(105b) Gas Fired Oven Explosion

A natural gas fired industrial oven exploded during an attempted restart a few hours after a cold start. The explosion was caused by accumulation of natural gas during the hours prior to the restart. The oven had a fully automated control system, and the start, shutdown and restart were controlled by process controls and the flame safeguard system. A malfunction of the flame detector allowed the automated controls to complete the start sequence and establish normal operating status without a flame present on the burner. The UV flame sensor gave a false positive signal. The investigation explored the cause of the incident and the failure modes of the flame detector.