Green Fleet

What Happens When Your CNG Tanks Expire?

Industry experts continue to explore options for possible CNG tank recertification, as well as safer tank disposal, a federal CNG vehicle registration system, and government grant help.

May 2010, Government Fleet - Feature

by Cheryl Knight - Also by this author

Utilizing compressed natural gas (CNG) in vehicles has benefited fleets around the world for decades. Compared to vehicles fueled with conventional diesel and gasoline, natural gas vehicles (NGVs) can produce greatly lower amounts emissions and reduce operating costs up to 50 percent, while helping wean the nation from dependence on foreign oil, according to NGVAmerica, the industry trade ­association.

NGVAmerica estimates about 110,000 NGVs are in use in the United States today, displacing about 360 million gasoline-gallon equivalents (GGE) per year. More than 11 million NGVs are operated worldwide, with the numbers growing quickly throughout Europe, South America, and Asia.

According to NGVAmerica, replacing an older vehicle with an NGV reduces:

  • Carbon monoxide (CO) by 70-90 percent.
  • Nitrogen oxides (NOx) by 75-95 percent.
  • Particulate matter (soot) by up to 90 percent.
  • Greenhouse gases by 20-30 percent compared to diesel or gasoline vehicles, respectively.

While the benefits of CNG vehicles are well documented, one issue looms over the industry that must be addressed: What happens when a dedicated CNG vehicle fuel tank reaches its expiration date?

Standards Pre-Determine CNG Cylinder Life

In the 1990s, the NGV industry created CNG cylinder certification standards. Cylinders built to meet the original (1992) version of Standard NGV2 were designed for a service life of 15 years, with labeling requirements setting a "Do not use after" date. A 1998 revision extended allowable cylinder life certification to 20 years. The 2007 revision raised that figure to allow a 25-year lifespan.

Most countries have adopted similar CNG cylinder standards. Tanks cannot be recertified after reaching the expiration date set at time of manufacture and must be taken out of service. That leaves vehicle owners two options: retire the vehicle or replace the cylinders.

"Most NGVs are retired well before their cylinders expire," says Stephe Yborra, director of market analysis, education, and communications for the Clean Vehicle Education Foundation (CVEF), a nonprofit working on NGV industry technology research, development, and design, and codes and standards. He acknowledges a small, but growing number of vehicles built in the mid-to-late 1990s "still have life in them," but their CNG tanks don't. "Like any other major item that needs replacement, you have to decide whether it makes economic sense to make the investment," said Yborra.
"With the 1998 and 2007 cylinder-life certification extensions, we expect this problem to diminish or go away completely through attrition of older vehicles," said Yborra."

The dilemma has surfaced primarily in California, where early adoption of NGVs in the 1990s was strongest and a mild climate has prompted a growing number of school buses, municipal trucks, and some light-duty vehicles outlasting their CNG cylinder's 15-year lifespan.

"The challenge before us right now is how can we help fleets that have well-maintained 15-year-old CNG vehicles keep them on the road," said Yborra."

What alternatives do these higher-mileage fleets have? According to Yborra, current options are limited. Although NGV standards officials initially considered a process for recertifying older tanks, liability and technical challenges scuttled the idea.

"Our organization's number one priority is safety," Yborra said. For CNG cylinders, it starts with certification standards, he added.

"Next is in-use inspections of cylinders," said Yborra. The National Highway Traffic Safety Administration (NHTSA) mandated notices be affixed to all CNG cylinders for vehicles produced after Dec. 2, 1996. The notices state the cylinders should be inspected for damage or deterioration every 36 months or 36,000 miles, whichever comes first, or after a fire or accident. A CNG cylinder safety inspection protocol and inspector certification program is in place, and a variety of community colleges and other organizations provide training.

"Last is the timely removal and proper disposal of a cylinder when it reaches its full useful life or if it's damaged," Yborra explained.
He believes some owners may keep CNG cylinders in service after expiration because either they don't realize tank life has expired or the tank may appear safe.


  1. 1. David Rodriguez [ May 08, 2012 @ 07:26PM ]

    So where do you go to get new tanks?

  2. 2. Lawrence S. Thomas [ February 12, 2013 @ 06:52AM ]

    Hello i am a small business owner who has 10 cng vans with wheel chair lefts. The tanks have just expired but i dont and the $5000 to replace each tank. I have a non emergency medil transportion company and i would love to keep it going but we ethier need funding of new cng van . Please if there is any info give us a call 559.481.0311 or 559.776.3448

  3. 3. Thi Dao [ February 12, 2013 @ 09:29AM ]

    Lawrence, I have directed your question to the San Joaquin Valley Clean Cities Coalition to see if they can help.

  4. 4. R. Bitter [ July 29, 2013 @ 12:12PM ]

    There appears to be a section missing at the end of page 2 - can you post it or send the missing text?

  5. 5. Thi Dao [ July 29, 2013 @ 01:05PM ]

    Thanks for your comment! There isn't any text omitted from this online version of the article. In case you can't access the third page, I'll send you a PDF of the article as it ran in the print magazine.

