The Useful Life of a CNG Cylinder

Compressed natural gas (CNG) cylinders aren’t like your run-of-the-mill fuel tank. They are pressurized, which means users must take special precautions when it comes to proper maintenance, replacement and disposal. Because of the potential safety hazards of improper care, it’s important to know the useful life of a CNG cylinder and to strictly follow recommended replacement times.

Determining Useful Life

The most obvious way to determine the useful life of a CNG cylinder is the expiration date listed on the cylinder’s label.

“All CNG cylinders for use in road vehicles in the U.S. must be labeled with a ‘do not use after’ date. This is a uniform federal regulation applying throughout the U.S. and is contained in Federal Motor Vehicle Safety Standard (FMVSS) 304 Compressed Natural Gas Fuel Container Integrity,” said John Dimmick, director of technology for the Clean Vehicle Education Foundation. “FMVSS 304 does not specify a certain life but requires the cylinder manufacturer to determine the life and apply the label.”

Typically, CNG cylinders have a useful life of 15 to 20 years, as determined by the ANSI NGV2 (natural gas vehicle) standard — a voluntary industry standard for CNG cylinders that is more comprehensive than FMVSS 304 but is not generally a legal requirement.

“The useful life of a CNG cylinder is dependent on the number of years it has been certified for, as per NGV2, and what environment the tank is being used in. Most CNG cylinders for onboard fuel storage are certified to either 15 or 20 years as per NGV2,” said Randolf Wollgiehn, marketing manager for CNG Cylinders International. “A cylinder can be safely used until its expiration date as marked on the cylinder label provided there is no damage, corrosion, or other issues with the tank.”

Factors considered in the NGV2 guidelines include cyclic fatigue, a failure that is the result of accumulated fill-empty pressure cycles and a more unusual stress-rupture failure that depends on the time at pressure and not the pressure cycles, Dimmick said.

Muthuramalingam Krishnan, project consultant at Gruntech Polymer Consultants, also said the useful life determination depends on the materials from which CNG cylinders are made. “The life of the CNG cylinder is based on the material and its properties,” he said. “The factor of safety also varies by material type, such as steel, glass fiber, aramid, and carbon fiber.”

Of course, if a CNG cylinder is damaged, its useful life can be cut short. Damage can include abrasions or impacts from the road or debris, as well as corrosion from de-icing salts.

Likewise, used CNG cylinders will have shorter lifecycles, which fleets should pay close attention to. “If a used vehicle is being purchased, the manager should make sure that the remaining cylinder life supports their plans. They should also assure that the cylinders have recently been inspected,” Dimmick said.

He also warned of cylinders whose manufactured dates don’t match the dates vehicles arrive in house. “In some cases, the manufactured date of the cylinder can be significantly earlier than the delivery date of the vehicle,” he said. “A good purchase specification would be for a minimum cylinder life on the delivery date of the vehicle, since a cylinder with a 20-year design life that is not delivered until it is two years old is really an 18-year-old cylinder.”

When purchasing a new natural gas vehicle, Dimmick strongly suggested ensuring the cylinder life is at least as long as the expected useful life of the vehicle.

“Cylinder manufacturers have provided cylinders with a range of design lives, and matching the vehicle and cylinder life should be a consideration in the purchase decision,” he said. “If resale value is important, the specified cylinder life should recognize this. A used NGV with expired or nearly expired cylinders has very little residual value in the market.”

Douglas Horne, PE, president of the Clean Vehicle Education Foundation agreed that fleets should be careful to consider cylinder life when purchasing NGVs. “The up-front education of end users on cylinder life and the economic decision of cylinder maintenance and eventual replacement as part of an informed purchase decision are very important,” he said.

Performing Proper Inspection & Maintenance

While the life of a CNG cylinder can’t be extended past the manufacturer’s expiration date, fleets can still take steps to make sure the cylinder is able to be safely used for its entire life, until it reaches its expiration date. These include:

• Proper installation.
• Proper shielding from road debris.
• Shielding from impact damage such as curb or road rash, especially in under-vehicle applications.

Following NGV2’s guideline of inspecting cylinders at least every 36 months or 36,000 miles by a certified CNG fuel system inspector.

“There is currently no recognized process to extend the life beyond the labeled life, but companies can assure that the inspections are performed on schedule so that problems in the installation and use can be caught and corrected before the cylinder is too damaged to save,” Dimmick said. “It is also necessary to use maintenance procedures that will not damage the cylinders, using instructions provided by both OEMs and cylinder manufacturers.”

