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Safe allowable vacuum for L.P. tank

Solid_Fuel_ManSolid_Fuel_Man Posts: 1,711Member
edited October 2017 in Gas Heating
What would you pull for a vacuum on an L.P. tank to return it to service. This is a tank I refurbished and has been purged with air and open to the atmosphere for a few days. Replaced relief, rebuilt service valve, and replaced fill valve. I own this 500 gallon tank, and would rather use my vacuum pump then fill with vapor 6 times. Just don't want a tank implosion like on mythbusters..... I'm thinking 28"hg should remove a sufficient amount of air/moisture. Not doing anything like 500 microns.

Not sure how deep the new vacuum purged L.P. tanks are. But I'm thinking nothing deeper than 26-28"
Serving Northern Maine HVAC, and Controls

Comments

  • Jamie HallJamie Hall Posts: 11,195Member
    Can you purge it with nitrogen instead? Then you really wouldn't need to pull a vacuum on it at all -- since what you are really worried about is getting into the explosive range. Seems to me, anyway.
    Jamie

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • Solid_Fuel_ManSolid_Fuel_Man Posts: 1,711Member
    edited October 2017
    Wouldn't that still end up with nitrogen in the tank instead of propane? I was thinking that same thing...

    Ideally I'd treat it just like any other closed system, vacuum to micron level, break vacuum with propane vapor and ready for service. I just don't want an implosion of the large vessel designed for 250psi.
    Serving Northern Maine HVAC, and Controls
  • EBEBRATT-EdEBEBRATT-Ed Posts: 6,050Member
    Pull whatever vacuum you want it's not going to implode. Depends on the temperature of the tank when you evacuate. Your trying to get the moisture out. The lower the temperature the more vacuum you need to remove the moisture.

    I would think 28' is fine. What I would do is evacuate to 28" bring it to zero with nitrogen. Evacuate again and go for the propane.
  • ChrisJChrisJ Posts: 10,147Member
    edited October 2017

    Wouldn't that still end up with nitrogen in the tank instead of propane? I was thinking that same thing...



    Ideally I'd treat it just like any other closed system, vacuum to micron level, break vacuum with propane vapor and ready for service. I just don't want an implosion of the large vessel designed for 250psi.

    Yes, it would.
    I recall guys pulling a vacuum on 20 pound BBQ tanks before filling.

    Purging with N2 makes no sense as it'd result in the same thing as just filling it with air in this situation.




    This says 26" HG is safe.



    http://www.propanetank.com/assets/purging-lp-gas-containers.pdf


    Not sure why they say to inject methanol?
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Jamie HallJamie Hall Posts: 11,195Member
    Methanol combines with any remaining water (if there isn't too much...). Think DryGas
    Jamie

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • EBEBRATT-EdEBEBRATT-Ed Posts: 6,050Member
    @ChrisJ , The reason for nitrogen right or wrong is that gas can't cause an explosion unless air is present. If he evacuates and dilutes with nitrogen the amount of air is negligible.

    I have welded a few gas lines. Shut of gas , purge it (the gas), purge with an inert gas (Co2 or nitrogen) then weld. No air is present....no explosion.

    It may be overkill but if you start a big commercial job with natural gas your "supposed " to fill the pipe with nitrogen to purge all the air first. Then when you start up the burner can only get gas or gas nitrogen mixture (not gas & air) so no puffback, no explosion. hopefully

    Purging a big pipe (or a tank) you could have air mixed in with the gas
  • Solid_Fuel_ManSolid_Fuel_Man Posts: 1,711Member
    edited November 2017
    Well in the end I pulled to 28.5"hg, broke vacuum with L.P. vapor, and then pressurized to 10psi. Blew off vapor to 1psi or so, and then filled with vapor the remaining contents of my 100# cylinder which was half full to begin with. Doused 100#er with hot water until vapor transfer stopped and then connected it all up and performed leak check on all new piping etc.

    Hopefully there will be no detrimental effects of (if any) remaining moisture in the tank.

    Now I can officially shop around L.P. companies for the best price since I own the tank! 400 gallons should last several years as well.
    Serving Northern Maine HVAC, and Controls
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