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KNOWING THE REAL-TIME CLEARANCE BETWEEN BLADE TIPS AND THE CASING CAN RECLAIM ENGINE EFFICIENCY
The clearance between blade tips and the inner casing of a turbine combustion chamber is directly related to fuel efficiency. When clearances are reduced, efficiency increases so fuel consumption and emissions decrease. However, turbine operators normally allow for some uncertainty in actual clearances, thereby forgoing some degree of efficiency.
Mismatches between casings, shafts and blades can affect clearances. Degradation from engine rubs can permanently increase clearances and decrease fuel efficiency. Rubs raise turbine exhaust temperatures as less energy is converted to turn the motor, gases stay hotter and this can degrade downstream components.
Other challenges originate from not knowing actual clearances. Hot restart risk is serious and appears usually after emergency engine shut down and sometimes after an aborted start-up. When the engine restarts, blade rub is likely as engine components do not cool at the same rate. To prevent a rub event, the safest course is to wait up to 24 hours before restart.
Rotor bow is another source of delay that must be mitigated. After a prolonged shut down, the rotor of a large turbine will bend. The shaft has to be rotated at low speed to unbow before it can safely rotate at high speeds without risk of rubbing.
Proximity probes and capacitive sensors are often used to measure casing clearance. Proximity probes (eddy current sensors) offer a cost-effective solution for blade monitoring in the cool stages of a compressor, but need to be calibrated using a sample of the target material.
Capacitive sensors are used in engine test beds as they provide fairly accurate measurements. However, they have a limited life span at high temperature and are susceptible to water entry. They also require calibration as the capacitance measurement is not linear and a correction must be calculated.
A key issue with both approaches is that they can not withstand permanent installation in the harsh environment of a gas turbine’s first stage, which is subject to vibration, fluctuating pressure, extreme temperatures and build-up of combustion byproducts.
One alternative is a microwave sensor, which can withstand gas path temperatures of 1,600°C or more. A microwave sensor works similarly to radar: The sensor emits a high-bandwidth signal against a metallic surface and measures what is reflected. The phase of the reflected signal is compared to a reference, as the signal’s phase difference is proportional to the wavelength, which is proportional to the distance between sensor and blade (Figure 3).
Such sensors can measure clearances up to 25 mm with an accuracy of 0.1 mm. The measurement system takes 50,000 samples per rotor revolution. It samples one rotation per second, which is sufficiently frequent that it is not necessary to measure every rotation.
Microwave sensors have been used in a test conducted by gas turbine manufacturer Zorya-Mashproekt, on a 25 MW-rated DM80 aeroderivative turbine. Eight sensors were installed on the high-pressure stage of the turbine to characterize tip clearance in different operating modes. Before starting the engine, 10 of the 86 rotor blades in the turbine stage were shortened by varying amounts, up to 1 mm. Three operating modes were monitored: Start up, hot restart, and use of a clearance control system (Figures 1 and 2).
Zorya-Mashproekt tested an active clearance control system that utilizes compressor-discharged air to cool the casing and squeeze it around the rotor. Tip clearance measurements were taken to determine the minimum single blade clearance for different modes of operation. The engine clearance control system was activated at about 6 MW output power. As a result of an average clearance reduction of 0.36 mm, rotor speed decreased by about 100 RPM while maintaining output (aerodynamic efficiency on the highpressure stage rose 1.29%. Efficiency increased an average of 1.5-2.0% for different operating modes).
Knowing the real-time clearance between blade tips and casing, therefore, is a means of reclaiming engine efficiency. By running the turbine with as tight a clearance as possible, it is possible to boost efficiency without danger of blade tip-to-case contact.
But an individual blade’s clearance is also an indicator of its structural health in the form of blade damage and wear from root cracking, creep, untwist, and cooling problems. Therefore, small changes in tip clearance can provide warning signs of impending failure. Monitoring tip clearance can also help identify more complex causes such as case deformation and rotor dynamics.
While condition monitoring using blade tip clearance measurement is currently unproven in the field, future studies will focus on how microwave sensing technology can be used to monitor the health of individual blades and prevent catastrophic failure. The promise of microwave sensors is advance warning that enables the condition-based maintenance of turbine blades.
Michael Hafner is Product Manager,Microwave Systems at Meggitt Sensing Systems. For more information, visit www.meggittsensingsystems.com