Types of hydraulic oil: HL, HLP, HVLP, HLPD and HFDU, which to use in each system

The type of hydraulic oil is written in the letters of its name (HL, HLP, HVLP, HLPD), and each letter tells you one protection. Most industrial machines run on HLP, which protects against wear. The other types come in when one condition takes over the system: cold and heat in the same day (HVLP), a little water you cannot drain (HLPD) or the risk of fire from a leak (HFDU, a fluid that is harder to burn).

What is hydraulic oil for?

Hydraulic oil carries force from one point of the machine to another and, along the way, lubricates, cools, protects and cleans the circuit. It is the same principle as a car brake: the foot pushes the fluid, and the fluid pushes the brake pad.

In the plant, the pump does the pushing, and the oil moves the press cylinder or the hydraulic motor of the injection molding machine. The oil travels in a closed circuit, always in the same order:

1. the reservoir, the tank where the oil rests;

2. the pump, which draws the oil from the reservoir through the suction line, where pressure is low, and pushes it into the pressure line;

3. the directional valve, which chooses which way the oil goes;

4. the cylinder, the actuator that turns pressure into movement; the oil leaving the other side of the cylinder goes back through the same directional valve; this stretch is called the return;

5. the filter, which holds back dirt on the return;

6. the reservoir again.

Along the way, the relief valve opens when pressure goes past the limit and sends the oil back to the reservoir, which protects the pressure line.

Figure 1. The path of the oil: the pump draws from the reservoir through the suction line and pushes through the pressure line to the cylinder; on the way back, the oil passes through the directional valve again and then the filter.

While it makes this round trip, the oil does five jobs at once:

• it transmits force from the pump to the cylinder;

• it lubricates the parts that slide inside the pump, the valves and the cylinder, and seals the gaps between them so pressure does not leak through the inside;

• it carries heat from the circuit to the reservoir;

• it protects the metal from rust;

• it carries dirt to the filter, and air and water to the reservoir, where they can separate.

All five apply to every industrial hydraulic oil, from the simplest to the fire-resistant ones.

If every hydraulic oil does the same five jobs, one question is left: why so many types?

Why are there different types of hydraulic oil?

Because each circuit punishes the oil in its own way, and each type brings protection against the attack that weighs most in that system.

Almost everything in a hydraulic oil is base oil, the liquid that makes up most of a lubricant: mineral (from petroleum), synthetic or vegetable.

The rest is a small share of additives: chemicals that give the oil a quality it would not have alone or reinforce one it already has. The article on what base oils are explains where this base comes from.

Inside the circuit, the oil faces seven enemies:

1. Heat and oxygen. Oxygen from the air reacts with hot oil, which thickens, darkens and forms sludge, a sticky mud. This chemical reaction is called oxidation.

2. Moisture and air. Humid air attacks metal and rusts it: this is corrosion.

3. Entrained air. The oil carries air when it splashes agitated into the reservoir, or when the suction line pulls air through a loose joint or a low tank level. Bubbles and foam form, like a shaken soda: the air is compressed and the cylinder moves in jerks. Good oil releases the air in the reservoir, and the antifoam additive breaks down the foam on the surface.

4. Pressure. Between the parts that slide, what keeps metal from touching metal is a thin layer of oil, the oil film. The pump pressure squeezes this layer, and where it breaks, the metal wears.

5. Temperature. The thickness of the oil changes with heat. This thickness is called viscosity: how much the liquid resists flowing. Honey from the fridge flows slowly; warmed up, it flows fast.

6. Water in the oil. It gets in through condensation (the humidity of the air that turns into drops inside the reservoir when it cools), through a leaking water cooler or through splashes of cutting fluid, the liquid that cools machining.

7. Fire. Near a furnace or molten metal, a leak can catch fire.

Each protection fights one enemy, and almost all of them come from additives. There are two exceptions. Resistance to the change of thickness with temperature can also come from the base oil, and resistance to fire calls for another base (a synthetic fluid or a water-based one), which appears in the fire section. Among mineral types, then, the difference lies mainly in the additives.

