Criticality Analysis of Steam Turbine Electro-Hydraulic Control (EHC) Systems

Introduction

Electro-Hydraulic Control (EHC) systems play a crucial role in the operation of steam turbines in power plants and industrial facilities. These systems regulate the turbine's speed, load, and emergency shutdown functions, ensuring safe and reliable operation. Given their importance, a criticality analysis of EHC systems is necessary to assess potential risks, failure modes, and mitigation strategies.

This article provides a comprehensive criticality analysis of steam turbine EHC systems, covering their function, failure consequences, risk assessment, and reliability strategies.

1. Understanding the EHC System in Steam Turbines

1.1. Function and Importance

EHC systems in steam turbines are responsible for:

• Speed Control: Adjusting the turbine’s speed during load changes.
• Load Control: Ensuring the turbine delivers the required power output.
• Trip Protection: Rapidly shutting down the turbine during abnormal conditions.
• Valve Actuation: Controlling steam flow to the turbine using servo-actuated control valves.

These systems utilize hydraulic power, typically provided by phosphate ester-based fluids, to actuate steam valves via servomechanisms. The reliability of the EHC system directly affects the turbine’s performance, efficiency, and safety.

1.2. Key Components

• Hydraulic Power Unit (HPU): Supplies pressurized fluid for actuation.
• Control Valves & Servo Valves: Modulate steam flow to the turbine.
• Filtration System: Maintains fluid cleanliness and removes contaminants.
• Accumulator System: Provides emergency hydraulic power during failure conditions.
• Trip Block & Solenoids: Enable emergency shutdown when required.
• Reservoir & Cooling System: Stores and regulates fluid temperature.

Any malfunction in these components can lead to loss of control over the turbine, potential equipment damage, and power generation disruptions.

2. Criticality Analysis of EHC Systems

Criticality analysis evaluates the impact of EHC system failures based on their severity, probability of occurrence, and detectability. The Failure Mode, Effects, and Criticality Analysis (FMECA) method is commonly used.

2.1. Failure Modes & Consequences

Each failure mode has severe operational, safety, and economic implications.

3. Risk Assessment and Criticality Ranking

3.1. Risk Matrix for EHC System Failures

A risk assessment matrix helps prioritize failure modes based on severity and occurrence probability.

From this assessment, servo valve sticking, fluid degradation, and filtration failure rank as the highest risks.

4. Reliability Strategies and Mitigation Plans

4.1. Fluid Condition Monitoring

Since EHC systems use phosphate ester fluids, proper monitoring is essential. Recommended tests include:

• Acid Number (AN) Monitoring: Identifies fluid degradation.
• Water Content Analysis: Ensures moisture does not cause sludge formation.
• Resistivity Measurement: Tracks contamination affecting fluid electrical properties.
• Varnish Potential Testing: Predicts the likelihood of deposit formation.

4.2. Filtration and Contamination Control

• Fine Filtration (≤ 3 μm) to remove particulate contaminants.
• Electrostatic and Ion-Exchange Filters to maintain fluid chemistry.
• Desiccant Breathers to prevent moisture ingress.
• Push Dry Nitrogen Gas to the EHC fluid tank to remove water and not allow moisture to come in

4.3. Preventive Maintenance

• Servo Valve Inspection & Cleaning: Periodic cleaning or replacement to prevent sticking.
• Accumulator Pressure Testing: Ensures sufficient backup hydraulic power.
• Solenoid and Trip Block Testing: Verifies emergency shutdown functionality.

4.4. Predictive Maintenance Using Condition Monitoring

• Vibration Analysis: Detects abnormal conditions in servo valves.
• Oil Analysis Trends: Predicts failures before they occur.
• Infrared Thermography: Identifies overheating in trip blocks and control valves.

4.5. System Redundancy & Emergency Planning

• Redundant HPUs: Provides backup hydraulic pressure.
• Dual Servo Valves: Ensures continued operation in case of failure.
• Emergency Shutdown Protocols: Clearly defined response plans to minimize damage.

5. Conclusion

The EHC system is one of the most critical control elements of a steam turbine. Failures can lead to catastrophic shutdowns, efficiency losses, or safety hazards. A structured criticality analysis helps identify high-risk failure modes, allowing for targeted maintenance strategies.

By implementing fluid condition monitoring, filtration improvements, predictive maintenance, and redundancy measures, steam turbine operators can enhance the reliability and safety of their EHC systems.

Read More:

D8323 Standard Guide for Management of In-Service Phosphate Ester-based Fluids for Steam Turbine Electro-Hydraulic Control (EHC) Systems

What Routine EHC Fluid Testing Testing Fails to Detect

Revolutionizing Phosphate Ester Fluid Filtration

ECR? 10000 EHC Fluid Conditioning | EPT Clean Oil

You Can't Manage What You're Blind To - Life and Death for Lubricants