Thermal Response of Gas Pipeline Metal to Gas Decompression

During natural gas pipeline processes that involve severe depressurization (e.g. blowdown), the gas experiences very significant cooling. The general impression in the industry has been that the adjacent pipeline metal also experiences cooling to a comparable extent. Should this actually be the case, the metal would be rendered susceptible to embrittlement. This would increase the possibility of fracture, thus compromising the integrity of the pipeline. To avoid the perceived possibility of fracture, pipeline design specifications tend to recommend special materials that can withstand low temperature. Such materials are often very expensive. However, recent experimental and analytical investigations into the heat transfer effects during pipeline decompression have shown that although the gas does undergo considerable cooling during events such as blowdowns, the metal is not cooled to nearly the same extent. These investigations resulted in a model of the blowdown. The model was based on the finding that the thermal response of the pipeline metal at a particular location is largely determined by the formation of a sharp negative spike in the gas temperature as the decompression wave passes that location. The present paper offers a more detailed version of the blowdown model, taking into account the transient temperature variations through the thickness of the pipe wall. The additional investigations offer insight into the phenomenon of ‘thermal shock’ in the pipeline metal. It is found that the metal response to a thermal ‘spike’ differs markedly from that to a thermal ‘shock’ imposed on the surface of the metal. It is shown that the possibility of damage due to unequal expansion/contraction in the material across the pipe wall thickness is minimal during a blowdown.