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Maintenance of Phosphate Esters-Based Fluids

WHITE PAPER © 2017 AGC Refining & Filtration LLC

AGC REFINING & FILTRATION

MAINTENANCE OF PHOSPHATE ESTERS-BASED FLUIDS 2

Contents Summary 3

General 3

Fluid Chemistry 3

Maintenance 4

References 8



Summary

Phosphate esters are polar fluids with excellent lubricating properties that can operate under extreme conditions such as electro-hydraulic compounds for steam and gas turbines, aircraft carriers, as well as steel mills and foundries. However, phosphate esters require strict control in order to extend their useful lifespan.

This article covers the requirements of the strict maintenance that is necessary to prevent the untimely destruction of phosphate ester fluids using equipment which AGC Refining & Filtration, LLC, has designed for over half a century.

General

Organo-phosphate esters are made by condensing an alcohol with phosphorus-oxychloride in the presence of a metal catalyst to produce tri-alkyl, tri-alkyl-aryl, or tri-aryl phosphates. For the aryl-phosphates, phenol or mixtures of alkylated phenols (e.g. isobutylated phenol: a mixture of several t-butyl phenols) are used as the starting alcohols to produce potentially very complex mixtures of organophosphate esters. Some phosphate esters (e.g. tri-cresyl and tri-xylyl phosphates) are made from phenolic mixtures such as cresylic acid which is a complex mixture of many phenolic compounds. The composition of these phenols varies with the source of the cresylic acid as does the resultant phosphate ester.

There are certain differences in the physical properties between different manufacturers of the same phosphate ester. Some phosphate esters contain additives that offer resistance to oxidation, resist foaming, or separate easily from water. Others consist of an invert (water-in-oil) emulsion where a continuous oil phase is surrounded by finely divided water droplets that are uniformly dispersed throughout the mixture.

Organo-phosphate ester fluids are used where fire-retardant properties are needed such as in aircraft operations, in marine applications, in electro-hydraulic control (EHC) systems of steam turbines, and in industrial systems where leaking fluid might come in contact with an ignition source. Organo-phosphate esters react with hydrocarbon-based materials such as those used in seals, hoses, paints, coatings, and elastomers that are used in machinery and other equipment.

Phosphate esters are degraded by temperature, oxidation, and hydrolysis. They are also subject to auto-catalytic hydrolytic degradation which means that the products of hydrolysis catalyze (promote) the hydrolytic process itself causing it to proceed at an accelerating rate. Hydrolysis is promoted by acids, bases, and salts and is pronounced even at low water concentrations. This process results in increased acid formation and increasing acid numbers (TAN) of the oil. Foaming and air retention are additional problems caused by poor fluid maintenance. Entrained air contributes to fluid oxidation while phosphate salts are known to cause foaming. Anti-foam agents can also create air entrainment problems.

Fluid Chemistry

The reaction can be formulated as:

(ΩO)3 PO + H2OH+

-> (ΩO)2P.O.OH + ΩOH (1)

where Ω represents C6,C7, C8-phenyl and alkyl substituted phenyl groups.

The reaction may proceed to yield the di-acid mono ester ΩO.PO (OH)2 and ultimately phosphoric acid.

The reaction with water results in ever increasing acid numbers (TAN). If the acidity is allowed to get out of control, it will proceed to attack machinery surfaces such as the Babbitt of bearings, while continuing to rapidly deteriorate the fluid. Simultaneously the resistivity of the fluid will decrease significantly, indicating the accumulation of ionic hydrolysis products.



Thus the two most important indicators of phosphate ester fluid health are the acidity level and the water content of the fluid. These two parameters are interdependent and must be controlled simultaneously. Third is fluid cleanliness which should be monitored by particle size distribution analysis. Viscosity measurement is the fourth indicator.

Maintenance

The control of acidity and water in phosphate ester fluids must be aggressively controlled within a very narrow range. The allowable acidity range is shown in table 1.

Fluid Conditioning

The hydrolysis of phosphoric acid esters and water creates phosphoric acids. If uncontrolled the acidity accelerates rapidly. Fuller’s earth has proven to be the most effective in controlling acidity in phosphate esters provided that the fluid acidity has not advanced beyond 0.05 mg KOH/g.

