Technical Bulletin: Reliable Temperature Compensation Is Critical to CNG Vehicle Safety

The Clean Vehicle Education Foundation (CVEF) has issued a technical bulletin, Reliable Temperature Compensation Is Critical to CNG Vehicle Safety, which addresses the potential hazards created by the failure of compressed natural gas (CNG) dispensers to accurately compensate for temperature. Proper temperature compensation is essential to safely fuel CNG vehicles, regardless of station type, as serious incidents have resulted from the overfilling of CNG containers at both fast-fill and time-fill facilities.

The bulletin makes three recommendations:

1. Station operators should contact their fast- and time-fill dispenser suppliers to make sure they have—and are using—the latest set of written instructions and maintenance schedules as required in NFPA 52-2013.

2. Station operators should schedule annual temperature and pressure calibrations and function checks in early fall to allow time for any needed corrective action before winter temperatures.

3. Station operators should also ask their dispenser suppliers whether existing dispensers comply with the requirements of NFPA 52-2013.

The bulletin explains that limiting the maximum pressure in the cylinder is not as simple as limiting the pressure dispensed to the vehicle. The temperature of the gas in a CNG container may increase after fueling if the temperature of the environment increases before the fuel is used. This may occur with warming outdoor temperatures or when parking indoors. In these situations, there is the possibility of the pressure actuated PRD used on some vehicles to rupture and vent down the cylinder if temperature was not properly compensated for during fueling. This issue is especially important to address prior to the cold temperatures many parts of the country experience during winter months.

The bulletin is available on the AFDC Technology Bulletins website:
http://www.afdc.energy.gov/bulletins/2014_09_18_CNG_Temp.html

 

We encourage you to share this information with your stakeholders.

For questions about any of the information and recommendations contained in the bulletin, please contact John Dimmick at 262-549-1894 or JDimmick@cleanvehicle.org

As a reminder, the AFDC has a new interactive animation developed to help coordinators and stakeholders learn more about how outside temperature and fill speeds affect the final fill volume in CNG vehicle tanks. The animation can be found at the following link http://www.afdc.energy.gov/vehicles/cng_tank_animation.html.

 

Thank you.

Sandra F. Loi
Project Leader III – Market Transformation Group
Clean Cities Program

 

National Renewable Energy Lab  (NREL)
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