Severe accidents may occur if gas-fired ovens are not inspected, tested, and maintained properly. In a recent article in Process Safety Progress, authors Quentin A. Baker, Daniel J. Benac, and Douglas B. Olson, discuss an explosion that occurred inside an industrial oven at a manufacturing facility during an attempt to restart the gas burner. The explosion was caused in part by a malfunctioning flame detector that allowed natural gas to accumulate during the hours before the oven was restarted. The incident resulted in the death of one employee.
The manufacturing facility shut down the oven every Friday night and restarted it on Sunday evenings. In this incident, an employee started the oven three hours before the first shift and observed that the control system showed that all was normal. However, as workers began to arrive for the first shift, they reported the smell of gas. A second employee was then sent to check the oven, shortly after which, the oven exploded, killing the second worker.
According to the article, the explosion was determined in part to be the result of a malfunctioning flame detector, which sent a false positive to the control system. This allowed the main burner gas valve to remain open, even though the burner had not been successfully lit by the igniter. Unburned gas, therefore, flowed into the oven for approximately 3 hours.
It was a second unrelated error that led to the fatal explosion. The second employee saw that the oven had not heated up, so he pressed the stop button on the control system and then pressed the restart button. A proper control system would have initiated a post-ignition purge cycle after the stop button was pressed to remove any unburned gas from the oven enclosure. Then after the restart button was pressed, the control system should have initiated a pre-ignition purge cycle to remove any unburned gas before the igniter would be allowed to energize. The investigators determined, however, that the control system allowed the igniter to energize immediately after the restart button was pressed, bypassing both the post-ignition and pre-ignition purge cycles. This allowed the igniter to energize while the oven was full of unburned gas, causing the fatal explosion.
The burner control systems on your industrial ovens and dryers are carefully designed and programmed by the equipment manufacturer to enable safe start-up and shut-down sequences. Look out for a future blog that will discuss 5 key things you need to do to help ensure proper burner control function throughout the life of the equipment.
By Jeff Harrington, CEO and Founder of Harrington Group, Inc.