Fire-Resistant Hydraulic Fluid Seal Compatibility Chart by Class

Each value holds only at the temperature, water content and fluid chemistry stated with it, so check the fluid maker's seal list for the exact product and confirm each kept compound by ISO 1817 immersion testing.

• ISO 12922:2020 specifies minimum requirements for HFAE, HFAS, HFB, HFC, HFDR and HFDU fluids and treats most HFDU fluids as "less flammable" rather than fire-resistant (ISO, 2020).

• ISO 6072:2011 defines five reference elastomers, NBR 1, NBR 2, FKM 2, EPDM 1 and HNBR 1, for baseline comparisons of hydraulic fluids (ISO, 2011).

• The water content of HFC fluids can vary between 35 and 50%, and their tribological properties change with it (Wilke and Jordan, 2016).

• Parker lists ethylene propylene compounds for aircraft (alkyl) phosphate esters and fluorocarbon compounds for industrial (aryl) phosphate esters (Parker ORD 5700, 2021, Table 3-15).

This fire-resistant fluid seal chart covers elastomer seals in industrial hydraulic systems running ISO 6743-4 fire-resistant fluids. It does not settle aircraft hydraulic systems, which follow the airframe maker's material lists, PTFE and other thermoplastic seal grades, hoses, paints and metals, or a seal that has already failed and needs a failure analysis.

When should you use a fire-resistant hydraulic fluid seal compatibility chart?

A fire-resistant hydraulic fluid seal compatibility chart is used at four points: when seals are specified for a new system on a fire-resistant fluid, when a system converts from mineral oil, when it changes from one fire-resistant category to another, and when seals are replaced on a system whose fluid has changed since the last overhaul. Maintenance engineers, hydraulic system designers and fluid suppliers' technical service staff use it to decide which seal compounds to keep, change or send for testing.

Conversion is the most common trigger. ExxonMobil lists the compatibility of each circuit component with the fire-resistant fluid to be introduced as a key check point, naming "seals, packings, hoses and coatings" (ExxonMobil, 2023). ISO 7745:2024 specifies the factors to consider when selecting a fire-resistant fluid and gives procedures for replacing one fluid with another from a different category (ISO, 2024).

The chart is a screening step, not an approval. Parker states that its compound listings "are intended only as general guides" and that users must test under their own operating conditions to determine the suitability of any compound (Parker ORD 5700, 2021, Table 3-15). Use the chart to build the short list of seal compounds that need the fluid maker's confirmation and an immersion test, and to spot the compounds that should come out of the system before the new fluid goes in. An Excel copy of the chart with a seal check log, and a CSV of the log columns, are at the end of this page.

Which fluid and seal details do you need before reading the fire-resistant fluid chart?

Before reading the fire-resistant fluid seal chart, you need the fluid's ISO 6743-4 class, its base chemistry, its water content, the temperature at the seal, the seal compound and duty, and what the previous fluid left behind. A trade name alone does not fix the row: ExxonMobil notes that brand names may be confusing and that the fluid supplier should be consulted on the exact category in service (ExxonMobil, 2023).

• Fluid class: HFAE, HFAS, HFB, HFC, HFDR or HFDU, from the supplier's data sheet or written statement.

• Base chemistry: for phosphate esters, industrial aryl or aircraft alkyl types, which Parker lists separately (Parker ORD 5700, 2021, Table 3-15); for HFDU, polyol ester or polyalkylene glycol, the examples ExxonMobil gives (ExxonMobil, 2023).

• Water content in %: HFAE and HFAS typically above 80%, HFB above 40% and HFC above 35% water (ExxonMobil, 2023).

• Temperature in °C at the seal, including start-up and peak values, not only the reservoir set point.

• Seal compound and duty: polymer family, compound or supplier grade, static or dynamic, and whether it is a rod, piston or valve seal.

• Residue: the previous fluid and how the system was drained and flushed.

The residue entry decides more rows than it seems to. EPDM is not compatible with mineral oil products (Parker ORD 5700, 2021, section 2.2.4), so an EPDM compound that suits a pure phosphate ester can fail in a system that still holds mineral oil from before the conversion.

Fire-resistant hydraulic fluid seal compatibility chart by fluid class

The fire-resistant hydraulic fluid seal chart below decides which seal compound to keep, change or test when a system runs or converts to a fire-resistant fluid. Every entry is a typical value from a published chart or test at the conditions stated, not a result for a specific fluid brand.

