32-39 tlt feature 10-05 - stle...32-39 tlt feature 10-05 9/26/05 4:26 pm page 33. water is one of...

32 O C T O B E R 2 0 0 5 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

32-39 tlt feature 10-05 9/26/05 4:26 PM Page 32

T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y O C T O B E R 2 0 0 5 3 3

CONTINUED ON PAGE 34

‘Water, water everywhere,Nor any drop to drink.’By Dr. Alan C. Eachus


Water is one of the two most widespreadand destructive contaminants in lubri-cants, second only to solid particulate con-tamination. Damage resulting from watercontamination is not as immediatelynoticeable as is that from particulate con-tamination, but it can be more systemic.Drew Troyer, senior vice president of Glob-al Services Operations for Noria Corp. inTulsa, Okla., lists five other, less-prominentlubricant contaminants as air, heat, fueland coolant/antifreeze (in engine lubri-cants) and radiation.

Water found in lubricants can enter in anumber of ways and will be present in theform of dissolved, emulsified (suspended asthe internal phase of a water-in-oil emul-sion) or free water, with the latter two formsbeing the most dangerous for lubricatedequipment.

How it gets thereWater can gain access to lubricant systemsthrough improper vents, defective seals orother housing openings or from internal heat

exchangers or oil coolers that are leaky orcorroded. Loose or even missing storage-tank caps or hatches, or transfer equip-ment such as hoses and pumps leftunprotected when not in use can permitincidental or weather-related wateringress. And in the case of leaked-insteam or splashed-in washdown clean-ing solutions, even harsher chemicalcontaminants can be introduced.

Water is a normal combustion prod-uct, and blow-by exhaust gases from

combustion processes can sometimesenter lubricant systems. Some lubricants

are hygroscopic enough to absorb watervapor directly from the air, and large tem-perature changes inside a lubricant systemresult in pressure cycles, permitting humidair to enter an imperfectly sealed oil reser-voir in which moisture can then condense.Additionally, virgin lubricants already may

be contaminated with some level of waterwhen they are delivered by the manufac-

turer. Water can exist in new oil as aresult of refining, manufacturing or

blending operations, or it can infiltrate inthe course of the supplier’s transport proce-dures, handling practices or storage condi-tions. Bulk-delivered lubricants will be moresusceptible to this than will drum-suppliedones. (For information about preventing wateringress from these sources, see “Prevention Meth-ods—How to Keep It Out” on page 37.)

Adverse effects—the damage it causes Although the ultimate manifestations ofwater contamination in lubricants are corro-sion, excessive wear and premature failureof lubricated metal surfaces, this destruc-tion is caused in a number of different ways.Water can act directly on metal surfaces andalso can impair lubricant effectiveness.

Bubbles of water vapor (or of entrainedair) can cause micropitting of bearing sur-faces during the course of pressure-causedimplosion in which they condense back toliquid phase; this is known as cavitationdamage. Moreover, the presence of water ina lubricant can enhance air entrainmenttherein, setting the stage for more cavita-tion yet. Water can be adsorbed directlyonto hydrophilic metal surfaces, displacingthe protective oil layer and even some lubri-cant additives. This permits direct exposureof the now-unprotected metal surface tocorrosive conditions and particulates andresults in excessive wear. Free water is espe-cially prone to this sort of behavior.

Another effect of water on metal surfacesis known as hydrogen embrittlement. Watermolecules can enter microscopic cracks inmetal surfaces through capillary action.Once there, the combination of extremepressures and free-metal surface contactcan cause an almost explosive decomposi-tion of these water molecules into compo-nents, one of which is atomic hydrogen. Thehydrogen can accumulate in cracks andalong metal grains, building up pressurewithin the metal under conditions of tensilestress and resulting in crack propagationand eventual spalling.

The adverse effects from contaminantwater on the lubricant itself can be mani-

Water is one of

the two most

widespread and

destructive

contaminants in

lubricants, sec-

ond only to solid

particulate

contamination.

