News and Views, Volume 54 | Combustion Turbine Compressor Hygiene, Part 2
Figure 1. Lower compressor casing after large forward blade liberation, causing a forced outage and catastrophic damage to aft components.
COMPONENT ASSESSMENT AND ANALYSIS
By John Molloy
Structural Integrity (SI) has decades of experience dealing with compressor degradation’s short-term and long-term effects, forced outages, and unforeseen costs regarding Combustion Turbine (CT) compressor hygiene. The inlet package and compressor condition can strongly influence unit performance, efficiency, reliability, risk, and maintenance costs. Controlling what enters the inlet and mitigating the effects of accumulated foreign material can significantly impact O&M activities and costs.
Additionally, in the new paradigm of cyclical operation and potentially longer downtime between operational periods, additional considerations and actions are incumbent upon O&M to prevent irreversible damage to various CT flow path components (compressor, rotor, etc).
In the 51st edition of News & Views, we discussed the CT compressor hygiene regarding component longevity. This follow-up article will dive into the assessments and analyses.
SI is pleased to offer a comprehensive suite of inspections, analyses, and O&M recommendations to help mitigate these debits to reliable, profitable, and safe operation.
Figure 2. Forward compressor blades with corrosion pitting on the leading edge, platform radius, providing nucleation sites for corrosion fatigue and cracking.
Inlet Package and Forward Compressor Detailed Site Survey
The inlet package and forward compressor site survey is a detailed visual inspection of the inlet filter house/inlet ducting assembly. The inspection allows for documenting potential sources of foreign objects, evidence of sealing issues (floor/wall/roof construction, penetrations, and filter seating), pooling water, corrosion, cracks, cleanliness, and contaminant/debris presence.
The in-depth inspection also examines:
- Water injection/misting systems for leaks/contaminant accumulation
- Bearing region and compressor leak entry passages for contaminants/debris
- Pre-filter deposits/debris collection to determine local environmental loading conditions and corrodents
Figure 3. Pitting on compressor blade.
The SI team additionally collects Inlet Guide Vane (IGV) and forward blade, stator vane, rotor, and carrier deposits/residues to establish the presence of contaminants and or corrodents on the compressor, and compares this to the finding in the pre-filters to establish the efficacy of the filtration system. Particular attention is given to the compressor’s lower half, which is usually the area most affected. Photographic documentation of the typical forward compressor (IGV/R0/S0/R1) pitting conditions is completed using a macro lens (Figure 3). In the final step of the survey, the team performs a mold replication on typical pitting and leading edge (LE) erosion.
Data Collection, Analysis, and Review
Figure 4. Leading edge chloride pitting and fracture on a vane.
During the compressor hygiene testing and analyses, there are several areas where data can be collected. This includes ambient and unit flow path humidity trends, ambient and CT inlet temperature/dewpoint, water injection operation, compressor performance, and an annual borescope inspection.
Other data elements potentially collected or reviewed include water wash frequency, duration, and water wash quality checks consisting of pH and chemistry constituents review of the demineralized water source, water wash solution, and drain. In addition, the IGV calibration report, pollution, contaminant producers, and environmental wind condition survey (drift) can be reviewed if the previous findings merit this effort. Finally, the team correlates the observed pitting locations with known risk profiles for blade/vane excitation. Nodal response stresses can be evaluated for a more detailed assessment.
Laboratory Support
Figure 5. Metallographic cross-section shows leading edge erosion channeling morphology on a forward compressor blade, which facilitates fatigue crack initiation.
SI’s world-class laboratory is leveraged to provide tailored analyses for compressor assessments.
This generally includes:
- Inlet prefilter contaminant/debris species analysis
- IGV/R0/S0/R1/Wheel/Case surface contamination species analysis
- Pitting and/or erosion depth assessment
Blade and vane metallurgical evaluation
When parts are available for analysis, the laboratory analyses can provide specific damage mechanisms affecting the components, which allows for better decisions on mitigation strategies. The goal of the assessment and analysis is to close the loop between the on-site unit findings, the operational assessments, to provde the client with best-in-class combustion turbine availability, and performance.