The key row is HFC: NBR and HNBR are the usual seal families, while FKM, which is the default for many other fluids, can change significantly in water glycol at 70 °C and above (Simpson and Bucci, 2016). The second row that decides most conversions is HFDR, where EPDM suits only phosphate ester free of mineral oil (Parker ORD 5700, 2021, Table 3-16). Treat every cell as typical: Parker states that users must test under their own operating conditions (Parker ORD 5700, 2021, Table 3-15), and MP Filtri's codes describe its own filter housings and element seals, not every component in a circuit.

How do temperature and water content change fire-resistant fluid seal compatibility?

Temperature and water content change fire-resistant fluid seal compatibility in two ways: heat drives water out of water-containing fluids and speeds attack on the seal polymer, and the water share itself changes how the fluid lubricates a dynamic seal. A seal rated compatible at one temperature and water content can wear or degrade at another.

Water-containing fluids rely on their water content to prevent fire, and Parker limits their working temperatures to between 50 °C and 65 °C because water evaporates easily at higher temperatures (Parker ORD 5700, 2021, section 3.9.17.1). ExxonMobil gives typical ranges of +5 to +50 °C for HFAE and HFAS and -20 to +70 °C for HFDR and HFDU, and notes that above +100 °C special materials may have to be used for seals, packings and hoses (ExxonMobil, 2023).

The seal polymer responds to the same heat. In Trelleborg's 90-day immersion tests in water glycol fluids, HNBR showed relatively small changes in hardness, volume and strain up to 70 °C and more pronounced changes at 135 °C (Simpson and Bucci, 2016). Polyurethane is listed as resistant to water only up to 50 °C (Parker ORD 5700, 2021, section 2.2.12), so a PU rod seal that works in an HFA press at 40 °C has no margin at 60 °C.

Water content matters for wear. The water content of HFC fluids can vary between 35 and 50%, and the tribological properties of the fluids change with it (Wilke and Jordan, 2016). Trelleborg's rod seal tests found that seal materials usually used with mineral oil HLP fluids showed significantly higher wear in HFC (Wilke and Jordan, 2016). Record the measured water content and the peak seal temperature with every compatibility result, or the result cannot be compared with the next test.

Which seals work with water glycol HFC hydraulic fluid?

Water glycol HFC hydraulic fluid usually works with NBR and HNBR seals, and FKM and polyurethane need the fluid maker's confirmation and a test before use. HFC fluids are solutions of polyethylene and polypropylene glycols in water (Parker ORD 5700, 2021, section 3.9.17.5); published water contents are more than 35% (ExxonMobil, 2023) and between 35 and 50% (Wilke and Jordan, 2016).

Parker states that most elastomer compounds compatible with mineral oils also can be used in HFC fluids, NBR for example, and that "certain FKM compounds are not compatible with HFC fluids" (Parker ORD 5700, 2021, section 3.9.17.5). The Trelleborg and MacDermid test program reached a similar point for high temperatures: water-based HFC fluids significantly changed the properties of FKM at 70 °C and 135 °C, so FKM would not typically be recommended with these fluids (Simpson and Bucci, 2016). In the same program, HNBR gave the best combination of compatibility and property retention up to 135 °C (Simpson and Bucci, 2016).

Polyurethane is the compound most often carried over from a mineral oil system as a rod seal. Parker lists glycols and hot water among the media polyurethane is not compatible with (Parker ORD 5700, 2021, section 2.2.12), so PU seals on an HFC conversion belong on the test list, not the keep list. Butyl resists polyglycol HFC fluids but is not compatible with mineral oil (Parker ORD 5700, 2021, section 2.2.5), which matters where oil residue remains.

The fluid formula also counts. Trelleborg notes that extra additives added by operators were not part of its tests and may lead to seal material incompatibilities (Simpson and Bucci, 2016). Ask for the HFC supplier's seal list for the exact product, and test the actual seal compound in the actual fluid at the system's peak temperature.

Which seals work with phosphate ester HFDR hydraulic fluid?

Phosphate ester HFDR hydraulic fluid in industrial systems usually works with FKM seals, and EPDM or butyl only where the phosphate ester is free of mineral oil. Parker describes the HFD group as water-free synthetic fluids whose compatibility "with most seal materials is rather limited" (Parker ORD 5700, 2021, section 3.9.17.6).

