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OT: Propane Tank Fill Percentage And Safety

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  • OT: Propane Tank Fill Percentage And Safety

    I got home last night to fine that my gas company had been out to fill up the tank. OK, good. I didn't have to call and they were on the spot.

    But the gauge read about 91 percent full. Now it is marked with cross lines between 80 and 100 percent to indicate that it should not be filled any more than 80 percent. And I have been told by more than one gas company employee that you must leave some space in the tank for expansion. So I called the gas company and the lady there says that the weather is getting warmer so it is perfectly OK to fill to 90 percent.

    I'm not sure of the exact reason for the 80 percent thing so perhaps some of the knowledge here can help out. My suspicion is that with the warmer weather, lower prices will follow and they are just trying to sell as much as they can at the higher rates before summer.

    Any thoughts? Is it really safe? Or is she full of it?
    Paul A.
    SE Texas

    And if you look REAL close at an analog signal,
    You will find that it has discrete steps.

  • #2
    I imagine the 80% fill limit is based on filling at the lowest temperature likely in the area. The propane would be at it's minimum volume and allowance must be made for expansion at the highest temperature likely. As the average temperature increases with the season a higher fill percentage is tolerable because there is less difference between the fill temperature and the maximum expected temperature. It is already expanded part of the way. In theory, one could fill the tank to 100 percent safely if the propane was already at the highest temperature expected such as on a very hot summer day.

    Note: Propane is stored above ground so it will be close to whatever the current average ambient temp is when delivered.
    Last edited by Evan; 03-10-2007, 01:54 AM.
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    • #3
      Fill percentage

      Do not trust the gauge, they are to get a general idea of what volume remains in the cylinder for the end user. When the cylinder is filled they are not supposed to refer to the gauge when filling. There is a height calibrated tube (about 1/16" ID) that is generally soldered into the bottom side of your valve assembly. The tubes length is such that it's end is at the height of the tank that is the 80% full capacity level. If you look at the valve and picture the discharge of the valve at 6:00 and the pressure relief at 12:00 there is a discharge for this tube at the 3:00 or 9:00 position. The valve usually has a small thumb wheel that is opened by hand. On a 20# barbecue grill it is a straight bladed screw driver to used to open it. This is what is used to determine fill percentage.

      The person filling the tank is supposed to do the following. They hook to the valve and start filling. They open this bleeder and keep filling while propane vapor escapes from this tube. When the tube starts to discharge LIQUID they are supposed to stop.

      You should never fill a propane tank to 100% no matter what the conditions. The relief valve is designed to relieve vapor pressure. If you vent liquid out the valve it could freeze open due to the rapid cooling of the liquid as it boils to gas. The most dangerous siuations for a liquified compressed gas is the BLEVE. A BOILING LIQUID EXPANDING VAPOR EXPLOSION is what you see on video as the sudden and catastrophic failure of the vessel. With flame impingment on a correctly filled tank the relief valve is designed to vent increasing pressures and reseat upon venting. The relief valve can not relieve LIQUID fast enough to prevent a BLEVE.

      The lower the temperature the higher you can fill the cylinder. The vapor pressure curve dictates this. As a liquified gas, the liquid boils until it reaches a temperature specific pressure and then just moderates until you use some gas. This pressure is plotted as vapor pressure curve on an X,Y axis graph. As the temp rises so does the internal pressure of the cylinder. Have you ever noticed that most cylinders are painted white? Helps in keeping the pressure down in the summer.

      As long as the next couple of days are generally the same temp wise and you are using propane there is not a problem.

      The fun part is getting the gas out when the valve does not work.

      Rick

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      • #4
        The lower the temperature the higher you can fill the cylinder.
        That is true in principle. The accurate statement is the lower the temperature the more propane that can be put in a tank. It doesn't mean you should.

        Liquid propane changes volume by about 1 percent for every 6 degrees Fahrenheit change in temperature. That means propane filled with a temperature of zero F will occupy 10% less volume than the same weight of propane at 60F. The lower the temperature of the propane the LESS the tank should be filled.
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        • #5
          Hey, Paul if you want to know if this is bad just call the state regulating agency for propane or call your local fire marshall. Tell them the gauge is reading 91% and see what they do. :O
          Jim, By the river enjoying life...

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          • #6
            Lower the temp

            Evan,

            The lower the atmospheric temperature the higher you can fill the tank. This is related to the vapor pressure curve and lack of need for expansion space. I routinely work on railcars that are at 90% in the dead of winter. If it is not going to expand you do not need room for expansion. It is all about being liquid full and the relief valve. When one of these vents when you are getting ready to tap it, you can literally hear a certain muscle of mine tighten up.

            Rick

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            • #7
              The colder it is the more propane by weight you can stuff in the tank. That is NOT how it's done, if done according to regulation. Allowance must be made for temperature and the lower the temperature the less the percentage fill allowed.

              From NIST:

              The effects of temperature expansion of the liquid product must also be considered in determining how much product should be delivered to a receiving tank. In general, the colder the liquid that is delivered, the greater the amount of head space that should be allowed for expansion. The reason for this is that if the product delivered is colder than the tank and the surrounding air temperature, it will expand as it warms. This process will be gradual, and it may take a number of hours before the product has warmed -- and expanded -- fully. For this reason, sufficient vapor space must be preserved in the top of the receiving container to permit expansion of product. Provisions of National Fire Protection Association (NFPA) 58 (“Storage and Handling of Liquefied Petroleum Gases”) and Department of Transportation (DOT) regulations (“Hazardous Materials Regulations,” 49 CFR 170-179) for filling cylinders, storage tanks, tank trucks, and tank cars allow adequate vapor space for liquid expansion as the result of a change in atmospheric temperature.


              http://ts.nist.gov/WeightsAndMeasure...ptr2May-05.doc
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              • #8
                Thanks guys. I think it was in the high 40 the day he filled it and I doubt it will reach over 70 in the near future. That's a possible 35 degrees or so difference. So, if Evan's 1%/6* is correct it gets to about 97%. Sounds like I am OK by 3%. But I don't like such small safety margins. I think I may call the Fire Dept. on Monday.
                Paul A.
                SE Texas

                And if you look REAL close at an analog signal,
                You will find that it has discrete steps.

