2 pipe steam risers not heating up

JStockMAC

I have a boiler fed steam radiator system at a 22 story apartment complex. The system operates at 5psi. The system is set up so that there is a main distribution steam header that supplies steam risers to the individual apartments. Each riser has 2 radiators per floor, with a thermostatic valve and operator and a steam trap on each. The steam trap drains to a condensate return header which feeds back to a boiler feed tank. The risers have a drip leg and a F and T steam trap which also feed back to the condensate return header.

The issue I am having is that 2 of the risers ( which are next to each other) keep losing all heat. I have replaced the F and T trap and drained them down and they will work for a short time before they fill up again. The 2 risers are at the beginning of the steam distribution header and the end of the condensate return header. What could be causing the trap not to drain the riser down?

nicholas bonham-carter

Are all the radiators fin tube like the photo?
Is there a reason for the pressure to be so high? The Empire State Building operates fine at 3 psi.
A line drawing would make it easier to visualize the layout of the piping for the system, and how the air is supposed to escape when steam is rising.
Somehow the air is not getting out from these risers, and preventing the steam from getting in, maybe from a failed trap allowing the returns to fill with steam, and closing adjoining traps. —NBC

Jamie Hall

Are the risers vented? They need to be. Perhaps more important, are the dry returns vented -- the high level "condensate returns"? They need to be, too. F&T traps are wonderful gadgets, but they are not vents. The air has to have some other way to get out.

Check and see that nothing is pressurizing those condensate returns -- such as steam coming from a failed trap somewhere.

And as @nicholas bonham-carter said -- consider reducing the working steam pressure to perhaps 2 psi.

EBEBRATT-Ed

@JStockMAC

I assume you checked the strainers. And yes a near by trap that is blowing steam would do it.

Why not disconnect the union down stream of the trap and see what you get! Let the trap dump into a barrel and see if it works. If the return goes to a boiler feed tank you shouldn't get any back pressure with the union disconnected, if you do that could be part of your issue and you can cap the union temporally to see the trap function

JohnNY

Lower the pressure and start blowing down or replacing those Y strainers. People tend to gloss over them like they're not even there.

JStockMAC

The strainers were being blown down regularly, and there would be material coming out and then clear water, so the assumption was that it was ok. After further checks, there was a pluggage just prior to the strainer which still let some condensate flow through but not enough to keep from backing up. I appreciate the suggestions. They helped in pinpointing the issue. Thanks