A chemical mechanical polishing system includes a steam generator with a heating element to apply heat to a vessel to generate steam, an opening to deliver steam onto a polishing pad, a first valve in a fluid line between the opening and the vessel, a sensor to monitor a steam parameter, and a control system. The control system causes the valve to open and close in accordance with a steam delivery schedule in a recipe, receive a measured value for the steam parameter from the sensor, receive a target value for the steam parameter, and perform a proportional integral derivative control algorithm with the target value and measured value as inputs so as to control the first valve and/or a second pressure release valve and/or the heating element such that the measured value reaches the target value substantially just before the valve is opened according to the steam delivery schedule.