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The end-of-main F&T trap is causing a problem

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Published
July 10, 2009 Subscribe to RSS feed

The trap is too small.
In a two-pipe steam system, the end-of-main trap has to handle just the condensate that forms when the main goes from room temperature to steam temperature. Usually, that's not much condensate because the condensate from the radiators returns to the boiler through the radiator traps and separate lines.
Most one-pipe steam systems have gravity-returns and no steam traps. But if the system has a condensate- or a boiler-feed pump, the end of the main will need a trap to keep steam from showing up in the receiver. With a pumped, one-pipe system, all the condensate (from both the pipes and the radiators) returns through that single end-of-main F&T trap. You have to size that trap for a much greater load than you would if you were using it on a two-pipe system. Don't base this trap on the line size. Line size may not be big enough in this case.
If the trap is too small, condensate will back into the main and cause water hammer. The water hammer will damage the F&T trap. Check the size of the trap carefully.

The trap isn't sized for the right pressure differential.
A one-pipe steam system operates best at low pressure. To make the end-of-main F&T trap drain quickly, size it for the lowest possible pressure differential. This will give you a larger trap (or a series of traps piped in a manifold), but that larger trap can return the condensate to the boiler quickly. That will keep the condensate from backing up in the system.

There's a water seal between the end-of-main F&T trap and the condensate- or boiler-feed pump.
Air can't vent through a water seal. Take a close look at the line running between the end-of-main F&T trap and the condensate- or boiler-feed pump's receiver. Does the line drop below the level of the receiver's inlet? If it does, you have a water seal. Either correct the piping so it pitches continuously to the receiver's inlet, or install a main vent on the outlet side of the end-of-main F&T trap.
The vent works best on the outlet side of the trap because, in this position, it will serve as both an air vent and a vacuum breaker. That's important because flash steam from the trap can create a partial vacuum between the trap discharge and the water seal. That vacuum can keep condensate from draining. By installing the air vent on the outlet side of the trap, you'll break that vacuum.