Boiler and Furnace Control System

Boiler and furnace control requires special methods and systems to properly light and operate the steam boiler and its burner. The burner fuel can be oil or gas, and also a combination of those and other fuels. A boiler can also operate with coal and other solid fuels such as wood pulp.

The light off sequence for each of these fuel types will vary and is regulated by the requirements of NFPA 85. Incorrect sequencing of fuel and ignition and combustion air during the light off phase can result in a boiler explosion, sometimes called a boiler puff. The puff occurs as a relatively low pressure explosion in the firebox when accumulated fuel ignites in a single burst. The fuel should not accumulate due to the pre ignition purge cycle required by NFPA 85. The lack of adequate purge can result in the accumulation of oil or gas fuel in the firebox.

Excess fuel can also accumulate due to inadequate combustion air during the light off phase as well as during normal operation of the boiler. Problems in boiler operation are often related to low water level in the drum. Boiler accidents can also occur from too much steam pressure coupled with failure of a relief valve or other overpressure protection device. Redundant relief valves are often used to prevent overpressure problems. Redundant level sensors or level transmitters are also used to prevent level control accidents that lead to destruction.

Ongoing operation of a boiler and burner requires different action than the light off phase. Once a boiler is operating the timing and limits of NFPA 85 are different than during the light off phase. The use of a flame sensor also, called a fire eye, is done to detect loss of flame after the initial ignition. The flame detector is also used during lightoff to detect pilot flame and main flame as part of the startup sequence.

Arthur Zatarain can provide forensic engineering and expert witness for incidents involving boiler and burner. The burner management system BMS is of particular importance in boiler explosions. A BMS can be fabricated using a programmable logic controller or a dedicated BMS electronic controller. A BMS can also be made with a distributed control system DCS if specific safety design features are adequately addressed. Other devices such as dedicated Honeywell BMS devices can also be used to control lightoff and operation of a boiler.

Burner Management System - the BMS

Burners for steam boilers are controlled by a burner management system. The BMS is used to purge the boiler firebox before initial light off. The BMS then controls the lighting sequence that includes the pilot and main burners. NFPA 85 is the usual governing document. The burner management system can be fabricated with standard BMS controllers from Honeywell and others. Alternately, the BMS can be designed with a dedicated programmable logic controller PLC. Or. the BMS can be implemented with the DCS that controls the plant served by the boiler.

Safety Instrumented System - SIS

A boiler control system and burner management system can be fabricated using the principles of safety instrumented systems, known as SIS. The governing document is ISA84, which is also known as IEC 61058. These documents differ only in a "grandfather clause" that is included in ISA84.