The CO2 compressor is one of the key equipment items in urea production. Its main function is to compress CO2 from purification section in the ammonia plant and send it to the high pressure synthesis system in the urea plant.
The compressor has been put into operation after its installation in 2007. On March 5, 2010, overpressure of the fourth stage of the CO2 compressor appeared. During overhaul inspection of the outlet valve of the fifth stage (aside the midbody), the inspector found penetrative crack in the valve cavity. A dye penetration test was carried out for confirmation.
The valve cavity of the HP cylinder is most prone to failure. According to data related, it is known that cylinder cracking caused by unreasonable design accounts for 74% of total cracking accidents. Therefore, the design department should fully recognize the importance of such problem, optimize structurally to avoid stress concentration and reserve sufficient safety factor allowance. Finite element analysis and calculation should be carried out if possible. Manufacturers should carry out warehousing quality inspection strictly to guarantee the quality of forgings, conduct machining as per requirements of drawings to avoid stress concentration of key components during the machining process. Only by giving common concern from all parties concerned and being strict in product quality, can the reoccurrence of such accident be avoided.