  6. 6. Mel Galbraith [ September 13, 2013 @ 10:30AM ]

    This article doesn't come close to answering the question in the title. So, what does happen when tank life is exceeded. We are going to remove the tanks of bi-fuel vehicles and disable the CNG. Is there going to be a re-certification program offered by someone?

  7. 7. Thi Dao [ September 13, 2013 @ 04:12PM ]

    Hi Mel,
    It seems like the options are limited -- you most likely have to replace them or take them out of service, like you did. I received another question about re-certification recently -- it's not possible and/or not recommended, depending on who you talk to. I'll send the full response your way via email.

  8. 8. Cory Hadaway [ September 20, 2013 @ 02:45PM ]

    We have a large number of CNG tanks to dispose or sell. Can you provide any suggestion on who to contact regarding this? Please call me at (847)233-7198

  9. 9. TonWilson [ November 30, 2013 @ 12:29AM ]

    Tanks cannot be re certified after reaching the expiration date set at time of manufacture and must be taken out of service. That leaves vehicle owners two options: retire the vehicle or replace the cylinders.

  10. 10. Shalini [ December 09, 2013 @ 04:18AM ]

    Yes, the CNG system gives more benefit in vehicles as well as natural. The information about the CNG is useful.

  11. 11. Donald E. Cusson [ March 18, 2014 @ 10:45AM ]

    How is the lay person going to know about the testing and checking of the tanks in their vehicle. How is the lay person going to be able to self-serve at the stations. It has taken a long time to get people to self-serve with gasoline let alone now CNG. How about the autos parked in the garage attached to the home?

    What about weights and measures, how is this being checked, or measured?

  12. 12. MarieArizona [ October 09, 2014 @ 08:34AM ]

    I need to find out how to dispose of CNG tank taken out of a 2002 Ford truck. I have searched the net and cannot find any waste management sites that mention CNG. Is it considered "Hazardous Material". Please let me know if there is a place in Arizona.

  13. 13. Cheril-Arizona [ February 28, 2015 @ 07:56AM ]

    I also have the same problem as MarieArizona. I have a cng tank that I need to get rid of. Any suggestions on how or where I could do that?

  14. 14. ashfaque [ April 27, 2015 @ 10:00AM ]

    i have car and faber cng cylinder is fixed, it is produced in dec 2007, what will be expiration date of it and after how many years i need to go for service

  15. 15. Jeremy [ June 14, 2015 @ 06:51PM ]

    I'm in California is it illegal to run a expired cng tank ? I am only using it for performance so it will no be my primary fuel sorce

  16. 16. wayne [ June 27, 2015 @ 03:58PM ]

    the tanks are good for 15 yrs. need to be inspected every 3 yr's or 36000 miles , by a certified mechanic , we can take the old tanks ,given we deplete them , and yes we do charge for that , questions e-mail wayne a proson can go to u-tube and click on 33 ford gasser to see some of the work being done on cng .

  17. 17. Bobby [ August 14, 2015 @ 05:49AM ]

    why can't the dedicated CNG vehicles be converted to LPG, is anyone doing any work on that?

  18. 18. Levi [ October 18, 2015 @ 05:22PM ]

    Yes I'm from California too and I don't see any depository for expired CNG tanks, inspection centers, or even anyone asking or policing whether my tank is expired or not. I just don't think it exist in practice but I do know there is an expiration for both tank. In fact on ebay they are selling expired tanks? am I missing something? In reality if the tanks looks safe and inspected to be without any leaks or corrosion whats the problem? I got into CNG to do all the moral reasons but I also did it for practical reasons, to lower my fuel cost. But if we need to replace the tank every 15 years that reason no longer make sense. BY the time I replace the tank the cost of replacement, labor and disposal will eat up all the savings. I don't use my vehicle like a business just personal use so I don't rack up a lot of miles. We should get some kind of government assistance for using alternative fuel. That's what I think anyway, what do you think?

  19. 19. Eddy [ December 12, 2015 @ 07:12PM ]

    One thing clearly not explained is what exactly is the risk? I'm considering buying a used older cng for cheap commute. I am noticing some have current tank, some dont. Is the thing going to blow up? I've never heard of that happening? Could I get a used tank with say 5 years left or so, then install it? I'm fairly mechanically inclined, worked on cars, houses- gas & plumbing lines & don't see much trouble with that. What am I missing? I anticipate they may corrode from inside? I certainly wouldnt want to drive a dangerous car, but there also is not clear information.

  20. 20. David [ April 11, 2016 @ 07:00PM ]

    I'm totally opposed to the whole "time based" standard....somebody designed these tanks to perform a job. Some calculated margin of error was certainly factored in to account for anomalous behaviors,(temperature swings, overfilling, mis-handling etc).

    I'm guessing, the answers could be found in any national or international fuel gas code or an engineering text on pressure vessels.

    Somewhere an agency or institution certified them for use in the first place, and had to perform testing to do so........if they can pass the same tests, shouldn't they still be suitable since they meet the standard?

  21. 21. MID-DEL AUTO PARTS [ November 04, 2016 @ 09:46AM ]



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