Calculating Main Venting

Abracadabra

I've got 2 locations I want to calculate the main venting for, and I've come up with some questions in trying to calulcate the main venting required.















Location 1, I've attached a sketch of the main piping layout in the building.  I'll leave location 2 out for now as it's piping is a bit more complictated.



I've come up with the following calcs  of what vents vent what main.



V1 = 33' 3.5" (2.2cf), 18' 3" (.9cf) , 70' 2" (1.53cf) Total cu.ft to be vented: 4.63



V2 = 15' 2.5" (.51cf), 43' 2"  (.94cf) Total cu.ft. to be vented: 1.45



V3 = 37' 3" (1.81cf), 10' 2" (.21cf),  31' 1.5" (.38cf)  Total cu.ft. to be vented: 2.40 



I have not included the mains that are vented by more than 1 vent.  This is where I'm kind of stuck. How do I include those in my calculations?  The 12' 5" main (1.63cf) is vented by all 3 vents.  The 54' 4" (4.7cf) main is vented by 2 vents (V2 + V3).



was thinking to somehow add a portion of that to each vent in proportion to the venting that each vent has to do.



ie. If we take the smallest amount of venting to be done (ie. V2) and calculate ratios to somehow to somehow include the venting of those shared mains into the venting requirements for the vents.



V1 = 4.63/1.45 = 3.2x shortest main 



V2 = 1.45/1.45 = 1.0x shortest main



V3 = 2.40/1.45 = 1.65x shortest main 



So, main question is: How do I add in the capacites of the shared mains into the venting requirements for the main vents? Split evenly? Split by ratio? Just add to the longest/biggest run?.....????



Also I've found these specs for venting capacities of Gorton #1 and #2. These are correct? I'm assuming at 2oz pressure?



Gorton #1 0. 330 cu.ft. per minute



Gorton #2 1.100 cu.ft. per minute.







Thanks!

Abracadabra

another thought

thought about this a bit more.. Since we are trying to get steam to fill the mains all at the same time, I think I can forget about the 12' 5" line since steam will fill that "at the same time" I have to think about once it splits left and right from there.







So thinking about filling the branches of the mains equally, the left branch (vented by V3) has to fill at the same time as the 2 branches on the left (vented by V2 and V1).  Dividing the pipe capacity by the venting rate of the vent on that branch gives us the time to fill the main with steam.

2.41/V3 is the time to fill the left branch.  This should equal the amount of time to fill the middle and right branches. The capacity of the 54' 4" shared main vented by V2 and V1 is 4.7cf.

The amount of time to get steam to V2 is

Shared Main + middle branch: (4.7cf/V2+V1) + (1.45/V2) 

The amount of time to get steam to V1 is

Shared Main + right branch: (4.7cf/V2+V1) + (4.63/V1)

So now we've got 2 equations:

 2.41/V3 =  

(4.7cf/V2+V1) + (1.45/V2) 





 2.41/V3 =  

(4.7cf/V2+V1) + (4.63/V1)







Solving gives us:

(1.45/V2)  =  

(4.63/V1)



Let's assume 1 gorton #1 for V2. That's 4.4 minutes to get steam to the main vents.

4.4 = 4.63/V1.  We need 1.05 cfm for V1.  That's 1 gorton #2

2.41/V3 = 4.4. That gives us 1.85 cfm for V3.  That's .54 cfm. That's 2 gorton #1

Do my calcs sound right?

Dave in QCA

3 minutes is a good target

You generally want the steam to get to the end of all 3 mains, 3 minutes after the boiler starts to steam.

I would divide the volume to the stretch that serves all 3 mains by 3 and add that 1/3 to the volumet of each branch.  The same with the part that is shared by 2.  Add up the total cubic feet, divide by 3 minutes, that is the cfm that you need to vent at each vent point. 

Your specs for the Gortons are correct, but at 1 oz pressure, and that is what I would use.  Another very common vent is the Hoffman #75.  It vents at .5 cfm at 1 oz.



This should give you fast venting and even steam distribution.  Is this a one pipe or two pipe sytem?

Abracadabra

venting

Dave,







Thanks! This is a 1 pipe system. With regards to splitting up the 3-way shared main evenly, wouldn't the fact that each branch is going to have different venting speeds, alter the ratio of how much I add to each branch? Or does "close enough" work?

Right now, I've got a gorton #1 at V3, a gorton #1 at V2, and 2 gorton #2 at V1.

The reason I'm going thru all of this is that the radiators off the right most branch (vented by V1) are getting steam much later than the rest of the building. My gut feeling (without going thru all these calcs) is that I need to slow down venting at v2 and v3 and add more at v1.  But the calcs are telling me different and that I need to add a main vent to a branch that is already getting heat very quickly. I'm confused.

jpf321

you want to ...

vent the mains .. but you also have to take into account the radiation ...



additionally, 3mins might be a great target for hot pipe .. but for pipe which is cooler than steam-hot, you have to understand that steam-creep factors very heavily. the steam must use most of its energy (condense) warming the pipes and this is why you should also need to have well insulated (at least 1" thick insulation) on the pipes. keep those pipes snuggled in and they will stay warmer and steam-creep will be less of an issue.



so vent the mains to the point where you have the equivalent of bare-open pipe (don't forget that a 1/2" open pipe can only vent so much)



if you need to fiddle with the timing of the steam to the rads, address that via the rad venting.



you can grab my excel venting worksheet which is in my signature and based on the work by Gerry Gill & Steve Pajek.

Abracadabra

tweaking required

Well, I've gone thru the entire building and replaced all main vents and all radiator vents.







Building is still heating unevenly.  I think all these calculations are just a starting point, and I'll have to do some additional tweaking of main vents.

I currently have 3 Gorton #2 at V1. 2 Gorton #1 at V2, and 1 Gorton #1 at V3.

V2 and V3 get steam in about 2:45 after boiler starts steaming.

V1 doesn't get steam until about 5:00 after boiler starts steaming.  That's over a 2 min difference, and in 2 minutes the radiators fed by the V2 and V3 mains have already started getting hot, while the V1 radiators are still cold.

Calculations were showing only 2 Gorton #2's required at V1, so I'm already overventing that branch.  I'm thinking about maybe underventing V2 and V3 to get steam pushed to the V3 branch maybe.