Identifying and Addressing Oxidation Challenges in a Aeroderivative Gas Turbine

Article Summary

• Excessive scrap rates of high-pressure turbine (HPT) blades in a aeroderivative gas turbine, which drives pipeline compressors, were investigated by Liburdi.
• The cause of the high scrap rate was identified as oxidation on the aftmost row of film cooling holes on the pressure side of the blades, leading to a loss of thickness and base material and impacting gas turbine engine overhaul intervals.
• Differences between the aeroderivative HPT blade and its aeroengine counterpart were identified, including variations in cooling design.
• Liburdi used a compressible flow network analysis (CFNA) model to analyze internal airflow rates and pressure drops, revealing the potential for cooling hole starvation and hot gas ingestion during transient or off-design conditions.
• The recommendation was to upgrade the gas turbine blade design to introduce more cooling flow to the trailing edge circuit, reducing the risk of oxidation and improving turbine blade longevity.

Operators discovered an industrial aeroderivative gas turbine was experiencing costly excessive scrap rates of high-pressure turbine (HPT) blades. The generator drives pipeline compressors. Upon investigation, Liburdi concluded that oxidation on the aftmost row of film cooling holes at mid-airfoil on the pressure side caused the high scrap rate.

The oxidation attack generated a significant loss of thickness and base material, shown in Figure 1, limiting part life and impacting the gas turbine engine overhaul interval.

Figure 1: Aeroderivative High Pressure Turbine Blade Film Cooling Hole Oxidation

A detailed comparison between the industrial aeroderivative high pressure turbine blade and its aeroengine version identified differences. The aeroderivative turbine blade has two tip cavity holes and more film cooling holes on the pressure side of the airfoil. The inclusion of a metering plate on the aeroderivative cooling air inlet (Figure 2) shows the differences.

Figure 2: Comparison of High Pressure Turbine Blade Cooling Design

Using the aeroengine configuration as a baseline, Liburdi developed a compressible flow network analysis (CFNA) model to understand the turbine blade's internal air flow rate, flow distribution, and pressure drop along the flow path.

The CFNA model predicted a significant pressure drop across the metering plate of the aeroderivative design compared to the aeroengine design shown in Figure 3. The low internal pressure difference across the row 3 film cooling holes on the pressure side of the airfoil could become zero. This could cause no flow (cooling hole starving), or even negative air flow, resulting in flow reversal (hot gas ingestion) during transient or off-design conditions.

Figure 3: Comparison of Pressure Drops Along the Flow path

In the laboratory, dimensional and airflow rig testing confirmed suspicions; The inlet metering plate combined with a large total exit flow area of the aeroderivative high pressure turbine blade design reduced cooling flow, altered internal flow pattern, and reduced internal pressure.

Liburdi concluded that hot gas ingestion during off-design or transient conditions could cause oxidation of the HPT blade row 3 film cooling holes. As mitigation, Liburdi recommended upgrading the design to introduce more cooling flow to the trailing edge circuit and reduce the risk of hot gas ingestion. Liburdi's complete analysis included a review of the impact of additional cooling air extraction on the other components in the Aeroderivative engine.

You can read the complete study in the ASME Turbo Expo paper GT2008-50431.

To see how Liburdi can save you money by maximizing your equipments operation and extending the life of your parts, contact Bob Tollett at rtollett@liburdi.com.