Intake valves from natural gas-fired reciprocating engines displaying “torching” were examined to determine their failure mechanism. The principal features of the “torched” valves include a relatively thick black deposit on the tulip area of the valve extending to the sealing surface, partial loss of those deposits in various locations, and localized metal loss, oxidation and/or surface cracking in the spalled regions. Electron microprobe, scanning electron microscopy, and optical microscopy were employed to characterize the deposit formation and metal loss mechanisms. The initial cause of the torching appears to be due to the localized spallation of a loosely adherent (Ca,Zn) phosphate oil deposit adjacent to the valve/seat seal which creates a channel of hot, high velocity combustion gases. Within the torched area, significant metal oxidation and metal recession due to erosion/corrosion was observed on the valve sealing face, creating a relatively wide gap where a valve/seat seal should be. In areas where torching is not evident on the valve sealing surface, no appreciable metal recession (but limited metal oxidation) was observed.

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