And these protections are written in the name of the oil, letter by letter.

What does each letter of DIN 51524 mean?

Each letter of the name indicates one protection, and the letters, in general, add up: HLP has everything HL has, plus protection against wear.

A standard is a public document that lists the tests (laboratory trials) an oil must pass in order to carry a name. DIN 51524, a German standard, defines mineral hydraulic oils in three parts. Each letter answers one of the enemies above:

• H: hydraulic.

• L: protects against oxidation and corrosion.

• Air and foam (no letter of their own): the standard requires the oil to release air and not foam, already from HL on.

• P: antiwear, or AW, from anti-wear. It protects the metal where the oil film breaks.

• V: viscosity that changes less with temperature.

• D: detergent. It keeps dirt loose in the oil until the filter and keeps water spread in drops so fine that it does not gather in a pool at the bottom, where rust begins. It is the exception to letters that add up: D swaps fast water separation for water held in fine drops.

Fire sits in another group, with another standard. Fluids that are harder to burn carry the letters HF, from fire-resistant. There, the D has nothing to do with the detergent of HLPD. In the code HFDU, the D marks a synthetic fluid without water, and the U, a composition that is not a phosphate ester, usually an ester, a synthetic made from acid and alcohol.

Figure 2. The letters, in general, add up: each one adds a protection (D swaps water separation for water in fine drops). HFDU stands apart, in another group.

For those who want the technical name: HL is in part 1, HLP in part 2 and HVLP in part 3 of DIN 51524. All three require low foam and fast air release (DIN ISO 9120). HLPD does not appear in the standard; it is a market name for an oil that meets part 2 minus demulsibility, the ability to release water. ISO 6743-4 calls HLP "HM" and HVLP "HV" (see the frequently asked questions).

The letters, however, are only half of the name of the oil.

What does the number on hydraulic oil mean, as in HLP 68?

The letters tell the protections; the number tells the thickness of the oil. It is the ISO VG viscosity grade: the middle value of a range of thicknesses, measured with the oil at 40 °C, a reference temperature. Higher number, thicker oil.

Thicker oil forms a thicker layer between the parts; thinner oil, a thinner layer.

Most industrial systems use 32, 46 or 68. The right number is the one that keeps the oil, already hot inside the pump, in the thickness range the pump manufacturer accepts, without being too thick at a cold start.

• A system that runs hotter calls for a higher number, so the film does not thin out.

• A machine that starts cold calls for a lower number, so the pump can draw the oil.

The decision belongs to the machine manual or the pump manufacturer. The article on the difference between hydraulic oil 46 and 68 shows this choice in a common case.

For those who want the technical name: the grade measures kinematic viscosity in cSt (centistokes). Under ISO 3448, the grade is the midpoint of a range that goes 10% above and below: ISO VG 68 lies between 61.2 and 74.8 cSt at 40 °C.

The range the pump accepts applies to the oil at the temperature where it works, so the check is made with the oil already hot.

With the letter and the number understood, we can see what happens when one of them is wrong.

Why does a hydraulic pump lose efficiency with the wrong oil?

Because, with the wrong oil, the pump wears from the inside or lets oil escape where it should not, and delivers less to the machine.

To see how, look at the parts. In the vane pump, blades fitted in slots of a rotor turn with it and slide loose, resting against a ring. In the piston pump, small pistons run inside bores of a rotating block, and a tilted plate makes each piston go in and out of its bore on every turn.

Between these parts, only the oil film separates metal from metal. The wrong oil attacks this film in three ways.

Figure 3. Inside the two pumps: in each one there is a point where only the oil film separates metal from metal.