CONTINUED FROM PAGE 33

While not as life-threatening as it was to SamuelTaylor Coleridge’s Ancient Mariner, the ubiquitousand damaging nature of water can be a true men-ace to lubricant users.



fested through physical, chemical andmicrobiological processes. Since the viscos-ity of water does not increase with pressurelike that of a lubricant, free or emulsifiedwater contained in oil can decrease theeffective lubricant viscosity, resulting ininsufficient elastohydrodynamic film thick-ness and inadequate fluid-film strength.This diminished boundary-layer protectioncan allow rolling or sliding metal surfaces tocome into direct contact with each otherunder high pressure (as in bearing wiping)and cause adhesive wear due to momentaryspot-welding of the contacting surfaces.

The overall effect is a lowering of load-carrying ability and a decreased fatigue lifeof the lubricated surfaces. Moreover, thespot-welded areas are typically sheared offand add to the damaging particulate load.Additionally, a too-thin oil film permitssmaller particulate contaminants, whichmay have passed through an inadequatelysized filter, to come directly into contactwith moving metal surfaces. The formationof emulsions in a lubricant can lead to cre-ation of sludge and deposits, which canstress pumps and clog orifices, nozzles andjets and blind filters.

Oil oxidation is initiated by the heat, airand pressures encountered in lubricated sys-tems; the presence of water increases thisoxidation rate by several times, and the prod-ucts of oxidation are often acidic and, there-fore, naturally corrosive. Further, the pres-ence of particulate contamination togetherwith water contamination can accelerate oxi-dation rates by at least 50 times.(1)

Apparently the metal content of particu-late contaminants (iron and copper),together with the surface area of these par-ticulates, provides efficient catalytic promo-tion of basestock-oxidation reactions. Suchenhancement of oxidative conditions alsodepletes antioxidant additives far more rap-idly than would be the case otherwise. Ini-tial oil-oxidation products, such as car-boxylic acids and hydroperoxides, will reactfurther, resulting in more sludge anddeposits. Unlike mineral oils, some synthet-ic lubricants are themselves susceptible tohydrolysis. Generally, diesters and phos-phate esters are especially prone tohydrolytic decomposition, while polyolesters and polyalkylene glycols are less-readily hydrolyzed; polyalphaolefins (PAOs)demonstrate the best hydrolysis resistance

among synthetic lubricants. Additives are incorporated into lubri-

cants to serve as viscosity-index improvers,antiwear and extreme-pressure (EP) addi-tives, corrosion inhibitors, antioxidants andfoam suppressants. The incorporation ofwater into oil can change the solubilizationcharacteristics of that oil. Some additivesmay be de-solubilized and, therefore, pre-cipitated by this change, adding to sludgeformation; this physical depletion also ren-ders them unavailable to function effective-ly in the lubricant. Other additives may bepreferentially solubilized by water and“washed out.” The incorporation of water-resistance polymers such as high-molecu-lar-weight polyisobutylenes can improvegrease washout resistance, notes Wickliffe,Ohio-based, Lubrizol Corp.’s Karen Allen,global business manager for Industrial Busi-ness; Debra Light, commercial manager forHydraulics and Industrial Gear Lubricants;and Matt Sivik, technical manager forGrease Additives.

Even worse, water can react with someadditives, not only making them chemicallyunavailable but also transforming them intosuch harmful materials as acids or moresludge. As an example, some sulfurizedadditives can undergo hydrolysis and beconverted eventually into such acidic prod-ucts as hydrogen sulfide or sulfuric acid,leading to etching of bearing surfaces. Sim-ilar effects will occur when moisture in acrankcase reacts with the sulfur in fuel-com-bustion products. Amine-based corrosioninhibition agents can provide significantutility by acting on metal surfaces to helpcombat acids and prevent the formation ofdamaging rust particles. Certain hindered-phenolic antioxidants and organophosphiteor -phosphate corrosion inhibitors also aresensitive to hydrolysis.

The antiwear additive zinc dialkyldithio-phosphate (ZDDP) can be destroyed by


(Ph

oto

s co

urt

esy

of P

all C

orp

.)