For industrial aryl phosphate esters, Parker's compound table lists fluorocarbon compounds, and its FKM description lists non-flammable hydraulic fluids (HFD) under chemical resistance (Parker ORD 5700, 2021, Table 3-15 and section 2.2.7). MP Filtri's table gives FKM seal codes for generic HFDR and for the industrial HFDR brands it lists (MP Filtri, 2026). Parker also lists phosphate ester HFD-R fluids under the chemical resistance of both EPDM and butyl (Parker ORD 5700, 2021, sections 2.2.4 and 2.2.5), but limits EPDM for HFD to "pure (mineral oil free) phosphate-ester (HFD-R)" (Parker ORD 5700, 2021, Table 3-16).

NBR is not among the seal materials Parker gives for the HFD group, which are FKM and EPDM (Parker ORD 5700, 2021, Table 3-16), and Parker lists esters among the media polyurethane is not compatible with (Parker ORD 5700, 2021, section 2.2.12). On a conversion from mineral oil, both families belong on the change list unless the HFDR supplier names them for its fluid.

Temperature narrows the choice further. FKM is normally limited to about -26 °C in static service, with certain compounds suitable down to -46 °C, and to -15 to -18 °C in dynamic service (Parker ORD 5700, 2021, section 2.2.7), while ExxonMobil gives a typical HFDR range of -20 to +70 °C (ExxonMobil, 2023). A cold-start system at the bottom of that range needs a low-temperature FKM grade or a test.

Why do seal charts list FKM for some phosphate esters and EPDM for others?

Seal charts list FKM for some phosphate esters and EPDM for others because they cover two different fluid chemistries and two different system histories under one name. The sources agree once those two conditions are separated, and they differ in what they call HFDR.

The first difference is chemistry. Parker lists aircraft types (alkyl phosphate esters) separately from industrial types (aryl phosphate esters), with ethylene propylene compounds for the aircraft types and fluorocarbon compounds for the industrial types (Parker ORD 5700, 2021, Table 3-15). MP Filtri files three aviation phosphate esters under HFDR with an EPDM seal code, while generic HFDR and the industrial brands carry FKM codes (MP Filtri, 2026). ISO 6743-4:2015 does not include aircraft hydraulic fluids in its classification (ISO, 2015), so a chart that puts aviation fluids in an HFDR row is mixing a fluid outside the standard into the class.

The second difference is mineral oil. EPDM is not compatible with mineral oil products (Parker ORD 5700, 2021, section 2.2.4), and Parker restricts EPDM to pure, mineral-oil-free phosphate ester (Parker ORD 5700, 2021, Table 3-16). ExxonMobil notes at least partial miscibility between mineral oils and HFD fluids, so a small amount of residual mineral oil can remain in a converted system (ExxonMobil, 2023). Industrial systems are often converted from mineral oil; aircraft systems are built for the phosphate ester. This likely explains why industrial charts default to FKM, which Parker lists for both mineral oil and HFD fluids (Parker ORD 5700, 2021, section 2.2.7), while aviation lists default to EPDM. Read the chart row that matches both the fluid chemistry and the system's oil history.

Can nitrile seals stay in service when switching to an HFDU fluid?

Nitrile seals can stay in service when switching to an HFDU fluid only if the HFDU maker's seal list names NBR for that exact fluid and a test of the installed compound confirms it. HFDU is a broad class: ExxonMobil gives polyol esters and polyalkylene glycols as examples (ExxonMobil, 2023), and ISO 12922:2020 states that most HFDU fluids are more appropriately considered "less flammable" fluids rather than fire-resistant ones (ISO, 2020).

The published charts split within the class. MP Filtri gives FKM seal codes for generic HFDU, and among the six HFDU brands it lists, two carry NBR codes and four carry FKM codes (MP Filtri, 2026). A natural-ester HFDU data sheet from one supplier states that the fluid is compatible with most standard seal materials and lists medium to high nitrile rubber with more than 30% acrylonitrile in its seal table, for systems with a maximum operating temperature of 70 °C (U.S. Lube, 2017). The acrylonitrile condition matters: Parker notes that higher acrylonitrile content gives better oil resistance at the cost of elasticity and compression set (Parker ORD 5700, 2021, section 2.2.1).

The mineral oil left after the switch does not harm NBR, which is listed for mineral oil (Parker ORD 5700, 2021, section 2.2.1), but ExxonMobil notes that residual mineral oil may impair the fire resistance of the HFD fluid (ExxonMobil, 2023). Keep NBR only with the HFDU maker's written seal list, the NBR grade's acrylonitrile content, and an immersion result at the system's peak temperature. Replace polyurethane seals or test them, because Parker lists esters among the media PU is not compatible with (Parker ORD 5700, 2021, section 2.2.12).

