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Hydraulic Fluids

There are many types of hydraulic fluid used today each type has its own distinctive attributes and applications for use. Some of the hydraulic fluids on the market today are:

• Petroleum Oil – used in most applications
• Fluids With High Water Content – primarily used where there is a high risk of fire.
• Invert Emulsion Fluids – is a petroleum base fluid with up to 40% water content, which acts as an oil.
• Glycol Based Fluids – mixed with water and are used in extreme cold conditions.
• Synthesized Hydrocarbon Hydraulic Fluid- used in applications where there is a wide range of temperature from hot to cold.
• Vegetable or Grain Oils – used in food or pharmaceutical industries.

Selecting The Right Fluid

Most hydraulic systems will function adequately using a range of fluids. But which type of fluid is best for a particular application? The type of hydraulic fluid to choose is based on its quality and the fluids capacity to perform the required function. The most widely used is petroleum based fluids, however due to its flammability the application for its used needs to be taken into consideration and used where the fire hazard risk of low. While it is not possible to make one definitive suggestion that covers all types of hydraulic equipment in all applications, the following are some of the factors to consider when selecting a hydraulic fluid.

• Viscosity – its rate if pour at a given temperature. The recommended viscosity for the system will be based on the pump manufacture’s specifications.
• Viscosity Index – the fluids change in viscosity over a given temperature range.
• Pour Point – this is the lowest temperature at which the oil will flow. The pour point should be approximately 20 Deg. F. below the lowest temperature that is expected.
• Thermal Stability – this is the fluids ability to resist chemical and physical changes at high temperatures.
• Resistance to Oxidization – at high temperatures and pressures hydraulic fluid can oxidize very quickly and can produce soluble and insoluble products. Products such as acids and sludge. Inhibitors are added to counteract this effect.
• Resistance to Rusting – when water is present in the system the ferrous parts are susceptible to rusting. The water is formed when the air that is entering the system reservoir condenses. Inhibitors are added to counteract this effect.
• Resistance to Air Foaming – the oil in the system does contain air in solution and when the pressure is greater the air is absorbed more. When air is being compressed it generates heat, so when oil that has air bubbles suspended in it will generated heat as it get compressed during pumping action. Therefore when the pressure is decreased the air comes out of the oil and produces foaming. Anti-foaming additives are added to aid in a faster separation of air and oil, to allow the bubbles to break away.
• Lubricating Qualities – the mechanical moving parts of the system require lubrication as well, therefore the oil that is selected need to assist in this function as well. The oil need to maintain an oily film between all the contact surfaces, regardless of the temperature and pressure. Extreme Pressure (EP) additives are used when there is a increase of temperature or pressure or metal to metal contact.
• Long Life – this is the life span the oil has, between system oil changes.

Cost – oil that has a longer life span may cost more than other oils, but over time can be a beneficial cost savings especially if the system has a large volume of oil. The other point to consider too is the cost to have the system shut down in order to perform the system oil change.

Fire Resistant Fluids

Some hydraulic systems have to operate in harsh environments. They could be located in a foundry where the surrounding equipment is extremely hot or flames are present. In this situation fire-resistant oils need to be considered. There are four basic types of fire-resistant hydraulic fluids:

High Water Content Fluids (HFA)

HFA fluids are soluble oil or chemical mixture (oil in water). They contain low amounts of oil around 5-10%. When looking at the solution you will notice that the oil in water will appear to be cloudy and white (milky). The synthetic blend will appear to be more, clear. One of the main beneficial factors to this type of fluid is the cooling abilities it has.

Since these types of fluids are primarily comprised of water, it in important to operate them a low temperature to avoid excessive evaporation and deterioration (Maximum 40 deg. C, 120 deg. F). Another important factor to keep in mind regarding temperature is to prevent the fluid, from freezing. If the solution freezes it could result in the separation of the oil and water mixture. Additives are integrated in the mixture to assist in rust prevention, fluid separation and also, in aiding with lubrication.

Due to the properties of these fluids, some seals, metals and protective coatings are not compatible. Be sure to refer to the manufactures information sheet.

Water in Oil (HFB)

HFB fluids are invert solutions and contain more oil than water. In most instances they only contain approximately 40% water. A regular inspection of the fluid needs to be maintained. As time goes on the ration of oil and water will change due to evaporation. This in turn will influence the fluid viscosity.

Much like the HFA fluids, HFB fluids have relatively the equivalent operating temperatures and additives.

Water Glycol (HFC)

HFC fluids are made up from 30-40% water that is dissolved in Glycol. These fluids too need a regular maintenance regime, to maintain the correct water-glycol ratio.

Glycol is from the same family as that of permanent anti-freeze ethanol. This is what makes them such an excellent choice for environments that are cold and therefore resist freezing. The additives that are formulated for these fluids have lubricating and anti-foaming properties along with improved viscosity.

Pump selection and system design are important to keep in mind, if this type of fluid is being implemented. Due to the characteristics of HFC fluids, suction lift needs to be taken into consideration, because these fluids are heavier than oil and a shorter suction lift is required. The system might need to have a special inlet design or the fluid level may need to be above the pump inlet.

Newer synthetic seals available such as neoprene and Buna-N are compatible with petroleum oil, are also HCF compatible. HFC however does not react well with galvanized (Zink), cadmium-coated (bright-finish), parts and most of the common paints used in hydraulic systems. HFC will attack these finishes and painted surfaces. They will also attack magnesium, aluminum and die-cast alloys.

Synthetic (HFD)

HFD fluids are comprised of various special compounds. These compounds are designed not to be combustible. These fluids are used for high temperatures, because they do not contain water and thus do not evaporate. One major key factor about HFD fluids is there weight. They are a heavy fluids and require special pump inlets or key pump placement. Due to the chemical characteristics of these fluids, seal selection is very important.

The fluid will attack and destroy seals that are designed for petroleum based fluids. If the system is getting converted to synthetic fluid, cost analysis should be taken to calculate, replacement of seals thought out the entire system, and bearing in mind that the system must be completely flushed out to eliminate any excess petroleum fluids, or other fluid types, the system tank then must be painted with a special epoxy paint.

Note: Do not forget about the spare valves that in stock these too need to have the seals changed.