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

Mineral hydraulic oils fall into three classes of the German standard DIN 51524: HL (protects against oxidation and corrosion), HLP (with an anti-wear additive, the industry standard) and HVLP (an HLP whose viscosity changes less with temperature). HLPD is a detergent HLP that holds water and dirt in suspension. HFDU, based on ester (a synthetic oil made from acids and alcohols), is for fire risk. Choose the class by the condition that punishes the system most, and the grade by the operating temperature.

Hydraulic oil works like the brake fluid in a car: the foot pushes the fluid, and the fluid pushes the pad. In a plant, the pump pushes the oil, and the oil moves the press cylinder or the injection molding machine motor. Industrial hydraulic oils differ from one another in their additive package, and each additive exists to prevent a specific failure in the circuit.

Which hydraulic oil should you use in each system?

Use HLP in most industrial machines and change class only when one condition dominates: a wide temperature swing calls for HVLP, a little water without frequent draining calls for HLPD, and a fire risk calls for HFDU. In the product column, the ISO VG number is the viscosity grade: the thickness of the oil at 40 °C, explained right below the table.

The class defines the chemistry; the ISO VG grade defines the thickness of the oil. ISO VG is the mean kinematic viscosity at 40 °C, in cSt (centistokes, the unit of that measurement), with a tolerance of 10% up or down under ISO 3448: an ISO VG 68 falls between 61.2 and 74.8 cSt at 40 °C. ISO VG 32, 46 and 68 cover most industrial hydraulic systems, almost always in HLP oil. The hotter the system runs, the higher the grade needed to keep the oil film. The colder the start and the higher the speed, the lower the grade, so the pump suction is not strained. When in doubt, follow the grade the pump manufacturer specifies.

What is hydraulic oil for?

Hydraulic oil transmits power: it carries the pressure from the pump to the actuators, which are the hydraulic cylinders and motors. At the same time, it does four other jobs:

• lubricates the sliding parts inside the pump, valves and cylinders;

• takes heat out of the circuit and releases it in the tank;

• protects the metal parts against corrosion;

• carries contaminants (particles, water, air) to the filter and the tank.

Each class of hydraulic oil reinforces one of these jobs, and Figure 1, further down, shows where in the circuit each property acts. In a car's automatic transmission the fluid also works as a hydraulic fluid, but with friction requirements of its own: that case is covered in the article on ATF hydraulic fluid.

What does each DIN 51524 designation mean?

In the mineral classes, each letter adds a property to the previous one: H for hydraulic, L for protection against oxidation and corrosion, P for an anti-wear additive, V for a high viscosity index and D for detergent.

HL: circulation and bearings

Where the main duty is circulation and bearing lubrication, the HL class requires protection against oxidation and corrosion. Oxidation is the aging of the oil caused by oxygen and heat, which thickens the oil and forms sludge.

HLP: severe pressure

HLP is HL oil with an anti-wear additive, under DIN 51524 part 2, and it serves the great majority of industrial systems, including pumps under severe pressure.

HVLP: temperature swings

The viscosity of every oil drops with heat and rises with cold. The viscosity index (VI) measures how much it changes: the higher the VI, the smaller the change. A conventional mineral oil, made with Group I and II base oils (the American Petroleum Institute base oil classification, in API 1509), has a VI between 80 and 120. The Supra HP line is formulated with a high viscosity index. In practice, HVLP stays pumpable at a cold start and keeps the oil film at hot operating temperature, where an ordinary HLP would be too thick when cold or too thin when hot.

HLPD: a little water, no draining

The detergent and dispersant additive keeps water finely emulsified (mixed in very small droplets) and contaminants in suspension until the filter, instead of letting them settle in the tank and in the valves. It is the choice when a little water gets in, through condensation or cutting fluid splashing into a machine tool, and the tank is not drained often. DIN 51524 does not define HLPD: the letter D comes from DIN 51502, the standard for lubricant designations, and HLPD oil usually meets the HLP requirements of part 2, except for demulsibility (the ability to separate water), because by design it keeps water dispersed. The Supra AWD line has an ashless additive package (no metallic additive, such as zinc), according to the technical bulletin.

