Oils · Site
Fire-Resistant Hydraulic Fluids: HFC vs HFDU for Steel and Mining
HFC uses water to resist fire and gives up a lot of lubrication and materials compatibility to do it. HFDU is a water-free synthetic, usually a polyol ester, that lubricates more like an oil and fails by hydrolysis if water gets in. ISO 12922 names the families. A brand name does not.
Direct answer
HFC fluids are water-glycol. The water is what resists fire, and it is also what punishes pumps, paints and the wrong seals. HFDU fluids are water-free synthetics, commonly polyol esters: they resist fire by chemistry and they lubricate more like an oil, but they hydrolyse if water is allowed to accumulate. ISO 12922 is the family specification. A product name is not. Mineral AW 46 practice does not carry over to either one.
- ISO 6743-4 and ISO 12922 name the families. HFC and HFDU are not two viscosities of the same oil.
- HFC is a composition you maintain: water and alkalinity. HFDU is an ester you keep dry.
- Fire performance is a cited test — spray, wick or manifold — not the words fire-resistant on the drum.
- Conversion is seals, paint, zinc, filters and a ban on mixing. It is not a top-up.
- Lubechem formulates toward those methods. Holding a Factory Mutual or mining approval is a separate certificate we do not imply.
HFA / HFB / HFC / HFDU in one table
ISO 6743-4 classifies fire-resistant hydraulic fluids in family H. ISO 12922 is the specification buyers should name, including the fire tests the category requires. The short map is enough to stop a bad purchase. HFAE fluids are oil-in-water emulsions, mostly water, used where fire risk is extreme and lubrication demand is modest. HFAS fluids are chemical solutions in water. HFB fluids are water-in-oil invert emulsions, now a legacy choice in many plants. HFC fluids are water-glycol solutions: less water than an HFA, far more water than an oil, viscosity that swings hard with temperature, and a real but limited lubrication life. HFDR fluids are phosphate esters, strong on fire resistance and aggressive to seals and paints. HFDU fluids are other water-free synthetics; in steel and die-casting conversations that usually means a polyol ester, sometimes a polyglycol, sold as a fluid that still builds a film.
Do not collapse these into “the fire-resistant grade”. An HFC at ISO VG 46 and an HFDU at ISO VG 46 can share an ASTM D445 number at 40 °C and share nothing else: seal swell, paint attack, pump wear, bacterial or evaporative water loss, and what “top up” is allowed to mean. The formulation brief for either family starts on the fire-resistant hydraulic fluid page, after you have picked the family.
Steel caster vs underground mining vs die-casting
Near a caster or a furnace, the fluid may see a molten-metal ignition source. That is why plants leave mineral oil. It does not decide HFC versus HFDU. HFC has been the traditional water-containing answer where the system was designed for it and the fire case is a spray of fluid onto a hot surface. HFDU is the answer where the pumps and valves were built for oil-like lubrication, where water maintenance has proved undisciplined, or where the OEM now names a water-free synthetic. Die casting adds a similar radiant-heat and spray risk around molten aluminium, with the same materials question: what the seals and the hoses were actually qualified on.
Underground mining adds a regulatory layer that this article will not pretend to clear. Protocols such as the 7th Luxembourg report, or a Factory Mutual approval, are certificates held by a fluid that was tested, not adjectives a formulator may print. If your buyer requires one of those, say so in the brief and expect the cost and the calendar of that programme. Mining product context, without that certificate implied, is on the mining lubricants page. Cite the fire test ISO 12922 points you at. Common methods in this family are spray ignition under ISO 15029, wick persistence under ISO 14935, and manifold ignition under ISO 20823. Name the clause. Do not write only “fire resistant”.
Seal, pump and reservoir conversion checklist
HFC and HFDU do not mix, and neither is a top-up for mineral oil. Conversion is a drain, a flush compatible with both the old and the new fluid, a filter change, and a cleanliness target you can state in ISO 4406. Elastomer compatibility belongs to ISO 6072, run on the actual rubber, not guessed from “nitrile usually works”. HFC is known to attack some paints and zinc-lined or galvanised surfaces; a reservoir that was fine on mineral oil can shed paint into the first HFC fill. HFDU esters swell and shrink a different set of seals from mineral oil and from water-glycol. Change the seals the OEM lists. Do not discover them as leaks.
Pumps need the same honesty. ISO 20763 vane-pump testing is a hydraulic wear method used on these fluids; a four-ball result borrowed from an engine-oil habit, ASTM D4172, is the wrong headline. Check the pump maker’s list for water-containing fluids before you put HFC into a unit that was built for mineral oil. The mineral baseline you are leaving is described on the AW 46 hydraulic oil page so you can see what not to copy: zinc antiwear treat, demulsibility as a virtue, and the assumption that water is contamination rather than part of the formula.
