3. AGC REFINING & FILTRATION
MAINTENANCE OF PHOSPHATE ESTERS-BASED FLUIDS 3
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
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.
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
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.
The reaction can be formulated as:
(ΩO)3 PO + H2O
-> (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.
4. AGC REFINING & FILTRATION
MAINTENANCE OF PHOSPHATE ESTERS-BASED FLUIDS 4
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.
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.
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
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.
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.
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.
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)
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
5. AGC REFINING & FILTRATION
MAINTENANCE OF PHOSPHATE ESTERS-BASED FLUIDS 5
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
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
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
6. AGC REFINING & FILTRATION
MAINTENANCE OF PHOSPHATE ESTERS-BASED FLUIDS 6
Having examined AGC Refining & Filtration’s system, the discussion now turns to alternative methods of
handling phosphate esters.
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.
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
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
7. AGC REFINING & FILTRATION
MAINTENANCE OF PHOSPHATE ESTERS-BASED FLUIDS 7
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
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
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
8. AGC REFINING & FILTRATION
MAINTENANCE OF PHOSPHATE ESTERS-BASED FLUIDS 8
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
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,
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-
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.