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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.