A large, hydrogen-cooled generator is driven by steam turbines at a power station. During maintenance shutdowns, the hydrogen cooling loop in the generator is purged with carbon dioxide. After CO2 concentrations are measured with a densitometer to verify the complete removal of hydrogen, the generator is purged with air and the maintenance is performed.
This purging procedure was used prior to the explosion. The CO2 reading was reported to be 100 percent CO2 at the top of the generator. The cooling system was then purged with air and a 1/2 inch pipe in the cooling loop was cut to install some new instrumentation. When the pipe was cut, pressurized gas was emitted at the opening. Workers assumed the gas was either carbon dioxide or air and proceeded with the new instrument installation. Unfortunately, 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. The explosion damaged ventilation baffle plates and auxiliary equipment in the generator, which caused the plant to be out of service for 26 days.
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This incident illustrates how difficult it is to completely purge hydrogen out of a large, complex piece of equipment. Uniform mixing and dilution is unlikely in all the partially enclosed spaces, crevices, etc. If a hazardous operation such as welding must be performed with an air atmosphere (instead of inert gas) in the equipment, reliable gas concentration measurements should be obtained at several different locations. In the case of the generator, a direct measurement of hydrogen concentration may well have been more reliable than the 100 percent CO2 reading on the densitometer. Furthermore, the gas composition should have been determined at the welding site as well as the top of the generator.
The importance of purging hydrogen piping and equipment is discussed in the Lessons Learned Corner on this website.