The performance recovery ability of disturbance observer based control (DOBC) is revealed based on equivalent transfer function transformation. It is found that, with the help of DOB, the open-loop characteristics of perturbed condition can be reshaped as similar to the nominal one. A nonlinear fluidized bed combustor is used to validate the ability. To implement the DOBC in practice, the bumpless switch is necessary for safety. A practical bumpless switch strategy is first proposed based on MATLAB/SIMULINK. Finally, a bumpless DOB enhanced PI controller is configured in Ovation DCS.