Controlling drum level of a utility boiler during boiler feed pump runback using related rate concept

Steam boiler provides appropriate quality and quantity of steam to the turbine for power generation. In drum type boiler, water and steam flow in the drum is controlled in such a way that the water level in the drum is maintained fairly at a preset value. While very low water level in the drum may lead to starvation and explosion of boiler, too high level may cause carryover of water particle and damages the super-heaters. Therefore Drum level is an important parameter for safe operation of a boiler. Drum level in a drum type boiler is influenced by number of factors, such as sudden change in load demand and changed in heat input to the boiler etc. During Boiler Feed Pump (BFP) runback, the boiler steam generation is reduced through fuel cut off while steam out flow to the turbine is controlled by pressure controller. Under such circumstance, due to mismatch between steam out and feed water in, it becomes difficult in maintaining water level in the drum to its pre set value. This paper has suggested an innovative drum level control by adopting the concept of related rate variation. The related rate module uniquely co-relates two independent time varying parameters namely steam flow and feed water flow in real time and accordingly apply correction of steam flow to match steam and water flow during BFP runback. Simulation result shows real time correction of steam flow maintains drum level within its safe operating limit during BFP runback. The innovative related rate module can be used as a control block in the future boiler controls for coordination between many parameters varying independently with respect to time.