The AMPTIAC Quarterly, Volume 7, Number 1 11

MaterialE A S E

UNDERSTANDING AND REDUCING THE EFFECTS OF CONTAMINATION ON HYDRAULIC FLUIDS AND SYSTEMS

Brigitte Battat(formerly AMPTIAC Technical Staff )

and

Wade BabcockAMPTIAC Technical Staff, Rome, NY

INTRODUCTION

At least 75% of all hydraulic systems degrade and fail due to contaminated oraging hydraulic fluid.[1] Contamination causes aging/degradation of fluidsand ultimate failure of hydraulic systems for a number of different reasons. Inaddition to increasing internal leakage (which lowers the efficiency of pumps,motors, and cylinders,) it also decreases the ability of valves to control flow andpressure accurately, thus wasting horsepower and generating excess heat.Furthermore, it causes parts to stick due to sludge or silting, or they can seizewhen large amounts of contaminants accumulate in the clearances. Sourcesof contamination can include: (1) the manufacturing process, (2) hydraulicfluids, (3) environmental exposure, (4) system wear, (5) and servicing.[2]

Many military systems rely on hydraulics, and as such these systems areput to work in every conceivable environment and are among the highest con-tributors to maintenance and service workloads. This ubiquity is a double-edged sword; the systems’ ever-present nature exposes them to continuous usemaking them a common maintenance item, and the common maintenanceand diverse operational environments offer many opportunities for contami-nation. In this issue of MaterialEASE, we seek to present some of the basicsregarding hydraulic fluids and how they become contaminated, and expandupon the deleterious effects this contamination has on hydraulic systems. Wewill also present some of the chief causes of fluid contamination, and the relatively simple measures which can be taken to lessen the problem.

Contaminants

While hydraulic system contaminants can be either solid particles or liquids,water is the most common. Solid particles contaminate by chemically reactingwith the fluid, or fouling the system through accumulation. For instance, anew hydraulic fluid can contain 500,000 particles per 100 mL that are 5 µmor larger, which exceeds the level recommended by most manufacturers fornormal operation.[3] Worse than this, 0.5 µm particles can be harmful tomost systems. Due to the wide range of hydraulic fluids, there are many standards for cleanliness which compounds the problem. For example, MIL-H-5606F requires that in 100 mL of fluid there be no more than 10,000particles ranging in size between 5-15 µm, no more than 1,000 particlesbetween 16-25 µm, and no more than 5 particles over 100 µm.[4]

The behavior of water as a contaminant varies from system to system.Water sometimes forms an emulsion with the fluid, or it might be partiallyimmiscible, floating on the surface or settling to the bottom of the fluiddepending on the relative density. The presence of water ultimately causes corrosion of the system components and resultant contamination of the fluidby corrosion products. Water ingress may be caused by design flaws, service

environment, maintenance activities, internal generation, and methods offluid servicing. Improper storage of open fluid containers and inadequatetransfer of fluid to the system may cause serious contamination problems.Contaminants also enter hydraulic systems through the reservoir breather(when used) and contaminated replacement components. Environmentalcontamination enters the system through ingestion, and can be preventedthrough use of (1) sealed reservoirs (2) vacuum breakers (relief valves),and/or (3) high efficiency breather filters (such as desiccators) in places wherehumidity is high enough to become hazardous.[5] Contamination fromstored containers may be avoided through proper storage and handling proce-dures. Large containers are likely to include a high level of particles; hence,fluid filtering is suggested prior to use.

In general, moisture is deleterious to hydraulic systems, with the exceptionof aqueous-type hydraulic fluids and the systems designed to utilize them.Moisture enters waterproof containers through “breathing” caused when acontainer is exposed to wide temperature ranges. Therefore, containers storedoutdoors should be placed on their sides to prevent water from accumulatingon their tops. Furthermore, container lids should be periodically checked fortightness.

Hydraulic systems may even be degraded by the additives included inthem to fight environmental attack and corrosion (i.e., through corrosion bygalvanic action, or due to slime and ultimate malfunctioning). For exam-ple, an additive in certain MIL-H-5606 fluids contains a contaminant that is soluble in the additive itself but is not soluble in the finished hydraulic fluids.[6] Hydraulic fluids containing corrosion inhibitors will often form aslime when moisture is introduced. Some mahogany sulfonate rustinhibitors containing calcium chloride and calcium sulfate (inorganicwater-soluble salts) cause degradation of hydraulic systems, through stain-ing and galvanic corrosion of steel.

