Resistivity in EHC Phosphate Ester Fluids: The Parameter Many People Underestimate

In steam turbine EHC systems, the fire-resistant fluid is not only a hydraulic medium. It is part of the turbine control reliability chain. It transmits command signals hydraulically to stop valves, control valves, intercept valves, and reheat stop valves. Because these systems often use high-response servo valves with very fine clearances, the fluid must be chemically stable, clean, dry enough, and electrically suitable.

One property that is often misunderstood is resistivity.

Many people treat resistivity as “just another oil analysis number.” This is wrong. In EHC systems using phosphate ester FRF, resistivity is directly connected to the risk of servo valve spool electro-kinetic wear, internal leakage, poor valve control, and possible turbine trip.

ASTM D8323-24 specifically includes volume resistivity as a routine monitoring parameter for in-service triaryl phosphate ester fluids in steam turbine EHC systems, especially for higher-pressure systems with servo valves. It references ASTM D1169 as the recommended test method, with IEC 60247 as an alternative method.

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1. What is resistivity?

Volume resistivity is the ability of the fluid to resist the flow of electrical current through its volume.

In simple terms:

High resistivity = the fluid is more electrically insulating.
Low resistivity = the fluid is more electrically conductive.

The inverse of resistivity is conductivity.

So, when an EHC phosphate ester fluid has low resistivity, it means the fluid contains more electrically active or conductive species. These may include acids, water, chlorides, metals, metal soaps, degradation products, soot, varnish precursors, fine particles, or other polar contaminants.

This is why resistivity is not only an electrical property. It is a chemical cleanliness and degradation indicator.

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2. Why phosphate ester FRF resistivity matters more than people think

Phosphate ester fluids are used in steam turbine EHC systems because of their fire-resistant characteristics. However, these fluids are chemically active compared with many mineral turbine oils. They can degrade through hydrolysis, oxidation, and thermal stress.

ASTM D8323 explains that phosphate esters can degrade by three main mechanisms: hydrolysis, oxidation, and thermal degradation. Hydrolysis produces acidic species and phenols, while oxidation and thermal stress can produce varnish precursors, dark degradation products, and other polar compounds.

All these degradation products can influence resistivity.

That means resistivity is not an isolated test. It is connected to:

Acid number
Water content
Phenol content
MPC varnish potential
Metals
Chlorine/chloride contamination
Soot and thermal degradation products
Fluid cleanliness
Adsorbent media condition
Servo valve leakage behavior

This is exactly why ASTM D8323 states that low resistivity must be handled with a holistic approach, not by looking at one number alone.

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3. Why resistivity is critical for servo valves

Servo valves in EHC systems are precision hydraulic control components. The clearances between the spool and sleeve are extremely small. The spool must move smoothly, accurately, and repeatably. Any corrosion, erosion, deposit formation, varnish, or abnormal leakage can disturb valve response.

ASTM D8323 highlights that EHC systems with servo valves may suffer from servo valve spool corrosion due to electro-kinetic wear. It also states that when volume resistivity is too low, leakage across the servo valve spool can increase. This is serious because system pumps may not be able to maintain the required flow demand, creating a risk of EHC system shutdown.

This is the key point:

Low resistivity does not only mean “bad fluid.” It can mean the servo valve is moving toward a mechanical-control reliability problem.

When the spool/sleeve interface is damaged by electro-kinetic wear, internal leakage increases. Once leakage increases, the system may need more flow to maintain pressure and actuator response. If leakage exceeds what the pumps can compensate for, the EHC system can lose control margin.

For a steam turbine, that is not a small lubrication issue. That is a turbine control system risk.

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4. What is electro-kinetic wear?

Electro-kinetic wear is a complex damage mechanism associated with electrical effects, fluid chemistry, and fluid movement through fine clearances.

In servo valves, phosphate ester fluid passes through very small flow paths at high velocity and pressure differential. Under certain conditions, electrical charge separation, conductive contaminants, corrosive species, and fluid degradation products can contribute to attack on the spool and sleeve surfaces.

ASTM D8323 does not present volume resistivity as the only cause of electro-kinetic wear, but it states that volume resistivity is currently used to predict conditions related to this type of wear. It also lists many contributors, including stronger and weaker acids, phenols, soot particles, wear debris, dirt, metal soaps, leached metals, chlorides, moisture, thermal degradation products, and organic varnish.

