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Vacuum breaker placement suggestions

I've been reading the wall for a few days now, and have picked up quite a bit of information to fill the holes on what I knew (and didn't know) about steam heat, I have to say this has been the largest collection of "free" information on steam heat I've ever come across.



Now that that's over with, I'll fill in some details. My grandparents own and operate a 32 unit, 3 story apartment building. It has a 1,500,000 BTU natural gas boiler on a single pipe system. At one point, it was a 10,000,000+  btu coal system that fed two other buildings not even on the same block. Now, it is relieved of even heating the 11 apartments on the end of the north wing. Currently, it has two 6" outbound mains, one about 55-60 feet long, the other about 120 feet long, though the last 30 feet of the longer main has been unused for 25 years. 90% of the footage of the mains is well beyond accessibility, as the crawlspace starts off about 3 feet tall, and by the end of the first 50 feet, is less than 2 feet tall, the rest drops off to near nothing soon after it makes the bend into the north wing, where the building has settled and the dirt foundation has grown. The last time a human being was able to see the end of the north wing main, was when the asbestos insulation was removed 40 years ago, about the same time the coal boiler was to be removed from service in favor of its first gas replacement. The vents are one each on the 4 dry returns, two 2" and two 3", one of which (we don't currently know which one, no one remembers) is no longer used as it was for the north end of the north wing. The vents are two hoffman #76's, and 2 hoffman #75's. I'm going to be replacing those all with gorton #2s, but I'm going to build three or four port manifolds for each of them, so I can add additional venting as funds allow.

The problem we have, is when the boiler shuts down, the vacuum pulls all the steam from the radiators. As an experiment, I had my grandfather run the boiler on direct til my apartment radiators were all completely hot, then i shut each of my valves off, and had him shut the boiler off..an hour later, all the other radiators in the building were cold, except mine, which were still very hot to the touch. A local steam fanatic, who has completely overhauled steam systems in two similar apartment buildings, one of which my family used to own, made the suggestion to install Watts vacuum breakers on the mains. My question is, could I install them in my vent manifolds i'm going to build, or do I need to drill and tap 3/4" pipe threads into the 6" mains, or less likely to ever happen, have bungs welded into the large mains?



Thanks in advance, sorry for the long rambling post, but I spent the evening gathering information about our system, and wanted to get it all down somewhere concrete before it slipped out of my little pea brain.

Comments

  • Steamhead
    Steamhead Member Posts: 15,657
    Were the pipes

    re-insulated? If not, that can cause vacuum. Insulation is the best investment you can make. 
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • Werewolfx
    Werewolfx Member Posts: 16
    Yep

    The mains were reinsulated, atleast as far down as I can see.
  • Werewolfx
    Werewolfx Member Posts: 16
    1000 words? maybe

    I'll take some pics tomorrow after I get off work, that way there's a visual for anyone willing to offer suggestions to go from.
  • Jamie Hall
    Jamie Hall Member Posts: 19,966
    Step one

    is going to be replacing those Hoffman 76s with Gortons or 75s.  The 76 will hold a vacuum while the 75s or Gortons will break the vacuum.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • steam vacuum problems

    when you say that the vacuum pulls the steam out of the radiators, does that mean they never get hot to begin with?

    jaimie is right in suggesting a switch to non-vacuum gorton #2's, and i would say from the size of your layout that several per dry return would probably pay for themselves in a short time.

    on the subject of vacuum breakers-use them to reduce the noise of vacuum on shutdown. i don't think that system efficiency is improved by them-only silence. therefore spend your money on plenty of main vents, otherwise you will pay the gas co. to get the air out through tiny little constipated vents.--nbc
  • Werewolfx
    Werewolfx Member Posts: 16
    They get plenty hot..

    The radiators get plenty hot, but they cool down WAY too fast when the boiler shuts off, in less than 30 minutes of shutting down the boiler, they are room temperature. If you shut the valves off on the radiators just before you shut the boiler down (its thermostatic controlled, but has a direct operation we use for skimming the rust and testing), they stay hot or at least warm to the touch for hours.
  • Werewolfx
    Werewolfx Member Posts: 16
    Pictures as promised.

    All of these pics were taken from the catwalk. There are 5 6" flanges on the steam header, and one that looks to be 3 or 4", it used to run to the other buildings.
  • cold rads

    usually the steam does not hang around in the rads at shutdown, but just quickly condenses. i suppose that a lot of air drawn by vacuum into the rad could cool it down.

    when you get an adequate venting arangement in place for all the dry returns, i am sure you will find that the vents themselves will relieve any vacuum. if there is still whistling from the rad vents at shutdown, then you could add a 3/4 inch horizontal check valve to the vent "antler"; and that will open on vacuum  allowing air in to the pipes. this presupposes that you have the ideal low steam pressure.--nbc
  • Werewolfx
    Werewolfx Member Posts: 16
    more pics, and another question.

    The steam pressure is pretty low, the needle on the pressure gauge barely moves past the 0 mark, well below the 1 mark. I'd guess around 4 oz. I've got a few more pics, I don't remember what the green devices are on the dry return (I THINK its still above the water line here), but they're a semi- recent addition, i think about 15 years ago..any ideas?..A lot of these pics are somewhat random, the sight glass on the tank is stained with rust badly enough you can't see through it, but I recall my grandfather saying that tank has to be kept pretty well full, i don't remember the reason why though.
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