There are limits. Above about 0.1% water, HLPD loses protection and ages faster. Because it carries dirt to the filter instead of leaving it at the bottom of the tank, check that the filter area can cope. When a lot of water gets in, the way out is different: HLP with good demulsibility and a drain at the bottom of the tank, because separating and draining the water is better than living with it.

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

Because the wrong oil reaches the pump in two ways. With a viscosity grade too low for the operating temperature, the oil leaks through the internal clearances and the pump delivers less flow from day one. With oil that has no anti-wear additive in a high-pressure pump, the oil film breaks and the metal wears, and the clearance grows over time. At the other extreme, oil that is too thick at a cold start strains the suction and can cause cavitation: with low pressure at the suction, vapor and air bubbles form, and they implode when they reach the pressure side of the pump, tearing material off the parts. The wear sequence is well known:

1. High pressure thins the oil film between vane and ring, or between piston and bore, to its limit, which leads to metal-to-metal contact at the critical points of the pump.

2. The contact tears material off vanes, plates and pistons (adhesive wear), which leads to metal particles circulating in the oil.

3. Wear widens the internal clearance, which leads to internal leakage: part of the oil flows back from the pressure side to the suction side.

4. Internal leakage lowers the volumetric efficiency (the pump delivers less flow at the same speed) and turns into heat.

5. The system slows down before it breaks, and the lost production costs many times more than the price difference between the oils.

The anti-wear additive breaks this sequence at the first step. It reacts with the heated metal at the contact point and forms a sacrificial chemical film exactly where the oil film breaks, and it is this film that wears in place of the part.

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

Figure 1. The oil path in the hydraulic circuit: the pump draws oil from the tank and sends it under pressure to the directional valve, which connects the pressure (P) to the cap-end chamber of the cylinder (A) and the rod-end chamber (B) to the return (T); the return passes through the filter and goes back to the tank, and the relief valve protects the pressure line. At each point, the property of the class that works there: anti-wear at the pump (HLP), demulsibility and anti-foam in the tank, detergency up to the filter (HLPD), viscosity index in the whole circuit (HVLP) and fire resistance near a hot source (HFDU).

When should you use a fire-resistant hydraulic fluid?

Use a fire-resistant fluid when a leak under pressure could meet a hot surface or a flame, such as near a furnace, an ingot mold (the mold where liquid steel solidifies) or a die casting machine. A small hole in a high-pressure hose sprays the oil as a mist, and mineral oil mist ignites easily. At these points, the fire-resistant fluid exists to reduce the fire risk.

ISO 6743-4 splits these fluids into two groups, and ISO 12922 sets the requirements for each:

• with water: HFAE and HFAS (water with a little oil or additive), HFB (water-in-oil emulsion) and HFC (water and glycol);

• water-free: HFDR (phosphate ester) and HFDU (other synthetics, usually esters).

HFDU works without water, so it does not need the water-content control that water-based fluids require, and it lubricates in a way closer to an anti-wear oil. Fire-resistant means the fluid burns less readily than mineral oil; it is still combustible, and ISO 12922 itself treats most HFDU fluids as "less flammable" fluids. Protection of the hot point still applies.

Kelpen Oil's Supra HFDU FR 68 is formulated with organic esters, completely free of water and mineral oil, and was developed to replace mineral anti-wear hydraulic oils in applications with a fire risk. The bulletin records corrosion protection and anti-wear performance. According to the bulletin, it can also be used where environmental protection is a process requirement. Converting a system from mineral oil to HFDU calls for draining, cleaning the circuit and checking the seals with the Kelpen Oil Technical Team.

What is the best hydraulic oil?