Table 1: Control Range of Acidity

Tan 0–0.05

Ideal control range, acid formation must be controlled with filtration or vacuum dehydration and fuller’s earth adsorption. Fluid water content must be maintained at less than 10 weight parts per million (wppm) total water. This can only be done consistently by a vacuum distillation system.

Tan 0.05–0.1 Range of concern, acid formation is rising. If water was previously not controlled with filtration or vacuum distillation, then the acidity will leach out metal salts from fuller’s earth to further deteriorate the fluid.

Tan 0.2–1.0

Accelerating hydrolysis and acid formation and presence of dissolved metals such as Mg, Sn, and Zn, indicating advancing Babbitt damage and the creation of a gel-like substance. Water adsorbing filtration or vacuum distillation or fuller’s earth adsorption will no longer be able to reduce hydrolysis. Ion-exchange could possibly lower acidity temporarily, however continued use of ionic resins that scavenge acids will ultimately result in a renewed increase in acid formation.

Tan 1.0–2.0 Possible permanent phosphate ester and machinery damage. Regeneration by ion exchange or acid scavenging is unlikely.

Fuller’s Earth Adsorption

Fuller’s earth—or montmorillonite—is a product composed of calcined opaline clay, a naturally occurring material also called atapulgous clay. It encompasses types of clay minerals formed from very small imperfect crystals that have the capacity to adsorb acids. Interlayer water or cation exchange occurs naturally, accompanied by large changes in one dimension (swelling).

The reaction of acid control is:

(ΩO)2HO.PO + MgO, or

Mg(OH)2 2 PO.OMgOH + [(ΩO)2 PO.O]2 Mg (2)

Magnesium Salts

These magnesium salts have been identified in the gel-like substance that forms when acidity is allowed to rise beyond 0.1 mg KOH/g. The gel results from the hydrolysis of the phosphate ester and the reaction of the hydrolysis products with the fuller’s earth which contains magnesium oxide and sodium aluminum



silicates. These will rapidly destroy the oil.

Thus to maintain the phosphate ester fluid in good condition the TAN must not exceed 0.05 mg KOH/g.

Control of Moisture

Using a Filter System

The moisture content of phosphate esters must be controlled within an equally narrow range. This can be done by combining water adsorbing filter elements with fuller’s earth adsorption followed by a post-solids filter to prevent fuller’s earth detritus from escaping.

Figure 1: An Allen Moisture Adsorption and Fuller’s Earth Filter System

The first phase of this system consists of a multi-element vessel with moisture adsorbing filter elements. These elements are capable of adsorbing a limited amount of moisture and solids from the phosphate ester. The material swells as it adsorbs moisture and once that adsorptive capacity has been exhausted, the elements must be changed.

The post-filter vessel usually contains 5-micron solids filter elements. These are Allen disc-type filters uniquely designed with a very large surface area and a high dirt-loading capacity.

Using a Vacuum Distillation System

The best means to optimize moisture removal from the phosphate ester fluid is to use a vacuum distillation system.

Figure 2 shows a typical Allen system. It uses solids filtration and thermal vacuum distillation of the phosphate ester to remove 100% of free and emulsified water and dissolved water down to 5 wppm or less.

At these water levels—combined with strict control of acidity—phosphate esters can maintain their useful life for 10 to 20 years with only minor amounts needed to top up the reservoir.

Figure 2: An Allen Vacuum Distillation System



Having examined AGC Refining & Filtration’s system, the discussion now turns to alternative methods of handling phosphate esters.

Ion-Exchange Conditioning

Experiments with strong base ion-exchange resins seemed to indicate that these were capable of significantly reducing the acidity of degraded phosphate esters. Ion exchange resin conditioning of used, highly acidic fluid initially caused a rapid decline in acidity. This was accompanied by a concurrent increase in transfer of water from the resin.

Ion exchange resins contain 65 to 70 percent by weight of water, which is how they are made and stored. This water enters the fluid being treated and will cause adverse side reactions.

