Simulation of steam superheater operation under conditions of pressure decrease

Abstract

A decrease in the live steam pressure at the turbine inlet results in a pressure decrease in the boiler and enables an immediate increase in the steam mass flow rate at the boiler outlet. Assuming that the rate of the pressure-decrease process is known, the correct value of the target pressure decrease can only be determined computationally. This value should ensure the required total increment in the steam mass flow rate at the boiler outlet which is necessary to achieve the requested increase in the power-unit load. The total increment is the sum of increments which occur in live steam superheaters and in the evaporator. Therefore, a robust mathematical model of the steam superheater is essential for the correct computation of the increase in the steam mass flow rate at the boiler outlet which occurs as the pressure decreases. In this paper, an efficient in-house model of a superheater is proposed. It is a distributed-parameter model based on the solution of the equations which describe the principles of mass, momentum, and energy conservation. The model enables the determination of the value by which the pressure must be decreased to ensure the required increment in the steam mass flow rate. Simulation computations related to the increase in the steam mass flow rate at the superheater outlet were performed. Different rates of the pressure-decrease process and different values of the target decrease in the pressure were considered. The obtained results were compared with the results of the simplified calculations for steady states. The developed model is intended for the computation of the live-steam mass flow rate increments at the boiler outlet when rapid increases in loads are required within a short time. The computations are necessary for the generation of the modified sliding curves of the power unit.