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Why are CNG tanks not re-hydrotesed or re-certified?

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  • Why are CNG tanks not re-hydrotesed or re-certified?

    We are coming to a cross road in the CNG industry. May I remind you, that anyone who owns/uses a CNG vehicle is "in" the industry. In 1998 Honda introduced the first Civic GX. This vehicle came with a CNG tank certified for 15 years. Also in 1998, Chevrolet made the Cavalier with a CNG option. Ford was making the Contour and F150 in 1998 both with CNG options. Well, now 15 years later, 2013 is almost in its final month. All these tanks have expired or are going to expire in December according to the CNG tank specifications to which they were manufactured (DOT FMVSS 304 and NGV2). A question that has come up concerning re-certification or re-testing of these tanks to extend their life. I will do my best to address this issue. I would also ask the other experts on this forum to also include their thoughts on the matter.

    First of all, CNG tanks are not like any other pressure tanks. They are not like any other steel tanks and they are not like any other composite tanks. CNG tanks are designed and manufactured under a totally different regulation and specification.

    CNG tanks certified to FMVSS 304 and NGV2 have a safety factor of 2.25. That means that if your tank is a 3,600 psi tank, the minimum burst pressure is 8,100 psi. Test pressure is 1.5 times 3,600 = 5,400 psi. So, test pressure is 67% of burst pressure. Keep that in mind.

    Composite paintball and Self Contained Breathing Apparatus (SCBA) tanks are designed and certified to DOT CFFC and now ISO 11119. DOT CFFC tanks have a safety factor of 3.4. So, for a 3,600 psi tank, the minimum burst pressure is 12,240. The test pressure is 5/3 (1.67) times 3,600 = 6,000 psi. That is only 49% of burst pressure (not 67% like NGV2).

    DOT 3AA and DOT 3AL tanks are all metal tanks and require a specific stress level in the wall to be below the minimum yield strength of the steel (3AA) or aluminum (3AL). They also require a specific chemical composition of steel (3AA) and aluminum (3AL). DOT 3AA does not allow high strength steel like NGV2, which means DOT 3AA has a higher safety factor than 2.25. If CNG tanks were designed to DOT 3AA, the wall thickness of a 16 inch diameter tank would be more than 5/8” thick. Can you imagine the weight of that tank and the cost to manufacture such a thick wall? For comparison purposes, the wall thickness of a 16 inch diameter composite CNG tank is less than 1/3” thick.

    CNG tanks are not allowed to be re-hydrotested or re-certified due to the fact that during the hydro test the cylinder will be pressurized to 67% of its capability, thus one risks damaging the cylinder even more. Also, DOT CFFC tanks (paintball, SCBA, etc) also have a 15 year service life, but are required to be hydrotested every 3 to 5 years (Remember their hydrotest pressure is only 49% of their minimum capability). NGV2 tanks are required to be visually inspected for damage and deterioration every 3 years or 36,000 miles.

    So, why don't CNG tanks have a higher safety factor and thus reducing the strain on the cylinder during hydro test? My answer to that is simple: If you increase the safety factor, you increase the wall thickness (i.e. you increase the amount of carbon fiber). And people already complain about the cost of current CNG tanks. Can you imagine how people would react if CNG tanks were required to have more carbon fiber on them?
    Jared.
    Mountain Green, Utah
    2003 CNG Cavalier
    2003 CNG Silverado 2500HD

  • #2
    Re: Why are CNG tanks not re-hydrotesed or re-certified?

    Well said Jared!
    Last edited by John Mitton; 11-27-2013, 09:35 AM. Reason: removed long quote from opening post

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    • #3
      Re: Why are CNG tanks not re-hydrotesed or re-certified?

      The technical explanation is sound. The true crossroads will be whether users abide by the expiration or continue to drive on them. My guess is the latter, since many in this seem to be in it for total cost of ownership, at all costs, as it were...

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      • #4
        Re: Why are CNG tanks not re-hydrotesed or re-certified?

        Okay that clears it up for me. Still don't like the fact that my van 2001 Dodge has only 81,000 miles on it and the tank,s still look new but they need to be change 5/2015.

        Jared when you have a safety inspection in Utah do they check the tank expiration date,s. ??

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        • #5
          Re: Why are CNG tanks not re-hydrotesed or re-certified?

          Originally posted by amoparacer View Post
          Still don't like the fact that my van 2001 Dodge has only 81,000 miles on it and the tank,s still look new but they need to be change 5/2015.
          No one knows the history of their tank(s). You don't know if the previous owner of your van removed the tank from the van and installed it on another vehicle only to reinstall it back on the van prior to resale. The sad thing is that odometers and speedometer clusters can be changed and tampered with (I'm not saying that your van has more miles than 81,000 on it). You don't know the history of your tank. Tanks may look new after 15 years of service, and that is a good thing because you know that your tank has seen zero to minimal exterior damage or deterioration. However, the fatigue on your tank from use (pressurizing and depressurizing over and over again and again) is what will cause your tank to fail (assuming there is no external damage). And that kind of fatigue is not visible on the outside and even if you can inspect the inside of the tank, it is very hard to detect until failure occurs.

          Originally posted by amoparacer View Post
          Jared when you have a safety inspection in Utah do they check the tank expiration date,s. ??
          When I perform a tank inspection, the first thing I check is the expiration date. If that date has passed, then my inspection is done and the tank fails the inspection. In Utah, I would have to say that most shops that do the vehicle safety inspection check the tank expiration date either by themselves or through the inspection report from the tank inspector.
          Jared.
          Mountain Green, Utah
          2003 CNG Cavalier
          2003 CNG Silverado 2500HD

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          • #6
            Re: Why are CNG tanks not re-hydrotesed or re-certified?

            I have a question about DOT tank differences between DOT tanks in vehicles and in stationary use.

            If a DOT tank requires a physical inspection every 3 years (36,000 miles) when in use in a vehicle, what reasoning is there for hydo-testing a DOT tank used in a stationary setting?

            Is a stationary tank classified as a 3AA vs a vehicle tank is NGV2 the difference?

            I know of a manual fast fill module with DOT tanks in use and the question of hydo-testing came up. He was under the impression of it having ASME tanks when purchasing it. (eBay honesty)

            The module is only filled to 3600 psi temp compensated. I really wonder what the extra standard is? Are the tanks to a lower metal quality as used in the 3AA tanks as stated above? (disassembly of the module and transporting the tanks to a test station and vice versa would be a royal pain, especially every 3 years.

            Would replacing the tanks with NGV2 tanks be an option? (maybe too expensive) Then they would only require physical inspections. ASME tanks would surely be cost prohibitive.

            Thoughts? Guidance?

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            • #7
              Re: Why are CNG tanks not re-hydrotesed or re-certified?

              There are people on this board that are far more knolagable about fuel systems than me. On modern NGV system the tanks used are NGVII, DOT cylinders are used for transporting compressed natural gas not used as motor fuel. Although 3AA tanks were used on vehicles from the 70's to the very early 90's to the best of my knowlage they are only used as temporary ground storage in commercial stations. In Ca. Only ASME tanks are to be used for percent ground storage.
              Last edited by cngmike; 03-10-2014, 08:32 PM.

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              • #8
                Re: Why are CNG tanks not re-hydrotesed or re-certified?

                Use of ASME in ground storage is an OSHA requirements in most states.

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                • #9
                  Re: Why are CNG tanks not re-hydrotesed or re-certified?

                  dot tanks are every 5 years and if you keep the water out last almost forever

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