Vent the Entire Pipe System or just vent up to the last take off?

JUGHNE

I have a one pipe system that loops around the basement and then drops at the boiler.
Total length is 165' boiler to EOM drop near boiler. This will be gravity return.
The last take off is 70' from the boiler.....the rest of the 95' is untapped steam main....some refer to it as "dry return" or extension of steam main.

So in my mind, I believe I have to only consider the 70' of 6"/4" pipe for calculations for air vents which are at the EOM in boiler room.

Does the steam "front" push the "air mass" ahead of it and once the steam reaches the 70' point it will not matter how slow the rest of the air vents out as the steam is where it needs to be???

So for main vents at the EOM I need to only calculate the first 70'.

The total air volume for all 165' would be 12.45 cubic feet of air.

6 Gorton #2 @3 oz will vent 13.2 cubic feet.

But to vent only the first 70' would be about 3 G2 air vents.

So at about $100 per vent would the last 3 G2 be a waste of money?

I like to keep the vents in the boiler room where visible, the other choice is above the ceiling of a classroom.

Gsmith

I think you can vent just up to the 70' point. the purpose of main venting is to get the air out of the way of the steam as it goes to the radiators. After the last takeoff, if air still remains in the dry returns (or main extensions depending on your desired terminology) it can just sit there, the condensate will return below the air with no problem. Steam is lighter than air so it won 't want to go into the main after the 70' point as long as it can get to a radiator vent or a radiator that will condense it.

Jamie Hall

Agree with @Gary Smith . No point in venting the rest of that steam return. However... that said, you do still need to insulate it.

dopey27177

Your rating on Gorton vent valves is wrong!

A Gorton # 1 Vent, vents 4 cubic feet of air per minute, A Gorton # 2 vent, vents 16.32 cubic feet of air per minute.

Based on your calculations you need only one Gorton # 2 Vent valve.

Jamie Hall

It might be worth noting that venting rates vary dramatically with pressure at the vent. While the rates cited by @dopey27177 are correct, at lower pressures the rates are somewhat less. A Gorton #2 at 3 ounces measured at the vent -- not the boiler -- vents 2.2 cfm, not 16 cfm.

In the initial stages of bringing a steam system -- one pipe or two pipe -- the boiler pressure shouldn't be more than a few ounces, and the gauge pressure measured at the far end of the main where the vent is certainly shouldn't be more.

EBEBRATT-Ed

I am always in the minority on this.

I vote to vent the whole thing.

To me there is air in the remaining pipe on start up.

Where does this air go it it's compressed by steam?

To me it is going to get pushed to the end where the pipe drops.

As condensate and steam go down the pipe the air has to go somewhere. it's not going down to the boiler. It is going to be pushed backwards against the flow towards the vent

I say vent it get it out of there

dopey27177

For the purpose of venting steam system (not vapor systems with very low steam pressures) Most 1 pipe stem systems operate at 1/2 psi to 2 psi the capacities of the Gorton vent valve as shown are correct and should be followed.

JUGHNE referenced very low operating pressure, What benefit in his installation is gotten by not running the system for five minutes longer to raise the pressure to 1 1/2 psi where he can save a couple hundred dollars by using one gorton # 2 vent valve, additionally if the piping is covered the boiler run time may be less than two minutes.

Also from a cold start air is driven out of the boiler as the air is first heated and pressurized before steam is formed added to that all the vents on all the heating elements are venting air through the vent valves on the radiators.

Yes it's true that air and steam are compressible and steam will ride on top of the air but both flow at about 35 miles an hour, almost all the air will be vented by the #2 vent and rad vents, what is of greater import is the speed at which condensate is pushed to the end of steam main and how fast that water will drain out of the steam main to the boiler.

The velocities of the steam and water is controlled by the size of the piping and formation of condensate by the heat transfer of the piping. Un-insulated piping creates more condensate than insulated piping and that item is the primary concern in system efficiency and fuel savings.

Jake

ethicalpaul

EBEBRATT-Ed said:

I am always in the minority on this.

I vote to vent the whole thing.

To me there is air in the remaining pipe on start up.

Where does this air go it it's compressed by steam?

To me it is going to get pushed to the end where the pipe drops.

As condensate and steam go down the pipe the air has to go somewhere. it's not going down to the boiler. It is going to be pushed backwards against the flow towards the vent

I say vent it get it out of there

I would say the air after the last takeoff/main vent is not compressed by the steam. The steam continues to flow up that last takeoff and if anything, it will pull a tiny bit of vacuum at that point, no?

dopey27177

That being said, nearly all the air is vented out of the system by multiple vent valves in the system. Vacuum is hard to form in a 1 pipe system as the vent valve allow air air back into the system to prevent a vacuum from being formed. If a vacuum was formed wouldn't you get banging due to flashing of steam in the system.

Jake

JUGHNE

The vent location is at the EOM-dry return where the pipe drops down to the wet return.
It seems that these vents will remain open until steam gets to that point.
With only the minimum vents installed, yes it will take longer for them to close.
But in my mind the steam has reached the last take off, heating the building, and is still pushing air out to the end for a few more minutes.

Previous there were a G2 and Hoffman 75 on the EOM. They were quite noisy passing air until steam got to them.
Also this return passed thru a F&T then dumped into a cond pump....this would also be venting air thru the pump.
(Pump was shared by the other 2 pipe system in the building......they are now separate......the one pipe will be gravity return to it's own boiler so the air vents will be the only path for air removal.

I will simply put the G2 and 75 back, with provisions to add more.
(IIRC, a 3/4" pipe can handle up to 4 G2 main vents)
So winter mission is to time how long to get steam to last take off....could do the open 3/4" pipe test also.
Playing with the FLIR gun is fun.

dopey27177

There will always be air in one pipe steam systems.
Simple reason when steam condenses it shrinks in volume, by doing so a vacuum can be formed, the vent valve allows air to enter the system to prevent a vacuum from forming.

Why is every body crazy about how fast air is removed from the system when in the old days in most buildings a Hoffman type 40 vent was used, or a Hoffman 75 was used as the end of line vent.

90% of the questions asked here are in building with heating systems under 250,000 BTUH load. In my experience from a cold start steam would be in all radiators in 15 - 20 minutes.

I am talking about a steam system that has operating vent valves, properly pitched, pipe covered with insulation and operating between 1/2 - 1 1/2 psi.

There are exceptions to how fast air needs to be removed and those are multiple family buildings with long runs of large pipe, in these buildings higher capacity vent valves will need to be used in the basement. Also the operating pressure needs to be considered because radiator vent valves have the proper drop back pressure.

Jake

ethicalpaul

I agree with Jake about it not being worthy of concern generally. (But I disagree about the vacuum part--my system where I'm not even trying to hold a vacuum holds onto it after the heating cycle for several minutes)

dopey27177

To prevent vacuum from forming in the system install a vacuum breaker on the boiler trim.
A soft seated check valve is the cheapest install to break system vacuum.

Jake

ethicalpaul

I desire a vacuum