Optimization of clearances is the key to high turbine efficiency

During load operations, a gas turbine is characterized by cyclic transients and long dwell times. As a result, variation in clearance occurs. Tight clearances in both the compressor and the turbine section are the key to high efficiency. However, some clearance must be maintained in order to avoid contact between rotating and stationary parts.

The optimization of clearances during operation, therefore, is a vital step in improving engine performance. A Hydraulic Clearance Optimization (HCO) system is one way to control turbine blade-tip clearances. An HCO shifts the rotor against the flow direction of the engine, consequently reducing radial gaps above the turbine blade tips. An efficiency of 0.2 percent, and the increase in power output of 1 percent have been attained using this method.

This approach has been implemented on a heavy-duty Ansaldo Energia gas turbine which is made using the splined-disk rotor concept. This features a single central tie-rod assembling the individual bladed disks together by radial facial serrations known as Hirth-teeth. Without transmitting any torque, the central tie-rod holds the multi-disks rotor tightly together via compressive pre-stress force.

The torque developed by the power turbine is transmitted to the compressor and then to the coupling by the Hirth teeth which connect adjacent disks. The evaluation of rotor-stator radial and axial clearances during operation is one of the most important topics in turbomachinery for heavy duty gas turbine power and efficiency.

The minimum value of clearance, however, does not always occur during base load. Therefore, clearance calculations must be performed in all transient conditions in order to avoid rubbing between rotating blades and the casing. This must encompass: radial clearance between tip rotor blade and casing; radial clearance between vane and disks in sealing cavity; and axial clearance between vane and disks in sealing cavity.

Through the action of an HCO, the gaps between turbine blades and the casing become narrower and the gaps in the compressor are increased. The sum of these two effects increases GT efficiency and power output. Note though that shifting may only be performed when the GT is sufficiently heated.

(Read more in the September/October issue of Turbomachinery International)