Blade-Tip Monitoring with Through-the-Case Eddy Current Sensors

摘要

A new eddy current sensor can monitor turbine blade-tip timing and clearance through the housing, in the presence of contaminants, and at temperatures up to 1000°F. Augmented with a nonintrusive stress measurement system, the sensor can detect both synchronous and asynchronous vibrations as well as the onset of blade fatigue. Blade-tip sensors are widely used to monitor the behavior of bladed disks in rocket engine turbo pumps, gas turbine engines, and steam turbines. They can also diagnose problems and help predict how much more service life to expect from the rotor. The sensors are commonly integrated with a nonintrusive stress measurement system in the laboratory and in test cells to measure blade resonance (high-cycle fatigue), detect cracks in blades and disks (high- and low-cycle fatigue), detect damage from foreign objects, investigate rotating stall, and analyze blade flutter, along with other rotational anomalies. The equipment also has the potential to perform real-time monitoring of an engine during flight. The conventional practice has been to mount strain gauges on the blades or in the engine casing (usually by drilling holes through the case). Depending on the sensor target location on the blade, such measurement devices can protrude into the gas path. This article describes the development of a novel sensor that can make the same detailed measurements while mounted on the outside of the engine case. No holes, and no interruption in the gas path. The three leading candidate sensor technologies for blade vibration measurement at present are capacitance, optics, and eddy current. Capacitance works well when clear access to the blade tip is available and the dielectric of the fluid in the gap between sensor and blade is constant. But these sensors tend to be unreliable at medium-to-high temperatures, they cannot be used when oil contamination is present, and special materials are needed for operation at cryogenic temperatures. Optical devices also require clear access to the blade tip, and the fluid in the gap must be transparent. These sensors can be used at very high temperatures but not in the presence of contaminants. Eddy current sensors can be used whether access to the blade is available or not (i.e., they can "see through the case"), and are unaffected by the presence of oil and other contaminants. Hood Technology's eddy current sensors (see Figure 1) can operate at up to 1000°F, and recent tests in a liquid nitrogen bath have shown that the signal is not affected by cryogenic temperatures. They can measure both tip timing and tip clearance of each blade in real time at very high resolution. There are, however, limits to the thickness and material of the case that these sensors can see through.