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  1. #21
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    You are working off some incorrect assumptions.

    LPG in a container follows a simple pressure/temperature relationship as long as there is liquid in the tank. (If there is no liquid there can be no BLEVE though there can still be an explosion.) If LPG is 132 psi at 70 degrees, it does not matter that it was 200 degrees and 400 psi a minute ago. Its pressure is solely dependent on its temperature. If any vapor is lost out the relief valve it will instantly be replaced by the boiling of the liquid. And the liquid will boil in this manner all the way down to -44įF. The only thing that changes is the pressure that it operates at.

    Birken
    Last edited by BirkenVogt; 07-18-2006 at 09:54 AM.


  2. #22
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    Those of you who would apply water to the liquid space should check the facts: cooling the the tank may have no effect on the temperature of the liquid left in a tank. LPG acts as a heat sink and basically protects itself. Failure of the liquid space is (according to NPGA) virtually non-existent. The same principles of heating a container can be used for LPG as the FF 1 class demonstration. Fill a styrofoam cup half full of water and put a flame to the bottom- doesn't burst in fact you can boil the water! Now put it to the top where there is no water- instant melt!

    Putting water on the point of direct flame impingement is to keep the steel from reaching its failure temp, not to cool the liquid. As for cooling the inside contents, you ust apply water to the vapor space above the liquid line.

  3. #23
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    17th edition of the Fire Protection Handbook (FPH) (pages 3-67 to 3-68): "As shown in Figure 3-7A, the strength of carbon steel steadily decreases with temperature increases above about 400F (204C). Figure 3-7A is based upon a typical low-carbon steel. The curves will vary quantitatively with other steels, but the loss of strength with increasing temperature is valid for all common metals, and the critical temperatures are well below those attainable in a fire." My take: as the steel is heated it loses strength.

    FPH: "Figure 3-7A also shows why entirely satisfactory performance of a spring-loaded relief valve to design parameters cannot prevent BLEVE. By its nature, such a valve cannot reduce the pressure to atmospheric but only to a point somewhat below its start-to-discharge pressure. Therefore, the liquid will always be at a temperature above its normal boiling point, pressure will remain insided the container, and the container structure will be stressed in tension. This stressed area is shown as a shaded area in 3-7A for a common type of LPG container having a pressure relief valve set for 250-psi. Again, while this area will vary for different steels or pressure vessel relief valve design characteristics, it is evident that if the metal is heated above this range (which is quite possible in the event of direct flame contact), the metal will not withstand the stress and the container will fail." My take: since LPG has such a low boiling point, the material within in the vessel cannot be cooled to a point of vaccuum. There will always be pressure (although I have experienced -60F and I am curious if this still holds true).

    FPH: "It is extremely difficult to significantly heat the container metal where it is in contact with liquid because the liquid conducts the heat away from the metal and acts as a heat absorber. For example, when the relief valve cited in this example is discharging, the propane liquid stays in the 120F to 140F range. As a result, the metal temperature is well within its safe limits. This situation does not exist for the metal in the vapor space of the container, as vapor is relatively nonheat conductive and has little heat-absorbing capacity." My take: cool the vapor space because the liquid space will not be heated to failure.

    "In most BLEVEs where the failure is due to metal overheating, it originates in the metal of the vapor space, and is characterized by both the metal stretching and thinning out and the appearance of a longitudinal tear which progressively gets larger until a critical length is reached..."

  4. #24
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    Though cooling the liquid space is definitely the wrong thing to do in a venting tank scenario, it does have an effect on the temperature of the liquid inside the tank, only slowly.

    Once the relief valve has closed, the water stream should be continued to bring tank pressure down as low as possible.

    I have used the application of water to the outside of LPG and other compressed gas/liquid tanks for a variety of applications. For instance heating one barbecue cylinder (upside down) with hot water to transfer liquid over to another. Or using room temperature water, from a fire engine, on a burn demonstration propane tank to warm the contents so that it does not freeze up due to the rapid rate of draw-off by the burn apparatus.

    A side note, if you have a propane or other tank that has had the valve sheared off and is at atmospheric pressure, you will get a rapid release of propane as the liquid boils until it reaches its zero vapor pressure temperature of -44įF. At this point the tank will be covered in frost. The only gas escaping will be due to what little heat is being put into the liquid from the surrounding air. If you hit it with a water stream, you will warm the liquid much more effectively than the atmosphere was doing and it will result in a rapid increase in rate of vaporization of the fuel. Now if you are hitting the tank incidentally with a hose stream that is there to protect your men who are moving up to close a valve or something then that is a judgement call you have to make at the time.

    Anyway I have gotten off track.

    Birken

  5. #25
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    I'll admit it, I was wrong. Birken made me "see the light", and I finally found my stuff from the training class I had on tankers and figured out what had me twisted around. Had the wrong stuff in my head mixing one thing with another.

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