This paper proposes the design of a PI robust controller with which to regulate the steam pressure in the steam drum of a bagasse-fired boiler. The dynamic behavior of this process was identified by means of experimentation. This identification procedure yielded an equivalent third order plus time delay model, and showed wide process static gain variations. We, therefore, propose a new method with which to design fractional-order robust controllers for this kind of processes. This method is based on the exact attainment of certain nominal time specifications while using one of the parameters of the controller to maximize the gain margin. The controllers attained were shown to significantly outperform the robustness achieved by using PI or PID controllers, in the sense of reducing the Integral Absolute Error (IAE) and improving the steam pressure uniformity.