Level control of a steam drum using double feed back loop control strategy along with set point filter

Abstract

Regulating the level of liquid in a boiler steam drum is of prime concern. The boiler is so manufactured that the drum level must be in the specified range. According to the demand of steam in process industries, steam drum provides steam of definite quality. The shrink and swell effect results in false level of water that leads to the measurement of three parameters including water level of drum, flow of feed water and flow of steam. This work deals with the level control of steam drum using double feedback loop method along with set point filter. In double feedback loop method, the inner loop consists of a proportional controller and is tuned with the help of Ziegler Nichols (ZN) method while the outer loop consists of a proportional integral derivative (PID) controller and is tuned by internal model control (IMC). The internal loop is employed for attaining the process stability while the external loop helps in fine tracking of set point. Also a comparative analysis between different control schemes such as conventional PID controller, single loop IMC based PID controller and double feedback loop method is done in respect of distinct time domain performance specifications such as rise time ( r t ), settling time ( s t ), percentage overshoot ( p M ), integral of square error (ISE) and integral of time absolute error (ITAE). The simulation results show that the single loop IMC based PID controller is more efficient than conventional PID controller as there is an improvisation in rise time and settling time. Furthermore, the double feedback loop method shows better performance than both conventional PID controller and single loop IMC based PID controller in respect of percentage overshoot, settling time, ISE and ITAE.