Qualifying gas turbines

December 8 2012 - TI Staff

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Steam and gas turbines were first developed for power generation – a pioneer industry in many aspects – and then adapted for mechanical drive applications such as compressor or pump drive services. As a result, many well-known gas turbine mechanical drive models have been adapted from power generation.

(A LM6000-PF mechanical drive aeroderivative gas turbine)

This adaption or qualification process should be continued because only a limited number of gas turbines have been qualified for mechanical drive. There is a constant need for new mechanical drives for larger power ranges and those between available options (considering each gas turbine model covers a limited power and speed range). However, new gas turbines are more complex with more power density and the qualification processes for mechanical drive application present more challenges.

There are many successful gas turbine models for power generation applications (usually for a limited speed variation range). To use these gas turbines as compressor drivers, for example, an extensive qualification process (including studies and shop tests) must be performed. A gas turbine for mechanical drive service should be able to drive mechanical equipment (usually a compressor) by providing sufficient string torque across the entire specified speed range, under all ambient air conditions and all operational situations.

Gas turbine components are subject to a number of failure mechanisms, such as crack initiation from low-cycle fatigue, cycling crack propagation, disc burst and creep, all of which should be evaluated across the speed variation range. In particular, the following components should be assessed for a variable-speed range application:

• Axial compressor rotating and stationary blades

• Turbine rotor and buckets

• Combustion systems

Accurate FEA (Finite Element Analysis) dynamic-thermal models are required to calculate temperature distributions, transient behaviors, aero-mechanical loads, dynamic inertia loads, and other aspects of the operation in a variable-speed, mechanical drive application. The ability of air-compressor rotor and stator blades (and assemblies) to operate over the full variable-speed range is also important, particularly for single-shaft gas turbines, where the air-compressor shaft is directly coupled to the turbine shaft and driven equipment rotor assemblies.

A ping test (a physical test to determine the natural frequencies of an assembly) is useful for assessing a gas turbine’s variable speed application. It consists of installing measuring devices on the machine and tapping it (usually with a special hammer). The assembly will then vibrate at its natural frequencies. A ping test can be employed to validate the analytical dynamic model of gas turbine assemblies (particularly the natural frequencies and natural modes of vibration).

A Variable Speed Drive (VSD) electric motor is often used as a starter-helper electric motor for a gas turbine in a mechanical drive application because the gas turbine cannot provide sufficient torque at the start-up of the train. The VSD electric motor is necessary for some gas turbines (especially single-shaft gas turbines) to start the rotation and bring the train to a sustained speed. Torque ripple effect contributions from the VSD electric motor should be assessed and considered in the gas turbine qualification process, particularly for a single-shaft machine.

The dynamic analysis often reveals that resonance may happen in a defined variable speed range. For example, resonance could occur between the natural frequencies of the turbine section bucket and the excitation frequencies of a rotor, when operating in a variable-speed range. This could limit the variable-speed range. For a single shaft gas turbine, the minimum threshold of the machine’s variable-speed range is limited to around 90 percent of the normal gas turbine speed, due to this effect.