Compressed natural gas (CNG) cylinders are not like ordinary fuel tanks. They are pressurized, which means users must take special precautions when it comes to proper maintenance, replacement and disposal. Since improper maintenance can pose a safety hazard, it is important to know the service life of CNG cylinders and strictly adhere to the recommended replacement frequency.
The most obvious way to determine the service life of a CNG cylinder is the expiration date listed on the cylinder label.
In the United States, all CNG cylinders used in on-road vehicles must be marked with a 'do not use again' date. This is a uniform federal regulation that applies throughout the United States and is contained in the Federal Motor Vehicle Safety Standard (FMVSS) 304 Compressed Natural Gas Fuel Container Integrity.FMVSS 304 does not specify a certain life span, but requires the cylinder manufacturer to determine the life span and label it.
Typically, CNG cylinders have a service 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 not typically required by law.
The service life of a CNG cylinder depends on the number of years it has been certified under NGV2 and the environment in which the cylinder is used. Most CNG cylinders used for on-board fuel storage are certified for 15 or 20 years according to NGV2. As long as the cylinder is not damaged, corroded, or otherwise faulty, the cylinder is safe to use until the expiration date indicated on the cylinder label.
Factors considered in the NGV2 guidelines include cyclic fatigue, failure due to cumulative fill-empty pressure cycling, and the more unusual stress fracture failure, which is dependent on pressure time rather than pressure cycling.
The determination of service life depends on the material used to manufacture the CNG cylinder and its properties. The safety factor also varies depending on the type of material, such as steel, fiberglass, aramid and carbon fiber.
Of course, if a CNG cylinder is damaged, its service life may be shortened. Damage may include abrasion or impact from roads or debris, as well as corrosion from de-icing salts.
Again, used CNG cylinders will have a shorter life cycle, and fleets should pay close attention to this. "If buying used, managers should make sure that the remaining cylinder life supports their plans. They should also make sure the cylinders have been recently inspected.
Cylinders whose manufacture date does not match the date the vehicle arrived at the house may, in some cases, be significantly earlier than the vehicle's delivery date. A good purchase specification is a minimum cylinder life on the date of delivery of the vehicle, as a cylinder designed to last 20 years is not delivered until two years later, effectively making it an 18 year old cylinder.
When purchasing a new natural gas vehicle, we strongly recommend ensuring that the cylinder life is at least as long as the expected life of the vehicle.
Cylinder manufacturers already offer a range of cylinders designed to last, and matching the vehicle and cylinder life should be a consideration in the purchase decision. If resale value is important, then the specified cylinder life should recognize this. NGVs with expired or near-expired cylinders have little to no residual value in the marketplace.
Fleets should carefully consider cylinder life when purchasing NGVs. As part of an informed purchase decision, it is important for the end user to be educated up front about cylinder life and the economic decision of cylinder maintenance and eventual replacement.
While the life of a CNG cylinder cannot exceed the manufacturer's expiration date, fleets can still take steps to ensure that the cylinder remains safe throughout its life cycle until it reaches its expiration date. These include.
✴ Proper installation.
✴ Proper shielding from road debris.
✴ Prevention of impact damage, such as curb or road rash, especially in under-vehicle applications.
Follow NGV2 guidelines and have cylinders inspected by a certified CNG fuel system inspector at least once every 36 months or 36,000 miles.
There is no accepted process for extending service life, but companies can ensure that inspections are performed on a schedule to identify and correct problems in installation and use before the cylinder is damaged beyond salvage. It is also important to use maintenance procedures that will not damage the cylinder, using instructions provided by the original equipment manufacturer and the cylinder manufacturer.
Of course, starting with a quality CNG tank will also help ensure that it is ready to operate before its expiration date.
Consistent appearance and consistent surface quality are the hallmarks of a quality product. Whether the cylinder is properly marked with important information and the manufacturer's contact information is also a good indicator. In addition to the appearance of the products, we recommend looking into how they are manufactured. The machinery used to produce metal cylinders needs to be reliable and durable. Don't compromise on materials, machinery or skilled technicians.
Since vehicle life cycles are usually shorter than 15-20 years, CNG cylinders are not often replaced; however, if a CNG cylinder is damaged or if the vehicle is equipped with used cylinders, it is critical that the CNG cylinder be replaced at the right time.
The first guideline for replacing a CNG tank is obviously its expiration date. But how does the fleet know if the tank should be decommissioned before that date? Timely inspection is the best place to start.
The simplest indication that a replacement is needed is when the date marked 'do not use after' has arrived. Cylinders can also be damaged or deteriorated during use, which is why the FMVSS 304 label also requires that they be inspected for damage or deterioration at least every 36 months or 36,000 miles. These inspections should be performed in accordance with the NGV OEM or cylinder manufacturer's instructions.
The fleet should also do the following.
◮ Look for visible damage or corrosion on the tank surface, including the neck or valve mounting area if the tank has a neckless design.
◮ Check the condition of the valve, as bent piping or damaged surfaces may indicate a sustained impact, which requires further, more detailed inspection.
◮ Look for damage or deterioration such as impact damage from collisions, cuts and abrasions, de-icing salt corrosion, exposure to fire, and/or overpressure from defective stations.
If there is any damage to the cylinder, the tank manufacturer's cylinder inspection manual should be consulted to determine the extent of the damage to the tank. The extent of damage will determine if the customer can make minor repairs, if the cylinder needs to be returned to the manufacturer for repair, or if the cylinder needs to be scrapped and taken out of service.
Guidelines for the proper replacement time for CNG cylinders have been developed for a reason: Continued use of a cylinder after its expiration date or operation of a damaged cylinder can have serious safety consequences.
Under no circumstances should expired cylinders be used on CNG vehicles because the manufacturer cannot guarantee that the cylinders will safely withstand the pressures and stresses applied to the cylinders during refueling and de-greasing. CNG cylinders on the U.S. market have an operating pressure of 3,600 psi (250 bar). Needless to say, you can't gamble with safety when dealing with such pressures.
The Clean Vehicle Education Foundation has seen compressed natural gas tanks fail. And, while these incidents are rare, they are serious in nature.
CVEF believes it is important to follow the safety standards and codes for CNG cylinders. The failure modes that drive the FMVSS 304 and NGV2 tests are CNG leaks and cylinder ruptures, and these tests are based on limited design life. The Clean Vehicle Education Foundation assists in the investigation of serious accidents involving NGVs, including some ruptures. In some cases, cylinders have exceeded their nominal life, but it has not been determined that this was the cause of the rupture. From a statistical standpoint, a relatively small number of compressed natural gas cylinders have been used long enough to exhaust their design life.
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