Compressed natural gas bus safety: a quantitative risk assessment

Area of Science:

• Transportation Safety
• Risk Assessment
• Alternative Fuels

Background:

• Compressed natural gas (CNG) is an alternative fuel for vehicles, including school buses.
• Assessing the fire safety risks of emerging technologies like CNG buses is crucial due to limited historical data.
• Diesel school buses represent a mature technology with available historical safety performance data.

Purpose of the Study:

• To assess the fire safety risks of compressed natural gas (CNG) vehicle systems, specifically school buses and their fuel infrastructure.
• To determine the sensitivity of fire safety risks to component failure rates and fire event consequences.
• To identify critical components and subsystems contributing most to fire safety risk.

Main Methods:

• Utilized a probabilistic risk assessment (PRA) approach to model and predict fire safety risks for CNG buses.
• Employed generic failure data, engineering judgments, and assumptions due to limited real-world experience with CNG bus fleets.
• Compared predicted risks with historical fire safety data from diesel-fueled school buses.

Main Results:

• Predicted mean fire fatality risk for typical CNG buses is approximately 0.23 fatalities per 100-million miles (all persons) and 0.16 fatalities per 100-million miles (passengers only).
• Diesel school buses have a mean fire fatality risk of 0.091 (all persons) and 0.0007 (passengers) per 100-million miles.
• CNG buses exhibit a 2.5 times higher fire fatality risk than diesel buses, with passengers being over two orders of magnitude more at risk.

Conclusions:

• CNG school buses present a significantly higher fire fatality risk compared to diesel buses.
• Failure rates of pressure relief valves, CNG cylinders, and fuel piping are the most influential factors in CNG bus fire risk.
• Further safety enhancements for CNG bus systems should focus on these critical components.