Minimum variance feedforward control of a once-through steam generator

Reliable operation of once-through steam generators (OTSG) of the continuous-tube type, as used for gas turbine heat recovery, requires tight regulation of final steam temperature, using feedwater flow as the controlled variable. The feedwater flow controller typically utilizes feedforward information concerning gas temperature and flow rate, as well as feedback of the measured steam temperature. The time-response characteristics of feed forward and feedback control elements must be designed to allow for thermal mass and transit time lags. The results presented in this paper were obtained by using a model of OTSG internal processes to identify the process response characteristics for various inputs, then using these transfer functions to find dynamic compensators that, when applied to stochastic feedforward signals, cause the variance in output steam temperature to be minimized. Simulation results show tighter regulation of final steam temperature, with step changes in gas temperature and flow, than is achieved with tuning of the conventional PI controllers.