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Maintenance of Hydraulic Oils with

Best Available Technology

WHITE PAPER © 2017 AGC Refining & Filtration LLC

AGC REFINING & FILTRATION

MAINTENANCE OF PHOSPHATE ESTERS-BASED FLUIDS 2

Contents General 3

Types of Hydraulic Fluid 3

Applications 4

Contamination 5

Purification and Reuse 6

Best Available Technology 6

Maintenance of Hydraulic Fluid 7

Control of Fluid Quality 8

Summary (for Synthetic or Petroleum-based Fluids) 10

References 10

Appendix 1: The Case of Phosphate Esthers 11



General

Designing equipment that requires fluid power requires consideration of factors such as:

Speed and accuracy of operation

Surrounding environmental conditions

Availability of replacement fluid

Operating pressures and temperatures

Safety

Life expectancy of the equipment

When selecting the hydraulic fluid, many similar factors are considered. Other important fluid properties are:

Viscosity

Lubricating properties

Chemical stability

Acidity

Flashpoint

Toxicity

Density and compressibility

Foaming tendency

Cleanliness

Currently, fluids being used include mineral oil, phosphate ester, water-based ethylene glycol compounds, and silicone. The three most commonly used hydraulic fluids are petroleum-based, synthetic fire-resistant, and water-based fire-resistant.

Types of Hydraulic Fluid

Petroleum-Based Fluids

Most mineral oil hydraulic fluids are made from dewaxed paraffin-based crude oil.

These fluids contain numerous additives that protect system metals from corrosion, reduce foaming tendencies, and improve viscosity, among others. They are flammable under normal conditions and can become explosive when subjected to high pressure and a source of ignition.

The carbon number range will vary between C15 to C50, depending on the application. The higher the carbon number, the higher the viscosity. More highly refined oil will have better viscosity properties, i.e. higher viscosity index.

Synthetic Fire-resistant Fluids

Phosphate Ester Fire-resistant Fluids

The vast majority of industrial organophosphate esters are based on triaryl phosphates without halogenated compounds.



These fluids will burn if sufficient heat and flame are applied, but they do not support combustion. Drawbacks are that they will attack and deteriorate many types of insulation, seals, and paint. In addition, organophosphate esters are also used as anti-wear additives in hydraulic fluids.

Silicone Synthetic Fire-resistant Fluids

These are used for hydraulic systems that require fire resistance but have only marginal requirements for other chemical or physical properties common to hydraulic fluids. They do not have the effects of phosphate ester fluids, nor do they provide the corrosion protection and lubrication of phosphate esters.

Light-weight Synthetic Fire-resistant Fluids

In applications where weight is critical, these fluids are used. They are generally of the low-viscosity type fluids most used in aviation support equipment.

Polyalphaolefins

A typical Polyalphaolefin is a combination of a large number of isomers. They are often classified by their kinematic viscosity at 100°C. The higher the viscosity, the longer the average chain length of the Polyalphaolefin is. These fluids are known for a high viscosity index, high oxidative stability, and relatively longer service life.

Water-based Fire-resistant Fluids

These are classified as water-glycol mixtures and water-synthetic base mixtures. They contain additives that protect against oxidation, corrosion, and microbiological growth. The fire resistance of these fluids depends on the vaporization and the smothering effect of steam generated from water.

These types of fluid are not amenable to purification by high-vacuum dehydration and will not be discussed further.

Applications

Mineral Oil Fluids

These are used most widely in numerous industrial applications such as:

Automobile automatic transmissions

Power steering units

Elevators

Farm equipment

Mining equipment

Organophosphate esters

These hydraulic fluids are used where fire resistance is needed such as:

Aircraft systems

Marine systems

Electrohydraulic steam turbine controls

Aircraft carrier hydraulic systems

Heat transfer fluids



Polyalphaolefins

Polyalphaolefins (PAO) have properties comparable to the most effective components in mineral oil and are used in identical applications. In addition these are used in:

Aircraft

Missile hydraulic systems

Tank and large gun recoil and hydraulic systems and aerospace test stands

Contamination

Contaminants in hydraulic fluids are of three types: solid, liquid, and air. The result of contamination can be:

Abrasive damage

Component leakage

Decreased efficiency

Rapid filter clogging

Internal corrosion

Premature system failures

Solids Contamination

Since tolerances between mechanical components in modern hydraulic systems are sometimes extremely tight, solid contamination is one of the most prevalent causes of malfunction and wear.

