Dear Satish,
Thermal Bowing occurs when your pipe experience different temperature between Top of pipe and bottom of pipe, the example of this is is rundown line og LNG TAnk piping system, or heaters pipes in the radiant section of a furnace source. The philosophy is that when there is uneven temperature distrubution then you must be aware of thermal bowing case as the thermal stress might lead to fatigue failure of the affected area.
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Tengku_Syahdilan
"From Failure we Learn"