Consequential Damage of Blade Failures in Single Shaft Gas Turbines

Abstract

In history of single shaft gas turbines many blade failures have happened. In most cases the consequential damage has been limited to the blading in the downstream gas path. This is unavoidable because of the impact of debris. But in some cases the related unbalance of the rotor has caused additionally extended damage during the run-down of the engine. In the worst publicly known case the result was a complete destruction of the engine with disintegration of the rotor. The general rotordynamic literature indicates features and criteria to prevent such a high sensitivity to unbalance. Some manufacturers use internal design criteria for this purpose. A sometimes mentioned criterion in this respect is: “To keep the rotation speed below the frequency of the first bending mode of the rotor”. The aim of this paper is to track generally the history of gas turbine blade failures and to explain the design features of rotors and bearing support structures, which lead to high or low risk of secondary damages after blade failures. This is not new but a description from a gas turbine design point of view is missing so far. Both theory and experience indicate that an exact prediction of the response amplitudes to a large unbalance from a blade failure is very difficult. On the other hand the most important criteria for a robust behavior in this respect are simple to describe. And they could easily be specified and verified by manufacturers, customers or insurers.