This is important because many people wrongly think:

“Acid number is okay, so the EHC fluid is okay.”

Not true.

A fluid may have acceptable acid number but still suffer from low resistivity because of conductive degradation products, chlorides, metal soaps, water, phenols, soot, varnish, or fine contamination.

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5. The correct limit: be careful with units

The image shows:

Resistivity >10 GΩ·cm

This is a common way of expressing the limit.

ASTM D8323 Table 2 expresses preferred resistivity in MΩ·m. It gives preferable levels of ≥50 MΩ·m or ≥100 MΩ·m, depending on OEM design, requirements, and field experience. The warning limits are <50 MΩ·m or <100 MΩ·m, again depending on OEM/system requirements.

The unit conversion is:

1 MΩ·m = 0.1 GΩ·cm

Therefore:

100 MΩ·m = 10 GΩ·cm
50 MΩ·m = 5 GΩ·cm

So the “>10 GΩ·cm” value in the image is essentially equivalent to >100 MΩ·m.

But this should not be used blindly. ASTM D8323 clearly indicates that the required level depends on OEM design, requirements, and operating experience. Some constant-flow EHC systems may struggle to maintain resistivity above 50 MΩ·m due to the stress imposed on the fluid.

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6. Temperature is a major source of misunderstanding

Resistivity is highly temperature-dependent.

ASTM D8323 states that ASTM D1169 is the recommended test method and that a test temperature of 20 °C is generally used for this application. It also warns that test temperature must be reported because temperature can significantly change the result, and higher temperature reduces resistivity.

This is a practical issue in the field.

You cannot properly compare:

A resistivity result at 20 °C
with a result at 25 °C
or with an online sensor reading at operating temperature
or with a lab using a different method

without understanding the temperature basis.

A common mistake is to panic because one report shows a lower resistivity than another report, while the test temperature or method was different.

For EHC fluids, the proper question is not only:

“What is the resistivity?”

The correct question is:

What is the resistivity, by which method, at what temperature, and what is the trend?

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7. What causes low resistivity in FRF fluids?

Low resistivity usually means the fluid contains conductive or polar contaminants. In EHC phosphate ester fluids, the main contributors include:

Acids

Hydrolysis of phosphate ester produces stronger acidic phosphate species and phenols. Oxidation can also form weaker organic acids. Acids increase the ionic character of the fluid and reduce resistivity.

ASTM D8323 notes that acid number is a very important parameter for EHC fluid condition, and that acid formation can become autocatalytic, accelerating degradation.

Water

Phosphate ester fluids are hygroscopic. Water accelerates hydrolysis, producing acidic degradation products. Water can also directly reduce electrical resistance.

ASTM D8323 gives a preferred water content range of 300–500 mg/kg and notes that water above typical OEM maximum levels requires action. It also warns not to over-dry below 300 mg/kg because that can negatively affect fluid condition.

Phenols

Phenols are produced during hydrolysis and can also be present from phosphate ester chemistry. ASTM D8323 links phenols to varnish formation potential and notes that alkylphenols may affect resistivity.

Chlorides / chlorine contamination

Chloride ions are particularly important for servo valve electro-kinetic corrosion/erosion. ASTM D8323 states that higher resistivity and lower chlorine values significantly reduce electro-kinetic wear.

Metals and metal soaps

Metals such as sodium, calcium, magnesium, iron, copper, and chromium can contribute to poor fluid behavior. Metal soaps may form when metals react with acidic degradation products. ASTM D8323 states that metal soaps can negatively affect foaming, air release, and volume resistivity, and can accelerate degradation.

Soot and thermal degradation products

Micro-dieseling, static discharge, hot spots, and internal valve leakage can produce dark degradation products and soot-like contamination. These can increase MPC values, darken the fluid, increase submicron contamination, and reduce resistivity.

Organic varnish and fine particles

Varnish precursors and insoluble color bodies can deposit on servo valve surfaces and also contribute to abnormal particle counts. ASTM D8323 connects MPC deposits and varnish with valve sticking and filter plugging.

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8. Why low resistivity can become a hidden failure mode

Low resistivity often does not immediately shut down the turbine. This is why it is dangerous.