Contamination with liquid other than water can occur with both miscibleand immiscible fluids. Contamination of this type takes place when onehydraulic fluid is replaced with another without thoroughly cleaning the system. Nevertheless, certain military specifications indicate when two fluidsare compatible and may be mixed. The two mixing fluids are compatible inthe sense that together they do not form resinous gums, sludges, or insolublesolid materials, but they are still contaminated because in a mixture they donot preserve the properties they have individually. Individual characteristicsmay still be required in applications that need pure fluids.

Microbiological contaminants grow in fluids where other contaminantsare present (e.g., water). This problem can be reduced or eliminated with theuse of biocides, but the source of the contamination must be removed.

21AMPTIAC

APPROACH TO FLUID CONTAMINATION – EXAMPLE ONE

Naval Air Technical Data and EngineeringService Command (NATEC)

The Naval Air Technical Data and EngineeringService Command (NATEC) carried out a con-taminant analysis through automatic particlecounting.[15] Particle counters used by the Navystudy included devices such as (1) HIAC Royco8011, (2) HIAC Royco 8012, (3) UCCCM20.9090.USN, (4) Millipore Patch Test Kit,and others. These are electronic particle countersthat provide the number of particles of varioussizes. Counts exceeding the maximum allowablein any size range render the fluid unsuitable foruse. The class of contamination is based uponthe total number of particles in any size rangeper 100 ml of hydraulic fluid.

Sampling techniques employed during thesetests were based on “flushing away” the contam-inants in both the sampling line and those gen-erated by mechanical operations, by allowing aninitial quantity to flow into a waste receptacle.Without interrupting the flow of the tested fluid,the sampling bottle was rinsed with variousamounts of the fluid that were discarded.Inspection of the samples for free water, cloudi-ness, haziness, visible particulate or a pink con-dition was conducted. Samples exhibiting any of

the abovementioned conditions were consideredunsatisfactory. Fluid was resampled to verify thiscondition, before taking corrective action. Table1 contains the cleanliness standards establishedby the Navy (hydraulic fluid contamination SAE749 D Navy Standard Class).

Contamination was defined according to thefollowing rules:• Class of contamination was based on the total

number of solid particles in any size range,per 100 ml. If the allowable particle count inany size range was exceeded, the next higherclass level was usually assigned.

• Class 5 represented the highest acceptablelevel of contamination.

• Class 5 level of acceptability had to be met atthe inspection interval specified for the equip-ment under test.On the basis of this approach to contamina-

tion control, the NATEC program concluded thatthe factors affecting the accuracy of solid particlecounts include (1) the type and cleanliness of thecontainer, (2) the sampling technique, (3) thesample preparation, (4) the Standard OperatingProcedure (SOP), and (5) the calibration.Calibration methods are changing due to a haltin the production of Air Cleaner Fine Test Dust(ACFTD, size-graded particulate) and becausethe US Military is using standards of cleanlinessnot used by the international community.

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MaterialE A S E

CONTAMINATION EFFECTS

The effects of contamination typically begin with the hydraulic fluid,extend into the system, and can even continuously worsen in a closed orsealed system. The primary initiator is corrosion products (chips and par-

ticles) that cause wear on internal components, exposing clean surfacesthat subsequently become corroded.[6] Additionally, contaminants suchas water, other liquids, and even purposely added chemicals (additives)can cause solids or slime to accumulate in the system, gradually reduc-ing the performance of valves and other tight-tolerance components.These vicious cycles will continue to quietly rob a hydraulic system’s per-formance, long after decontamination measures are taken.

Solid particles cause damage according to their size. High concen-trations of small particles (≤10 µm) form silt which erodes the interiormating surfaces of valves and renders them inoperable. Contaminatingsolid particles that are equal in size to the clearance between two movingsurfaces can cause both jamming and wear. Cycling the valve may clearthis contamination, but when it jams clearance can only be achieved bydisassembly. Larger contaminating particles block ports and orifices, andcan cause transient malfunction (“coincident jamming”) when they aretrapped by a mechanism moving from one position to the next (e.g., a

Table 1. Hydraulic Fluid Contamination Classes (according to SAE 749 D[13]).

