Vapor to traditional steam

Fpipes

Hi. I have noticed in my area that many of the old residential two pipe vapor systems in My area with thin 1/2 inchsupplys and adjustable valves with steam traps now just work like a regular two pipe but not optimal. Is there a way to make it work on vacuum again or just make it work better as a traditional set up? 

Steamhead

"Vapor" refers to the operating pressure. These systems were designed to run on roughly 8 ounces of steam pressure at the boiler. Some could go into vacuum to allow the boiler to continue making steam as the coal burned down over the course of a few hours. This doesn't always work well with the shorter firing cycle of an oil or gas boiler.

If you have a Vapor system that doesn't heat well, make sure the air can get out of the system quickly. These systems typically have one air vent located near the boiler, and the mains are vented through steam traps at the ends.

If you haven't already, get a copy of "The Lost Art of Steam Heating", here:

https://heatinghelp.com/store/detail/the-lost-art-of-steam-heating-revisited

Chapter 15 covers Vapor systems.

Jamie Hall

You absolutely have to distinguish between "vacuum" and "vapour". They aren't the same thing at all. The term vapour refers to steam systems which were designed to operate on a very small pressure differential between the steam supply lines and the dry returns. Specifically, less than 8 ounces (usually more like 6 ounces).

Vacuum may refer to one of two styles of operation -- vapour systems which are allowed to drop into a vacuum throughout the system, but maintain the pressure differential, and systems on which a vacuum was applied only to the dry returns, usually to speed air removal from the system.

Chances are that the systems which you are looking at really were vapour systems. If so, there are really only four things needed to get them running well (other than the obvious stuff, like pipe pitch!). First, make sure all the radiator traps are working properly. They aren't really needed that much if the valves are set right -- but that's not a given. Second, almost all of these systems had crossover traps. Make sure they are still there and they are working (it's easy to tell -- run the system, and the inlets should get steam hot or close to it, while the outlets should be no more than warm). Third, make sure that the main venting is working and adequate. If there are crossover traps, the only vents should be where the dry returns join before returning to the boiler, but they need to be generous.

Then fourth and last, the boiler will need a vapourstat as the control (keep at least one pressuretrol as your safety), set to a cutout of 6 ounces and a 3 ounce differential. Check the pressures with a reliable low pressure gauge -- modern vapourstats are precise enough, but the calibration is none to accurate.

Now if you have reason to suspect the system may have been played with, there are a few other things which might have gotten messed up. Boiler water line is one. Make sure that all your wet returns are below the water line, especially where drips come down from steam mains or dry returns. If not, lower the wet returns if you can -- or come up with a false water line. I someone got creative and put in vents instead of crossover traps, you may have some strange balance problems if they neglected the main vents on the dry returns. That can be fixed by making sure the proper main vents -- on the dry returns -- are working and adequate.

Note particularly if someone put vents on the steam mains that they will have defeated the pressure differential control devices characteristic of vapour systems; this makes the vapourstat all that more important.

JUGHNE

Were there previously vacuum pumps on these systems?

The 1/2" supplies might be limited delivery without pumps?
IDK, just curious of the small supply size.

Jamie Hall

On the 1/2 inch pipe -- it does seem as though it should be too small, but most of the smaller radiators which Cedric powers use 1/2 inch. The larger ones are most 3/4 inch.

Fpipes

Thanks for the info. This was a hoffman vapor system for sure. The only vents are on the dry return and below that are swing checks. I will start by replacing the radiator traps as the elements are missing and new main vent as well as a new swing check as it is corroded. I'm still curious as to how the vapor can be achieved with this as is. A diagram of the cross over trap would help. Thank you guys for all the help I have been working with steam for a long time in New england with success but want to learn more about the vapor set ups as they seem like an ingenious dying art and I don't know what products are available today to make if function best.

mattmia2

Since the steam trap elements are gone anyhow, you could carefully set up the inlet valves to only admit as much steam as the radiators can condense at the cut out point of the vaporstat. (or use orifice plates if some of the vapor radiator valves are missing on those radiators)

Jamie Hall

The check valves aren't needed -- and may present troubles. They were sometimes added in the hope that they would prevent water from backing up into the dry returns -- which they won't. But you do need to keep the pressure down.

