Turbine Oil Purification: How to Solve EHC Fluid Degradation Issues

How to Restore Degraded EHC Fluid in Steam Turbine Control Systems

The Electrohydraulic Control (EHC) system is critical to the safe and precise operation of a steam turbine. It uses fire-resistant phosphate ester EHC fluid to transmit hydraulic power and control steam valves through high-precision servo valves.

But what happens when EHC fluid becomes contaminated or chemically degraded?

Water contamination can accelerate hydrolytic degradation, while thermal and oxidative degradation can generate acidic compounds, sludge, varnish, and fine contaminants. These degradation products can interfere with servo-valve operation and reduce the reliability of the turbine control system.

So, can degraded EHC fluid be restored instead of being completely replaced?

In many cases, yes—but the answer depends on the actual condition of the fluid.

Condition-based EHC fluid purification can help control moisture, acidic degradation products, particles, and varnish-related contaminants without immediately replacing the entire fluid charge. However, the appropriate treatment depends on the fluid formulation, laboratory test results, turbine OEM requirements, and fluid manufacturer's recommendations.

ASTM D8323-24 provides guidance for managing in-service phosphate ester fluids in EHC systems, but OEM specifications take precedence.

This guide explains:

• What causes EHC fluid degradation

• How water, acidity, particles, and varnish affect turbine control systems

• Which tests should be performed before purification

• How different purification technologies address different contaminants

• How to choose an EHC fluid purification system

• How to operate an offline EHC fluid purification process

• When EHC fluid should be purified or replaced

What Causes EHC Fluid to Degrade?

Phosphate ester EHC fluids can deteriorate through several interconnected mechanisms. The most important factors include moisture contamination, hydrolytic degradation, thermal degradation, oxidation, and the formation of acidic or insoluble degradation products.

1. Water Contamination and Hydrolysis

Water is one of the most important contaminants to control in phosphate ester EHC systems.

When moisture enters the hydraulic circuit, it can contribute to hydrolytic degradation of the phosphate ester fluid. As degradation progresses, acidic compounds can accumulate and the fluid's chemical condition can deteriorate.

The result may include:

• Increasing Total Acid Number (TAN)

• Reduced fluid stability

• Corrosion-related concerns

• Formation of degradation products

• Increased risk of servo-valve contamination

This is why keeping EHC fluid clean and dry is an important part of long-term fluid management.

For phosphate ester EHC fluids, moisture control should not be considered separately from acidity control. Water removal may help limit further hydrolytic degradation, while already-formed acidic products may require a separate treatment process.

2. Thermal and Oxidative Degradation

EHC fluid operates in a demanding environment. Localized high temperatures, air entrainment, pressure effects, and other operating conditions can accelerate fluid degradation.

Thermal degradation can produce insoluble or semi-soluble contaminants that may later deposit on hydraulic components.

These contaminants are particularly important in EHC systems because the hydraulic circuit relies on high-precision servo valves with very small clearances.

Even relatively fine contaminants can therefore contribute to:

• Valve sticking or restricted movement

• Increased component wear

• Deposits on control components

• Reduced hydraulic control accuracy

3. Acid Formation and Chemical Degradation

As phosphate ester fluid degrades, acidic products may accumulate.

A rising Total Acid Number (TAN) is therefore an important condition indicator. However, TAN should not be evaluated in isolation.

A complete EHC fluid condition assessment should also consider:

• Water content

• Particle cleanliness

• MPC or other varnish-related indicators

• Resistivity

• Fluid chemistry

• Operating history

ASTM D8323 provides guidance for monitoring and managing in-service phosphate ester fluids used in steam turbine EHC systems.

4. Varnish, Sludge, and Fine Contaminants

EHC fluid degradation can also generate fine or semi-soluble contaminants that may contribute to deposits on hydraulic components.

Membrane Patch Colorimetry (MPC) has been investigated for phosphate ester EHC applications. MPC can help detect varnish and thermolysis-related degradation products, although its interpretation must account for the different chemistry of phosphate ester fluids compared with conventional turbine lubricating oils.

These contaminants can become particularly problematic around servo valves, where deposits or particles may interfere with precise valve movement.

This means that an EHC fluid purification system should not be selected based only on its nominal micron rating. The treatment technology must match the actual contamination mechanism.

How Does Degraded EHC Fluid Affect Steam Turbine Control?

Different contamination problems require different treatment methods.