Wear Particles

These can consist of parts of o-rings, seals, gaskets and hoses, which are semi-solid. However, wear metal particles and others such as sand and dirt can cause major damage due to their abrasive action.

Oxidation

Oxidation occurs with higher system pressures and temperatures. Oxidation products appear as organic acids, asphaltenes, gums, and varnish. These combine with other solids to form sludge, at time also changing the viscosity.

Biological growth

The presence of air and water promotes the growth of microorganisms, which in turn produces acids and sludge.

Liquid Contamination

Moisture

Water in the form of free, emulsified, and dissolved water is a serious contaminant of hydraulic systems. It can aid in the formation of acids and corrosion of metal parts. In marine hydraulic systems, salt water contamination is an added hazard.

Solvents

Chlorinated solvents in the presence of water create highly corrosive hydrochloric acid. The resulting corrosion particles add to the sludge formation.



Air Contamination

Air can enter the system through defective seals, scored piston rods, improper maintenance or system design. Air will cause the system to become sluggish, contributes to foaming, oxidation, microbiological growth, and reduces the viscosity.

Purification and Reuse

The target is “to get the oil as clean as possible with the best available technology, while ensuring no degradation to the fluid and its additives.”

Best Available Technology (BAT)

BAT is the latest stage of development (state-of-the-art) of processes, facilities, or methods of operation that indicate the practical suitability of a particular measure for the purification of hydraulic oil.

BATNEEC is the Best Available Technology Not Entailing Excessive Cost, as defined by the U.S. Environmental Protection Agency.

Allowable limits of contamination

It follows that with BAT, the currently prevailing limits of contaminants in hydraulic fluid would seem

unrealistic. For instance, for non-emulsifying hydraulic oil (Military symbol 2075TH

and 2190 TEP) the specification for new fluid is 0.05% or 500 wppm of water.

Since 1948, Allen Filters, Inc. has easily enabled the achievement of contamination levels that are considerably lower than these current standards. This has contributed to longer fluid life, all the while using existing technology. The end result has been less waste discharge and lower operating costs. It begs the question: Why be satisfied with 500 wppm water in your hydraulic fluid if 10 wppm or less is easily achieved with very little additional cost?

The answer is that most manufacturers of purification equipment design their low-cost equipment to barely achieve current purification standards (if at all). To obtain better purification entails an incrementally higher cost and is mostly unavailable.

Best Available Technology

For a considerable time, Allen Filters, Inc. has marketed Allen Oil Conditioner as being the best available and most cost-effective technology.

The Allen Oil Conditioner system consists of a heavy-duty vacuum pump capable of maintaining in excess of 10-3 Torr of vacuum. This is coupled with a Vacuum Distillation column with patented internals that facilitate vaporization of contaminants. An integral part of the system is pre- and post-filtration of 5 micron and 0.5 micron respectively.

The system is compact, designed to operate continuously without operator attendance, and is available in many capacities.



Maintenance of Hydraulic Fluid

With proper maintenance of hydraulic fluid, the following can be achieved:

Useful life of the original batch of more than 10 years

Little or no additive depletion

Minimal addition of new fluid (topping up)

Minimal waste (sludge, used filter elements, etc.)

Minimal operator involvement

Considerable savings in cost of new fluid, disposal, storage space, etc.

It has been proven in field studies that with the proper maintenance, even in extreme climates such as that of Saudi Arabia, that this holds true. Proper maintenance consists of the following:

Frequent sampling and analyses of the fluid

Tight control of significant fluid parameters

Continuous purification by filtration and high-vacuum dehydration

Sampling and Analyses

Acceptance Testing

Hydraulic fluid distributors use tanker trucks that are used to haul many different fluids, their assurance to the contrary notwithstanding. To insure quality of the incoming fluid, a sample must be taken from every incoming batch and the results carefully examined.

