Big system, no venting

FlyingScotsman

Hello! Long time listener, first time caller.

I am looking at a big system (100+ radiators up and down the building) dating back to about 1900. Looks to have been a vapor system to start (we found some trapless radiators that had been missed in retrofits-gone-by). They run on city steam; condensate is used to heat their loading dock (nice touch) but then goes to drain.

Discrete portions of it had (at least partial) pipe replacements refits and had the big old condensate returns taken out, skinny ones put in, and a local vacuum loop added, while the bulk of it still runs on gravity return.
The vacuum pumps were deactivated about 20 years ago, and so the crew run the whole system at about 4psi, which they argue, is needed to get to the top of the system.

They have thermostatic control valves and thermostatic traps on most of the radiators right now. There are blowing and plugged traps all over, so the 4psi isn't doing them any favors, (I guess, from reading here, that the blowing traps might be responsible for a few of the apparently plugged ones) and we're working on switching those out.

Now, here's my question. There are no vents anywhere on the steam mains, risers...nothing.
Any amount of piping could have been replaced over the life of the building, but there aren't any supply side vents. All the air must be being pushed through the traps right now.

If I want to get them down to 1psi or less, do I introduce vents? It's a very big system...I keep thinking of all that air being pushed up into the risers, and getting caught ahead of closed thermostatic valves, or plugged traps...or at the tops of risers (I found the top of a riser in front of a closed attic control valve that went from 230F to 65F in the space of about 6 feet as you went up).

Three years from now, when there are a bunch of failed traps again, the air removal for the system will be bad, and they'll crank the pressure again...

Jamie Hall

You do need to get the pressure down... if it really was a vapour system, it will help a lot!

And you do need to vet all the traps. That high a pressure has done them no favours, and you need to check them all.

Now. Vents. You will probably get as many different opinions as there are answers to your query. That said... are there returns near, like really near, the mains? If there are, then the question is, are those returns piped so that they drain freely (I like the loading dock touch!)? If they are -- no water traps anywhere -- look around and see if there is a way to introduce crossover traps at the tops of all the risers. A crossover trap is just an ordinary radiator trap -- though you may want to use large ones -- piped so that air can get to it, but not water. It is connected to the steam main -- or riser -- at the inlet, and the outlet is connected to the freely draining (dry) return. They have a huge venting capacity, and they are cheaper than vents.

If you can't do that, yes you will have to add main vents to the tops of the risers. Size them in the usual way (and, again, you will get as many opinions as there are replies...)

nicholas bonham-carter

What about restoring the vacuum pump?
Steam rushing into a vacuum, travels very quickly. The traps may spring back to life when the highest pressure is mere ounces.--NBC

FlyingScotsman

Jamie- thanks for the reply! The steam supply and the condensate return are typically in the same chase, with legs out to and from radiators making the connections left and right as needed.

At the moment there are F&Ts at the base of the supply stacks that tie into the dry condensate return (for condensate forming in the supply pipe up, I assumed).

For a crossover trap...you're talking about introducing a loop into the top of the system that vents air through another (larger) trap into the return line? Makes sense.

FlyingScotsman

Nicholas - thanks for the reply.
Oh yes, we have to do that for those areas with the smaller condensate returns.
I think it's easy to say 'we need a vacuum pump back on' but tougher to say why, other than the condensate pipe is likely now undersized.

I was trying to figure out how to prove that.
Beyond calculating the EDR and associated volume of condensate produced in pounds...water volumes are always so low I'm left asking 'really? that few gallons should be okay in a 3/4"...shouldn't it?'

nicholas bonham-carter

Now that i think of it, how would the condensate drain out from a vacuum???
Maybe the building had it's own boiler before going on district steam.--NBC

FlyingScotsman

nicholas bonham-carter said:

Now that i think of it, how would the condensate drain out from a vacuum???
Maybe the building had it's own boiler before going on district steam.--NBC

It did. Old school coal-fired boilers making 200psi which they used for DC electrical generation, and once they'd done that, they were left with (slightly greasy) 1-2psi steam.

Jamie Hall

FlyingScotsman said:

nicholas bonham-carter said:

Now that i think of it, how would the condensate drain out from a vacuum???
Maybe the building had it's own boiler before going on district steam.--NBC

It did. Old school coal-fired boilers making 200psi which they used for DC electrical generation, and once they'd done that, they were left with (slightly greasy) 1-2psi steam.

That does date back. I'll bet they were steam engines... pity you don't still have them!

And yes, the loop at the top is exactly what I was talking about. So long as the air can get out of the return somehow, they work just fine.

Steamhead

Check out those "trapless" radiators. You're looking for original radiator shutoff valves and/or devices in the radiator return connections where traps would be mounted. Do any of these have the manufacturer's names or trademarks on them?

Here is a system which is larger than yours, where we found such things and could therefore ID the original system. This helped us get into the heads of the Dead Men while working with it. And look at how we vented the main...................

http://forum.heatinghelp.com/discussion/142217/the-king-of-all-crossover-traps

Where are you located?

FlyingScotsman

Steamhead - Thanks, and grateful to have your esteemed input! That looks like an interesting job. I'm going to go check out the double-manual radiators this week - only just had word of them, but the top-and-bottom of stack hardware is not as old. I'm excited to see them, because - as you say - I want to get into the heads of the Dead Men. Hence the research on the building's old coal cogen system!

I like your approach to venting the main. Clever in how it's structured, but also unlikely to lead to failure in the critical components. Good for systems that aren't likely to get a lot of maintenance.

I'm running some calcs on system volume now...will share my draft approach later!

This is in NYC.

Steamhead

Double-manual radiators? If you mean shutoffs on both the steam and the return side using standard radiator valves, it might have been 2-pipe air-vent.

But if there is a manual, key-operated valve on the return side, it might be a Moline.

Post pics please!