117.What is the meaning of lead/lag control with respect to Boiler Master? What is cross limit and cross bias?

The Boiler Master Controller controls Boiler outlet Steam pressure by regulating Fuel (Coal/Oil/Gas) and Air required for combustion. For any combustion to be effective , the volume of air required has to be equal to atleast the Schiometric ratio. However, it is necessary to prevent Fuel rich conditions in the bolier as it can lead to unburnt carbon and un-controlled combustion resulting in possible explosion. Hence, the control system design shall ensure an air rich condition always. to accomplish the same, whenever there is a demand to increase fuel, the actual airflow shall be increased first and then fuel flow (Air leads Fuel). When the Boiler Master demands decrease in fuel flow, first fuel has to be reduced and then air flow (Air lags Fuel). The Cross limiting circuit ensures the above to happen--actual Airflow and Fuel flow measurement shall be used to cross limit increase/decrease (thru Max/Min selectors) to ensure air rich condition always.