The best hydraulic oil is the one that matches the dominant condition of the system: HLP for pressure, HVLP for temperature swings, HLPD for a little water without draining and HFDU for fire risk, always at the viscosity the operating temperature calls for. That is why the question only has an answer when it comes with another one: which condition punishes your system most? Pressure, variable temperature, water or fire. Each answer leads to a different designation.

Within the same class, the designation guarantees the standard's minimum. Two HLP 68 oils from different manufacturers are not automatically equivalent: the standard sets the minimum, and testing shows the real result. In the same machine, other points call for another oil: the gearbox, for example, uses an extreme pressure (EP) additive, the subject of the article on choosing oil for an industrial gear reducer.

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

DIN 51524 defines a battery of tests. An HLP demonstrates, among other requirements, anti-wear protection (the standard calls for the FZG gear test and a vane pump test), demulsibility, foam behavior and corrosion protection. In the in-house laboratory in Arujá (SP), no batch of the Supra line is released without approval, and part of these behaviors is measured there: demulsibility, foam, copper strip corrosion and viscosity.

One caution when reading the report: the result is interpreted by the class of the oil.

• Demulsibility: the same test (NBR 14172) has opposite readings. In HLP, water must separate from the oil in a short time. In HLPD, water stays dispersed by design. An HLPD read with the HLP criterion looks like a failure without being one; an HLP that has stopped separating water goes unnoticed if read as HLPD.

• Anti-wear additive: ICP, a spectrometer that measures chemical elements in parts per million (NBR 14786), tracks phosphorus and zinc, the elements of the classic anti-wear additive in HLP oils (zinc dithiophosphate). A falling level in oil in service suggests additive depletion or dilution with another oil. Zinc showing up in a system filled with ashless oil may come from mixing with another product or from wear of brass parts; the copper level, which follows brass wear, helps tell one case from the other.

The same laboratory serves outside companies: any company can send the hydraulic oil it uses today, from any manufacturer, and receive a technical report with viscosity, water content, acidity and particle contamination. It is a paid service, and it is the fastest way to find out whether the system's problem is in the oil or somewhere else.

Frequently asked questions

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

Is HM hydraulic oil the same as HLP? They correspond in function. The ISO 6743-4 classification and the ISO 11158 specification call an oil with anti-wear HM and an anti-wear oil with a high viscosity index HV; DIN 51524 calls them HLP and HVLP. The "L" in "L-HM 46" indicates the lubricant class in ISO. Each standard has its own tests, so use the one the machine manual cites.

Can you mix hydraulic oils of different classes? It is not recommended. An HLP mixed with an HLPD tends to lose its ability to separate water, and different additive packages 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 is classified by ISO 15380 (categories HETG, HEPG, HEES and HEPR); fire resistance, by ISO 6743-4 and ISO 12922. An ester fluid can combine both properties, but each is proven with its own test.

Glossary

• ISO VG: viscosity grade of the oil, equal to the mean viscosity at 40 °C, in cSt (ISO 3448).

• Viscosity index (VI): a number that shows how much viscosity changes with temperature; the higher it is, the smaller the change.

• Anti-wear additive (AW): a compound that forms a sacrificial film on the metal where the oil film breaks.

• Demulsibility: the ability of the oil to separate water, which then settles at the bottom of the tank.

• Detergency and dispersancy: the ability of the oil to keep dirt and water in suspension, without depositing in the circuit.

• Volumetric efficiency: the flow the pump actually delivers, compared with the theoretical flow at the same speed.

• Flushing: washing the circuit with cleaning oil before filling with new oil.

• Ester: a synthetic base used in fire-resistant fluids and in fluids for applications with environmental requirements.

Talk to the Technical Team

The Kelpen Oil Technical Team evaluates pump type, working pressure, start-up and operating temperature, the presence of water, contamination history and fire risk, and confirms the right class and grade for each system. If the oil is already in service, 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.