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Tanks typically had 15-year lives stamped on their labels . . . thus a 98 Civic GX or bi-Fuel Ford pickup would have tank with expiration dates printed on the tank label saying like Expires Sept 2013. Older cng vehicles like Dodge vans from early 90's (let's say a 1992 Dodge) would have tank label saying Expires Oct 2007.
FYI to Steven86 in Houston . . . there are also different "Types" of tanks ranging from Type 1 (steel) to Type 4 (full composite). Many old school cng guys feel that Type 1 steel tanks (when installed and taken care of properly) likely could have much longer life than stated 15 years . . . . reason being is that DOT bottles (typically 6 gge) that are used for storage in some public cng stations (ie bunch of bottles daisy-chained together) can have lives longer than 15 years if they are inspected and repainted to gain an additional 5 years at a time.
However, recent tank manufacturing techniques are getting better and better and now vehicle installed tanks generally have 20-year life stamped on the tank label for all types (1 through 4).
One of the primary issues with cylinder life is exposure to liability by the cylinder manufacturer. In many cases, the cylinders have not changed appreciably in design but their insurer underwriter bacame more willing to accept a longer life/greater risk due to the increasing number of cylinders on the market. This is why many newer cylinders have an increased life rating. As for the steel type 1 cylinder, it has not changed since the 80's, yet they are rated for 3600 psig and a longer life than the late 80's 2400 psig 5 year life with hydrostatic pressure testing. The cylinder manufacturer underwriter has been willing to accept the longer life even though there are no appreciable changes in the cylinder design since there have been relatively no problems with the design.
Any modification to the original tank design by the cylinder manufacturer by extending the life by lowering the pressure or just keeping the cylinder in service after the expiration date would leave absolutely no support by the manufactuerer and the vehicle/cylinder owner would bear ALL liability IF there were an accident of any type, even if not related to the cylinder.
One case I was involved as an expert witness, the plantiff's counsel demonstrated that the defendant showed a disregard for safety by circumventing a manufacturer designed safety system (he had defeated the antilock brake system by hydraulically bypassing the control unit). The case did not involve the brakes, but the plantiff's counsel was able to successfully shift the focus to the defendants history. In court, its called "Exploration", where by a through a series of related investigations, other issues are uncovered which may have some effect on the outcome of the case.
In the case of CNG cylinders, any attempt to circumvent the manufacturers rating would likely shift the focus directly to the owner even outside of the cylinder issue.
I have a 1996 Ford F-250 Bi-Fuel and the fuel door says that the tanks expire in May 2011. Other than that, the truck is fine. There's no certified CNG inspectors or mechanics within 60 miles of me and even if there was, it seems the cost of new tanks is just too much for me. So, here are my questions:
If I just run the engine using CNG until the tanks are "empty," can I just forget about the CNG and run the truck on unleaded til it dies? Or, is there some special procedure necessary to depressure the expired tanks?
Yes Michael you can run on gasoline until it dies. You're on the right track about running the tanks down on CNG until the engine dies. Yes, there is a procedures for venting the tanks to zero pressure and should be done by a trained individual. When the tanks expireCGA C-6.4 requires that the tanks have at one 1/2 hole drilled into the tank (after it is vented to zero pressure). I would suggest removing the tanks; why carry the weight?
When the tanks expireCGA C-6.4 requires that the tanks have at one 1/2 hole drilled into the tank (after it is vented to zero pressure).
Don't forget to purge the tanks before you drill the 1/2" hole. Do not purge the tank with air, use water. Fill and drain it several times with water. Usually what I do is drill two 1/2" holes so that they overlap each other and make kind of a figure eight that way this is not chance at all of someone finding the tank and putting a pipe plug in it and trying to re-use it.
I have been an oxygen acetylene dealer for many years and think CNG it the future. Being in the state of Missouri there are no CNG filling stations, It seems the tanks are the biggest cost and there are allot of expired ones out there so anyway I was speaking to my gas supplier about tanks and he told me they would could recertify them. Am I missing something??
From what I know of the ridiculous DOT and EPA regualtions surrounding this issue... the only way the tanks might be legitimately re-certified is if the manufacturers were willing to make the effort. Obviously, none of the manufacturers have any sort of vested intereset in doing so when they can simply sell new tanks.