The National Highway Traffic Safety Administration (NHTSA) has issued a notice of recall for more than 67,000 pressure relief valves that are used with compressed natural gas (CNG) fuel tanks in natural gas vehicle applications.
Circle Seal Controls manufactured the valves between July 2011 and December 2013 and supplied the products to numerous customers, including Hexagon Lincoln, Agility Fuel Systems, Hoke Controls and several others. The valves are typically integrated into CNG tank systems that are used on medium- and heavy-duty NGVs, such as transit and shuttle buses and refuse vehicles.
Late last year, Hexagon Lincoln informed Circle Seal that one of its CNG tank customers, New Flyer Industries Inc., observed some relief valve failures when the outside temperature dropped below -13 degrees F. Upon failure, CNG leaked from the tanks.
Circle Seal halted valve shipments and began investigating the issue. Testing revealed that a design defect led to the failures – specifically, one of the o-rings used in the valves was manufactured with a rubber compound that ‘did not perform as required’ at very low temperatures. Further testing confirmed that the o-rings do function properly at temperatures above -5 degrees F, ‘as long as a user does not run the vehicle until the CNG tank is completely empty,’ according to the NHTSA.
‘In the unlikely scenario that the tank is completely emptied and the o-ring is, therefore, no longer subject to any pressure, tests indicate that the o-ring can fail at temperatures of 20 degrees F and below when the tank is being refilled from its completely empty state,’ the agency's report reads.
Circle Seal said it is unaware of any incidents resulting from the defect, and the Hexagon Lincoln/New Flyer incident was the only one reported. The company is informing CNG tank manufacturers of the problem, and NGV owners will likely be notified by the OEM or the supplier of the CNG system. Circle Seal does not sell its pressure relief valves directly to any vehicle manufacturer, so the company does not know exactly which NGV makes and models are affected by the defective equipment.
For more details, click here. Any fleet operator or CNG system professional can also call the NHTSA with questions: (888) 327-4236.