Air Release in Turbine Oils: A Detailed Technical Overview

Air Release in Turbine Oils: A Detailed Technical Overview

Turbine oils are critical to the operation and reliability of steam, gas, and hydraulic turbines. Among their many properties, air release is a key factor that influences system efficiency, reliability, and safety. Understanding air release—its causes, measurement, and management—is vital for ensuring optimal turbine operation.

1. What Is Air Release in Turbine Oils?

Air release refers to the ability of a lubricant to expel entrained air within a specified timeframe. Entrained air consists of tiny bubbles dispersed throughout the oil, which, if not released, can cause operational and reliability issues. Unlike dissolved air, which is molecularly integrated into the oil, entrained air can form foam or coalesce into larger bubbles under certain conditions.

2. Causes of Air Entrapment in Turbine Oils

Air can become entrained in turbine oils due to several factors:

• High Turbulence: Rapid fluid flow in pumps and pipes can trap air within the oil.
• Poor Reservoir Design: Inadequate separation zones or high agitation in the oil reservoir can lead to persistent entrained air.
• Oil Characteristics: Low viscosity or poor air-release properties exacerbate the issue.
• Contamination: Water and particulate contamination can stabilize air bubbles, making them harder to release.

3. Why Is Air Release Important?

Entrained air in turbine oils can lead to the following problems:

• Cavitation: Entrained air can cause cavitation in pumps, leading to wear, noise, and reduced efficiency.
• Reduced Lubrication Efficiency: Air bubbles in the lubricant reduce its ability to form a stable oil film, increasing wear and friction.
• Foaming: Persistent entrained air can lead to foaming, which hampers lubrication and leads to oil overflow.
• Oxidation: Air bubbles accelerate oil oxidation by exposing a larger surface area to oxygen, reducing oil life.
• Measurement Inaccuracy: Air bubbles can skew pressure and flow readings, leading to incorrect operational decisions.

4. Measurement of Air Release in Turbine Oils

The air release property of turbine oils is measured using standardized test methods:

• ASTM D3427: This method measures the time required for entrained air to dissipate from oil under controlled conditions. Shorter times indicate better air release properties.
• DIN 51381: Similar to ASTM D3427, this European standard evaluates air release in lubricants.

Test Setup:

• Oil is heated to a specified temperature (usually 50°C or 75°C).
• Air is introduced into the oil at a controlled rate.
• The time taken for entrained air to reduce to a certain volume is recorded.

5. Factors Affecting Air Release Performance

• Viscosity: Higher viscosity oils generally have poorer air release properties.
• Additives: Anti-foam agents can improve air release but may compromise other properties.
• Temperature: Elevated temperatures typically reduce air release time by lowering oil viscosity.
• Base Oil Type: Synthetic oils often exhibit superior air release compared to mineral oils due to their uniform molecular structure.

6. Improving Air Release in Turbine Oils

To enhance air release properties, consider the following strategies:

• Oil Selection: Use turbine oils specifically formulated for excellent air release.
• System Design: Optimize reservoir design to minimize turbulence and allow adequate residence time for air release.
• Filtration: Use high-efficiency filters to remove contaminants that stabilize air bubbles.
• Maintenance Practices: Regularly monitor and maintain proper oil levels to prevent excessive agitation.
• Deaeration Systems: Employ deaeration equipment to mechanically remove air from the lubricant.

7. Practical Example of Diagnosis and Management

Scenario: A steam turbine system experienced erratic pressure fluctuations and increased noise in the lubrication pump. During routine inspection, foam was observed in the oil reservoir.

Diagnosis Process:

1. Visual Inspection: Foam formation and bubbles in the reservoir suggested air entrapment.
2. Oil Analysis: Lab testing confirmed prolonged air release time beyond acceptable limits as per ASTM D3427.
3. System Assessment: High turbulence near the pump suction line and a poorly designed reservoir baffle were identified as contributing factors.
4. Contamination Check: Water contamination (150 ppm) was detected, further stabilizing the bubbles.

Corrective Actions:

• Installed a redesigned reservoir baffle to reduce turbulence.
• Replaced the oil with a premium-grade turbine oil with superior air release properties.
• Installed a high-efficiency coalescing filter to remove water contamination.
• Introduced a deaeration system to aid in bubble removal during high-load operations.

Outcome: Post-implementation, air release time improved significantly, cavitation noise disappeared, and system pressure stabilized.

8. Monitoring Air Release in Operational Turbines

Regular monitoring of air release performance is critical in identifying potential issues early:

• Visual Inspection: Check for foaming or air bubbles in the oil reservoir.
• Oil Analysis: Test air release properties periodically as part of routine oil analysis.
• System Performance: Monitor pump noise, pressure fluctuations, and temperature changes as indirect indicators of air entrapment.

9. Conclusion

Air release is a vital property of turbine oils that directly impacts the reliability and efficiency of turbine systems. By understanding the causes of air entrapment, measuring air release, and implementing strategies to improve this property, operators can significantly enhance system performance and reduce operational risks. Selecting the right turbine oil and maintaining a well-designed lubrication system are key to managing air release effectively.

References:

• ASTM International, "ASTM D3427: Standard Test Method for Air Release Properties of Petroleum Oils."
• DIN Standards, "DIN 51381: Determination of Air Release Properties of Oils."
• Industry case studies and operational guidelines on turbine oil maintenance.