Water contamination cancause a host of problems,including fluid breakdown,reduced lubricating filmthickness, accelerated me-tal surface fatigue and cor-rosion. When oil becomesmilky in appearance (as inthe oil on the right), thesaturation limit at the oiltemperature has beenexceeded, indicating thatboth dissolved and freewater are present.

CONTINUED ON PAGE 36



traces of water at temperatures above about180 F (82 C). However, Lubrizol personnelpoint out that varying the dialkyl moiety ofZDDP can assist in stabilizing it againsthydrolytic destruction. The impairment oflubrication properties can result in higheroperating temperatures, which further exac-erbates already-damaging conditions andcorrosive wear; an 18 F (10 C) increase inexcessive temperature will roughly doubleoxidation-reaction rates. Moreover, sometypes of oxidation create more water as areaction product.

Bacteria and fungi may need as little as500 ppm of water in the oil environment togrow. The less-than-sterile conditions oflubricant systems, combined with darkness,moisture, moderate (75-120 F, or 24-49 C)temperatures and nutrients supplied bylubricant additives can permit microorgan-isms to thrive in oil systems. The microbescan degrade additives, induce corrosion andproduce biologically derived sludge anddeposits that can be as detrimental to sys-tem performance as chemically derived ones.

Determination of water—how much is there? Because water has such devastating effects,it is vital for the lube user to test for and

quantify water contamination and takeappropriate steps to remove the waterfrom the oil. The level of moisture thatcan be tolerated in lubricants beforewater starts to cause serious problemsvaries widely. For instance, water con-tent of less than 200 ppm (0.02%) willnegatively affect roller-element bear-ing systems, says Troyer; he adds that,“This extreme water sensitivity includes

the rolling contact of gear-tooth pitchlines.” Conversely, a few thousand ppm

of water is needed to seriously degradeengine crankcase oils, with additives playinga major role in permitting contaminationtolerance. Many technical reports providedata relating reduced lubricant-moisturecontent to increased bearing and system life.What is virtually universally agreed is thatwater levels should be maintained, if possi-ble, below the saturation point of the lubri-

cant, since dissolved water is less damag-ing than emulsified or free water.

The simplest way to evaluate watercontent is to visually inspect the oil in

question—a hazy or cloudy appearance

can indicate the presence of emulsifiedwater, and free water can be readily noted.Another easy test to perform on-site is the“crackle” test, where a dropperful of lubri-cant is dropped onto a hotplate preheatedto above 100 C. An audible crackling noise,accompanied by bubbles, indicates that thewater present is being rapidly driven out ofthe oil. While trained personnel can quanti-fy water content to well below 1,000 ppmusing this procedure, it is probably betterused as a qualitative test for the presence offree or emulsified water.

Fourier transform infrared spectrometry(FT-IR) is a well-known procedure for quan-tifying water content of lubricants and iseasy to perform in an appropriate laborato-ry setting. While some sources suggest thatthe lower limit of detection for this proce-dure is about 1,000 ppm, ASTM subcommit-tee D02.96 is developing a standardized FT-IR test method (currently designatedWK6623) that is said to be able to measurelubricant water-levels as low as 50 ppm. Themost sensitive test method for water con-tent is the coulometric Karl Fischer titration,which can readily determine moisture levelswell below 100 ppm. However, its accuracycan be interfered with by some additives.The current ASTM standard test method forthis procedure is designated D6304-04ae1.Other procedures used to determine waterlevels in oil include measuring the volumeof water separated from the oil by centrifug-ing a sample or distilling off the water in aDean-Stark apparatus (ASTM D95-05).