It can silently move the EHC system through a degradation path:

Fluid degradation begins
Acids, phenols, moisture, metals, soot, or varnish increase
Resistivity decreases
Servo valve electro-kinetic wear risk increases
Spool/sleeve surfaces deteriorate
Internal leakage increases
Control response becomes unstable or sluggish
Pump flow demand increases
Filters and servo valves become more stressed
Trip risk increases

The final failure may be seen as:

“Servo valve problem”
“Actuator response issue”
“EHC pressure instability”
“Pump cannot maintain pressure”
“Turbine trip”

But the root cause may have started months earlier as a fluid chemistry and resistivity problem.

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9. Why resistivity must be interpreted with other tests

A resistivity number alone is not enough. It must be interpreted with the full EHC fluid health profile.

A good EHC FRF report should include at least:

Acid number by ASTM D664
Water by Karl Fischer
Volume resistivity by ASTM D1169 at reported temperature
MPC by ASTM D7843
Patch weight
Phenol content by LSV
Metals by ICP
Fluid cleanliness
Chlorine/chloride indicator
ASTM color
Appearance
Air release and foam if symptoms exist

ASTM D8323 includes these tests in its routine condition monitoring program and recommends trending rather than judging only the latest result.

This is where many reports fail. They show a table with “Pass/Fail” but no interpretation of interactions.

For example:

Low resistivity + high water = hydrolysis risk
Low resistivity + high acid = chemical degradation risk
Low resistivity + high chlorine = servo valve electro-kinetic wear risk
Low resistivity + high metals = metal soap / corrosion / media carryover risk
Low resistivity + high MPC = varnish and degradation product risk
Low resistivity + high phenol = hydrolysis and varnish precursor risk
Low resistivity + dark oil = thermal degradation / micro-dieseling / hot spot risk

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10. Corrective actions for low resistivity

ASTM D8323 lists typical corrective actions for low resistivity: reduce acidity, improve cleanliness, lower elevated water levels, and change the adsorption medium. It also notes that optimized ion-exchange resins can increase resistivity over long-term application, while partial or complete replacement may be considered for quicker correction.

A practical corrective action plan should include:

Step 1: Confirm the result

Repeat the test if the result is abnormal. Confirm method, temperature, sample bottle cleanliness, and sampling point.

Step 2: Check the trend

One low result is a warning. A decreasing trend is a stronger signal. A sudden drop is a red flag.

Step 3: Identify the conductive contaminants

Look at acid number, water, phenol, chlorine, metals, MPC, particle count, and color.

Step 4: Remove the root cause

Do not only replace fluid without understanding why resistivity dropped. Otherwise, the new fluid may degrade again.

Step 5: Review the purification system

Check whether the acid scavenging media is suitable, saturated, releasing metals, releasing water, or failing to control weak acids and varnish precursors.

Step 6: Protect servo valves

If resistivity is low and servo valve symptoms exist, inspect servo valve performance, leakage, response, filters, and actuator behavior. In critical cases, servo valves may need bench testing or replacement.

Step 7: Consider optimized ion exchange treatment

For phosphate ester EHC fluids, optimized ion-exchange media can target acids, metal soaps, and organic varnish species. ASTM D8323 notes that different resin types can remove stronger/weaker acids, metal soaps, and organic varnish, and that long-term optimized ion-exchange application can increase resistivity.

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11. The practical message for power plants

For a steam turbine, the EHC fluid is not just a chemical fluid. It is part of the turbine protection and control system.

When resistivity drops, the plant should not say:

“The oil is still clean.”
“The acid number is acceptable.”
“The color is not too bad.”
“The turbine is running, so no issue.”

The right technical view is:

Low resistivity means the FRF has become more electrically conductive, and the servo valve system may be exposed to increased electro-kinetic wear risk.

This is especially critical in high-pressure EHC systems with servo valves.

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Conclusion

Resistivity is one of the most important but least understood properties of phosphate ester FRF fluids in steam turbine EHC systems.

It is not simply an electrical value. It is an integrated indicator of fluid degradation, contamination, ionic species, water, acids, phenols, metals, chlorides, varnish, soot, and purification performance.

ASTM D8323 makes it clear that low volume resistivity places servo valves at elevated risk of electro-kinetic wear, and that this can increase internal leakage across servo valve spools. In severe cases, the EHC pumps may not keep up with system flow demand, creating a possible shutdown risk.

For Khash, the practical rule is simple:

In EHC systems, resistivity is not a laboratory number.
It is a servo valve reliability number.
It is a turbine control risk number.
It is an early warning signal before expensive and dangerous failures appear.