The Clean Vehicle Education Foundation (CVEF), a U.S. non-profit national organization whose mission is to assess and guide alternative fuels including NGV activities, has released the Maintenance Facilities Modification Project’s Phase I final report titled Analyses in Support of Risk-Informed Natural Gas Vehicle Maintenance Facility Codes and Standards: Phase I. For the report, a HAZOP analysis was used to determine creditable release rates and quantities for both LNG and CNG vehicles. A typical garage structure was then modeled for computational fluid dynamics (CFD) modeling of the releases.
The creditable releases included venting of an LNG tank due to pressure build from boil-off, venting fuel lines from the valved-off storage vessels for both LNG and CNG vehicles and, as a worst case, a pressure relief device (PRD) failure on the largest CNG cylinder plus 50 percent.
The first three releases indicate that the ignitable mixtures are confined to a small area around the release point and do not accumulate in the ceiling area. Also, ventilation has little impact on the release and concentration of the gas.
The last case (with the PRD failure) is of real concern and the existing code requirements may be ineffective. It looks like that the renewed use of rupture disc PRDs by some NGV OEMs and modifiers create a greater hazard due to their operation than what they were designed to prevent. The industry stopped using these PRD designs in the late 1990s to prevent these unintended releases, but they are now being used again.
CVEF will not be able to go forward with Phase II of the SANDIA project until the PRD issue is resolved. In the meantime it has issued a Safety Alert on valves and rupture disc PRDs on the CVEF website here: http://www.cleanvehicle.org/technology/ TechnicalBul9ServicePressureSafetyAlert.pdf.
The report has been posted on the CVEF website, available by clicking here. SANDIA National Laboratory will publish the report through the Office of Scientific & Technical Information (OSTI) sometime in May for public access. If anyone has any questions about this work or the seriousness of the issue, email Douglas Horne at CVEF or Diego Goldin at NGV Global.