(105f) Fatal Explosion Caused by An Intermittently-Used Fuel Gas Piping System

A fatal explosion occurred at a foundry during a routine test of an intermittently used propane standby system. Just prior to the explosion, maintenance personnel were preparing the propane standby system for a startup test. While preparing for the startup test, the group was alerted that employees in an adjacent building had detected the odor of propane. The test was stopped and the group conducting the test implemented emergency shutdown procedures. The explosion occurred during the execution of the emergency shutdown. The explosion resulted in three fatalities, numerous injuries, and extensive property damage.

The propane standby system was intended to supply synthetic natural gas in the event of an interruption of the natural gas supply from outside utility company. Synthetic natural gas is a mixture of propane and air that has similar combustion properties to natural gas. To ensure the reliability of the standby system, it was scheduled for two routine maintenance tasks each year: a preventative maintenance task and a startup task. When the piping was placed back into service for the test, flashing liquid propane discharged from an undetected hole in the pipe into loose backfill material in the soil. The propane vapor entered a building through a penetration in the building foundation and was subsequently ignited. Following the incident, the buried liquid propane line was excavated and a hole in the piping was discovered.

This accident was caused by the failure of the mechanical integrity of the propane piping. The root causes of the piping failure are discussed along with corrective actions that could have prevented this accident. The corrective actions include selecting an alternative material of construction for the piping, implementing proper construction practices for buried piping, following appropriate test and maintenance procedures for the existing cathodic protection system, and verification of piping integrity by leak testing with an inert gas before placing it into service.