Thanks for the answers. We have solved our problem (drum obstrution) by injecting water in the feed pipe on the drum bottom by a hose in a small flow because the obstrution. When we observed that the water reach the drum top we wait 4-6 hours (hot soaking). The temperature on the wall was below 110 degree C and the pressure below 0.1 Kgf/cm2. We opened the drum and we decoked it with no problem (hot spot). The water even in a small quantity cooled the drum.
Thanks by the helping