As a minimum, the following analyses should be performed:

Water (ASTM D-1533, Karl Fischer Method)

Acidity (ASTM D-974)

Viscosity (ASTM D-88, Saybolt Viscosity)

Particle size distribution (by laser particle counter)

Water (ASTM D-1533)

This test is used both for vendor qualification and acceptance. To avoid misleading results, sampling procedures require:

Clean, dry, glass sample bottles with plastic caps

Precautions to prevent external moisture (rain) contamination

Speed in shipment of samples to the laboratory

Low water levels minimize corrosion, sludge, and microbiological growth. Additives are also sensitive to water contamination. Water levels higher than 10 ppm are considered too high and a cause for concern. With Allen Oil Conditioner, levels below 5 ppm can be achieved and maintained without extreme effort.

Acidity (ASTM D-974)

This test is also used for vendor qualification and acceptance. It is vitally important to always start with a low acid content oil of less than 0.01 mg KOH/g to help attain the full life expectancy of the fluid as well as of the installed equipment. When dealing with Phosphate Esters, acidity should be tightly controlled between 0.025 and 0.01 mg KOH/g) by high-vacuum dehydration. (See Appendix A: The Case of



Phosphate Esters)

Viscosity (ASTM D-88, Saybolt Viscosity)

Viscosity influences heat transfer rates and temperature rise in equipment. It also influences the speed of moving parts as well as purification rates.

ASTM D-445 measuring flow through an orifice may be used for reference.

Particle Size Distribution (PSD; laser method)

This test is used for acceptance. It details the variations in size and amounts present of solids in the fluid. It will serve as a base-line measurement to gauge effectiveness of the purification system.

Note: Specifications of phosphate esters currently on the market generally have moisture levels of around 500 ppm and acid numbers of 0.03 mg KOH/g. Water and acidity creates more acidity, which will quickly deteriorate the fluid and will require costly remediation. If customers realized the harmfulness of such levels and insisted on tighter fluid specifications, the market would be forced to supply it. However, long fluid life obviously goes counter to the prevailing concept of selling as much oil as possible.

Control of Fluid Quality

Figure 1: An Allen Oil Conditioner

Whether working with mineral or synthetic fluid, the secret to long fluid and equipment life is tight control of contaminant levels.

The Allen Oil Conditioner system will remove solids, maintain moisture levels below 5 wppm, and reduce entrained air and acidity.

With an additional Fuller’s Earth polishing filter, it will control acidity indefinitely below 0.025 mg KOH/g.

Tight control means not allowing the acidity to rise above 0.025 mg KOH/g.

While normal flow does not include Fuller’s Earth treatment, if the acidity rises above the control point, the flow must be routed through the FE filter to reduce the acidity below the control point of 0.025 mg KOH/g.



There is a good reason for this:

Synthetic fluids undergo hydrolysis in the presence of water. This reaction is auto catalytic; in other words, with water present, it propagates itself creating more acid.

Prevention of this hydrolysis is the reason water content must be kept as low as possible. The Allen HVD system maintains total water level at less than 10 ppm, usually around 2 ppm.

Thus if there is minimal water, there will be a minimal amount of acid generated.

Of course, if allowed to go out of control, the fluid will hydrolyze until it becomes unusable, usually at about 0.8 mg KOH/g.

Fuller’s Earth should not be used for treating fluids with high acid numbers (> 0.05) since at this point metal salts will leach out and create scum and foaming.

Some manufacturers claim to be able to recover the fluid with costly proprietary ion-exchange techniques. However for all practical purposes, the cure is worse than the disease, because other types of metal salts are released into the oil.

For steam turbine electro-hydraulic control fluids, and on board ships where space is at a premium, the Allen HVD system can be custom-designed to fit in available space.

On submarines, hydraulic systems often consist of a 50 gallon reservoir, while the system volume is about 300 gallons. The Allen Oil Conditioner system’s PLC controlled process continuously purifies the reservoir contents. Signals on an alarm panel indicate process conditions such as high filter differential pressure, at a glance. Several Allen Oil Conditioner systems strategically placed will keep the entire ship’s hydraulic fluid inventory clean, thereby reducing waste and cost of replacement fluid.