Unless removed by water adsorbing filters or vacuum distillation, this water will remain in the system and—with continued circulation through the resin—will cause the acidity to rise again, presumably because hydrolysis increases due to the increase in water content. Subsequent decreasing of the water content will cause a leveling of the acidity at a higher value than when the treatment started.

A side issue is that when a vacuum distillation system is used to dehydrate the fluid, the resin will become ineffective due to the efficient water removal by vacuum distillation. Thus, the use of ion exchange alone may only be temporarily effective in recovering used, phosphate ester with high acidity.

Acid Scavenging

The principle of acid scavenging is the reaction of hydrolysis products with a fluid-soluble, basic, or neutral compound to produce a neutral non-ionic compound, which can be removed from the fluid.

When applied to a used phosphate ester with high acidity, an immediate but moderate reduction in acidity will result. However, relatively quickly the acid number will rise again to its initial level. Undesirable by-products produced by this method are sludge generation and an increased pressure drop across solid filters.

The use of acid scavenging compounds will provide only temporary relief from high acid numbers and may ultimately completely deteriorate the fluid beyond recovery.

The Allen Solution (Best Available Technology)

The ideal approach and most often the only solution to get long life from phosphate ester fluid is to start maintenance when the fluid is new.

The systems that can properly achieve this and allow phosphate ester fluid to maintain a useful life of over 20 to 30 years are described below:

An Allen Filtration System

An Allen filtration system has been previously described (see figure 1).

An Allen Vacuum Distillation System

This system (see figures 2 & 3) is an integrated, self-contained, and fully automatic system that consists of the following:

1. 5-micron multi-element solids pre-filter vessel

2. Vacuum distillation vessel for degassing and water removal

3. Multi-element fuller’s earth adsorption filter vessel to remove any remaining acids and color

4. 1-micron multi-element solids post-filter to polish the fluid before returning it to the reservoir



These custom-designed systems with various capacities can be mounted on a steel skid or on a trailer to be used in various locations. The system is designed in accordance with ASME and API standards. It is fully automatic—controlled by a programmable logic controller—and can be made explosion-proof as the situation demands.

The heart of the Allen Vacuum Distillation System is a strong two-stage piston vacuum pump and a vacuum vessel that has been designed by our chemical engineers to function like a refinery fractionation tower.

Heated oil enters the top of the vacuum vessel and is distributed over specially designed internals in a thin film. The strong vacuum lowers the boiling point of the contaminants and allows these to be removed from the fluid, condensed, and discarded.

Figure 3: An Allen Phosphate Ester Purification System Utilizing Vacuum Distillation and Acid Adsorbtion



References 1. Al-Amoudi, A.A. and R.J. Simon. “Control oil Purification of CHD Steam Turbines” (Internal

Maintenance Engineering Project Report AEM-324-1989, Aramco-ExxonMobil Refinery, Yanbu Saudi Arabia).

2. Anzenberger, J. F., Sr. “Evaluation of Phosphate Ester Fluids to Determine Stability and Suitability for Continued Service in Gas Turbines.” Lubrication Engineering 43: 7, 528-532.

3. Brown, K. “Conditioning Monitoring of Phosphate Ester Hydraulic Fluids.” Machinery Lubrication, no. 200211.

4. Duchowski, J. K., Dr., Dr. D. J. Sutton, and B. S. Sinclair. “Ion Exchange/Vacuum Dehydration Treatment: An Improved Approach for Conditioning and Reclamation of Phosphate Ester Hydraulic Fluid.” Lubrication Engineering (April 2001).

5. Stark, L. R., “Status of Fire-resistant Turbine Lubricants.” Lubrication Engineering 33: 10, 535-537.

6. Troyer, D. and R. Wurzbach. “Will Ion Exchange Resins Remove Acid from Mineral Oil.” Machinery Lubrication, no. 2000211.

7. Wolfe, G.F. , and A. Whitehead. “Experience with Phosphate Ester Fluids as Industrial Steam Turbine Generator Lubricants.” Lubrication Engineering 34 (1977): 8, 413-420.

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