Of course, starting off with a quality CNG tank will also help ensure it is operable until its expiration date.

“A uniform appearance and consistent surface quality are signs of a good product,” said Wollgiehn of CNG Cylinders International. “Whether the cylinder is properly labeled with the vital information as well as manufacturer’s contact information is also a good indicator.”

In addition to the appearance of the product, Krishnan of Gruntech Polymer Consultants also recommended researching how they’re manufactured. “The machinery used to produce the metal cylinders needs to be reliable and durable,” he said. “Do not compromise on materials, machinery or skilled technicians.”

Signs Cylinders Should Be Replaced

Because vehicle lifecycles are typically shorter than 15-20 years, CNG cylinders aren’t often replaced; however, if a CNG cylinder is damaged or a vehicle is equipped with a used cylinder, replacement of CNG tanks at the proper time is critical.

The first guideline to replace a CNG tank is clearly its expiration date. But, how do fleets know if it’s time to retire a tank before that date?

Dimmick of the Clean Vehicle Education Foundation said timely inspections are the best place to start.

“The easiest indication that replacement is necessary is if the labeled ‘do not use after’ date has arrived. It is also possible that the cylinder has been damaged or has deteriorated while in service, which is why the FMVSS 304 label also requires inspection for damage or deterioration at least every 36 months or 36,000 miles,” he said. “These inspections should be carried out in accordance with the instructions of the NGV OEM or the cylinder manufacturer.”

Fleets should also do the following:

• Look for visible damage or corrosion to the tank surface, including the neck or the valve mounting area if the tank has a neckless design.
• Inspect the condition of the valve, as bent tubing or marred surfaces may indicate an impact was sustained, which would warrant further, more detailed inspection.
• Look for damage or deterioration, such as impact damage from a collision, cuts and abrasions, corrosion from deicing salts, exposure to fire, and/or over-pressurization by a defective station.

“If there is any damage to the cylinder, one should refer to the tank manufacturer’s cylinder inspection manual in order to determine which level of damage the tank has sustained,” Wollgiehn recommended. “The level of damage will determine whether a minor repair can be performed by the customer, whether the cylinder will need to be returned to the manufacturer for repair, or whether the cylinder needs to be condemned and removed from service.”

The Consequences of Waiting

Guidelines for the proper replacement times of CNG cylinders are in place for a reason: keeping cylinders in use after the expiration date or operating damaged tanks can have serious safety consequences.

“Under no circumstances should an expired cylinder be used on a CNG vehicle as the manufacturer is not able to guarantee the cylinder will be able to safely withstand the pressure and stress put on the cylinder during the fueling and de-fueling process,” Wollgiehn said. “CNG cylinders in the U.S. market have a service pressure of 3,600 psi (250 bar). Needless to say, when dealing with these kinds of pressures, one should not gamble with safety.”

Dimmick said the Clean Vehicle Education Foundation has seen CNG tanks fail. And, while these incidents are few, they are serious in nature.

“CVEF believes that it is very important to comply with the safety standards and codes for CNG cylinders. The failure modes that drive the tests in FMVSS 304 and NGV2 are a CNG leak and a cylinder rupture and these tests are predicated on a finite design life,” he said. “The Clean Vehicle Education Foundation assists in the investigation of serious incidents involving NGVs and these include a number of ruptures. In some cases, the cylinder had exceeded its labeled life but it has not been definitively possible to state that this was the cause of the rupture. From a statistical standpoint, relatively few CNG cylinders have been in service long enough to exhaust their design life.”

“We strongly discourage fleets from purchasing used CNG cylinders, since it is very difficult to determine their history, such as whether or not they were involved in an accident. Damage is easily disguised but can have detrimental consequences,” Wollgiehn said. “Pricing can also be an indicator. If it sounds too good to be true, it usually is.”

When in doubt, Horne of the Clean Vehicle Education Foundation said the best rule of thumb is to never deviate from code and manufacturer guidelines.

“As long as the CNG cylinder owners follow the code requirements and manufacturers’ instructions concerning the inspection programs and the end of life date noted on the cylinder labels, we will continue to have safe and reliable CNG storage systems,” he said. “As we move toward more OEM products these issues will be part of the OEM maintenance programs and notices for the end of life removal, decommissioning and proper disposal of cylinders.”