Problem No. 9: Unblocking pipelines after a urea plant shutdown

In several sections of a urea plant, pipelines can become easily blocked when temperatures become too low and crystallisation can take place. In the evaporation section, urea melt is present which crystallises at about 133°C when in the pure form. Furthermore, urea can also polymerise to biuret, triuret and poly-urea with high crystallisation temperatures when urea is kept for a longer period at high temperatures. In addition to pipelines becoming blocked, the internals of several equipment items can also suffer from these problems as shown in the picture. In the recirculation section ammonium carbamate is present which crystallises at about 153°C when in pure form. And in the high pressure synthesis section, urea and carbamate mixtures are present. During normal operation the risk of blocked lines is small, however, during a shutdown of the plant, blocking can occur if the right measures are not taken. This Round Table discussion discusses which measures to take and what to do when a line is blocked.