Fatima Fertilizer has been operating an Ammonia/Urea/CAN/NP and NA Complex since April-2010 at Rahim-yar-khan Pakistan. The Ammonia Plant was originally designed by CF-Braun and commissioned in year 1967 in Rosenberg Netherland. Ammonia Plant reengineering was carried by MWKL and was relocated to Green Field Project Fatima Fertilizer Pakistan in 2007. Ammonia Plant was successfully commissioned in April-2010 for 1500MTPD capacity.

On 21^st August 2012, fire occurred at ammonia plant synthesis compressor due to gas breakthrough owing to improper functioning and damage of casing seals. Seals damage mainly occurred due to transient surge in LP/MP casing in event of emergency trips since machine startup in 2010. Adequate surge protection was available at synthesis compressor in normal operation; however transient surge caused excessive thrust and damage to the seals and thrust bearings (no such damage experienced at HPC) which eventually led to the gas breakthrough and fire incident.

Various anti-surge schemes were studied involving individual anti-surge protection to each stage (LPC/MPC/HPC) which however was not feasible due to high cost of high pressure equipment procurement, piping modifications and limited available timeline prior to TA-2014.

An alternative scheme implemented successfully mainly involved installation of failsafe close control valve that fully opens on ESD to provide surge protection to LP/MP casing in trip case. Also it involved replacement of   low capacity vent valve at machine suction that caused suction pressure to increase in case of tripping.

The scheme proved to be very beneficial in protecting synthesis compressor against transient surges by establishing flow through LPC/MPC in 4 seconds as compared to previous 14 seconds in subsequent tripping. Machine also depressurized lot quicker than previous tripping and no flow fluctuations or surges were observed.

Conclusion: The modified anti-surge scheme not only enhanced system reliability against surge protection but also mitigated likelihood of gas breakthrough and fire hazards due to casing increased clearances and damage