Self-supported flare-stack vibrations in ammonia plant



Qatar Fertilizers 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 self-supported free-standing structure with a height of 70.5 m, located in the middle of the plant. Excessive vibrations/shaking of the flare and flare structure were observed due to mixing of NH3 and CO2 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°C) from the low temperature shift converter (LTS) outlet were lined up to the flare stack in small quantity to melt the deposits. Flare vibrations completely stopped 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. © 2010 American Institute of Chemical Engineers Process Saf Prog, 2010