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Refilling CNG tanks

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  • Refilling CNG tanks

    Does anyone know if it is possible and/or practical to refill a smaller relatively empty CNG tank from a larger relatively full tank without using a pump?

    I know very little about the physics involved, but it seem like the CNG would flow from the higher pressure full tank into the empty tank until the pressure equalized between the two tanks.

    Am I way off base on this?


  • #2
    Re: Refilling CNG tanks

    You would not get a lot, they would both equalize out to the same pressure, then minus the expansion of gas due to the transfer friction.
    Dave Clement


    • #3
      Re: Refilling CNG tanks

      Us old ranchers used to do things like that all the time. We always had an old 10 ply spare tire with 80 psi in it. When you had a slow leak you could hook the two tires together with a hose & let the pressure equalize, then drive home to fix the tire. We also used to carry a propane bottle and did the same thing with the tractor in the field so we could drive it in instead of taking the truck throught the field we were working. As a kid it was my fault most of the time because I was messing around and not paying attention -- but it sure made things easier a few times and saved me some grief.
      With cng the problem is building the setup & hooking them together to let them equalize.


      • #4
        Re: Refilling CNG tanks

        There was a pretty thorough discussion of this on the group. In order for the amount transferred to be significant the supply tank needs to be a lot bigger than the receiving tank.
        02 GX
        01 GX
        03 Crown Vic
        06 GX
        Home Fueler


        • #5
          Re: Refilling CNG tanks

          The ideal gas law allows you to calculate what pressure the system (2 tanks) would equalize to given your initial volume (pressurized tank) and final volume (tank 1+ tank 2) . See

          Navy 2008 Civic GX (wife's)
          Silver 2012 Toyota Prius
          Grey 2012 Civic Natural Gas (mine)


          • #6
            Re: Refilling CNG tanks

            Originally posted by freedml View Post
            There was a pretty thorough discussion of this on the group.
   is not a valid domain.

            ?Innovation is driven by having access to things.? -- Gleb Budman, CEO of


            • #7
              Re: Refilling CNG tanks

              I might be taking too simplistic an approach to this but here it goes...


              P = total pressure which is known before adding additional volume
              V = total volume which is known before and after gas transfer
              n = number of moles in the gas which will remain the same before and after
              R = universal gas constant (which is constant... ;-) )
              T = absolute temperature which we will assume will be constant before and after (my physics teacher assumes that a falling horse is spherical...)

              so P1V1 = P2(V1 + V2)

              If the volume is doubled then the pressure is halfed.



              • #8
                Re: Refilling CNG tanks

                On a fast fill station, we are talking 3 separate banks of say a minimum of 4000 cubic feet or any station for that mater. for 3000 PSI fills the storage should be set at 3650 PSI. for 3600, 4500 is required. If you want to fill smaller tanks from storage you need 3 vessels so you can cascade. fuel maker actually makes a unit for emergency filling.


                • #9
                  Re: Refilling CNG tanks

                  I have heard that Murphy (CNGMotors) actually has a cascade system for his fuelmaker. Maybe when he gets back from vacation, he can chime in on this.
                  Mountain Green, Utah
                  2003 CNG Cavalier
                  2003 CNG Silverado 2500HD


                  • #10
                    Re: Refilling CNG tanks

                    I was using a double volume only as an example. If I remember this stuff correctly, the equation would be:

                    P1V1 = P2V2

                    If P1, V1 and V2 are known, then P2 is known. Now you can start getting fancy with regulators making sure that P2 never gets above 3600 psi. With 3600 psi and an assumption of what V2 could be (say V2 is the vehicles fuel tank), then you can determine what P1 is (or has to be based off of the equipment you have) and then an upper limit of V1 as the refueling storage tank.

                    But... with the simplest example with no regulators:
                    P(initial) x BigVolume = P(final) x (BigVolume + LittleVolume) or

                    P(initial) x (BigVolume)/(BigVolume + LittleVolume) = P(final)



                    • #11
                      Re: Refilling CNG tanks

                      so, if p1v1=p2v2

                      Lets show some examples. Lets say that you have a 5 gge tank. and your goal is just to get that tank to 3000 psi. Lets pretend that you have an extra 10 gge tank that you will use as the "supply" tank.
                      10gge(x psi) = 15gge(3000 psi)
                      x = 4500
                      (where 15 is the new total volume once you attach your tank to the supply tank)
                      Your 10 gge tank would need to have a pressure of approximately 4500 psi in order to fill you cars tank to 3000 psi.
                      However, if you increased the size of your supply tank to 20 gge, then the pressure in the tank would only need to be 3750 psi, and if you filled it to 4500 psi, then you could get a 3600 psi fill.

                      On the other hand, lets pretend that you want to use a small 1gge tank as a backup, so that if you run out of gas, you can use the backup to give your 5gge tank a little extra fuel to get to the next station. If you could only fill the small tank to 3600 psi (at a regular station for example)

                      1gge (3600 psi) = 6gge (x psi)
                      x = 600 psi

                      So in this example you would only be able to give yourself a 600 psi fill... so you'd better hope that the station is pretty close!

                      Anyone see any errors in my math?



                      • #12
                        Re: Refilling CNG tanks

                        The Fuelmaker units are for manual cascade Fast-Fill systems. They have 6-9 DOT AAA or ASME bottles and a 3/4 way valve to dump/equalize from the three banks and/or top off from the Fuelmaker.
                        They cost about $25k from Fuelmaker. We had and sold two of them. One was from a system that had four FM4 units(replaced by the FMQ2-36) and one that had a FMQ8-36 Quad unit .
                        A while back, I sold Murphy a rack with 6 - DOTAAA bottles to cascade a FMQ2-36. I don't know if that's the system you're talking about.
                        Cascading a Fuelmaker for some people is thier only option because thier vehicles don't stay parked long enough to get a full slow-fill without the help of the cascade as a buffer.
                        If anyone is interested in a setup like this or larger (I've done a 40 bottle automatic priority/sequential cascade for an FMQ8-36 with a dispenser) let me know. I have 5 year certified DOT AAA bottles, Fill posts and hoses, and can fabricate what you need or just help you with your own design.
                        First read installation docs on Fuelmaker site for FF350 at:
                        and aquire and familiarize yourself with a copy of NFPA 52 before you start.
                        Be aware, once you add anything to a VRA, you are openning up another can of worms (Code, Inspection and Permitting wise).
                        Last edited by cnghal; 05-28-2008, 01:04 PM.
                        Your Friendly Nazi Squirrel Administrator


                        • #13
                          Re: Refilling CNG tanks

                          Sounds like the FM unit I have. 7 bottles in the cabinet with a manual cascade. 30gge storage, but realistically will only do one Civic since you only have about 20% usable storage at pressure. I'mm adding an 80gge sphers and cascade during my upgrade, along with the card reader, training post and Krause dual hose/dual pressure dispencer. I did however upgrade the compressor to an Angi/Rix 25hp/15.5gge/hr skid. We call it the sieve. They are out all the time trying to stop the gas leaks.


                          • #14
                            Re: Refilling CNG tanks

                            When did you say you were going to be in my neck of the woods again Hal?