Hydrogen explosion during maintenance of a turbine-generator of a power plant

This event took place in the power generation system of a coal-fires power plant. One of the 3 turbine-generators was taken offline because workers detected hydrogen contamination and suspected a leak. By using a portable leak detector, workers were able to find the location of the leak where the terminal plate was bolted to the bushing box. To repair the connection and solve the leak, the workers had to access the bushing box via a man-way. Before removing the man-way cover, the workers purged the generator housing with carbon dioxide (CO2) to remove hydrogen, and then with ambient air to remove CO2.
After purging, the man-way cover was removed and the possible presence of hydrogen was measured at a fixed location. Considering the situation safe, one worker entered the confined space with an electric drop light and an electric fan. At  that moment, a flash fire erupted and the worker was fatally burned.

The INITIATING CAUSE was the formation of an explosive atmosphere in the bush-box of a power generator, due to the presence of  residual hydrogen and the injection of air, o be able to access the box.

The IGNITION SOURCE was provided by non-explosion proof electrical tools (electric light and electric fan). 

The ROOT CAUSE was a very inadequate execution of the written procedure to be applied in this case, probably worsened by the lack of supervision of the activities . 

H2 coolant loop

A bushing box is a box or housing attached to the generator casing, contains generator lead terminals.

US OHSA investigators identified various deviation from the work practices as factors bringing to the flash fire:

• Plant written procedures foresaw the use of twelve CO2 gas cylinders to ensure a ful purge of the generator housing. This requirement was based based on the internal volume to be purged. Only six CO2 cylinders were used, not enough to fully purge the generator.
• Plant written procedures required a portable gas monitor to check hydrogen concentration after purging. On the contrary, a fixed gas detector was used, .located at the bottom of the generator housing base instead, rather than near the housing roof where the hydrogen vented and where the was the highest riak of accumulate. Consequently, workers could not identify residual hydrogen.
• The confined space assessment was performed three days before opening the bushing box man-way cover. Since the space remained sealed and closed for those three days, the measures were no more representing the reality. 
• Instead of using explosion-proof tools, workers produced an ignition hazard by using unsafe, spark-generating tools.