Part-Load Operation and Accelerated Gas Turbine Oil Degradation: A Field Case Study

Part-Load Operation and Accelerated Gas Turbine Oil Degradation: A Field Case Study

Introduction

A dual-fuel industrial gas turbine (natural gas and diesel), in service for approximately two years, was evaluated during a field lubrication survey. The unit was operating primarily under part-load and intermittent conditions, with frequent start-stop cycles and reduced injector demand.

Although OEM recommendations typically schedule major maintenance around five years of operation or defined operating hours, field observations indicated early lubricant distress.

“The turbine was still early in its lifecycle, but the lubricant had already begun showing accelerated degradation behavior”.

Customer Observation

The customer was proactively monitoring varnish formation using MPC (Membrane Patch Colorimetry) testing, which showed increasing values.

While MPC is a useful early warning indicator, it does not provide a complete diagnosis of oil health.

It confirms varnish potential, but not oxidation stability or remaining lubricant life.

System Overview

• Mineral-based turbine oil
• Part-load and intermittent operation
• Routine oil analysis + MPC monitoring

Advanced oxidation life diagnostics were not initially included in the monitoring program.

Sampling Approach

To ensure reliable results, representative sampling was implemented:

• Sump sample (before filtration)
• Post-filtration sample
• Return line sample (properly flushed)

Accurate diagnosis depends on representative sampling, not just laboratory testing.

“A bad sample doesn’t give bad data, it gives confidently wrong data, which is far more dangerous”.

Additional Diagnostics

To complete the assessment, the following tests were included:

• RPVOT – oxidation stability
• RULER – antioxidant depletion
• Karl Fischer – water content (baseline trending only)

These tests provide a complete view of varnish formation and oxidation conditions.

“Together, they turn confusion into clarity”.

Key Findings

• High MPC → active varnish precursor formation
• Low RPVOT → reduced oxidation resistance
• Low RULER → depleted antioxidants
• Elevated oxidation trend → accelerated oil aging

The lubricant was degrading due to operating conditions, not calendar age.

“Indeed, the oil wasn’t old, it was just overworked and underappreciated”.

Root Cause

The degradation was driven primarily by operating profile:

• Part-load operation → lower oil temperature and reduced stability
• Intermittent operation → repeated thermal cycling
• High cycling frequency → accelerated oxidation stress

These conditions promoted varnish formation within the system.

“Varnish rarely announces itself. It prefers to ruin your weeks silently”.

Risk

If left unaddressed, the condition could lead to:

• Servo valve sticking
• Increased filter differential pressure
• Reduced heat transfer efficiency
• Unplanned turbine trips

Corrective Action

The initial idea of adding antioxidants was rejected, as it does not remove existing degradation products.

Final approach:

• Continue MPC trending
• Introduce RPVOT and RULER for oxidation monitoring
• Implement ester-based varnish removal fluid with filtration skid to support for soluble and insoluble varnish cleaning from the system

Outcome

• Avoided premature oil change
• Avoided addition of antioxidants to the contaminated system
• Improved condition-based maintenance strategy
• Implemented active varnish control
• Enhanced understanding of operating impact on lubricant life

“Adding antioxidants to varnished oil is like putting air freshener in a dirty room, technically something changed, but the problem definitely didn’t leave”.

Conclusion

Gas turbine lubricant degradation is driven more by operating conditions than service time. Part-load and cyclic operation significantly accelerate oxidation and varnish formation.

With proper diagnostics and corrective action, lubricant-related failures can be prevented well before they impact turbine reliability.

“The biggest win was not the cleanup — it was the mindset change”.

Author Details : Muzaffar Hussain

https://www.linkedin.com/in/muzaffar-hussain-lubes