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# Calculating venting required on steam mains

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something I wish I knew too, but it seems(and rightly so), to be a "heavily guarded trade secret" that comes with years of experience. Oh, I`m sure if a person had the time to "lurk here" it could be figured-out, but I find the people who do know are always willing to help out when given the proper specifics, so I just ask them. That`s just another thing that makes them "pros".<BR><BR>Dave

• Member Posts: 3
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Calculating venting requirement on steam mains

Anybody know a formula to calculate the proper venting on steam mains in relation to the size pipe and run?How many minutes is ideal to explumge the air in the main?Thanks
• Member Posts: 77
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volume

Here you go guys Formula is 3.14 x radius of pipe squared x the length of pipe divided by 144 will give you the volume of air in a pipe in square feet. Example: 50 feet of 4 inch pipe? Answer 3.14 x the radius of pipe which is half the diameter, squared( 1/2 of 4 is 2)2 squared is 4 x length of pipe which is 50 will give you volume in square inches . Divide this figure by 144 to get the volume in square feet. 3.14 x 4 x 50 divided by 144 = 4.36 square feet of air. Then call Gorton and get the vent rates of their #1 and #2 main vents. They are rated in square feet per minute. I think the #1 vents 4 sfm at one pound of pressure and the #2 vents 16 sfm at 1 lb pressure. But here's the rub,upon a call for steam your boiler will make steam at very low pressure eg. 1 oz , it's at this time that you want to empty the mains. The lower pressure will vent the air at a slower rate then the above quidelines .So a rule of thumb is to double the venting capacity needed . No set time to vent the mains , just the faster the better -ALWAYS OVER VENT YOUR MAINS!
• Member Posts: 57
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Is there a rule of thumb for minimum venting? All the air in the main in less than a minute?

How about venting the radiators? In TLAOSH Dan mentions it's not a good idea to vent too fast (something I wish I knew before I bought 14 Varivalves). Is there a rough idea how fast is too fast?
• Member Posts: 9
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units

Looking at ?Jim's? calculations, there are some problems. To get the volume, you have multiplied inches squared x feet which doesn't get you cubic feet or cubic inches. I think it gets corrected later, when you multiply by 144. A cubic foot is 12 x 12 x 12 = 1728 cubic inches.
Basically you need to change all your units to inches or feet before doing your Vol.= 3.14 x radius squared x length.
So, 50ft of 4inch dia gets 7536 cu inches or 4.3cuft. I too would appreciate someone here suggesting how quickly one should vent this volume.
• Member Posts: 1,557
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I think there is a difference between venting the mains too fast and the radiators too fast. The varivents are very quick, I usually set them around 1/2 open and work from there. Don't worry so much with how fast, you want the heat balanced in the rooms. With the mains, as long as you have a parallel flow system, the vent speed should not be an issue. As opposed to a counter flow where venting to quickly will hold the condensate back.

I think 1 minute is a good average time for venting the mains.
• Member Posts: 2,440
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How about the Gerry Gill and Steve Pajek E-Book?

http://www.heatinghelp.com/shopcart/product.cfm?category=2-138
• Member Posts: 3
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Jim..thanks for you answer it was on the money.Still not sure how long is an ideal time required to vent the main but a 20ft and 200 ft main will not vent at the samel time.Question.Should the short main and long main vent at the same rate.ie 1 min,2 min etc..I cam across a book which seems informative and give an excerpt on venting the mains.http://steamupairoutwaterback.com

Regards,George
• Member Posts: 152
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thanks for noticing

Hi George,

Thanks for the plug. I recommend around three minutes or so for venting. I agree that when it comes to parallel flow steam mains, one or two pipe, the faster the better, within economic reason. If you can it out in a minute, that's great. The idea is get the steam to the end quickly, and let it go up all the risers at the same time. That way there is even heat.

I have a chart on page 85 of my field guide that gives you the capacity of different size and length mains. If you can get the venting rate from the manufacturer, its simple division to calculate how long it will take to get the air out. I had to wait almost 6 months to get the venting capacity of a Hoffman #75 when I was writing the book.

If you have uneven lengths, it is better to select vents to balance the venting, say both mains in 2 minutes or so.

Best regards, Pat
• Member Posts: 3
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Hi Pat..thanks again...you would think that Gorton or Dole would furnish some technical advice but they don't.I actually wrote to Gorton three days ago and have not received a response.I thought the response on the message board that "it is a secret about venting the mains " was funny...but actually there is some strange truth to it.I have two mains 25ft and 75 ft w/3inch piping.I plan to put one Gorton #1 on the short and three on the long.Regards,George
• Member Posts: 152
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mo venting, mo better

Hi Charlie,

The chart in my field guide has 50 ft of 4" at 4.42 cubic ft., real close to your calcs. With the typical rate of 1.4 cubic feet per minute for a #75 Hoffman, it would take 3.16 minutes. Two vents would halve it to 1.58 minutes. Mo venting, mo better.

The quick answer for "how quickly one should vent this volume" depends on how evenly the main(s) heat. The radiation connected to a main has to heat evenly along its length, and multiple mains have to heat evenly. If the mains are of uneven length, the short main might have to vent slower to even the heat in the building. Or, the long main might have to add vents to speed up in order to even the heat. Generally, we speed up rather than slow down.

Best regards, Pat
• Member Posts: 152
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vent rate?

Hi George,

I agree with you about Gorton, Dole or Hoffman. With technical advice on venting, most people are going to buy multiple vents which would increase sales. What are they, anti-capitalism? Educate and they will buy.

25 feet of 3" is 1.28 cubic feet
75 feet of 3" is 3.85 cubic feet

What is the vent rate of a Gorton #1? Anyone?

Best regards, Pat
• Member Posts: 77
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gnewshan

I would put two gorton #1 on the 25' main and a minimum of 1 gorton #2 on the 75'main.If it were mine I would put two #2 on the long main even though they are pricey they really pay dividends on system performance. Good Luck
• Member Posts: 6,688
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I'll second that

Gerry and Steve have done lab-quality measurements on various vents and published the results in the above e-book. All proceeds go to charity.

GNewshan, for your mains as described in your post above I'd use THREE Gorton #2 vents on the 75-foot main and one Gorton #2 on the short one. This is pretty straightforward since the long one has three times the air of the short one. This setup will make the steam reach the ends of both mains quickly and at about the same time.

The Gorton #2 is the biggest main vent available today. It puts the Hoffman #75 and everything else to shame on all but the shortest mains- for details see Gerry and Steve's e-book.

If you can't find Gorton vents in your area, State Supply (www.statesupply.com) carries them and I think PEX Supply does too (www.pexsupply.com). They're not cheap but will save lots of fuel by shortening the system's heating-up period.