because condensate pump receivers are vented to the atmosphere. Without traps, you'd turn the boiler room into a sauna. You can't plug the receiver vent because the receiver could explode. I've seen this happen. It's not pretty.

You also need F&T traps if the system you're working on is two-pipe, has no condensate pump, but does have dry returns.

You need them because you don't want the steam to move from the end of the steam main into the dry return main. If it does, you'll lose the pressure differential across the system. Steam will stop moving to the radiators and you'll get some BODACIOUS water hammer!

If you don't have a condensate pump or dry returns, you don't need F&T traps. You really don't.

Nevertheless, I constantly see F&T traps in places where they don't belong. They get put in those places because of the "Looksalotlike" Rule. You know how that works?

Two guys are working on a job. One says to the other, "Hey, this looks a lot like that other job we did. Remember?" Then the other guy says, "Yeah, let's use some F&T traps."

2) There Is No Temperature Drop Across An F&T Trap.

Float and "thermostatic" is really a lousy name for these things because it makes you think that they're sensing temperature. They're not.

Well, that's not entirely true. There is a thermostat inside the trap that allows air to pass and closes to steam. But once steam reaches the trap, the thermostat is closed and out of the picture.

Water doesn't cool off before it leaves an F&T trap the way it does before it leaves a thermostatic radiator trap. F&T traps discharge water at saturated steam temperature. That water is as hot as the steam that's entering the trap. Only the latent heat has been removed.

And since thermometers can't sense latent heat, you can't check the trap with one. Try it sometime. You'll get the same temperature on both sides. The trap could be good or bad. You can't tell.

You can't check them with those sexy sonic devices either because F&T traps discharge constantly. Those sensors work well on bucket traps because they cycle full-open to full-closed, but an F&T trap (to me at least) sounds about the same all the time.

The only way to check an F&T trap is to open up a valve on the discharge and see what comes out. If the trap is working, you'll get a mixture of water and vapor. If it's not, you'll get steam.

I'm partial to Hoffman Specialty's F&T traps because they have two discharge holes. This is just a piping convenience and you're supposed to plug one of them, but I always stick a nipple and a ball valve in that extra hole. It gives me an instant trap tester that building owners love because they can have the traps checked, in-line, as the years go by.

3) Much Of What Comes Out Of An F&T Trap Will Immediately Tum Back Into Steam.

It's called reevaporation, or "flash" steam. You'll see it coming out of the condensate pump's receiver vent if you have a trap anywhere nearby.

The higher the steam pressure, the more flash steam you'll get. Again, it's because you're releasing very hot water to atmospheric pressure. For instance, on a system running at 5 psig, 2270 water will come out of the trap. That's why it flashes.

Get enough of this hot stuff into the condensate receiver and the pump will cavitate. Cavitation is what happens when the water entering a pump is not under enough pressure to keep it liquid.

The water flashes to vapor as the pump tries to move it. And when it flashes, it expands about 1,700 times in volume.

These vapor "bubbles" get whipped out to the edge of the impeller where higher pressure causes them to suddenly collapse. Then the water surrounding the condensed "bubble" rushes in to fill the void at about 1,000 mph (no foolin'!) and tears the pump impeller to pieces.

Bad stuff, cavitation.

4) The Discharge From All F&T Traps Should Drain Completely Back Into The Condensate Pump's Receiver.

If you drop the trap's discharge piping below the inlet to the condensate pump's receiver you'll create a water leg through which air will not vent. I see this problem a lot when condensate pumps are replaced with boiler-feed pumps.

Since today's boilers contain so little water, we frequently have to use boilerfeed pumps to hold the water that used to be in the old boiler, but isn't in the new one.

The problem is that the inlet to the boiler-feed pump is higher than the inlet to the condensate pump. Most guys bring the low return up into the boiler-feed pump's receiver with an ell and a piece of pipe. This creates the water leg across the bottom of the system. The air can't vent through the water so the steam won't move out to the radiators.

The building goes cold and the owner tells you that, “EVERYTHING WAS FINE UNTIL YOU GOT HERE WITH THAT DAMN BOILER OF YOURS!”

So what do you do? Vent the air upstream of the trap with a decent main vent. And I also want you to use a vacuum breaker on the discharge side of the trap. This will break the vacuum that’s going to form between the trap and the water leg when the trap’s flash steam condenses.

This is important because that vacuum, when combined with the pressure drop through the water leg, can be enough to keep the condensate from flowing back to the receiver. You’ll swear the return is completely clogged.

But it ain’t.

For a similar reason, never, NEVER double trap a return line. This happens when a guy (usually a good hot-water man) decides that he really doesn’t need to fix all those traps out in the building. “One big one at the inlet to the condensate pump should do it,” he says. “Yup, that should protect the pump real good.”

But all it does is create this bizarre low-pressure situation that keeps the condensation from flowing. The water gets stuck between the two traps. Take the guts out of your new trap and everything starts to flow. Again, you’ll swear the trap is clogged.

But it ain’t.

(And think about this: If all the traps in the the building could be replaced with one gargantuan MASTER TRAP!, how come the first guy didn't do it?)

I learned about this vacuum business the hard way. The thermostat in the F&T trap I was troubleshooting kept blowing out from the water hammer that was created by the undrained condensate in the discharge line. As soon as the trap's thermostat failed, steam got into the water leg and, over an hour or so, heated the condensate to the point where it was causing the condensate pump to cavitate.

It was a real head-scratcher for a while. But I didn't have to learn the lesson more than once.

I hope you don't either.

5) Never Size F&T Traps To Line Size.

If you do, I guarantee they'll be oversized. Not only will you spend more than you have to on the thing, it will also fail in no time at all.

An oversized F&T trap barely has to open to discharge its condensate. That tiny opening speeds up the water as it leaves the trap. It's like putting your thumb over the end of a hose. The high velocity water rips up the trap's seat and kills it.

Learn how to size F&T traps. It's easy and it saves you money. Besides, manufacturers are dying to give you their trap-sizing literature. All you have to do is ask for it.

6) Always Match The Trap's Seat Pressure To The System Pressure.

A 15-psig trap has a pretty big seat. If you hit it with high-or medium-pressuresteam it won't stay closed.

A 75-psig trap has a much-smaller seat. If you use it on a 2-psig system there won't be enough steam pressure to push the condensate through the hole. It will back up into the heating equipment and cause all sorts of problems.

The trick is to match the trap pressure to the system pressure. Don't think that a 125-psig trap will work on any system up to 125 psig. It won't.

For now, I hope this helps.