We operate two, ABB three pressure HRSG's, down stream of GE 7EA gas turbines burning natural gas. Without seeing your PID's and with my experience with our HRSG's, I'd say the line from your IP economizer to LP drum is a vent header used for venting the air from your IP economizer harp assemblies upon start up. The steam line from your IP drum to your Deaerator is used for "pegging steam". During start-up there may be times when there is not enough heat making it to the rear of the HRSG and your Deaerator pressure will decay. Pegging steam flow is controlled by a pressure control valve set to your desired Deaerator pressure, 10 psig. Hope this helps.