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main vent sizing

If I want to vent the mains as fast as possible, why are main vents usually teed off 3/4"? Why wouldn't I just keep the tee the same size as the main and then put enough vents to equal the same size as an open 2" pipe or whatever size the main is? It would take a lot of vents but if the mains have to be vented as fast as possible then this would be the way to do it. I don't see it done this way I'm assuming because of the cost. How fast should a main heat up? Is there a a specific number of minutes? I'm asking because I don't really know how to size vents I usually just guess. From what I've been reading is that you want to match the equivalent of an open pipe. So if most vent tappings are 3/4" then shouldn't I just be using 5 Gorton number 2s on every system?


  • Fred
    Fred Member Posts: 8,542
    Typically four to five minutes from Header to end of main is pretty good. You have to remember that not only are you pushing air out of the mains but you also have to heat that main along the way so that it's hot enough to support the steam. It is condensing along the way. Enough vents to accommodate a 2" tapping is a waste of vents. Steam still won't get there any faster than you can heat the main piping.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    If you have a low pressure gauge, you can see what the resistance (backpressure) is when the air is being pushed out. It should be lower than 2 ounces, or less. Having more main venting is also a benefit in not collecting debris blown up into the vent. If the vent is too small there will be high pressure air velocity which blows debris into the orifices.
    You may not need, enough venting to equal the open pipe, but you do need to keep the backpressure down very low.—NBC