Keep in mind that "vapour" is always created -- it's simply a term used for very low pressure steam.

A crossover trap is simply a standard thermostatic trap -- almost always the same ones as used on radiators in the installation. I have no good way to draw a diagram, but they are piped as follows. At the end of the steam main -- or certainly very close to it -- there will be a T and a nipple -- usually about six inch, but it's not critical -- going straight up. Then a 90 and another nipple going horizontally over to the inlet of the trap. From the outlet of the trap there will be a pipe -- in many installations just a nipple of appropriate length -- going straight down to a T near the end of the corresponding dry return.

This arrangement allows air to escape from the main through the trap and go into the dry return, but will close when steam hits the trap.

Very often the steam main is piped parallel flow, and the dry return parallel to it and therefore counterflow. So there will be a pair of drips -- one from the steam main, one from the dry return, down to a corresponding wet return. This is a source of potential trouble, as the wet return is often laid on a slope back to the boiler (it doesn't have to be) and, as a result, a boiler replacement may have lowered the boiler water line to the point where the wet return at the ends isn't wet any more -- allowing steam to go around from the steam main into the dry return. This needs to be checked and corrected in some way if it is true.

You mentioned check valves above -- they aren't needed. However, the pressure must be kept low enough so that you have a minimum or 28 inches from the water line up to the dry return for each psi of boiler pressure. Since vapour systems run at half a pound or less, this shouldn't be a problem -- but if they have been modified to run on higher pressure, it can result in the dry return flooding, which is a very big problem. Check valves won't prevent this, but they do resist the flow of condensate out of the dry returns which can also result in the dry returns flooding. Further, to be of use they would have to be on every single drip in the system -- and they usually aren't. Get rid of them.

Fpipes

Thank you for all the input. I will keep everyone updated when I start my endeavor. 

Steamhead

@Fpipes , does this system have a Differential Loop mounted near the boiler?

Fpipes

Currently no. It is just piped as a two pipe steam. Possibly any device it had if any was removed. I would like to get it working better without the addition of any large piping arrangement. Can a differential loop still be purchased or something with a similar objective?

Fpipes

I figure with new rad traps and a nice return vent I should at least get heat to the 2 top fl rads that aren't working. Wish the supply risers weren't so small but I don't plan on changing that. I'm still trying to picture how these devices like differential loops or steam vapor generators create the magic of vacuum. Is it the higher pressure moving to the low at a modulated rate? How would today's vapor system look if it was done today?

Steamhead

Don't need the Loop now. It served to make sure water could return to the boiler if the steam pressure got too high. Nowadays we replace the Pressuretrol with a Vaporstat and set it to 8 ounces or so.

Fpipes

Thank you 

Jamie Hall

Note my comment on vacuum. Some vapour steam systems did drop into vacuum as the coal fire died. But that wasn't the essential or even necessary feature. It's the small pressure differential which is the key to their operation.

A brand new vapour system today would look very like what you have.

mattmia2

Look for returns that are steam hot, if there is steam in the returns, air can't vent from the radiators that aren't heating(also could be trapped water) then figure out where it is coming from. If you can track that down, you could possibly fix it with a little adjustment of some inlet valves instead of having to install a lot of radiator traps.

Jamie Hall

mattmia2 said:

Look for returns that are steam hot, if there is steam in the returns, air can't vent from the radiators that aren't heating(also could be trapped water) then figure out where it is coming from. If you can track that down, you could possibly fix it with a little adjustment of some inlet valves instead of having to install a lot of radiator traps.

Indeed you can. Get the pressure under control, and then work on adjusting the inlet valves to control the steam to the radiators -- and persuade people to kindly keep their mitts off them... then note which traps seem to be malfunctioning and fix them.