Marianne Duncanson, senior tech ana-lyst with ExxonMobil Lubes & SpecialtiesCo. in Friendswood, Texas, states, “By find-ing trace elements or chemicals in contami-nating water, oil analysis labs can oftendetermine its source—whether it is from asteam leak, cooling water, condensation oreven rainwater.” Sampling for moistureanalysis must be done carefully becausewater is not necessarily distributed through-out a lubricant system homogeneously.Water vapor released by heat at one pointcan condense in a cooler portion of the sys-tem. It’s difficult to determine water contentof used crankcase lubricants, for example,since the operating temperature of theengine and exhaust will drive off water. Also,extremely low temperatures can convertwater into ice crystals, which also may bemissed by sampling.

CONTINUED FROM PAGE 35

Because water has

such devastating

effects, it is vital

for the lube user

to test for and

quantify water

contamination

and take appropri-

ate steps to

remove the water

from the oil.



Since it is desirable to maintain moisturelevels below the saturation point of thelubricant, water-content data alone are notas useful as knowledge of the lubricant’smoisture-saturation value at an appropriateoperating temperature and the temperatureat various sampling sites, which would per-mit comparison with the measured systemmoisture. A lubricant’s saturation behaviorwill be determined by the baseoil (synthet-ic or mineral), the additive package and theoperating temperature. Aged oils will retainmore moisture than new ones; API type Ihydrocarbon basestocks generally willretain more water in the dissolved statethan will type II or type III stocks. Also, polaradditives—including detergents and dis-persants and some antiwear additives andcorrosion inhibitors—raise the saturationpoint. However, saturation levels at higheroperating temperatures can decrease atlower stand-by temperatures, permittingdissolved water to come out of the cooledlubricant as free water.

For typical mineral oils, saturation isoften about 200-300 ppm moisture, while

for some synthetic lubricants it can beclose to 1,000 ppm. Polar additives play amajor role in the ability of a lubricant tomaintain water in solution and to hinderdemulsibility. For example, the antiwearadditive ZDDP and some sulfur-phos-phorus EP additives are known to retarddemulsibility. For proactive mainte-nance, manufacturers of on-line mois-ture-monitoring equipment suggestthat action be taken when a lubricant’swater level reaches about 70% to 90%of saturation, often 500-1,000 ppmwater. Some saturation-meter systemsuse an oil-inserted probe, while others usesaturated relative humidity to calculatewater content.

According to Rojean Thomas, engineer-ing manager at Trico Manufacturing Corp.in Pewaukee, Wis., on-line sensors im-mersed in the oil give an immediate indi-cation if water contamination is enteringthe system with the oil such as from awater-contaminated lubricant fill or top-off. However, strategically placed airspace

According to Pall Corp., whichmanufactures the Ultipleat SRTfilter pictured here, up to 80% ofall hydraulic system failuresare directly related to dirt andwater contamination of fluids,resulting in wear and corrosionof components and leading tobreakdown of pumps andvalves. Filtration is a frequentlychosen solution to the problemof water contamination.

Prevention Methods—How to Keep It OutPreventing water ingress would be preferred to reliance on moisture removal or chemical additization to preserve func-tionality of a lubricant. However, few lubricant systems can be maintained in a completely water-free state, since they’revery seldom totally closed to the outside environment. Thus, minimizing water ingress is important.

Within a piece of lubricated equipment, differential pressures will occur because of thermal expansion and contrac-tion of lubricant fluid or in the course of emptying and refilling the lubricant reservoir. Dessicant breathers can beattached to reservoir and drum vents to prevent moisture-laden air from entering the system under these circumstances.These breathers usually contain silica gel impregnated with cobalt chloride, which acts as a color indicator by changingfrom blue to pink when the gel is depleted. It is common to also incorporate a particulate filter into such breather con-tamination-protection systems. In some cases, use of a flexible bladder-type expansion chamber is suitable rather thanan outside-environment vent.

The other major equipment-related opportunity for moisture ingress is at the housing seals around rotating shafts,which are meant to retain lubricant and exclude contamination. Lip seals, elastomers which press tightly around a rotat-ing shaft, will exclude moisture when they are new, but the contacting elastomeric surface will wear in the course of serv-ice, usually needing replacement after a few months of continuous use despite the presence of a thin film of lubricant atthe point of elastomer-shaft contact. It’s crucial to ascertain compatibility of not only the base oil but also the additivepackage, with the elastomeric material, since swelling of the seal will lead more quickly to its failure.