Number of particles per 100 ml of oil

Contaminant ExceedingClass 5-10 µm 10-25 µm 25-50 µm 50-100 µm 100 µm

0 2700 670 93 16 11 4600 1340 210 28 32 9700 2680 380 56 53 24000 5360 780 110 114 32000 10700 1510 225 215 87000 21400 3130 430 416 128000 42000 6500 1000 92

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AMPTIAC

particle trapped between a poppet and its seat prevents the closing of a reliefvalve).

System malfunctions caused by contamination may be classified into thefollowing three broad categories:• degradation (gradual changes over time in pump flow rates, valve leakage,

and wear of cylinder barrels causing cylinder speed decline, etc.)• transient (intermittent failures)• catastrophic (complete failure of a system or component)All of these can be reduced through a well-planned contamination control pro-gram.

Removal of contaminants is planned according to the sensitivity of the sys-tem to contamination. The various components are rated according to thissensitivity, also known as the omega rating of the components. It is often moreefficient to use a filter dedicated to the entire system which meets the specifi-cations of the system’s most sensitive component.

SOLUTIONS TO CONTAMINATION

In order to solve the contamination problem, contamination analysis is firstneeded to identify the type and concentration of the contaminant. A compre-hensive discussion on methods for contamination analysis is beyond the scopeof the present article. However, a few methods deserve being pointed out.Methods for gauging solid particle contamination involve counting, sizing,and/or weighing of the particles contained in a given volume of fluid.

Microscopic particle counting is conducted according to standards such as SAEARP-598[7] and ASTM F 312.[8] Automatic particle counting is conductedaccording to a national (ANSI B 93.28M) and an international (ISO 4402)standard that calibrates systems and establishes reproducibility of counters invarious laboratories within 6%.[9]

Liquid contaminant measurement is difficult, especially if the contami-nant is soluble or miscible in the hydraulic fluid. As water is the most commonliquid contaminant of hydraulic fluids, it can be separated from the fluid (ifpresent at a high concentration) through dilution with a solvent that makesthe water immiscible. This technique includes Federal Test Method 3001[10]and ASTM D 95.[11]

Chemical analysis and accurate recording of contaminant concen-trations over the life of the hydraulic systems can identify abnormal

conditions and potentially predict failing components. The followingprocedures for extracting fluid samples and analyzing them for con-tamination have been well-established and even standardized: 1. Proton-Induced X-Ray Emission (PIXE) is a nondestructive technique that

analyzes simultaneously all samples (i.e., powders, soil, aerosol filters, etc.)and assesses particles up to 150 µm in size.[12]

2. Infrared analysis is commonly used to identify chlorine or fluorine in thehydraulic system corrosion products.

3. Gas chromatography4. Spark source mass spectrometry5. Energy dispersive X-ray scanning electron microscopy

STEPS TO REDUCE CONTAMINATION AND FUTURE WORK

While the issue of contamination in hydraulic systems will not go away, thereare measures which can be employed to lessen its deleterious impact. Many ofthese are relatively simple handling and storage processes, and involve pay-ing careful attention to fluids and fluid combinations specified in systems.The long-term performance of critical systems depends upon placing arenewed emphasis on maintenance and handling procedures which are oftentaken for granted.

APPROACH TO FLUID CONTAMINATION – EXAMPLE TWO

Breakdown of Preservative Fluid MIL-PRF-46170 in Aircraft HydraulicSystems[14]

Several aircraft programs have linked the forma-tion of organic material and uncommandedflight control events to the presence of preserva-tive fluid in the hydraulic systems. Examplesinclude F-14 and F/A-18 aircraft where thehydraulic filters have shown buildup of organicmaterials. The V-22 hydraulic systems are preser-vative-free, and the filters have no organicbuildup. Also, the V-22 pumps stored for severalyears have shown no corrosion damage. By com-paring aircraft with and without fluid preserva-tives several programs have decided to eliminatethe preservatives from their hydraulic systems.

