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What is the pressure inside domestic LPG cylinders

  1. May 3, 2013 #1
    I have a domestic LPG cylinder.

    Composition - 20% Propane and 80% Butane.
    Temperature - Around 30-35 deg C.

    I wanted to know the temperature of the gas inside the cylinder or in other words, if i take gas out directly from the cylinder without using pressure reducing regulator, what will be the pressure of this gas?
  2. jcsd
  3. May 3, 2013 #2
    As long as there is liquid in the container that means any gas is in a vapor-liquid equilibrium. Also the evaporation of the liquid is going to cool the inside of the container noticeably (so the inside of the container isn't going to 30-35 degrees once it starts flowing). Also if it's LPG you should be aware that the concentration of the mixture is going to morph as it vaporizes (essentially you are performing a single stage distillation).

    ie .... not going to be very constant without a regulator.

    A regulator would be your best bet, but lpg ranges 100 psi - 200 psi when it's in the tank around room temperature when it's shipped.
    Last edited: May 3, 2013
  4. May 3, 2013 #3
    Assumption : no temperature drop.
    or obtaining the pressure at the beginning of the operation, when temperature drop is negligible.
  5. May 3, 2013 #4


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    Really? That low? :confused:
    My 412L medical oxygen cylinders are delivered at 2,000 psi; the red "refill" zone on the gauge starts at 500, at which point I have about 15 minutes to continue breathing.
  6. May 3, 2013 #5
    Seems low to me but that's regulation


    wiki says max is 320 psi at 55 C.
  7. May 3, 2013 #6


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    I put a pressure gauge on my 20 lb tank so I could tell when it's time to refill without leaving a half-cooked meal on the grill. It's 160 - 180 psi when freshly filled.
  8. May 3, 2013 #7


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    The database I have shows the saturation pressure for 100% propane (C3H8) would be 150 psig at 90 F (32.5 C). For 100% butane (C4H10) it gives 48 psig at 90 F. For a mixture of 20% propane and 80% butane it gives 57 psig at 90 F. That's saturation pressure of course.

    Hey Danger. The oxygen you have in your cylinders is pure gas, but LPG isn't. It's basically a boiling liquid with the vapor in equilibrium above the liquid kinda like the coolant in your car when it overheats. As soon as you pop the cap off your radiator, you get a snootfull of boiling liquid because it's right on the verge of boiling. When you remove the radiator cap, it starts boiling vigorously because the pressure in your radiator suddenly goes down. Same thing with the LPG. When you start removing some of the gas, the pressure in the tank drops and some of the liquid boils off and creates vapor. When it does, it cools slightly because some of the energy in the liquid goes into boiling and converting the small amount of liquid to gas. Of course, once it cools off a bit, heat from the environment gets in and warms it back up. But in the case of your oxygen cylinders, there's nothing but gas inside, so there's no boiling and the pressure is determined only by how much oxygen is in your cylinder.
  9. May 3, 2013 #8


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    Q, that is one of the best explanations of anything that I've run across in years, especially because of you tapping into my gearhead background. (I'll have you know, however, that I got antifreeze up my nose only once when I was about 8 years old. Never again. You always take off your T-shirt and wrap it around the rad cap before twisting. :biggrin:)
  10. May 3, 2013 #9


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    Your pressure gauge will indeed show you if the tank is empty, or very nearly empty... if there's no liquid left in the tank.

    You need what is called a "percent full", or float gauge to measure the liquid level, or volume of the tank. They're usually calibrated as a percentage of the total liquid volume capacity of the tank.

    As long as the tank still contains some liquid, your pressure gauge will continue to read 160 - 180 psi, due to vapor pressure. When the LPG in the tank has completely evaporated, the gauge goes to 0... fast... :frown:



    Last edited: May 4, 2013
  11. May 4, 2013 #10


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    OCR, I've never bothered with gauges for my BBQ because the gas station was a 30-second drive away. (It's gone now, but it doesn't matter because I don't drive any more, nor BBQ due to my oxygen supply. Nothing will ruin a good steak quite as much as exploding while trying to cook it.)
    Anyhow, I remember a few years back there was a "gauge" available that you could buy for about $5 and stick on the side of the tank. It was a self-adhesive strip that looked similar to the battery test strips included with Duracells, and supposedly measured the temperature of the tank at each "depth". It purported to determine the liquid level by how cold the shell was. Are those things of any use?
  12. Jun 28, 2015 #11
    how can we determine the ideal pressure for LPG? or how can we say this is already High Pressure or Low Pressure by just looking at the Pressure gauge in 50kg LPG?
  13. Jun 30, 2015 #12
    Good Danger, yes those things are worth it they are actually pretty accurate. They are essentially a strip thermometer. There is a definite difference in temperature with a defined line at the liquid/vapor interface. All you are seeing is an exterior indicator of the liquid level. Be aware that the cylinder being out of level will also affect the indicated level as it is liquid level you are seeing.
  14. Jul 27, 2016 #13
    Use weight scale .... Get a reading full filled and empty.... The difference is filled-in lpg
  15. Aug 2, 2016 #14
    Assumption: if it is in phase equilibrium (vapor and liquid) you can simply use daltons equation to find the total pressure that the total pressure of vessel= partial pressure of the constituents. assuming they the only pressure is the vapor pressure you can simply find it, as both are nonreacting gases.
    but this is a very big assumption you are making that the temperature drop is zero
  16. Aug 3, 2016 #15


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    This thread is dead, the OP posted in 2013.
  17. Aug 4, 2016 #16
    Oops sorry :p i didn't notice the date
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