No letter P in a high-pressure pump. The damage comes as a chain:

1. Pressure thins the film between the vane and the ring (or between the piston and the bore), which leads to

2. the film breaking at one point and metal touching metal, which leads to

3. metal particles torn off and loose in the oil, which leads to

4. a larger internal clearance and internal leakage: oil escapes from the high-pressure side back to the low-pressure side, which leads to

5. a drop in volumetric efficiency: the pump delivers less flow (less oil per minute) at the same speed, and the oil that leaks inside turns into heat. The machine gets slow before it breaks.

The antiwear additive acts at the second step. Where the film breaks, the heat of friction makes the additive form a thin layer bonded to the metal, the sacrificial film. It wears away in place of the part and renews itself as long as there is additive in the oil. This is one reason the additive content falls with use; heat and water also consume it.

Figure 4. Where the oil film breaks: without antiwear additive, the metal wears; with it, the sacrificial film wears in place of the part.

For those who want the technical name: the classic antiwear additive is ZDDP, a compound of zinc, phosphorus and sulfur, which comes back in the laboratory section. With it, the film is a layer of zinc and iron phosphates.

Number too low for the heat. Thin oil slips through the pump clearances, and the pump delivers less from day one.

Number too high at a cold start. Thick oil is slow to reach the pump, and the damage comes in steps:

1. Pressure drops at the pump inlet.

2. Air dissolved in the oil comes out as bubbles, like in soda when you open the bottle.

3. If pressure drops even further, the oil itself starts turning to vapor, the same way water boils at a lower temperature on a mountaintop, where pressure is lower.

4. When the vapor bubbles reach the pressure zone, they collapse all at once and tear pieces out of the metal. This damage is called cavitation.

5. Air bubbles do not collapse like vapor bubbles: they are compressed and heat up, and this heating darkens the oil and speeds up oxidation. Oil that does not release air well makes things worse.

Figure 5. At the suction side, low pressure makes bubbles appear; in the pressure zone, the vapor ones collapse and mark the metal (cavitation).

How this shows up in your plant: the cylinder takes longer to close the press at the same setting, and the oil in the reservoir runs hotter than normal. The filter clogs ahead of schedule, with shiny metal particles on the element, and the pump gets noisier.

For those who want the technical name: strictly speaking, aeration is the presence of air bubbles in the oil, whether entrained air or dissolved air that comes out at low pressure. Cavitation is the formation and collapse of vapor bubbles of the oil itself.

Now you can choose the type by the condition of your system.

Which hydraulic oil should you use in each system?

Use HLP in most machines and change type only when one condition dominates the system. The table shows the condition, the type and the Kelpen Oil line; the number that follows each line is the ISO VG grade of the products.

DIN 51524 has no requirements for the highest grades of the Supra and Supra AW lines, such as ISO VG 320. For those grades, talk to the Technical Team to confirm the type.

Three choices tend to raise questions.

HL or HLP?

HL is enough for circulation and for bearings without severe pressure, or when the machine manual calls for HL. A pump under high pressure needs the letter P. In the Supra line, Supra 68 RED is an HL (DIN 51524 part 1) in red, which helps find leaks.

Cold and heat in the same system: HVLP

All oil thins in the heat and thickens in the cold. The viscosity index (VI) tells how much: the higher the index, the smaller the change. HVLP, with a high VI, stays pumpable at a cold start and keeps the film in the heat. A plain HLP, in the same system, would be too thick in the cold or too thin in the heat.

A high VI can come from an additive, the VI improver, made of long molecules. It can also come from the base oil itself: mineral base oils with more severe refining, called Group III, are born with a high VI (the article on base oils explains the classification).

Figure 6. From cold to heat, HVLP changes less in thickness and stays inside the range the pump accepts.

For those who want the technical name: mineral base oils of Groups I and II have a VI from 80 to less than 120, and Group III starts at 120 (API 1509, Annex E). Part 3 of DIN 51524 includes a shear stability test, the cutting stress inside the pump. This stress can break the long molecules of the VI improver and make the oil thin out for good.

