The carbon steel casings of Heat Recovery Steam Generators (HRSGs) in a co-generation plant
were found to have developed cracks that originated on the inside surfaces. The HRSGs receive hot
exhaust gases from the primary Gas Turbines (GTs), which are fuelled with pipeline natural gas. The
GTs was initially operating without any suppression of nitrogen oxides (NOx).
After being in service for about over three years, some of the cracks were found to have
penetrated right through the ¼ in. thick casings. The casings are fitted with insulation on the inside (hot
gas side).
Examination of steel samples cut from the casing walls revealed (i) fine cracks propagating
through the steel from the inside surfaces, and (ii) the presence of relatively large amounts of nitrate
ion on the inside surfaces.
It was concluded that the nitrate ions originated from nitric acid that was condensing on the
relatively cool walls of the casings behind the insulation. Nitric acid was formed in the GT exhaust
gases by the combination of water vapour (formed from combustion of the hydrocarbon fuel and from
moisture in the combustion air) and nitrogen oxides (formed from nitrogen and oxygen in the
combustion air).