What happens when your CNG storage tank expires?

The use of compressed natural gas (CNG) in vehicles has been benefiting fleets around the world for decades. Compared to vehicles fueled by conventional diesel and gasoline, natural gas vehicles (NGVs) have significantly reduced emissions and operating costs by as much as 50%, while helping to wean the country off its dependence on foreign oil.

NGVAmerica estimates that there are approximately 110,000 NGVs in use in the United States, displacing approximately 360 million gasoline gallon equivalents (GGEs) annually. More than 11 million NGVs are in operation worldwide, with this number growing rapidly in Europe, South America and Asia.

According to NGVAmerica, replacing older vehicles with NGVs can reduce.

✴Carbon monoxide (CO) by 70-90%

✴Nitrogen oxides (NOx) by 75-95%

 up to 90% of particulate matter (soot)

✴20-30% reduction in greenhouse gas emissions compared to diesel or gasoline vehicles, respectively.

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

Standards Predetermine CNG Cylinder Life

In the 1990s, the NGV industry created the CNG cylinder certification standard. Cylinders manufactured to meet the original (1992) version of the standard, NGV2, were designed to last 15 years, with labeling requirements setting a "not to be used after" date. 1998 revisions extended the allowable cylinder life certification to 20 years. 2007 revisions increased that number to an allowable 25 years. Life.

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

Most NGVs are retired before their cylinders expire. a small but growing number of vehicles built in the mid to late 1990s "still have life" in them, but their CNG tanks do not. Like any other major item that needs to be replaced, you have to decide if it makes economic sense to make the investment.

With the extension of cylinder life certification in 1998 and 2007, we expect this problem to diminish or disappear altogether as older vehicles wear out.

This dilemma has arisen primarily in California, where NGV adoption was strongest in the early 1990s and where milder climates have contributed to a growing number of school buses, municipal trucks and some light-duty vehicles with CNG cylinder life in excess of 15 years.

The challenge now is how we can help keep fleets of 15-year-old CNG vehicles on the road.

What are the alternatives for these higher mileage fleets? The current options are limited. While NGV standards officials initially considered a recertification process for older tanks, liability and technical challenges derailed the idea.

Our organization's first priority is safety, and for CNG cylinders, it starts with certification standards.

Next came the in-use inspection of the cylinders. The National Highway Traffic Safety Administration (NHTSA) mandated that a notice be placed on all CNG cylinders manufactured after December 2, 1996. The notice states that cylinders should be inspected for damage or deterioration every 36 months or every 36,000 miles driven, whichever comes first, or after a fire or accident.CNG cylinder safety inspection protocols and inspector certification programs are in place, and various community colleges and other organizations provide training.

Lastly, the cylinder is removed and properly disposed of in a timely manner when it reaches its full life or is damaged

He believes that some owners may continue to use CNG cylinders after their expiration date because they either do not realize that the cylinder has reached the end of its life or the cylinder appears to be safe.

Appearances can be deceptive. A certified cylinder inspector is trained to look for bracket wear, chisel marks from road debris, and - less obvious to the naked eye - corrosion that could compromise the integrity of the cylinder. He cites examples of battery acid, industrial solvents and other chemicals that may be stored in vehicles or spilled from road leaks.

There is no official tracking system to ensure that all CNG cylinders are properly and regularly inspected for safety or decommissioned at set expiration dates.

We think the mechanisms in place in the industry are good - we have the right steps in place - but we have no way to ensure that everyone is following them.

One challenge is the lack of a national database of all CNG vehicles, whether OEM-built or modified. Each state has its own vehicle registration requirements, and there are only a handful of record-breaking fuel types.

With a more complete and accurate database of CNG vehicle registrations, he believes the industry can be more proactive in educating NGV owners about proper cylinder safety practices and sending out notices for older vehicles equipped with expiring cylinders.

The NGV industry is also addressing whether retired NGV cylinders are properly drained and rendered inoperable.

CNG cylinders are quite expensive. We suspect that some expired cylinders are being resold by unscrupulous stores, which could lead to dangerous situations. The resale of unexpired cylinders removed from vehicles, while legal, should always include a thorough inspection by a certified inspector.