Propane Tank Overfill Safety Advisory

Updated June 24, 2010

The use of propane vehicles can enhance our energy security and improve air quality. Today, propane vehicles are most often used in school and shuttle bus fleets, in mass transit and taxi fleets as well as light-duty truck applications throughout the United States. There has been some concern over reported cases of fuel tanks on propane vehicles being overfilled, potentially resulting in emissions from pressure release relief valves. A Tiger Team Project was initiated to evaluate this condition.

Project Activity Summary

Seven propane fleets, of varying sizes, were included in the LPG tank fueling evaluation. At each of these fleets a sampling of vehicles were observed during fueling (105 vehicles total). Vehicles were noted when their overfill prevention device (OPD) failed to activate and stop the fueling process. This was done to quantify the frequency of such an occurrence in practice in the field, which could lead to safety and environmental issues.


Of the 105 vehicles tested at seven fleets, roughly 16% had OPDs that did not operate properly to stop fueling at the appropriate level. While this number is significant, the overfill scenario is only one of three conditions necessary for a potential incident to occur from overfilling. These conditions include: 1) Overfilled tank that leads to a release of fuel; 2) Rising ambient temperatures; and 3) Ignition source.

Rising temperatures cause expansion of LPG, so if a tank is overfilled during a cool time of day and sits, without driving and consuming fuel, hot temperatures later in the day may cause the expansion of the fuel and could potentially lead to a fuel release or leak through the pressure release device (PRD). The PRD is designed, as a safety measure, to release propane gas if pressure in the tank rises above set safe levels. However, even if conditions result in a fuel release, an ignition source must be present for a fire or safety incident to occur. We are not aware of any incidents caused by overfilling of LPG vehicle tanks.

Next Steps

While 16% failure of OPDs is significant, given that motor vehicle tanks are all equipped with PRDs to ensure safe levels of LPG pressure in the tanks, and we are not aware of any incidents to date, the following steps are recommended:

  1. Inspection of OPDs in vehicle fleets — fleet operators using LPG vehicles should inspect the OPDs to ensure they are operating properly and repair or replace them as necessary. Also, fleets should develop a process for ongoing inspection as part of routine maintenance.
  2. Training and education of LPG vehicle users — users should be trained on proper fueling and maintenance practices, and awareness of overfill potential.
  3. Documentation of OPD maintenance — industry groups should develop and implement standard maintenance inspections and practices for OPDs to ensure they work properly.

The US DOE Clean Cities (DOE-CC) program is working with the National Propane Gas Association's (NPGA) Technology Standards and Safety Committee to ensure they are aware of this issue, and work is already underway in this committee on a proposal to examine this issue more closely. Underwriters Laboratory (UL) and the Propane Education and Research Council (PERC) are involved in this process. The National Fire Protection Association has considered adopting some of these measures through the work of its Technical Committee on Liquefied Petroleum Gases. It is expected that in June 2010, the association will approve the 2011 edition of NFPA 58 "LP-Gas Code," which will contain new requirements for the periodic testing of overfilling prevention devices on motor vehicles.

DOE-CC will support recommendations by NFPA and will work with NPGA, UL and PERC to develop training and maintenance practices and communicate them to fleets and stakeholder groups through webcasts, workshops, the Alternative Fuels and Advanced Vehicle Data Center website, and printed materials as appropriate.

As a best practice, all fleet managers, owners, and operators of LPG powered vehicles are encouraged to re-emphasize the importance of closely following the LPG tank manufacturer's fueling instructions as part of their ongoing training and safety programs. It is important to understand the safe and proper operation of LPG refueling equipment and integrated safety devices. Anyone who suspects or observes that an LPG tank has been overfilled or that an OPD or other safety device is not operating properly should not operate the vehicle and should immediately report the matter to their fleet safety manager or other appropriate official.