Other problems attributable to preservative

fluids include F/A-18 E/F hydraulic filters becom-ing prematurely clogged with green organicmaterial. (Navy engineers have suspected fluidbreakdown and Boeing engineers have assumedthat copper played a key role in filter clogging.)The incompatibility of the preservative fluid withthe hydraulic system at elevated temperatures, theimpossibility of drip draining actuators, and the

validation of hydraulic fluids as lubricants forcorrosion protection were but a few of the issues tobe solved. As a result of these problems, the “RedTeam” made of government and industry experts(representing the Navy, the Air Force, Boeing,Northrop, Vickers, Honeywell/Allied Signal, PTIIndustries, and Pall) was formed. At the initialmeeting, the members of the team suggested the

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MaterialE A S E

Proper Fluid Selection

Maintainers must pay careful attention to choosing the proper fluid specified for use in a particular system. It is critical to use the properfluid, and to avoid mixing fluids in a system. Even when a system speci-fies that more than one fluid may be used, maintenance personnelshould try to always use only one fluid in that system and not mix fluids.If a different fluid must be used, the system should be drained andflushed before adding the new fluid. Keep in mind that even if two fluidsare miscible, the mixture may not perform with the ideal properties ofeither component separately. Also, the combination of some fluids and/ortheir additives may be deleterious to the system.

System manufacturers and suppliers must be diligent in specifyingappropriate fluids for a system. These specifications should be clear aboutpotential incompatibilities if multiple fluids are used, and should rec-ommend proper system maintenance procedures for all environmentalconditions. Fluids should also be chosen with proper regard to environ-mental and hazardous materials regulations.

Proper Fluid Handling and Storage

Hydraulic fluids should be kept in their sealed containers until ready foruse. Both new and partially used containers should be handled andstored very carefully. Lids should be checked routinely and secured tight-ly, and drums should be kept in a relatively benign environment. Largetemperature swings cause the containers to “breathe” from expandingand contracting air and liquid inside. This, combined with moisturefrom rainfall or humidity, can cause the ingress of water into the con-

tainer. Some fluids are hygroscopic, meaning that they absorb moisturereadily when exposed to humid air, and thus require extra measures tobe taken to lessen the chance of them becoming contaminated.

System Maintenance

Hydraulic systems, because of their prevalence, are often in need ofrepair just because they are used so much. Standard operating proce-dures should be carefully checked for fluid contamination opportuni-ties, and system maintenance procedures should be similarly diligent inkeeping contaminants from systems. Much of this may be accomplishedthrough procedural changes identifying which fluids are appropriate forwhich systems, and the proper handling of those fluids and systems.Additionally, procedures should be adjusted to spell out what must bedone to replace a fluid or change from one specified fluid to another. Ingeneral, the system should be flushed of all the old fluid and potential-ly cleaned prior to installation of the new or replacement fluid.

Future Work

Plans for future hydraulic fluids include research to reduce their fire risksand the development of thermally stable hydraulic fluids. These will meetthe challenges of increased pressures and electro-hydrostatic actuationdue to higher temperatures. Novel purification methods that wouldenable the reuse of hydraulic fluids are also under consideration. Thecontinuation of laboratory simulations of the conditions that causedstuck valves in helicopters, hydraulic pump tests, and toxicity are expect-ed to be included in future programs.[2]

Table 1. Fluids Used for Aircraft Storage.*

Type of fluid Type of aircraft/engine

MIL-H-5606 B1B, C-130, C-135, E-3, E-4, E-6, F-5, P3C, U2RMIL-PRF-83282 F-110 (F-16, actuator), F-404, H-60, H-64, S-60MIL-PRF-6083 C-5A/B, F-117, F-16MIL-H-46170 AV-8, C-17, S-3A, B-2, F-15, E2C, F-18, H-53, H-60, S-60, V-22*Information from Parker Aerospace

REFERENCES[1] Norvelle, D.F., An Introduction to Fluid Power, West Publishing Company, St.Paul, MN, 1991[2] Gschwender, L., Snyder, C.E. Jr., Sharma, S., Flanagan, S, (MaterialsDirectorate, Wright Patterson Air Force Base), Advances in US Air ForceHydraulic Fluids, JSL 16-1 35 ISSN 0265-6582[3] Moore, L.C., The Cleanliness of New Hydraulic Fluids, Basic Fluid PowerResearch Journal, vol. 11, Oklahoma State University, Stillwater (October 1977)[4] MIL-H-5606F, Hydraulic Fluid Petroleum Base: Aircraft, Missile, andOrdnance, 26 August 1991[5] MIL-HDBK-118, 22 September 1993, pp. 7-3 to 7-22[6] Deane, T.N., The Effect of Contamination on Fluids and the Effect of theIngredients of the Fluids on Contamination, Proceedings of Aerospace FluidPower Systems and Equipment Conference, Society of Automotive Engineers,Warrendale, PA, 1965

