With the help of a dynamic mathematical model, the differential pressure across the waterwall in a PFBC (pressurized fluidized bed combustion) boiler is analyzed under abnormal conditions. The simulation tests have shown that the differential pressure will exceed design specification when the bursting diaphragm at the outlet pipe of the freeboard is ruptured and its capacity is larger than 50% of the flue gas flow rate of the boiler. (C) 2003 ISA-The Instrumentation, Systems, and Automation Society.