143.What is Runback?

In a large units, the auxiliaries will be split to two of 2 x 60% capacity. This will help to produce atleast 50% load during any such auxiliary. Loss FDF/IDF will reduce air supply. Loss of BFP will reduce feed water supply. Loss of Mill will reduce fuel supply. When a loss of auxiliary is sensed the load shall be reduced to below 50% as quickly as possible. Loss of Fan needs 100%/min rate. This control is initiated logically.This is called Runback. Runback for FDF can be achieved successfully for almost all the boilers. However the unit can survive for loss of BFP only in the case of assisted circulation boilers. Loss of coal Mill will transfer the coal flow control to Manual.The unit will go to Turbine follow Mode for Runback.