CNG Cylinder Inspection Interval
A transit agency has a 2015 CNG bus with 120,000 miles. The bus is used for daily service. The maintenance supervisor asks a technician to determine if the CNG cylinders need inspection or replacement. The cylinders are Type 4 (plastic liner with carbon fiber composite). The bus was manufactured in June 2015 and placed in service in August 2015. The agency follows manufacturer recommendations and applicable codes (NFPA 52, CSA). The technician reviews the maintenance records: the last cylinder inspection was done at 60,000 miles in 2018, with no issues. The bus operates in a region with moderate climate. The agency plans to keep the bus for another 5 years.
1. What inspection is due now, and what is the basis for the interval?
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For CNG cylinders in bus service, inspection intervals are typically every 3 years or 36,000 miles, whichever comes first, per manufacturer and codes like CSA/ANSI NGV 2 and NFPA 52. The last inspection was at 60,000 miles in 2018. Now at 120,000 miles, the bus has accumulated 60,000 miles since last inspection, which exceeds the 36,000-mile interval. Also, 3 years have passed since 2018. Therefore, a cylinder inspection is overdue. The inspection should include visual inspection for damage, corrosion, and abrasion, and may require hydrostatic testing per manufacturer. Type 4 cylinders have a limited service life (often 15-20 years), so check the manufacturer's recommended life. The technician should perform the inspection per the manufacturer's procedures.
2. What factors determine whether the cylinders can remain in service or must be replaced?
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The cylinders can remain in service if they pass inspection: no damage, no leaks, and within the manufacturer's service life. Type 4 cylinders have a specified service life (e.g., 15 or 20 years from date of manufacture). The bus was manufactured in June 2015, so it is now about 9 years old (assuming current year 2024). If the service life is 15 years, it can remain until 2030. However, if the inspection reveals damage (e.g., cuts, gouges, or impact damage to the composite), the cylinder must be replaced. Also, if the cylinder has been exposed to fire or extreme heat, it must be replaced. The technician should follow the manufacturer's criteria for condemnation. If the cylinders pass, they can be returned to service, but the next inspection is due at the earlier of 3 years or 36,000 miles from now.