Boiler Drum Level Control Using Internal Model Controller

The water level of boiler drum is one of the crucial control parameters for any process industry which reflects the control of mainly boiler load and feed water indirectly. If the level exceeds the limits, boiler water carryover into the superheater or the turbine may cause damage resulting in extensive maintenance costs. If the level is low, overheating of the water wall tubes may cause tube ruptures and serious accidents, resulting in expensive repairs, downtime, and injury or death to personnel.A rupture or crack most commonly occurs where the tubes connect to the drum. Damage may be a result of numerous or repeated low drum level conditions where the water level is below the tube entry into the drum. Boiler drum is one of the typical multi-input multi-output systems, and also shows high nonlinearities, time varying response and a transportation lag. Conventionally, such system has been controlled using PID (proportional, integral, derivative) controller. Due to presence of nonlinearity (dead time) boiler drum performance can be affected when it is controlled by PID controller.Moreover PID controller produces larger settling time. To have an effective tracking of set point, an internal model based control (IMC) methodology can be implemented.The transfer function for drum level is obtained using mathematical modeling and transfer function for steam flow rate is obtained using system identification tool box.single element control is implemented using PID and IMC controllers.Three element control is implemented using cascade structure in which PID is used in primary and secondary loops.From the performance criteria it has been analysed and found that the IMC controller has better settling time, rise time, peak time and peak overshoot compared to conventional PID controller.