Noise in One Pipe Steam

Ralph Blaine_2

My one pipe steam system is very quiet for the first week or so of the heating season. After that, one line in particular has a repetitive banging or thumping sound once the steam starts to move. The noise lasts for a minute of two (three?)as if something has to be cleared away and then it is more-or-less silent. The line that makes the noise comes up from the basement all the way to the third floor where it turns and feeds four radiators which I have turned off most of the time. I have never understood this line - it actually has two parallel lines. (See photos included.) Why does each of these lines have a valve on the return portion. Clearly the line on the left is being fed steam but it also has a return so why are there two lines and why is there an air vent on the line on the right which is obviously some sort of dedicated return line?

I should add that at one point I put a big (#1 or #2) Gorton vent in place of the shut off valve on the second radiator of this line(there were no vents on the mains) but I keep the regular air vent for this rad turned down most of the time so that no steam goes into the rad- but the steam does make it as far as the big vent. I can see the two pipes going up through a cupboard on the second floor but after that everything is hidden under the floor but I can see that only one pipe comes up to each radiator.

The noise was present even before I added the big Gorton vent and when I kept the rads in operation.

So to sum up my questions:

1. Why am I getting the noise - is it condensate that is building up somewhere in the line? The noise seems to be on the third floor itself.

2. What is the purpose of having two lines coming up from the basement and what is the purpose of the two shutoff valves and the air vent in the basement?


Finally, I know from Dan's book that the near boiler piping on my furnace in not correct - the two steam headers should not meeting head-on. It's been this way since the start (almost 20 years). How serious is this?


Thanks for any help