Need Help with Main Vents Sizing

Marc_17

My steam boiler has a single main pipe, 61 feet long, with an inner diameter of 2.5 inches. At 37 feet down the main, there's a Hoffman #41 vent up on a nipple. At 58 feet, there is a Hoffman #4A vent up on a nipple.

How quickly should the main be venting with this setup?

Would it be reasonable to replace the #4A with a #2 to speed the venting? If so, should I move the #4A to where the #41 is now?

(As a minor matter, the #41 tends to stick open when the system is steaming - I can rap it with my knuckles to close it. It would have to be cleaned or replaced if I kept a vent at that point in the system.)

Any comments or suggestions would be appreciated.

nicholas bonham-carter

vent

i would go with 1 big vent at the end of the dry return just before it drops down to wet.the 2nd vent appears to be in such a location, so use a big gorton [PROTECTED FROM WATER HAMMER].dans steam books, available on this site, discuss how to do this.cap the intermediate vent tapping.
also check your pressure it should be OUNCES not psi, and you will need a good low pressure gauge[0-5] to get an accurate reading [gaugestore.com].these small investments when i made them were returned in a couple of weeks!--nbc

Marc_18

Thanks for the reply. I plan to do as you suggest - remove the intermediate vent and replace the vent at the end of the main with a Gorton #2.

I just got the excellent "Balancing Steam Systems" booklet today and that is very helpful in selecting vents. From the chart, the Hoffman 4A that's now in place appears to take 10 minutes to vent the main. Seems very undersized - I don't know why the plumber who put in the heating system selected it...

Steamhead (in transit)

On that main

I'd use TWO Gorton #2 vents at the end. Plug the hole in the middle.

Why two? Well, the entire boiler output goes into that one main. If the mains were split, each would have its own vent and the boiler's output would have access to two vents.

I've often found on those long single mains, that doubling the usual vent size works quite well.



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Marc_18

Here's what I did last night--

Per the Gill/Pajek Venting Capacity write-up, I took off the Hoffman #4A from the end of the main and fired the boiler from a cold start. I would say that it took the header pipe about 12 minutes to become warm/hot. (By the way, all the near-boiler piping is currently uninsulated. I have 1" fiberglass pipe sleeve that I'll be installing. The main and the runouts and risers all have the original ~1" asbestos insulation which I've cloth-wrapped.)

After the header pipe became warm/hot, I monitored the 1/2" vent nipple for steam. It took a good ten minutes from that point to see steam! At 1 oz. of pressure, that open nipple should have fully vented the main in eight-tenths of a minute. One Gorton #2 should vent the main in under 2 minutes (1 oz pressure). The original Hoffman 4A should take almost 16 minutes (at 1 oz) to vent. One #2 has more than eight times the venting capability of the 4A. But the open 1/2" nipple still required as much as ten minutes to show steam.

Let's say that I misjudged by a factor of two the point at which steam started to enter the main. Then it took five minutes after the header pipe got hot for the open nipple to show steam - still longer than one Gorton #2 would take to vent the main.

Though I expect a speed-up after insulating the near-piping, I'm now not sure what a sensible main vent size would be. Even one Gorton #2 would seem to be overkill. Did I actually have much less than 1 oz of steam pressure by venting into the open nipple?