Time-Dependent Exergy Analysis of a 120 MW Steam Turbine Unit of Sapele Power Plant

Abstract

Time-dependent exergy model was used to assess the exergy losses that occurred in the major components of a 120 MW steam turbine unit of Sapele power station. Data used for the analysis were both base parameters and measured values recorded in the station operational logbook for the period of January 2007 to December 2011. Component’s exergy destruction increments as compared with its base value were highlighted and possible causes of the increment were identified. The boiler section had the highest value. The economiser had a maximumof 4.26 % in 2009 and minimum of 1.25 % in 2007. While the evaporator had a maximium of 5.02 % in 2009 and minimum of 1.50 % in 2008. The superheater had maximum of 4.64 % in 2011 and minimum of 1.48 % in 2007. For the reheater, the maximum was 3.57 % in 2011 while the minimum was 1.71 % in 2007. Tube fouling, defective burners, steam traps and air heater fouling were adduced for the increment. Upgrading components with better designs, optimizing system performance and elimination of conditions that degrade efficiency between maintenance outages were suggested for improving the performance of the boiler section. The analysis showed that for the three turbine stages, HP turbine had the highest increment while the LP turbine had the lowest. The loss in the three turbine stages were attributed to throttling losses at the governor valves and silica deposits at the nozzles and blades. Retrofitting of rotors, diaphragms or complete stator/ rotor modules (inner block) were suggested for improving the situation. The results generally showed that exergy loss increased with increased operation time. It was observed that deterioration and obsolescence may be the major problems and that plant rehabilitation is a feasible solution. It was noted that the suggested modification and refurbishment of Sapele power plant units is an attractive solution to improve the plant economy and keep production cost competitive in a restructured Nigerian power system.