Blade tip clearance is the distance between the tip of a blade and the casing. This clearance is an essential parameter in turbomachinery design: too little clearance and the rotor will rub against the casing; too much clearance and the turbomachine loses efficiency by allowing the gas path to bypass the blades.
Measuring Blade Tip Clearance
A variety of sensors exist to measure blade tip clearance, such as capacitive and Eddy current systems. Prime Photonics uses a patented optical method to measure blade tip clearance, which presents numerous advantages over other measurement methods. An optical system allows measurement of blades regardless of material, whereas other systems are limited to metallic blades. The system features superior accuracy and lower noise than other methods.
The principle of operation of our optical system is time-based, and not based on the amplitude of the signal. As a consequence, our system can tolerate dirty environments without accuracy degradation.
Using FOCISTM to Measure Tip Clearance
The FOCIS™ system measures average, minimum, and maximum clearance. It can measure the average clearance of all blades or look at individual blades. By placing several probes around the rotor, the system can output clearance data at specific rotor positions.
Measurement of Average Blade Tip Clearance
The screen capture below from FOCISTM software shows measurement of both blade tip clearance and speed.
This measurement shows average clearance (top graph) inversely tracking rotor speed.
We specify the FOCISTM system accuracy as 25 microns (0.001 inch), but the system has much better resolution and low noise which allows detection of even smaller clearance changes.
Measurement of Individual Blade Clearance
The FOCIS system can also measure clearance of individual blades. The graph below shows a typical blade-by-blade clearance chart.
Tip Clearance Output
The FOCIS system can stream clearance data in real-time via several output methods. It can also store clearance data in a CSV file for import in other data visualization or data analysis systems.