2013 Juneja Fabrication of Construction Materials in Urea Manufacturing Plants
Urea is produced from NH3 (Ammonia) and CO2 (Carbon dioxide). The reaction involves the formation of intermediate product known as ‘Ammonium Carbonate’ which further decomposes to Urea and Water. Elevated temperature and pressure is required for the above reaction to occur. The pressure typically varies between 140 and 210 bars and the temperature between 170oC and 200°C , depending on the process technology. Ammonium Carbonate, which is an intermediate product, is very corrosive. Equipment and Piping of High pressure (HP) synthesis section (where the above reaction occurs), need to be constructed with a corrosion resistant material. Stainless steels are the candidate materials but conventional SS grades such as SS 304, SS 316 do not withstand the corrosion due to carbonate. Earlier, SS 316L UG (urea grade) was used with high amount of oxygen supply. Recent trend for selection of MOC involves application of 25Cr-22Ni-2Mo, Safurex, DP 28W, depending on the process licensors, in HP section. Equipment such as Urea reactor, Urea stripper, Carbonate condenser, Carbonate ejector are constructed with Carbon steel lined or overlayed with a Protective layer of the above mentioned Stainless steel materials. Welding is used as prominent method for fabricating such huge equipment. Though weld ability is not a concern with respect to the above materials, quality of weld obtained during fabrication decides the service life of the equipment. Welding procedures are qualified by optimizing weld parameters in order to meet stringent. The qualification is adopted for job, with appropriate quality checks in production welds also. Recent developments in construction materials to curb corrosion mechanism operative in HP section is also integrated. The criticality involved in welding these materials, to meet the stringent client specification requirement is discussed in the paper.