The key point is that EHC fluid purification is not simply a matter of removing particles.

Depending on the condition of the fluid, an effective treatment strategy may need to combine:

Moisture removal + acid control + fine filtration + condition monitoring

For example, a conventional particulate filter may improve cleanliness but will not necessarily remove dissolved water or acidic degradation products.

EHC Fluid Purification System

What Should You Test Before Purifying EHC Fluid?

Before selecting an EHC fluid purification system, test the fluid and identify which parameters are outside the applicable operating range.

Key EHC Fluid Condition Parameters

Why Should You Avoid Using a Universal EHC Fluid Limit?

There is no single set of numerical limits that should automatically be applied to every EHC system.

Fluid formulation, turbine design, operating conditions, OEM requirements, and fluid manufacturer's specifications can all affect the appropriate control limits.

ASTM D8323-24 provides guidance for in-service phosphate ester EHC fluids, but it is not intended to ｒｅｐｌａｃｅ applicable OEM specifications.

For this reason, operators should evaluate trends as well as absolute values.

A single abnormal test result may indicate a developing problem, while a consistent trend can provide a much stronger indication of fluid deterioration.

How to Purify and Restore Degraded EHC Fluid

Once the fluid condition has been established, the purification process should target the actual contamination problems rather than relying on one treatment technology for every application.

A practical EHC fluid restoration strategy can be divided into four main treatment functions:

1. Remove moisture

2. Control acidic degradation products

3. Remove particles and fine contaminants

4. Continue condition monitoring after treatment

1. Remove Moisture with Vacuum Dehydration

When excessive moisture is present, vacuum dehydration can be used to remove free and dissolved water from the fluid.

A vacuum dehydration system heats the fluid under controlled conditions and exposes it to a reduced-pressure environment. This allows water to be removed at a controlled operating temperature.

For phosphate ester EHC fluids, the operating temperature and vacuum conditions should be selected according to the specific fluid properties and equipment design.

The objective is to remove moisture effectively without exposing the EHC fluid to unnecessary thermal stress.

A suitable moisture-removal system can be especially useful for continuous offline or kidney-loop conditioning, where a controlled portion of the EHC fluid is continuously circulated through the purification system.

What Does Vacuum Dehydration Remove?

Vacuum dehydration is primarily intended for moisture control.

It can be useful when the main problem is:

• Free water

• Dissolved moisture

• Moisture-related fluid deterioration

However, vacuum dehydration should not be treated as a complete solution for chemically degraded EHC fluid.

If TAN is already elevated because acidic degradation products have accumulated, an additional treatment technology may be required.

2. Reduce Acidic Degradation Products

Moisture removal alone does not necessarily correct an already elevated TAN.

When acidic degradation products have accumulated, an appropriate adsorbent or ion-exchange treatment may be required.

Ion-exchange treatment has been widely studied for phosphate ester EHC fluid management. It can play an important role in controlling acidity, but different resin types can interact with phosphate ester fluids differently.

Therefore, resin selection should be based on:

• EHC fluid formulation

• Current TAN

• Overall fluid condition

• Resin compatibility

• Required treatment capacity

• OEM requirements

• Fluid manufacturer's recommendations

Using an unsuitable resin or treatment medium can create additional fluid-compatibility problems.

For this reason, chemical purification should be treated as a fluid-specific engineering decision rather than a standard consumable-filter replacement.

3. Remove Varnish, Sludge, and Fine Contaminants

After moisture and chemical degradation have been addressed, the EHC fluid may still contain fine particles, sludge, or degradation products.

High-efficiency filtration can help remove these contaminants.

Depending on the application, treatment may involve:

• Fine-depth filtration

• Sub-micron filtration

• Electrostatic contamination removal

• Balanced charge agglomeration

• Specialized filtration media

The appropriate technology depends on the type and concentration of contaminants identified through fluid analysis.

For phosphate ester EHC fluids, MPC results should also be interpreted carefully because MPC can respond to both varnish-related material and thermolysis products.

How to Choose an EHC Fluid Purification System

The best EHC fluid purification system is not necessarily the system with the largest number of filtration stages.

The right system is the one that addresses the actual condition of the EHC fluid.

EHC Purification Selection Guide

What Should You Consider When Selecting the Equipment?

