Two Pipe Steam Questions

MHSteam

I am working on a two-pipe system that is not providing steam to many of the radiators, and the remaining radiators are only partially heating. The systems "B dimension" is 30". There are 3 zones, electric valve-controlled, on the system. It is gravity return, no boiler return pump. Two of the zones have a radiator type steam trap (not F&T) just prior to where the main enters the return, the third zone has no trap where the main enters the return and this is the poorest functioning zone on the system. There are NO main vents in the system. It is clear that some of the piping where the main and returns tie together has been changed recently. My concern is that traps and main vents have been removed over the years as contractors not familiar with steam have worked on the system. My questions are.
-Does each return require a "main vent" after the location where the main ties into the return?
-Should I be using an F&T trap on the mains, just prior to where they enter the returns, as is shown on many of the online two-pipe diagrams?
-Given the 30" B dimension, my understanding is that cut-out pressure cannot exceed 1#. My calculated pressure loss to the furthest radiator is 5 oz. Can I set cut-in and cut-out at 8 and 16 oz. respectively, or am I better off to add a boiler feed pump and use a slightly higher pressure?
Good advice would be appreciated.

Jamie Hall

Good grief. It's a wonder it works at all...

First and simplest, yes, change the pressure to 6 ounce cutin and 12 ounce cutout. Better yet, 4/8, if the pressure control will allow it.

Second, it's highly unlikely that you need a boiler feed pump. Possible, but very unlikely. Read on.

Now you need a wrench or two. At the end of each steam main, there needs to be a crossover trap to the dry return. They are usually normal radiator type vents. Piped as follows: nipple perhaps 6 inches straight up. 90 over, nipple to the inlet of the radiator trap. Then straight down from the outlet of the trap to the dry return. It is likely that you will need two drips, one from the steam main and one from the dry return, to a wet return. Some installations, where there is a good vertical drop, do use an F&T, but in that type of installation the dry return has to have adequate pitch back to the boiler. I also feel that that type of installation needs a main vent on the return, as an F&T really isn't meant to be a vent. Otherwise a main vent on the return is not needed (and in some systems definitely not wanted) as the crossover trap does the venting.

Now you also need a main vent. This should be located on the dry returns at the point where they join together before they drop to the boiler. You may need two at that location, if the piping is extensive.

Now to the zones. They are going to be a real headache. At each zone valve, you MUST also have a crossover trap on the boiler side of the zone as above, or a main vent, and you MUST also have either a drip to a wet return on that side unless the main pitches all the way back to the boiler. On the other side -- the side which can be shut off -- you will also need a main vent or crossover trap and a drip at the valve. If properly done, you don not need a valve on the dry return at that location, and you must NOT have a valve on the wet return, if any, serving that area. If there is no wet return and the dry return ptiches back to the boiler, you must NOT have a valve on it at that location, but you will need a water seal from the steam main to the dry return on the far (shut off) side of the steam main valve, located as close as possible to the valve.