Chapter 8 - Furnace control and safety

Abstract

This chapter concentrates on the special features of furnace control and assumes a basic knowledge of process control systems. Furnace control systems perform two basic functions, controlling the furnace process and ensuring safe operation. Pneumatic systems used for system controls are obsolete, and all new furnaces and retrofits either utilize PLCs for individual furnace control, or the furnaces are controlled through the overall plant Distributed Control System (DCS). Furnace control is usually based on an interpretation of product quality based on a raft of secondary measurements, which when occurring in the appropriate combination, provide conditions conducive to optimum product quality. Good instrumentation is an essential component of the furnace control system. If the furnace performance parameters cannot be measured reliably, then the furnace cannot be effectively controlled. Flue gas analysis is used for a wide range of processes. It is widely used to control and monitor excess air in combustion systems, as described above and is useful even where air/fuel ratio control is feasible because it is more accurate and maintains a check on the flow instrument calibration. Combustion control systems are used to control the operation of combustion plant, including the firing rate and airflow, and to maintain the air/fuel ratio required to meet the desired process conditions. The provision of combustion control improves process efficiency by maintaining an approximately constant air/fuel ratio under changing load conditions and relieving the operators of this task. Burner management systems are used to control start-up, shutdown and to supervise the safe operation of burners and combustion equipment. The burner management system performs a vital safety function preventing operator error leading to danger and causing the safe shutdown of the burner in the event of other equipment malfunction.