1. EHC Fluid Type

Confirm the exact phosphate ester formulation before selecting:

• Filtration media

• Resin

• Seals

• Hoses

• Pumps

• Other wetted components

Compatibility is particularly important for phosphate ester fluids because different formulations can have different material compatibility requirements.

2. Current Fluid Condition

Provide recent laboratory test results whenever possible, including:

• TAN

• Water content

• Particle count

• MPC or other varnish-related indicators

• Resistivity

• Relevant fluid chemistry data

This information helps determine whether the system needs primarily dehydration, particle filtration, acid removal, or a combination of technologies.

3. Required Flow Rate

The purification flow rate should be matched to:

• EHC reservoir volume

• Contamination level

• Available operating window

• Desired conditioning rate

• Offline operating requirements

A larger flow rate is not automatically better. The purification rate should provide effective circulation and treatment without creating unnecessary operational constraints.

4. Treatment Technology

Do not ｓｅｌｅｃｔ an EHC fluid purification system based only on nominal filtration accuracy.

A system designed only for particulate removal may not address:

• Dissolved moisture

• Acidic degradation products

• Varnish-related contamination

The treatment configuration should therefore be based on the laboratory diagnosis.

5. Fluid Compatibility

All filtration media, resin, seals, hoses, pumps, and wetted components should be compatible with the specific phosphate ester fluid.

This is particularly important when retrofitting a purification system to an existing turbine EHC system.

6. Offline or Kidney-Loop Operation

For many power-generation applications, offline purification allows EHC fluid to be conditioned without relying solely on the turbine's normal hydraulic circuit.

A continuous side-stream or kidney-loop approach can also support:

• Condition-based maintenance

• Continuous cleanliness management

• Moisture control

• Particle control

• Longer fluid service life

How to Use an EHC Fluid Purification System

A practical EHC fluid maintenance program can follow six steps.

Step 1: Test the EHC Fluid

Collect representative fluid samples and test the key condition parameters.

At minimum, evaluate the parameters relevant to the suspected contamination mechanism.

Step 2: Identify the Dominant Contamination

Determine whether the main problem is:

• Water

• Acidic degradation products

• Particles

• Varnish-related contamination

• Or a combination of these

Step 3: ｓｅｌｅｃｔ the Appropriate Treatment

Match the purification technology to the contamination mechanism.

For example:

Water → Vacuum Dehydration

High TAN → Ion-Exchange / Compatible Resin

Particles → Fine Filtration

Varnish-Related Contamination → Suitable Fine or Electrostatic Contamination Removal

This contamination-to-treatment approach is more reliable than selecting equipment solely by flow rate or filtration accuracy.

Step 4: Run Offline Purification

Operate the purification system as an offline loop or kidney-loop, following the equipment and fluid manufacturer's operating instructions.

The treatment process should be controlled to avoid unnecessary thermal or chemical stress on the phosphate ester fluid.

Step 5: Retest the Fluid

After treatment, test the fluid again to determine whether the target condition has been achieved.

Compare the post-treatment results with the original laboratory data and applicable OEM or fluid-manufacturer recommendations.

Step 6: Continue Condition Monitoring

EHC fluid management should not end after one purification cycle.

Regular testing and trend analysis can help identify:

• Recurring moisture ingress

• Increasing acidity

• Particle contamination

• Developing varnish-related contamination

• Other changes in fluid condition

ASTM D8323-24 provides guidance for ongoing management of in-service phosphate ester EHC fluids rather than treating purification as a one-time activity.

EHC Fluid Purification Equipment Options

Different EHC fluid contamination conditions require different purification configurations.

Vacuum Dehydration Oil Purifier

A vacuum dehydration oil purifier is designed primarily to remove free and dissolved moisture from contaminated hydraulic or EHC fluid under controlled vacuum conditions.

Best suited for:

• High moisture content

• Free water

• Dissolved water

• Moisture-related fluid degradation

Vacuum Oil Purification System

EHC Fluid Purification System

A multi-stage EHC fluid purification system can combine moisture removal, fine filtration, and suitable chemical purification media according to the actual fluid condition.

Best suited for:

• Multiple contamination problems

• Moisture + particles

• Moisture + TAN

• Particles + degradation products

• Comprehensive offline fluid conditioning

Resin / Ion-Exchange Purification

Resin or ion-exchange treatment is used to remove or reduce acidic degradation products from phosphate ester fluids when compatible treatment media are selected.

