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One Pipe Steam - What Is This?

Bent Member Posts: 5
I'm currently troubleshooting a one-pipe steam system in Manhattan. I came across what seems to be some sort of a loop seal to return condensate. The building has an 8" express steam main that goes up to the top of the apartment builidng and downfeeds various risers. At the bottom of the express main is a 1-1/4" pipe that is piped to the wet return through what looks to be a loop seal i'm just not sure why it is done this way. The pipe passes a tee which would allow water to go directly into the wet return, however there is a closed valve on this line forcing the condensate to flow through the loop seal which appears to go through the floor and then back up again in uninsulated pipe. The pipe then turns toward the door at about a 6-foot elevation and goes through the wall near the door. The pipe then turns down and tie's into the wet return. Any Ideas? I have never seen anything like this.


  • JStar
    JStar Member Posts: 2,752
    Loop Seal

    How high is the normal water line in that picture? It was probably acting as a steam trap to keep steam out of the returns.
  • Steamhead
    Steamhead Member Posts: 15,986

    it might be a False Water Line.

    More pics are needed.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Bent
    Bent Member Posts: 5
    One Pipe Steam - What is This?

    The water level is 66" above the floor. The "A" dimension is 20" and they operate the boiler at 3 psig. That was the best picture I had showing the entire loop so I made a sketch to indicate what they currently have.
  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 16,281
    False waterline

    I agree. It would be good to know the levels of what's beyond the boiler room.
    Retired and loving it.
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,253
    From your drawing

    that is a drip from the main supply. They dropped it to provide a trap to keep the steam from entering the return unless there was a high steam pressure.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • Bent
    Bent Member Posts: 5
    One Pipe Steam - What Is This?

    Thanks Charlie. If they are operating at 3-psig then that trap leg would have to be over 7'-0". I'm guessing they are blowing out that trap and getting steam into the return. They have water hammer issues and are dumping condensate because it is not returning to the boiler. I believe they are operating at too high a pressure. Any thoughts on this?
  • Steve_210
    Steve_210 Member Posts: 628
    Low pressure gauge

    Install 3 pound gauge get the pressures as low as possible and see what happens
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,573
    Over pressure

    Probably the maintenance crew have been raising the pressure to compensate for inadequate main venting.

    The existing vents could be replaced with some traps as others have done. Perhaps a trap at the top of the main riser would speed things up.--NBC
  • Bent
    Bent Member Posts: 5

    Thanks NBC, i agree they may have a venting problem. When you indicate replacing the vents with traps are you referring to steam traps? Wouldn't that screw up the "A" dimension?

    Would new vents be sufficient in lieu of traps. I'm thinking vents should be at the top of the express main and also at the bottom of each riser since it is a downfeed system.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,573
    edited June 2014
    Using steam traps on one pipe as main vents

    I tried to search for the thread showing that modification, but haven't found it yet.

    I think Dave QCA, and others have done this, saving space, and gaining capacity, using a Monash trap.--NBC

    Here it is:

  • Bent
    Bent Member Posts: 5
    Fail Safe

    It appears these traps are piped to atmosphere. Is there a different piping arrangement to reduce damage from a trap that could fail in the open position? The main express header is located above the ceiling on the top floor of an 8-story apartment complex. Would you recommend piping the vents to outside?
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,573
    Venting location

    I would be afraid of making any penetrations in the roof to vent the trap exit, because the exit might become blocked with ice in the winter, and the hole might leak. Could you drop a pipe down the same pipe chase? Pex could be used here.

    Alternatively, the venting could be increased on the dry returns so as to have capacity for the riser as well. Just keep the amount of venting on each dry return the same so each has the same resistance to the escaping air (as low as possible). Otherwise, you may have one return getting steam more quickly than its neighbors, leading to unbalanced heat. A 0-3 psi gauge will show you how close to the open pipe equivalent you are. With that size riser, you may need some big traps. Look in the venting guide available in the shop here for their capacities.

    Remember, " air is the enemy-to be chased out like the raccoon from the garbage can".--NBC