Qatar Fertilize's Company (QAFCO), Ammonia-3 is a 1,500 metric tons per day (MTPD) plant, designed, supplied. and commissioned by a company (Division of Thyssen Krupp Group) (UHDE) in 1997 The main gas flare stack is a sell-supported free-standing structure with a height of 705 m, located in the middle of the plant Excessive vibrations/shaking of the flare and flare structure were observed due to mixing of NH(3) and CO(2) rich gases in the presence of water Ammonium carbonate and bicarbonate sludge (salts) formed in the flare stack and increased resistance to the gas flow path The partial blockage increased the pressure at the flare-stack bottom and the sudden release of the gases gave vibrations (surging effects) to the flare stack The flare stack was drained after unplugging the stack-bottom drains Hot-process gases (167 degrees C) from the low temperature shift convener (LTS) outlet were lined up to the flare slack in small quantity to melt the deposits Flare vibrations completely slopped after 3 days Inspection of the weld joints and foundation bolts was done, and no abnormalities were found No vibrations were observed during all normal/emergency plant shutdowns faced after the incident The work describes the root cause of unique experience of flare-stack vibrations, and the way the problem was solved online (C) 2010 American Institute of Chemical Engineers Process Saf Prog 29 254-263, 2010