Explosion at the generator of a coal-fired power plant

The incident happened at a 500-megawatt, hydrogen-cooled generator driven by steam turbines. The generator was shut down for maintenance.
A purging procedure had to be executed before starting repair and maintenance works, consisting of (1) purging the hydrogen cooling loop of the generator with carbon dioxide, (2) measuring the CO2 concentrations by a densitometer to verify the complete removal of hydrogen, and (3) purging the generator with air, after which maintenance could be performed.

The CO2 reading was reported to be 100% CO2 at the top of the generator. The cooling system was then purged with air, after which a 1/2-in. (1.3-cm) pipe in the cooling loop was cut to install some new instrumentation. When the pipe was cut, pressurised gas was emitted. Workers assumed that the gas was either carbon di-oxide or air and proceeded with the new instrument installation. However, there was still at least some hydrogen in the pipe and the rest of the cooling loop. When the welder struck an arc, a flame developed at the pipe opening and flashed back into the generator. This caused a low-level explosion within the generator shroud.

[Zalosh and Short, 1978]

The INITIATING CAUSE was an explosion during maintenance works, due to residual presence of hydrogen despite the purging action performed before.

The ROOT CAUSE was an ineffective purge procedure, and the lack of ‘second opinion’ measure such a direct measure of gas composition.

generator and its internal components

The explosion damaged ventilation baffle plates and auxiliary equipment in the generator so that it was out of service for 26 days.

Zalosh and Short (1978, see References) concluded the presentation of this exemplary case, by remarking the difficulty to guarantee large and complex equipment free of hydrogen. Purging is based on the base of mixing and dilution, but it is improbable that it could occur uniformly and completely in closed spaces containing small diameter gas paths, curves and dead ends. Therefore, additional precautions must be taken, before operation such welding are performed in air. One of the precautions could consist in direct concentration measurements of hydrogen, not (only) CO2, performed at several locations, and in particular t the place where works are planned.