Water-Glycol Hydraulic Fluid and Seal Compatibility: Do Not Assume Your Standard Seal Is Suitable
Fire-resistant water-glycol (HFC) fluids are common in high-fire-risk environments, but a seal that works well in mineral oil is not automatically suitable. The exact compound and operating temperature matter.
Build a compound-specific compatibility request
- Identify the exact HFC fluid
- Identify every seal compound
- Record operating conditions
- Obtain application-specific review
On this page
- Build a compound-specific compatibility request
- Why water-glycol changes the selection problem
- Avoid blanket material rules
- What to collect before choosing a seal
- When an existing seal fails after a fluid change
- Information that changes the supplier answer
- Illustrative example: “PTFE seal” is not a complete specification
- Preserve evidence when failure follows a conversion
- Was this guide helpful?
Why water-glycol changes the selection problem
HFC fluids contain a substantial water fraction plus glycol and additives. Compared with mineral oil, that changes lubrication, water exposure and chemical compatibility. Trelleborg notes that high-temperature exposure in water-based fluids can significantly affect sealing materials, and SKF publishes separate compatibility ratings for HFC fluids.
Avoid blanket material rules
Two polyurethane grades can behave differently, and two FKM or HNBR compounds can also have very different HFC ratings. For that reason, statements like “Viton is always better” or “all PU is unsuitable” are too broad for engineering selection.
What to collect before choosing a seal
- Exact fluid brand and HFC classification.
- Expected water concentration or maintenance specification.
- Continuous and peak temperature.
- Pressure, speed and extrusion gap.
- Seal manufacturer compound code and compatibility rating.
When an existing seal fails after a fluid change
Keep the failed parts and compare hardness, swelling, cracking and wear against unused material if possible. A fluid conversion can expose seals, hoses, coatings and lubricated interfaces to a different environment, so investigate the system as a whole.
Related: seal material compatibility and swollen or softened seals.
To connect this inspection with the next repair decision, see Skydrol & Phosphate-Ester Seal Compatibility: Guide and Checker.
Information that changes the supplier answer
| Observed situation | Next verification |
|---|---|
| Fluid identity | Provide trade name, grade and current supplier documents, including the specified water-content maintenance method. |
| Complete sealing assembly | List the main ring, energizer, back-up and guide materials. A compatible main ring does not qualify the other components. |
| Exposure and duty | Record continuous/peak temperature, motion, pressure and dwell conditions. State what was measured and what is still assumed. |
Illustrative example: “PTFE seal” is not a complete specification
A supplier is asked to replace a mineral-oil seal after conversion to HFC. The proposed assembly is described only as PTFE. The missing information includes the ring grade or filler, energizer compound, exact fluid and operating temperature. Ask for a review of the full assembly at those conditions rather than a family-level yes/no answer. Trelleborg highlights the need to evaluate high-temperature exposure together with water-based fluids. The example supplies no compatibility rating, universal temperature ceiling or recommendation to change the machine fluid.
Preserve evidence when failure follows a conversion
- Record the fluid-change date, product identity and which components were changed.
- Keep labelled failed parts and available unused comparison parts; separate observations from a chemical-failure diagnosis.
- Check the machine and fluid supplier requirements for conversion, maintenance and approved components.
- Request the decision scope and remaining limits in writing before treating a proposed compound as qualified.
Technical references
Use the exact seal manufacturer and machine service data for final selection.
Need to narrow the fault?
Use the interactive tools after the inspection—not instead of it.
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