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                • #9
                  Turn up the heat Paul. You'll be fine.

                  BTW, the 1% per 6 F is correct. From the NIST document above:

                  In fact, for LPG products the temperature can make a very great difference, since among their physical properties is a high rate of change in volume with change in temperature. For example, commercial liquid propane expands or contracts by about 1 percent of its volume for each change of temperature of 6 °F.
                  Last edited by Evan; 03-10-2007, 11:51 AM.
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                  • #10
                    I haven't seen it on this thread, so FWIW...
                    boil the kettle, pour it over the cylinder.
                    You will see, and be able to feel, the liquid level in the container.
                    This applies from tiny portable stove cyls, to 100 lb+ domestic heat/cook.
                    Just got my head together
                    now my body's falling apart

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                    • #11
                      Assumption

                      Paul,

                      You are ASSUMING that the gauge is correct. You know what happens when we assume. My gauge on my one cylinder shows 71% when I get vapor out of the 80% valve. Would i fill it to 80% gauge in the summer? No. You can quote all of the stuff you want off of Google but you need to know the facts.

                      Message me if you have a problem checking it and I can walk you through it. Peace of mind is priceless.

                      Rick

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                      • #12
                        Originally posted by pressurerelief
                        Paul,

                        You are ASSUMING that the gauge is correct. You know what happens when we assume. My gauge on my one cylinder shows 71% when I get vapor out of the 80% valve. Would i fill it to 80% gauge in the summer? No. You can quote all of the stuff you want off of Google but you need to know the facts.

                        Message me if you have a problem checking it and I can walk you through it. Peace of mind is priceless.

                        Rick

                        Actually summer is the safest time of the year, in fact if your anywhere near close to the general guidlines then your going to be fine,,, Its actually winter that you have to be very careful with, all it is is the potentual measurement between the most extremes in temperature that the tank will see --- and its only the measurement in one direction that is of concern --- Cold to hot,,,, Hot to cold does not matter and actually works in your favor,

                        Your statement of; "The lower the atmospheric temperature the higher you can fill the tank. This is related to the vapor pressure curve and lack of need for expansion space. I routinely work on railcars that are at 90% in the dead of winter. " is a dangerous statement, listen to Evan on this one ----- It is actually the coldest of days that you need to pay close attention to how much you fill because these are the days that have the most for potentual variation should you get a really hot day in the middle of winter...

                        PS,,, none of this is off of "google" its just common sense.
                        Last edited by A.K. Boomer; 03-11-2007, 09:24 AM.

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                        • #13
                          You can quote all of the stuff you want off of Google but you need to know the facts.
                          I'm not quoting "off" Google, I'm quoting the National Institute of Standards and Technology. I am glad you aren't filling my propane tanks.
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                          • #14
                            Evan, worry not. I know we have disagreed in the past, but I do respect your opinion and in this case, it makes perfect sense. Boomer, yes I understand the temperture change thing. Problem is, it was a fairly cold day when they filled it and spring is coming on.

                            As for gauge accuracy, who knows. The tank was used about 10 years ago when they installed it. I have seen no reason to doubt the gauge but who knows what the original specs were. Could be +/-10%.

                            And I am not sure Rick was saying that winter was safer, he only used summer as an example.

                            I will be keeping an eye on it as the temperature rises. I hope we have enough cold nights to use up 10% or so before the really warm weather. I still suspect they were just trying to sell as much gas as possible at the higher rates, before the warm season kicks in.

                            Again, thanks to all.
                            Paul A.
                            SE Texas

                            And if you look REAL close at an analog signal,
                            You will find that it has discrete steps.

                            Comment


                            • #15
                              Timeout.

                              I think Pressurerelief deals with these tanks in a professional context on an industrial scale and I suspect he knows what he is talking about.

                              Evan, you've got the theory right but methinks your logic has a flaw in it. You're looking too rationally at a tank assuming fill would be determined by mass. The flaw is that 40 CFR 173.304 paragraph 4 states that the measurement method to be used legally for determining tank fill consists of a dip tube.

                              In general, you don't want to put in more propane as the temperature gets lower. But in practice the precise tank fill is a function of the measurement method. The official fill criterion is by volume thus the line of reasoning about mass is not the one on which tank fill is based. The dip tube is calibrated for a specific quantity at a specific temperature and so if you fill at a lower temperature, more propane can be put in.

                              The density of propane remains almost constant with an average of about 31.5 lbs/cubic foot although it goes up slightly with temperature varying between 27 and 36 over the temperature range of -40F to 140F. The dip tube will let you put more propane into the tank at low temperatures and this is accounted for in the margin of safety for the tank design and the dip tube design rather than by writing guidelines on tank fill vs. temperature.

                              When safety is at stake something idiot proof like a dip tube embodying an inexact engineering calculation tends to beat an engineering calculation that only needs to be made wrong once and then .

                              See http://www.access.gpo.gov/nara/cfr/w...cfr173_98.html for the CFR chapter and verse and http://www.standby.com/propane/thermo.html for the density of propane from a thermodynamics book from 1926.

                              --Cameron
                              PV=nRT

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