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.


"Appearances can be deceiving," Yborra noted. "Certified cylinder inspectors are trained to look for bracket wear, gouges from road debris, and less obvious to the naked eye corrosion that could compromise a cylinder's integrity." He cited examples of battery acid, industrial solvents, and other chemicals that may have been stored in the vehicle or splashed from a road spill.

Yborra also pointed out no official tracking systems are available to ensure all CNG cylinders undergo proper periodic safety inspections or are retired at set ­expiration dates.

"We feel the mechanisms the industry has in place are good we have the right steps in place but we don't have a way to make sure everyone follows those steps," Yborra said.

One challenge is the lack of a  national database of all CNG vehicles, whether OEM-built or converted. "Each state has its own vehicle registration requirements and only a few record fuel type," said Yborra.
Armed with a more complete and accurate CNG vehicle registration database, he feels the industry could be more proactive, educating NGV owners about proper cylinder safety practices and issuing notices for older vehicles equipped with soon-to-expire cylinders.

The NGV industry is also addressing whether retired NGV cylinders are properly defueled and rendered unusable, according to Yborra.

"CNG cylinders are fairly expensive. We suspect that some expired cylinders are being resold by unscrupulous shops, which could result in a dangerous situation," said Yborra. He added that the resale of unexpired cylinders removed from a vehicle, while legal, should always include a thorough inspection by a certified inspector.

CVEF Task Force to Address Expiration & Other Issues

CVEF is currently assembling a task force of cylinder manufacturers, fleet operators, state motor vehicle agencies (MVA), and other government agencies to address cylinder expiration, replacement, and ­related issues.

"The task force is an extension of our current role managing cylinder incident investigations," said CVEF president Doug Horne. The task force hopes to assemble NGV inventory data