[7] SAE ARP 598B-86, Determination of Particulate Contamination in Liquidsby the Particle Count Method, Society of Automotive Engineers, Warrendale, PA,1986[8] ASTM F 312-69, Method for Microscopical Sizing and Counting Particlesfrom Aerospace Fluids on Membrane Filters, American Society for Testing andMaterials, Philadelphia, PA, 1969[9] ISO 4402-77, Hydraulic Fluid Power – Calibration of Liquid AutomaticParticle Count Instruments – Method Using Air Cleaner Fine Test DustContaminant, International Organization for Standardization, Stockholm,Sweden, 1977[10] Federal test Method 3001.8, “Water in Petroleum and Other BituminousMaterials”, Federal Test Method Standard No. 791C, Methods of TestingLubricants, Liquid Fuels, and Related Products, 30 September 1986[11] ASTM D 95-83, Standard Test Method for Water in Petroleum Productsand Bituminous Materials by Distillation, American Society for Testing andMaterials, Philadelphia, PA, 25 March 1983[12] Eleftherakis, J.G., “Accurate Analysis and Description of Field-Induced and -Generated Contaminants”, Paper No. 901678, SAE Technical Paper Series, Societyof Automotive Engineers, Warrendale, PA, September 1990[13] Zdrojewski, W.S., NOAP Spectrometer Technical Report, Automatic ParticleCounting, Military Aerospace Hydraulic Fluids Workshop Proceedings, August2000[14] Moorman, J.W. (Naval Air Systems and Flight Controls), Breakdown ofPreservative Fluid MIL-PRF-46170 in Aircraft Hydraulic Systems, MilitaryAerospace Hydraulic Fluids Workshop Proceedings, August 2000[15] Ohlson, J. (Sabre Systems), F/A-18E/F Hydraulic System Red Team,Military Aerospace Hydraulic Fluids Workshop Proceedings, August 2000[16] Gschwender, L., Elimination of Barium Containing Fluids in DODAircraft Systems-Pollution Prevention Research Program, Military AerospaceHydraulic Fluids Workshop Proceedings, August 2000

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AMPTIACA D VA N C E D M AT E R I A L S A N D P R O C E S S E S T E C H N O L O G Y

following issues to investigate:• The Navy discussed the premature clogging of

F-18 E/F filters with organic plaque.• The Air Force presented data from in-service

breakdown of preservative additive in Armyhelicopters.

• Boeing presented data on Scanning ElectronMicroscopy (SEM) analysis of filters, demon-strating that filters showed organic materialbound to filter fibers, and that the materialcould be removed with toluene/xylene (thesame solvent used by the Air Force to cleanvalve deposits in Army helicopters). At later meetings, team members demonstrat-

ed that the rust inhibitor barium dinonylnaph-thalene sulfonate (BSN) in MIL-PRF-46170 is amajor contributor to filter clogging. As a result ofthese meetings, the Red Team recommendedeliminating the use of MIL-PRF-46170 as a pre-

servative hydraulic fluid in F/A-18E/F aircrafthydraulic systems (and all other Navy AircraftHydraulic Systems), and the use of operationalfluid MIL-PRF-83282 for system operation andshipping/storage of components. Table 1 containsa summary of fluids used in military aircraft.

Currently, BSN is successfully used as a corrosion inhibitor in two military fluids, MIL-H-6083 and MIL-H-46170. They are utilized opera-tionally by the Army in ground vehicles and asstorage fluids for components in the Air Force and Navy. The Environmental Protection Agencylimits the concentration of barium to 100 mg/laccording to the handbook 40 CFR, 261.24. Anyamount beyond this limit must be disposed of asa hazardous material.

Further efforts by the Red Team establishedthe need to initiate a pollution prevention re-search program to remove barium compounds

from fluids in DOD aircraft systems. The maincatalysts to this program included:• The aircraft components are stored with four

different fluids• Lack of documented reasons to use fluids with

corrosion inhibitors such as barium• Inventory is reduced, hence components have

shorter shelf life• Cost savings

The expected payoffs through the imple-mentation of this program include (1) theelimination of hazardous waste disposal, (2)curbing of operational problems from bariumcontamination, (3) reduction of AF inventoryby two fluids, (4) reduction of storage space,(5) saving charges passed on by componentsuppliers and overhaulers.