The flow rate (gpm) (red graph) is modeled as an on /off cycle with a period of about 5 minutes. This might represent the load on a commercial laundromat with several washers filling every few minutes.The second graph (white graph) represents the action of the steam control valve in response to changes in the water temperature in the water heater as represented by the third graph. (blue graph) The temperature set point that you are trying to maintain is 140 F.
Your goal is to adjust the proportional band (PB in yellow) until there is only a slight overshoot as the result of a step change in the load (gpm). Notice that the the temperature oscillates around the set point. With too little PB, the valve cycles continuously. Next add some integration (Gain in green) to ramp the proportionality error back toward the set point.