Large turbines are periodically torn down, inspected, and rebuilt. Turbine blade clearance to the case is one of the most critical measurements. Too close and the blades could expand and the turbine could be destroyed, too far away and efficiency is lost and power is sacrificed. This makes it vital to know the actual blade tip clearance.

The inquiry

A large multinational company approached MTI Instruments to provide a capacitive sensing system that could inspect blade tip clearance through existing inspection ports in a steam turbine case. The inspection ports had pre existing threads, so MTII’s probes had to fit a specified diameter.

Blade tip clearance measurement

MTII had to also be able to accurately sense the clearance to the manufacturers specially shaped blade tips.

The solution

MTI’s Digital Accumeasure TM is ideal for this type of application because very small probes can be mated to the amplifier, then digitally corrected for high accuracy (linearity) at what would previously have been considered too large a working gap with older analog capacitive products. MTI designed a special probe configuration to fit into the inspection port.

case to blade clearance

Analysis at MTI Instruments’ facility ensured that the probe could properly detect the narrow blade with no chance that it would be struck. After customer review of the data, they subsequently purchased the systems. Utilizing a MODBUS output interface, the customer was able to forward tip displacement results right to the maintenance engineer’s smart phones!

Two pilot systems were built and are undergoing field tests with excellent results. Analysis is performed using these specially designed probes at slower speeds during rebuild to ensure blade tips will not rub during high speed testing. Our application engineers will be pleased to provide you with the same degree of confidence in your clearance applications.
Probe customization is available for any of your demanding requirements, without breaking the bank.

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