Rotating labyrinth seals consist of two mating, but not truly contacting, parts that create a narrow, convoluted path-way through which moisture and particulates find it difficult to infiltrate. These types of seals should not be used withvertical shafts. Some designs employ an O-ring which lifts off a seat by means of centripetal force during operation andpermits expanding air to escape but which reseats to keep out incoming contaminants when the shaft stops turning.Some of the most efficient seals are magnetic seals, usually composed of a stainless steel face adhering magnetically toa polytetrafluoroethylene face; double-faced magnetic seals are even more effective than single-faced ones.

Other steps which can be taken to preclude moisture contamination involve storing drummed or bulk lubricants in aclean, dry and controlled-temperature environment to minimize possible exposure to humidity due to temperature-cyclebreathing. Lubricant users should examine their handling and sampling procedures for possible sources of contamina-tion, and caps, hatches and bungs should always be tightened.


sensors can give immediate warning ofvapor-phase moisture such as ingressaround a seal or through a vent. Anotherwater-analysis procedure uses the reactionof water with calcium hydride to stoichio-metrically produce measurable hydrogengas. No matter what the lubricant system,lower moisture translates into significantlylonger equipment life.

Removal methods—how to get it outSeveral methods, both mechanical andchemical, are available to lower the mois-ture levels in lubricants to less-destructivelevels. The simplest and least effective pro-cedure is settling, wherein free water isallowed to separate from the oil onto thebottom of a containment tank. Heating theoil will increase the rate of settling by low-ering the oil’s viscosity, but it also can stressthe oil by increasing its oxidation rate. Cen-trifuging the oil physically separates watermore rapidly than does gravitational set-tling but still cannot remove dissolvedwater. The usual force used here is severalthousand G’s. Bypass cartridge-type filtersalso can be employed to remove free andemulsified water from oil, using ahydrophilic fiberglass or derivatized cellu-lose medium with large surface area. These

filters work best with lower-viscosity oilsand at constant flow rates. It is importantto service filters regularly to ensure opti-mal functioning.

Two methods permit the removal ofdissolved water as well as emulsifiedand free water. Air stripping involvesintimately mixing heated oil with dry airor nitrogen in a high-velocity mixingnozzle. Moisture is removed from the oil

through its migration to the dry, flowing

gas, while the dried oil is returned to thelubrication system. Vacuum dehydrationcombines moderate heating of the oil and alarge mass-transfer area with a loweredambient pressure to efficiently “boil off” dis-solved water.

A variation of this technique uses less-severe vacuum and temperature conditions,together with a steadily flowing stream ofdry air into which the water transfers. Thelubricant can be repeatedly cycled throughsystems such as these until the desireddegree of dryness is reached. Unlike physi-cal methods of water removal, air strippingand vacuum dehydration will not removenon-volatile additives from the lubricant;however, they can remove dissolved gasesand volatile hydrocarbons that may be pres-ent as contaminants. On the other hand,they also might remove vapor-phase corro-sion inhibitors.

Another typical strategy for dealing withoil in water is chemical in nature. Oil-solu-ble additives called demulsifiers are formu-lated into many lubricants for which waterexposure is likely to be a problem. Demulsi-fiers are special surfactants that disrupt theoil-water interface surrounding each small,stable water droplet, causing them to coa-lesce into drops too large to remain sus-pended in the continuous oil phase (andthus rendering them more amenable to thedrying methods just outlined). An exampleof such chemistry is the class of ethoxylatednonylphenols. ExxonMobil’s Duncansoncautions users against adding demulsifiersto a lubricant after formulation, warning,“Demulsifier systems are specific to eachlubricant formulation. Moreover, excessiveamounts (usually more than 20-50 ppm) canproduce an opposite effect.”

32-39 tlt feature 10-05 - stle...32-39 tlt feature 10-05 9/26/05 4:26 pm page 33. water is one of...

Documents