In this paper results of analysis of process steam boiler operation with maximum continuous steam production of 75 t/h, steam pressure and temperature of 45,5 bar and 412 oC are presented. Steam boiler is predicted to operate within steam production facility of oil and gas refinery in Pančevo, Serbia. On the demand of purchaser steam boiler is enabled to operate with various fuels as well as with two or three fuels simultaneously. By fuels intake facilities it is enabled to introduce refinery gas, fuel oil and high-temperature waste gas to the boilers furnace. Waste gas is guided from FCC (Fuel catalyst cracking) facility and presents two-component fluid with polydisperse solid phase composed of particles of diameter smaller than 40 µm. Recent changes in steam consumption in refinery resulted in modifications in boiler`s operating regime and, further on, in significant changes in its operational parameters. At present, in new conditions, boiler operates with frequent pipe bursts on pipe bundles of boiler economizer which requires forced interruption in steam production as well as time and cost consuming operations for resuming boiler work. In this paper results of thermal calculations for design and new operational conditions are presented for the purpose of identification of cause of frequent unscheduled steam production interruptions. After analysis of presented results, recommendations on how to modify certain boiler operational parameters in order to eliminate or reduce possibility of such unfavourable occurance has been provided.