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Massively vent mains, too?

Brian_74 Member Posts: 237
I have a two pipe vapor system. I understand why it's a good idea to massively vent the dry returns to reduce back pressure. Should I also massively vent the mains? There's one vent there now. My concern is that by lowering the back pressure on the mains, the steam will just zip around the mains, back into the boiler. At least that's what's happening in my head as I picture this. (Just to be sure that I'm using the term correctly, by "mains" I mean the pipes that supply the steam from the boiler).

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  • Main Vents

    Hi Brian-  I think you're getting hung up on the "back pressure" idea.  The first objective is to get steam to the radiators and to do this, the air in the mains must be evacuated so that the steam can replace it.  In Two Pipe systems there are two standard methods to do this.

    Method #1: The air leaves the Steam Main via a pipe at the end of the Main that connects to the Dry Return and then out through the Main Vents on the Dry Return,  On the connecting pipe is an F&T trap  The "T" part means thermostatic which allows the escaping cold air to pass through but closes when steam reaches it. (Like a Main Vent)  The "F" part stands for Float which opens and allow condensate through the connecting pipe but not steam.

    Method # 2 :  There isn't a connecting pipe or an F&T trap so the Steam Main must have another means for the air to escape. This is done by mounting Main Vents on the end of the steam main.  Also as there isn't a connecting pipe, the condensate in the Steam Main must have a route back to the boiler. This is done using a drip pipe on the end of the main that drops down into the Wet Return. Dropping into the water in the Wet Return is a "Steam and Air Trap" as it doesn't allow the steam or air escape so it can't return to the boiler.

    After the Main Vents on the Steam Main shut, the steam routes its way through the radiators. Air in the radiators escapes through the radiator traps and down to the Dry Return. From the Dry Return the air escapes out the Dry Return Main Vents.

    As I remember your system uses Method # 2. If you don't have vents on your system main that would mean that all the air from the Steam Main would have to vent through the radiators and steam distribution would be a lot slower and uneven. 

    Remember steam travels from an area of high pressure to an area of low pressure so back pressure is the last thing you want. - Rod
  • Brian_74
    Brian_74 Member Posts: 237
    Thanks, Rod.

    I appreciate you explaining this to me, Rod. You're right that my system uses method #2. Your explanation made me realize that I was ignoring the trapping aspect of the vents. I forgot that they permit only air to escape. D'uh. Now I get it.

    Thanks for helping me end 2009 a little smarter. :-)

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  • Unknown
    edited December 2009
    The Steam Train

    Hi Brian- Glad to help. Learning steam is like trying to figure out a big jig saw puzzle.As you put more pieces together your concept of the finished picture changes. I don't know how many times I've read through "The Lost Art..." but know every time I do a few more pieces fall into place. 

     I found one of the best concepts to understand steam is to think of it as Dan calls it , "The Steam Train".    Steam Train carries heat (BTUs) "passengers" from the Boiler "station" to the Radiator "station" where it unloads the heat (BTUs) and returns empty (as Condensate) to the boiler "station" to pick up another load of BTU "passengers."

     Look at the system through the eyes of the engineer in the locomotive and through the eyes of the railroad's general manager. Your job in running this railroad, is to see that the train is able to make the route circuit as easily as possible and stay on schedule. You have to make sure there is no air on the "tracks" slowing the train down, make sure the grade of the tracks is beneficial to the train and are well maintained. You also have to have insulation around the tracks to stop the train dropping off "passengers" before it gets to the "station".  Wet steam is a thing you want to avoid as it prevents you getting a full load of BTUs on the train which makes the train's journey less profitable. Main vents are great as they clear the air off the tracks for the steam train. Traps on the mainline make sure the train goes into the radiator station. Traps on the radiators make sure the passengers get off the train in the station and not somewhere else. 

    I know it sounds like a "Sesame Street" approach to steam heating but you'd be surprised how well things fall into place if you think of it this way. 

    Best Wishes for the New Year!

    - Rod
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