A Fiber-Optic Tip-Shroud Deflection Measurement System

A Dutch utility faced the fact that a second stage set of turbine blades of a gas turbine had to be replaced long before the estimated lifetime as a result of tip shroud deflection. This deflection caused the risk of loss of support between the individual tip shroud segments. The goal of this paper is to find the cause of the problem, to see how it increases with time, and to take appropriate action to prevent the problem from occurring again. A fiber-optic tip-shroud deflection monitor has been developed and tested on a gas turbine to study this phenomenon in real time. The optical system is based on astigmatism to derive distance information from the measured optical signals. Characteristic features from the system are the good spatial resolution of about 1 mm, the distance resolution of about 0.1 mm and the distance from the probe tip to the target of about 20 mm. These specifications are difficult to achieve with, for example, capacitive sensors. The probe tip can with stand temperatures of about 500°C. The system can be calibrated in situ, given a stable operation of the gas turbine. This is accomplished by stepping the probe tip over some distance and recording the signals corresponding to a certain point on the tip shroud. The instrument has been used to monitor the tip-shroud deflection in a gas turbine at various loads and over a time span of several months. The results indicate that the deflection can be divided in a part depending on the load and a part that is a permanent deflection. Based on the results, it can be judged whether blades need to be rejected because of a too large deflection.