Best suited for:

• Increasing TAN

• Acidic degradation products

• Chemical degradation requiring acidity control

Fine Filtration System

A fine filtration system is designed to remove fine particles and other physical contaminants from EHC fluid.

Best suited for:

• High particle count

• Sludge

• Fine degradation products

• Servo-valve cleanliness control

Oil Purification Products

Frequently Asked Questions About EHC Fluid Purification

Can degraded EHC fluid be restored?

In many cases, contaminated or moderately degraded EHC fluid can be conditioned through appropriate purification.

The treatment depends on the type and severity of degradation.

If the fluid has severe chemical degradation or falls outside the applicable acceptance criteria, it should be evaluated against the fluid manufacturer's and turbine OEM's requirements before deciding whether purification or replacement is appropriate.

How do you remove water from phosphate ester EHC fluid?

Vacuum dehydration is one approach for removing free and dissolved moisture.

The operating temperature and vacuum conditions should be selected according to the specific phosphate ester fluid and purification equipment.

How do you reduce TAN in EHC fluid?

When TAN increases because of acidic degradation products, compatible ion-exchange or resin-based treatment may be used.

Resin selection should be based on fluid compatibility and actual fluid condition rather than using a generic treatment medium.

How do you remove varnish from EHC fluid?

Varnish-related and other fine degradation products may require:

• Fine-depth filtration

• Electrostatic contamination removal

• Specialized filtration media

• Other suitable contamination-removal technologies

Fluid analysis should be used to determine the nature of the contamination before selecting the treatment.

How often should EHC fluid be purified?

There is no universal purification interval.

A condition-based EHC fluid maintenance program is generally more appropriate than relying only on a fixed calendar schedule.

Testing results, fluid trends, operating conditions, and OEM recommendations should determine when treatment is required.

Should EHC fluid be replaced or purified?

If the fluid is primarily contaminated with water, particles, or manageable degradation products, purification may extend its service life.

If the fluid has severe chemical degradation or falls outside the applicable acceptance criteria, replacement may be necessary.

The decision should be based on laboratory analysis and the applicable OEM and fluid-manufacturer requirements.

Can a standard oil filter remove EHC fluid degradation products?

Not necessarily.

Conventional particulate filtration can remove solid particles, but it does not address every form of contamination.

For example:

• Particles → Mechanical filtration

• Moisture → Vacuum dehydration

• Acidic degradation products → Compatible ion-exchange/resin treatment

• Varnish-related contaminants → Suitable fine or electrostatic treatment

This is why EHC fluid purification should be designed around the actual contamination condition.

What information is needed to ｓｅｌｅｃｔ an EHC purification system?

The most useful information includes:

• EHC fluid type and brand

• Reservoir capacity

• Required flow rate

• Water content

• TAN

• Particle count

• MPC or other varnish-related test results

• Resistivity

• Turbine and EHC system information

• OEM requirements

• Existing servo-valve or hydraulic control problems

Providing recent laboratory test results can significantly improve the accuracy of the purification system recommendation.

Conclusion: Move from Reactive Fluid Replacement to Condition-Based EHC Maintenance

EHC phosphate ester fluid degradation is rarely caused by a single contamination mechanism.

Moisture, hydrolytic degradation, acidic compounds, thermal degradation, varnish-related contaminants, and fine particles can interact and affect the performance of the hydraulic control system.

For this reason, effective EHC fluid management should begin with fluid analysis rather than equipment selection alone.

A practical strategy is:

Test the fluid → Identify the contamination → ｓｅｌｅｃｔ the appropriate treatment → Purify offline → Retest → Continue monitoring

Vacuum dehydration can address moisture, compatible resin or ion-exchange treatment can help control acidic degradation products, and high-efficiency filtration can remove fine physical contaminants.

The exact combination should be determined by the fluid condition and applicable OEM and fluid-manufacturer requirements.

Need Help Selecting an EHC Fluid Purification System?

If your EHC system is experiencing high TAN, moisture contamination, increasing MPC, particle contamination, or servo-valve problems, the most useful starting point is your latest fluid analysis report.

Provide the following information:

• EHC fluid type and brand

• Reservoir capacity

• Required flow rate

• Water content

• TAN

• Particle count

• MPC results, if available

• Turbine/EHC system information

• Any specific OEM requirements

Our engineers can evaluate the contamination condition and recommend a suitable EHC fluid purification configuration based on your actual operating requirements.