Figure 2: The Allen HVD System

This Allen Oil Conditioner system was specifically designed for use on off-shore oil platforms for Texaco and has been operational on North Sea platforms for more than a decade.

The unit is explosion-proof and has pre- and post-filtration of 5 micron and 0.5 micron respectively.

A similar system in operation at the Saudi-Aramco-Exxonmobil Refinery in Yanbu, Saudi Arabia has allowed the refinery to operate for more than 12 years without wasting any oil from more than 10 major compressor-turbine trains.



Summary (for Synthetic or Petroleum-based Fluids)

Start with oil that is:

Low in acid content (less than 0.025 mg KOH/g)

Low in moisture (less than 50 wppm)

Low in solids content (ISO Code 13/10 maximum)

Control Parameters include:

Acidity at less than 0.025 mg KOH/g

Total water at less than 10 wppm

Solids by changing filter elements when indicated

Note: See also the AFI paper on Water Activity to learn how Relative Saturation influences the health of Hydraulic Oil.

References

1. Allen, A. S. “Design Consideration for Vacuum Distillation Purification,” 1956.

2. Allen, A. S. “Principles of Filtration and Applications,” 1960.

3. Allen, A. S. “Principles of Oil Conditioning & Vacuum Distillation,” 1962.

4. Allen, A. S. “Design of Vacuum Vessel Trays for Laminar flow of Oil,” 1963.

5. Bloch, H. P. & Aamin, A. “Optimized Vacuum Purification Methods for Lubrication Oil,” AFI Publication 107.

6. Al-Amoudi, A. O. & Simon, R. J. “Hydraulic Fluid Purification at the Petromin-Mobil Refinery,” Yanbu, Saudi Arabia. 1991.

7. Worrell, M. R. & Simon, R. J. “Cost Analysis of the Oil Purification Program at the Saudi Aramco-Exxon-Mobil Refinery,” Saudi Arabia, 1992.

8. Simon, R. J. “Evaluation of an Allen Vacuum Distillation Unit at the Udhailiyah Gas Plant,” ARAMCO Maintenance Engineering Dept., Udhailiyah, Saudi Arabia, 1985.

9. Simon, R. J. & Al-Motairi, F. A. “Evaluation of an Allen Compact Design Lube Oil Purification Unit for the Carrier Gas Compressors at ARAMCO Ain Dar GOSP 2, 3 and 4,” Southern Area Maintenance Engineering Dept., Saudi Aramco Oil Company, 1986.



Appendix 1: The Case of Phosphate Esthers Organophosphate esters are important.

They are used in electrohydraulic controls that govern steam turbine speed and steam controls. On aircraft carriers and other marine craft, they are used in multiple hydraulic systems. Their use in the U.S. Armed Services is wide spread.

They are also expensive, averaging around U.S. $30 per gallon. Some specialty esters such as Mobil Pyroguard used in hydraulic control of slide valves on Catalytic Hydro-crackers in refineries cost even more.

Acidity

The most important operating parameter for successful maintenance of phosphate esters is the control of fluid acidity.

Acidity control points have been described previously. These must have conscientious adherence to prevent the leaching out of magnesium and aluminum salts from the Fuller’s Earth. If allowed to occur, this will create gel and scum which adheres to the reservoir walls and will plug filters.

Should the oil become acidic (> 1.0 mg KOH/g), then the Fuller’s Earth filter should be bypassed. The Allen High-Vacuum Dehydrator provides optional ion-exchange treatment that can be selected to remove the acidity.

Ion-Exchange Conditioning

Experiments performed in the factory, using proprietary ion-exchange resins showed that it is important to keep the fluid going through the dehydration process. If only ion-exchange is used, after initially decreasing the acidity, the water released by the resin will start to increase the acidity once again to unacceptable levels.

Continued dehydration will remove the moisture that hydrolyzes the phosphate ester.

Figure 3: Variation of Neutralization Number and Water Content During Ion-Exchange Treatment



Figure 4: The Allen High-Vacuum Distillation System for Phosphate Esters

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