Blade Tip Clearance Measurement

FOCIS thumbThe FOCIS™ system measures blade tip clearance.  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.


Using FOCIS™ to Measure Tip Clearance

The FOCIS™ system measures average, minimum, and maximum blade tip 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 FOCIS™ software shows measurement of both blade tip clearance and speed.

Speed vs average clearance

Picture Above: This measurement shows average clearance (top graph) inversely tracking rotor speed (bottom graph).

We specify the FOCIS™ 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.

Blade by blade clearance

Picture Above: Average, min, and max blade tip clearance data is available for every single blade.

Tip Clearance Output

The FOCIS system can stream clearance data in real-time via several output methods including MODBUS.  It can also store clearance data in a CSV file for import in other data visualization or data analysis systems.


Measuring Blade Tip Clearance Optically

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.

More Information on FOCIS™ Blade Tip Clearance Measurement System