question on main vent piping size

jimmythegreek

on a large commercial building, I have 400k btus nat gas and 40+ rads.  Have 2 branches of mains, 4" cast iron, 75 feet one way, and 90 the other way.  Had only 2 main vents, both in right place just back of drop at the end of main.  They were old and painted shut, I put new temps on, gonna go with a couple gorton #2s on an antler.  Question is the pipe tapping there is 1/4" and I used a reducer upside down to go to 1/2" and go inside threads on jacobus 3/4 by 1/2 main vents(these are temp).  Is it ok to use that tap and add my antler with prob 3 gorton #2s and just do the union and reducer to enlarge pipe or do I have to tap to the size the gortons are?  I see some have 20+ main vents on larger systems, Im gonna try 3 on each side 6 total and go from there.  thanx for the help.....

Dave in QCA

Remember the limits of your vent piping...

Looking at the venting chart in Greening Steam, under the conditions of 2 oz. of pressure, The Gorton #2 will vent  1.75 CFM each.  An open 1/2" pipe will pass 3.4 CFM.  If you put 2 Gorton #2 vents on a 1/2 pipe, you will be venting 3.4 CFM, not 3.5 CFM because of the limitation of the pipe.  Also, 3 vents will still only vent 3.4 CFM because of the limitation of the piping.



Also, the 1/4" connection is going to be a restriction on the connection where it exists.  The chart does not include a number for 1/4" pipe, but it does have a figure for 1/8" pipe.  It is 2 CFM, so the 1/4" connection is going to be somewhere in between 2 and 3.4 CFM.  Actually, an orifice of 1/4" is going to pass a little more than a 6" long piece of 1/4" pipe, but we can only guestimate that that is going to be.



If you have 4" pipe, that would measure about 4 1/2" outside diameter, it contains .09 cubic feet / lineal feet of pipe.  So the 75 foot run would have 6.75 Cubic Feet and the 90 foot run would have 8.1 Cubic Feet.  Ideal venting is 3 minutes, so at 2 oz pressure, you would need to be able to vent 2.25 CFM on the 75 foot run and 2.7 CFM on the 90 foot run.  I don't know which run has the 1/4" connection....   but with 2 Gorton #2 vents on each, it looks like you will be fine.  Adding a 3rd Gorton #2 to either vent will have little or no effect at all. 

 Save your money.



Hope this is helpful to you.

Dave

jimmythegreek

very helpful

thank you for sharing your knowledge.  to clarify each run has a 1/4" vent tapping about 18" before the drop of the main's elbow, what Im reading is the right place.  One main (the 90 foot run) has room to retap to a larger size fairly easily, while the shorter one is only a few inches down from the ceiling joists, and has a cluster of floor joists right above, making it more difficult.  Your mathematics are super helpful, thank you so much.  I'm wondering based on your numbers, If im better off tapping the easy one to 1/2" and doing a 2 vent antler w two #2's and using the recommended 6" nipple to the antler, and plugging the other which is in a tough spot and going a little further back where I cant get into a bay and tapping 1/2" there for another double #2 antler. 2 questions I have:

1. is there anything I need to worry about tapping 4" 100 year old cast to 1/2" from nothing?  also from 1/4" to 1/2" any tips?  Im assuming go one size at a time slowly

2. on the tough one, is there another good place on the main to tap other than just before the drop? 

thanks again for the invaluable information..........6

Charlie from wmass

Do you know a good

pipe welder? If you do they can with caution add Threadolets where you need vents. I would put on a 3/4" or 1" if you can get pipe welder in there. Note said pipe welder not just welder.