- Identify the current family. Do not trust the drum colour.
- Confirm seals, hoses, paint and any zinc or galvanised surface against the new family.
- Drain, flush, change filters, fill, and sample. Do not mix HFC into HFDU or either into mineral oil.
- Reset the maintenance tests. They are not the mineral-oil set.
What Indian plants get wrong on water content and pH
HFC is a maintained composition. Water evaporates off hot casters; glycol does not leave at the same rate; viscosity and fire performance both drift. Topping with hard tap water adds chloride and calcium that the original blend did not contain. Topping with mineral hydraulic oil, a real shortcut when the HFC drum is late, destroys the family. pH and reserve alkalinity are control tests because the corrosion inhibitors are alkaline and they deplete. Use the method named on the fluid’s own control sheet. Do not assume ASTM D6304 Karl Fischer, which is the right water method for an oily HFDU, reads cleanly in a glycol-water HFC. Many plants use the supplier’s refractometer correlation for HFC water and reserve Karl Fischer for HFDU contamination. Write which one you mean.
HFDU fails quietly in the other direction. Esters pick up water from breathers and wash-down, then hydrolyse. Rising acid number, ASTM D974 or a method the buyer cites, and a hydrolytic-stability screen such as ASTM D2619 tell you the fluid is becoming corrosive. A polished ISO VG number at 40 °C will still look fine while that happens. Breathers, reservoir temperature and a ban on water-based top-up are the operating spec. None of this is fixed by copying an AW 46 zinc package into the ester.
What Lubechem will and will not claim
Lubechem will formulate an HFC or an HFDU against a written family, a viscosity grade, a fire-test target and a materials list. We will not claim that the fluid is Factory Mutual approved, Luxembourg approved, or certified for a named steel plant because a formulation project exists. Those are certificates with owners. If you need the mark, the project plan has to include the laboratory that can grant it.
We will also not claim a seal, a pump or a paint is compatible because a brochure for a different ISO VG said so. Send the elastomer specification, the pump model and whether the reservoir is painted or galvanised. The article is the selection. The kettle recipe stays in the project file under NDA, not on this page.
HFC and HFDU are different maintenance regimes
| Question | HFC water-glycol | HFDU water-free synthetic | Method to cite |
|---|---|---|---|
| What resists fire? | The water content, which you must keep | Chemistry and low volatility, often a polyol ester | ISO 12922, with ISO 15029, ISO 14935 or ISO 20823 as specified |
| What does lubrication feel like? | Weaker than oil; viscosity swings with temperature | Closer to a hydraulic oil, until it is wet | ISO 20763 vane pump, not a borrowed four-ball headline |
| What maintenance actually is | Water balance, pH or reserve alkalinity, no mineral top-up | Keep water out; watch acid number | Supplier water method on HFC; ASTM D6304 and D974 on HFDU |
| What conversion breaks | Wrong seals, paint, zinc, mixing with oil or HFDU | Wrong seals, water ingress, mixing with HFC | ISO 6072 elastomers; ISO 4406 cleanliness after the flush |
ISO 12922 names the fluid. A fire-resistant claim needs the test in that specification. Approvals such as Factory Mutual are certificates this article does not grant.
Questions plants actually ask
Can we switch from HFC to HFDU by topping up the reservoir?
No. They are different families and they do not mix. Drain, flush with a fluid compatible with both, change seals as the OEM requires, change filters and refill.
Is ISO VG 46 on the label enough to call two fire-resistant fluids equivalent?
No. ASTM D445 can match at 40 °C while water content, seal response and fire-test category do not. Name the ISO 12922 family.
Does a formulation project include Factory Mutual or mining approval?
Only if that certificate is in the scope and a qualified approval route is planned. Lubechem does not imply those marks. Formulation toward a fire test is not the mark itself.
Why not just keep mineral AW 46 and add a fire-resistant additive?
Fire resistance of HFC and HFDU is the fluid family, not a treat rate in mineral oil. Mineral AW practice, including zinc antiwear logic, does not transfer.
Literature and prior art
Citations below are orientation, not a freedom-to-operate opinion. Lubechem does not copy claimed compositions and does not treat a patent as a product endorsement.
- ISO 6743-4 and ISO 12922 — Classification and specification of fire-resistant hydraulic fluids. Cite the family and the edition.
- ISO 15029, ISO 14935, ISO 20823 — Spray, wick and manifold fire tests used in this field. The clause you owe is the one ISO 12922 names for the category.
- ISO 20763, ISO 6072, ISO 4406 — Vane-pump wear, elastomer compatibility and cleanliness. These are conversion and durability, not slogans.
- ASTM D445, D974, D6304, D2619 — Viscosity, acid number, Karl Fischer water in oils, hydrolytic stability. Do not point Karl Fischer at an HFC without checking the method.