A system includes a controller including a memory storing instructions and a processor that executes the instructions. The instructions cause the controller to receive a first input signal of a first temperature at an inlet of a gas turbine of a gas turbine and heat recovery steam generator (HRSG) system and a second input signal of a rotational speed of the gas turbine. The instructions also cause the controller to calculate the exhaust flow rate of the gas turbine and HRSG system based on the first input signal and the second input signal. Further, the instructions cause the controller to control the gas turbine and HRSG system to isolate a fuel source at a portion of normal operating speed of the gas turbine sufficient to achieve a predetermined purging volume during coast down of air flow through the gas turbine and HRSG system based on the exhaust flow rate.