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Need Help with impact of State Changes

  1. Dec 29, 2004 #1
    I need some help from the experts, because I am really having trouble simulating this in the real world which is where I can understand things.

    If propane gas was released from a container (typical metal propane tank or cylinder) where it had been stored as a liquid under pressure, it would go through a phase change from liquid to gas, and would produce a cooling effect. When this happens would the cooling effect have the greatest impact on the metal container, the liquid propane inside the container, or the propane gas inside the container? Or would the cooling effect be pretty much equal to the gas, liquid, and container?

    I am asking because I am a Firefighter on a Haz Mat Team, and we use Thermal Imaging Cameras to detect product levels inside of containers. For an example of what I am talking about you can go to http://cms.firehouse.com/forums2/showthread.php?threadid=19178 . Normally we are able to detect a product level inside a propane tank because the liquid cools the outside of the tank quicker than the air gap above which produces a temperature difference on the surface of the container that the thermal imager can detect and display. We recently had an incident where we had a tank that was leaking and we could not get a clear product level. We were wondering if this was happening because the contents inside the tank (liquid and gas) had been cooled to an “equal temp” or if the shell of the container/tank had been cooled to an “equal temp” or if something else was going on? We verified after we stopped the leak that the tank did actually contain both liquid and gas.

    Any thoughts on this would be greatly appreciated.

    Thanks,
    Mike Richardson
     
  2. jcsd
  3. Dec 29, 2004 #2

    Q_Goest

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    Propane is a liquid stored under pressure as a "saturated liquid" which means its always on the verge of boiling. It stays in thermal equilibrium with its environment, warming and cooling as air temperature changes.
    - As it warms up, some liquid boils, and the pressure inside the container increases.
    - As it cools, some of the gas condenses, and the pressure decreases.

    It's not unlike a car radiator, a liquid under pressure. And just like your car radiator when it's very hot, if you take the cap off and release pressure, the liquid boils. In the case of propane, you don't simply "take the cap off", you typically open a valve which lets some of the gas out that's above the liquid. This is the boiled off vapor, or "saturated vapor". It is at the same temperature as the liquid. But as you release some of the gas, the pressure drops slightly, causing the liquid to boil. When the liquid boils, it actually gets cooler. Think of it this way, the liquid is changing into a gas, and when it does so, there's a certain amount of energy needed to change it. That energy comes from the surrounding liquid and when it does, the liquid cools. The gas does not cool though, so there will be a cooler liquid below the slightly warmer gas. This is how you can tell the level in the container.

    Sometimes you can also find a temperature difference between the liquid and gas without venting any gas. Since the liquid has much more thermal mass than the gas, its temperature will change more slowly, allowing you to thermally see the level even when there is no change of state going on.

    Anyway, that's the basic physics behind it. If you had a container which was leaking and couldn't detect a temperature difference between the liquid and gas, I'd guess that was because the leak was very slight, and perhaps it had to do with ambient conditions. Was the air temperature dropping or rising? Perhaps if it was falling, the drop in the liquid temperature might have kept pace with the cooling affect on the gas. Just a guess. I'd think it would be most difficult to detect any differences in temperature between liquid and gas when the temperature the propane was stored at was held fairly constant and no gas was leaking from the container.
     
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