How to Change Over Heat Recovery Steam Generators After Gas Turbine Trip

Abstract

Heat recovery steam generators downstream of gas turbines are often used in combination with process steam applications. Because of the high importance of the required process steam, a boiler trip is more severe than in usual applications, where only electricity is produced. In most cases these boilers are equipped with a supplementary and fresh air firing system having the capacity of replacing the whole waste heat coming from the gas turbine or even more. A fresh air firing system offers the possibility to keep the boiler in operation without the gas turbine running. If the boiler has to stay in operation even after a gas turbine trip a change over from waste heat firing to fresh air firing has to follow immediately. Due to the very sharp breakdown of the gas turbine speed after trip, the change over procedure has to be carried out within a few seconds to avoid a boiler shut down. The problems are — on the one hand — not to have to switch off the supplementary firing, on the other hand not to exceed the backpressure of the gas turbine because of too fast closing of dampers necessary for fresh air firing. The first would cause a necessary purging with a certain time period without firing, the second would lead to damages of the gas turbine exhaust system. Backpressure and oxygen supply have to be managed carefully to provide a smooth and save change over. In addition it has to be considered, that the first time period after gas turbine trip, the oxygen supply of the boiler’s firing has to be ensured by the running out gas turbine. Special investigations allow to predict the amount of exhaust gas mass flow after gas turbine trip by using the speed behavior of a reference gas turbine trip. At an Austrian steel mill (VOESTALPINE) these procedures were investigated in detail, and a lot of measurements were done. Based on this the existing change over procedure was optimized and the possibility of a quick change over procedure was realized.