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Steam Traps

There is this job. It is a one-pipe steam system. We installed a new boiler and lost our A dimension. So we installed a condensate tank with traps at the end of the main and at every drip. We installed air vents on the outlet side of each trap. All of the traps and air vents are about 25 inches above the boiler's water line.

When we started it up, condensate was stacking in the returns. It was not flowing to the condensate tank fast enough. The condensate would build and build until water started squirting out of every air vent. The air vents were not breaking the vacuum quick enough. So, I raised each air vent about 2 feet above the outlet of the traps. Presto. This solved everything.

I thought that the return was open to the atmosphere so the highest the condensate would get would be the inlet to the tank. And when the tank got full it would pump the water into the boiler. Does anyone know why the air vents needed to be raised in order to break the vacuum?


  • Dave in QCADave in QCA Member Posts: 1,660
    Why did you do that?

    It is very unusual that a dimension A is lost with a boiler replacement.  Dimension A states that the lowest steam carrying pipe should be at least 28" above the boiler normal water line.  So, if you lost the dimension A, you put in a boiler with a higher water line than what was there when the system was built.  This is very unusual because modern boilers tend to be smaller, not larger.  At least, it would seem that you could have found a boiler that would have met the dimension A requirement.  Your decision to go another way has made a simple system unnecessarily complicated and more costly to maintain over time.

    As to the traps and piping. I assume that you cut every drip leg that existed in the system, and installed an F&T trap.  The F&T trap selection should be based on the maximum condensate flow at that point.  If they are too small, the condensate will back up.  Also, if the outlets are not propertly piped, they will back up.   From the outled of every trap, the piping sizes should be selected based on steam system guidelines.  Guessing won't do.  The piping should all be pitched continyously to the condensate recevier tank.  Make sure there are no dips or low points in the piping.  Not only does the piping have to carry condensate, it also has to carry air.  If it can't do both properly, it will do neither.  The condensate tank must be openly vented.  This allows for all air to get out of the way so that condensate can easily flow into the receiver.

    Vents on the condensate lines, after the traps, should not be needed, because these lines all are supposed to be venting through the tank.  But, piping errors, or mistakes in setting up the condensate tank would definitely cause problems.

    If the close piping of the boiler is not correct, and/or it has not been properly skimmed, or for whatever reason is producing wet steam or otherwise throwing boiler water up into the mains, that will also cause a backup, simply because of the volume of water that is being pushed into the mains.  Hopefully, that is not the case.  You should only have to deal with condensate.

    The problems you describe, should not be happening according to simple laws of physics.  Therefore, what you describe, and what is actually there must be two different things.  Can you take a bunch of pictures so that we can see what is going on?
    Dave in Quad Cities, America
    Weil-McLain 680 with Riello 2-stage burner, December 2012. Firing rate=375MBH Low, 690MBH Hi.
    System = Early Dunham 2-pipe Vacuo-Vapor (inlet and outlet both at bottom of radiators) Traps are Dunham #2 rebuilt w. Barnes-Jones Cage Units, Dunham-Bush 1E, Mepco 1E, and Armstrong TS-2. All valves haveTunstall orifices sized at 8 oz.
    Current connected load EDR= 1,259 sq ft, Original system EDR = 2,100 sq ft Vaporstat, 13 oz cutout, 4 oz cutin - Temp. control Tekmar 279.
  • Positive SolutionPositive Solution Member Posts: 19
    Steam Traps

    Thanks for your reply.

    The new boiler was installed on cinderblocks which brought the A dimension to about 24 inches.

    The original problem was water hammer in the mains. I believe the real problem here besides the A dimension was that the return lines and drip legs were undersized. Condensate would back up the drip legs and enter the mains causing water hammer. I also believe that is why the condensate was backing up and spitting out of the air vents. By raising the air vents about 2 feet, I bought myself a small reservoir of space to store the extra condensate and I gave the air vents a chance to break the vacuum.

    Someone told me that the condensate could never back up higher than the traps. Do you know if this is true?  If it is true, that would explain why the condensate stopped spitting out of the vents when I lifted them vents 2 feet higher than the traps.
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