A little water you cannot drain: HLPD

The detergent in HLPD keeps dirt loose until the filter, which holds it back, and keeps the water in very small drops spread in the oil, which is called an emulsion. The filter lets this water through. It works when little water gets in, by condensation or cutting fluid splashes, and the reservoir has no frequent drain.

HLP does the opposite: it has good demulsibility, the ability to release water, which sinks to the bottom of the reservoir and leaves through the bottom valve, the drain.

HLPD has limits:

1. It holds only a little water. Beyond that, the water separates as in any mineral oil.

2. Even spread out, water ages the oil faster and shortens bearing life. That is why the water limit the pump manufacturer accepts still applies.

3. Water held in fine drops does not leave through the drain: only dehydrating the oil (by vacuum, for example) or changing it removes it. The part that goes past the limit and separates does sink to the bottom.

4. It carries dirt to the filter, so the filter area must cope.

5. With a lot of water, the way is HLP with good demulsibility and a drain at the bottom of the reservoir.

For those who want the technical name: HLPD holds about 1 to 2% of water in fine drops, according to hydraulic equipment manufacturers.

The seventh enemy is left, fire, which calls for a fluid from another group.

When should you use a fire-resistant hydraulic fluid?

When a leak under pressure can meet a hot surface or flame: near a furnace, an ingot mold (the mold where liquid steel solidifies) or a die casting machine.

The greatest danger comes from the small hole: in a high-pressure hose, it sprays the oil into a mist, and a mist of mineral oil catches fire easily.

Three temperatures explain why one fluid burns less easily than another:

• Flash point: the temperature at which heated oil gives off enough vapor to flash when a flame passes.

• Fire point: the temperature at which the flame lights and stays lit.

• Autoignition temperature: the temperature at which the oil catches fire on its own, without a flame, only from the heat of the air or of a surface around it.

Ester, the base of HFDU, needs more heat to give off vapor that burns and to ignite on its own on a hot surface. In a mist, the flame tends not to hold when the heat source goes away.

Figure 7. The same mist before the flame: in mineral oil, the fire keeps going; in HFDU, it tends to go out when the source goes away. It burns less easily, but it is still combustible.

HF fluids, in ISO 6743-4 (the international standard that classifies hydraulic fluids), are divided into two groups: those with water and those without. In water-containing fluids, it is the water itself that makes burning harder; HFC, water with glycol and polymer, is common in die casting. Fluids without water are synthetic, like HFDU.

HFDU needs no control of the water content or topping up with water, as aqueous fluids do, and it lubricates in a way closer to that of an antiwear oil. In return, it must stay dry: water slowly breaks the ester down, which is called hydrolysis, and the fluid turns acidic.

Fire-resistant, here, means the fluid burns less easily. It is still combustible, and protection of the hot spot still applies.

We at Kelpen Oil formulate Supra HFDU FR 68, a hydraulic fluid of the HFDU class based on organic esters, with no water and no mineral oil, made to replace antiwear mineral hydraulic oil where there is a fire risk. Converting a system from mineral oil to HFDU calls for draining, cleaning the circuit and checking seals, the inner paint of the tank and filters with the Kelpen Oil Technical Team. Mineral oil left in the circuit reduces the fire resistance of the new fluid.

For those who want the technical name: ISO 12922 classifies as fire-resistant, under the ISO 5598 definition, the fluids with water (HFAE, HFAS, HFB and HFC) and HFDR. The same standard says most HFDU, although better than mineral oil, fall outside that definition and are better called "less flammable".

The figure below puts it all together: the same circuit from the start, now with each protection at the point where it works.

Figure 8. The same circuit, with each protection at the point where it works: antiwear at the pump (HLP), demulsibility (HLP), air release and antifoam in the reservoir, detergent to the filter (HLPD, instead of releasing water), viscosity index in the whole circuit (HVLP) and fire resistance near a hot source (HFDU).

One last question remains: are two oils with the same letters the same?