Reading the fire-resistant fluid seal chart for one system (illustrative)

Reading the fire-resistant fluid seal chart for one system means listing every seal compound in the circuit against the new fluid's row, then sorting each into keep, change or test. The example below is illustrative: the system, temperatures and seal list are invented to show the method, not taken from a real case.

A die-casting press converts from an HM mineral oil to an HFC water glycol with a reservoir at 45 °C and a peak of 55 °C at the cylinders (illustrative). The circuit holds NBR static O-rings, polyurethane rod seals on two cylinders and FKM O-rings in one proportional valve (illustrative).

The chart reading, in this illustrative case, is as follows. NBR O-rings go on the keep list, subject to the HFC supplier's seal list, because NBR is listed for HFC (Parker ORD 5700, 2021, section 3.9.17.5). The PU rod seals go on the change or test list, because glycols and hot water are among the media Parker lists for PU as not compatible (Parker ORD 5700, 2021, section 2.2.12). The FKM valve O-rings go on the test list, because certain FKM compounds are not compatible with HFC fluids (Parker ORD 5700, 2021, section 3.9.17.5).

The test plan then names the conditions, in this illustrative case: immerse each listed compound in the HFC product at 55 °C or above, by an ISO 1817:2026 method, and record volume, hardness and tensile change against limits agreed with the seal supplier before the test (illustrative). ISO 1817:2026 evaluates rubber resistance to liquids by measuring properties before and after immersion (ISO, 2026). Each row goes into the Excel log with the fluid class, water content, temperature and result.

Which columns does the fire-resistant fluid seal check log have?

The fire-resistant fluid seal check log has 24 columns, one row per seal position, and records the chart reading, the fluid maker's seal list, the immersion test and the decision for each seal. The Excel file holds the log with units in the header and an illustrative example sheet; the CSV holds the same header row. The table groups the columns as the header has them, and the example values are illustrative and match the PU rod seal above.

Fill one row for every seal position, including seals of the same polymer in different components, because duty and temperature differ between them. Enter the chart reading and its source before the fluid maker's seal list arrives, and change the decision only when the seal list or the test result supports it. Leave the volume and hardness columns blank until the test ends, and write the acceptance limits before the test starts, so the result is judged against limits fixed in advance.

Where does the fire-resistant fluid seal chart not apply?

The fire-resistant fluid seal chart does not apply to aircraft hydraulic systems, to temperatures above the fluid's typical range, to non-elastomer components, or to mixed fluids. In each of these cases the chart's typical values are outside the conditions they were measured at.

Aircraft hydraulic fluids are outside ISO 6743-4:2015 (ISO, 2015), and the airframe maker's material lists govern them. Above +100 °C, ExxonMobil notes that special materials may have to be used for seals, packings and hoses, with clarification from the equipment and fluid makers (ExxonMobil, 2023). PTFE and other thermoplastic seals behave differently from elastomers: in Trelleborg's HFC rod seal tests, PTFE-based materials were significantly worn after 200,000 cycles while two proprietary materials showed the least wear (Wilke and Jordan, 2016). Metals and paints need their own checks; ExxonMobil advises consulting both the fluid and the system makers on seals, metals and paints (ExxonMobil, 2023).

Mixed fluids are a separate question. Mixing a water-based fluid with an HFDR or HFDU fluid forms two layers, leading to potential pump failures (ExxonMobil, 2023), so no seal row covers that system. An immersion test also has limits: ASTM D471, as quoted in the Parker handbook, states that its accelerated test "may not give any direct correlation with service performance" (Parker ORD 5700, 2021). ISO 6072:2011 allows baseline comparisons of fluids with standard elastomers and does not provide formulations of elastomeric materials for actual service (ISO, 2011). Lubinpla's compatibility assessment treats chart values like these as typical values and judges each fluid and seal pairing from your own documents at its concentration, temperature and contact time, returning one cited document for your review.

Key takeaways

• Fix the fluid's ISO 6743-4 class, base chemistry, water content and peak seal temperature before reading any chart row.

• For HFC water glycol, NBR and HNBR are the usual seals; FKM and PU go on the test list.

• For industrial HFDR, FKM is the usual seal; EPDM fits only phosphate ester free of mineral oil.

• HFDU splits by fluid: keep NBR only on the fluid maker's written seal list and a test.

• Confirm every kept compound by immersion in the actual fluid at the system's conditions.