What does DIN 51524 require and how does Kelpen Oil check it?

DIN 51524 requires a battery of tests, and the letters on the drum are the manufacturer's declaration that the oil reaches the minimum in each one. The real result shows up in the test, and that is why two HLP 68 from different manufacturers are not automatically equal.

Among other requirements, the standard asks for resistance to oxidation, water separation, control of foam and air, and corrosion protection. In HLP and HVLP, it also asks for antiwear: the FZG test, on a gear machine that raises the load in stages until damage appears, and a test in a vane pump, the same one of the wear mechanism seen above.

In our own laboratory in Arujá (SP), no batch of the Supra line is released without approval. Part of these behaviors is measured there: demulsibility, foam, copper strip corrosion and viscosity.

For oil in use, the analysis comes in a report, the document with the results. Two cautions in reading it:

1. Demulsibility is read in reverse depending on the type. In HLP, water has to leave the oil; in HLPD, it stays by design. Read with the ruler of the wrong type, the result condemns good oil or lets bad oil pass. The test is the same (NBR 14172).

2. Phosphorus and zinc show the additive. ICP, an instrument that measures chemical elements in the oil (NBR 14786), tracks the zinc and phosphorus of ZDDP, the classic antiwear additive, which forms the sacrificial film. Falling content indicates consumption of the additive (by use, heat and water) or dilution by another oil.

For those who work with the standard, these are the tests of the used-oil report that the laboratory performs as a paid service:

The same laboratory serves outside companies, as a paid service. You can send the hydraulic oil you use today, from any manufacturer, and receive a report with viscosity, water content, acidity and particle count. It is a direct way to know whether the problem in the system is in the oil or somewhere else.

Frequently asked questions

What is the best hydraulic oil? The one that matches the condition that punishes your system most, in the ISO VG grade that the temperature calls for. In most machines, it is an HLP.

What does AW mean in hydraulic oil? AW comes from anti-wear: it indicates the oil has an additive against wear. "AW 68" is a commercial name; what defines the tests is the class in the standard, such as HLP in DIN 51524 part 2.

Is HM hydraulic oil the same as HLP? In practice, yes. ISO 6743-4 calls HM what DIN 51524 calls HLP, and HV the HVLP. The "L" in "L-HM 46" indicates the class of lubricants in ISO. ISO 6743-4 groups hydraulic fluids in the H family. HLPD has no ISO category.

Can you mix hydraulic oils of different classes? It is not recommended. An HLP mixed with an HLPD tends to lose its ability to release water, and different additives can form deposits in the filter. When changing class, drain the system and, if there is sludge, clean it first with flushing oil.

Is biodegradable hydraulic oil the same as HFDU? Not necessarily. Biodegradability has its own standard, ISO 15380; fire resistance, ISO 6743-4 and ISO 12922. One ester can meet both standards, but each asks for its own tests: being HFDU does not prove biodegradability, and being biodegradable does not prove fire resistance.

Glossary

• Oxidation: reaction of oxygen from the air with hot oil, which thickens, darkens and forms sludge.

• Viscosity: how much a liquid resists flowing; the thickness of the oil.

• Antiwear (AW): additive that protects the metal where the oil film breaks; it is the letter P in HLP.

• ISO VG: viscosity grade of the oil, the middle value of the range at 40 °C (ISO 3448).

• Viscosity index (VI): how much viscosity changes with temperature; the higher the index, the smaller the change.

• Demulsibility: the ability of the oil to release water.

• Cavitation: vapor bubbles that collapse in the pressure zone and tear out metal.

Talk to the Technical Team

The Kelpen Oil Technical Team evaluates the pump type, working pressure, start-up and operating temperature, the presence of water, the contamination history and the fire risk, and confirms the right type and grade for each system. If the oil is already in use, the Arujá (SP) laboratory performs the used-oil analysis, as a paid service. Save this article or send it to whoever handles hydraulic maintenance at your company.