Cavitation Explained and Illustrated

E. C. Fitch, Tribolics, Inc.

The phenomenon of cavitation consists in the disruption of continuity in the liquid where there is considerable local reduction of pressure. The formation of bubbles within liquids (cavitation) begins even in the presence of positive pressures that are equal to or close to the pressure of saturated vapor of the fluid at the given temperature.

Various liquids have different degrees of resistance to cavitation because they depend, to a considerable degree, upon the concentration of gas and foreign particles in the liquid.

Wear Mechanism

The mechanism of cavitation can be described as follows: Any liquid will contain either gaseous or vaporous bubbles, which serve as the cavitation nuclei. When the pressure is reduced to a certain level, bubbles become the repository of vapor or of dissolved gases.

The immediate result of this condition is that the bubbles increase rapidly in size. Subsequently, when the bubbles enter a zone of reduced pressure, they are reduced in size as a result of condensation of the vapors that they contain.

This process of condensation takes place fairly quickly, accompanied by local hydraulic shocks, the emission of sound, the destruction of material bonds and other undesirable phenomena. It is believed that reduction in volumetric stability in most liquids is associated with the contents of various admixtures, such as solid unwetted particles and gas-vapor bubbles, particularly those on a submicroscopic level, which serve as cavitation nuclei.

A critical aspect of the cavitation wear process is surface destruction and material displacement caused by high relative motions between a surface and the exposed fluid. As a result of such motions, the local pressure of the fluid is reduced, which allows the temperature of the fluid to reach the boiling point and small vapor cavities to form.

When the pressure returns to normal (which is higher than the vapor pressure of the fluid), implosions occur causing the cavity or vapor bubbles to collapse. This collapse of bubbles generates shock waves that produce high impact forces on adjacent metal surfaces and cause work hardening, fatigue and cavitation pits.

Thus, cavitation is the name given to a mechanism in which vapor bubbles (or cavities) in a fluid grow and collapse due to local pressure fluctuations. These fluctuations can produce a low pressure, in the form of vapor pressure of the fluid. This vaporous cavitation process occurs at approximately constant temperature conditions.

Cavitation Types

Two principal types of cavitation exist: vaporous and gaseous.

Vaporous cavitation is an ebullition process that takes place if the bubble grows explosively in an unbounded manner as liquid rapidly changes into vapor. This situation occurs when the pressure level goes below the vapor pressure of the liquid.

Gaseous cavitation is a diffusion process that occurs whenever the pressure falls below the saturation pressure of the noncondensable gas dissolved in the liquid. While vaporous cavitation is extremely rapid, occurring in microseconds, gaseous cavitation is much slower; the time it takes depends upon the degree of convection (fluid circulation) present.

Cavitation wear occurs only under vaporous cavitation conditions - where the shock waves and microjets can erode the surfaces. Gaseous cavitation does not cause surface material to erode.

It only creates noise, generates high (even molecular level cracking) temperatures and degrades the chemical composition of the fluid through oxidation. Cavitation wear is also known as cavitation erosion, vaporous cavitation, cavitation pitting, cavitation fatigue, liquid impact erosion and wire-drawing.

Cavitation wear is a fluid-to-surface type of wear that occurs when a portion of the fluid is first exposed to tensile stresses that cause the fluid to boil, then exposed to compressive stresses that cause the vapor bubbles to collapse (implode).

This collapse produces a mechanical shock and causes microjets to impinge against the surfaces, unifying the fluid. Any system that can repeat this tensile and compressive stress pattern is subject to cavitation wear and all the horrors accompanying such destructive activity.

Cavitation wear is similar to surface fatigue wear; materials that resist surface fatigue (hard but not brittle substances) also resist cavitation damage.

Cavitation Wear Process

Liquid is the medium that causes cavitation wear. Cavitation wear does not require a second surface; it requires only that high relative motion exists between the surface and the fluid. Such motion reduces the local pressure in the fluid. When the liquid reaches its boiling point and ebullition occurs, vapor bubbles form, which produces cavitation.

Each vapor cavity lasts a short time because almost any increase in pressure causes the vapor in the bubble to condense instantaneously and the bubble to collapse and produce a shock wave. This shock wave then impinges on adjacent metal surfaces and destroys the material bonds.

The shock wave first produces a compressive stress on the solid surface, and then when it is reflected, produces a tensile stress that is normal to the surface.

Figure 1. Vapor Bubble Collapse and the Birth of a Microjet

Figure 1 depicts the collapse of a vapor bubble and the birth of a microjet. Cavitation is generally found where a hydrodynamic condition, characterized by a sudden and gross change in hydrostatic pressure, exists. Because ebullition can occur the instant pressure drops, vapor bubbles form and collapse frequently and quickly.

Entrained air and dust particles in the fluid serve as nucleation sites for the formation of vapor cavities. These nuclei can be small gas-filled pockets in the crevices of the container or simply gas pockets on contaminant particles moving freely in the flow stream. Therefore, all confined fluids may contain sufficient impurities to produce cavitation.

Small voids near the surface or flow field, where minimum pressure exists, indicate that cavitation has begun. Once initiated, bubbles continue to grow as long as they remain in low-pressure regions. As the bubbles travel into high-pressure regions, they collapse, producing intense pressures and eroding any solid surfaces in the vicinity.

During the collapse, particles of liquid surrounding the bubble quickly move to its center. Kinetic energy from these particles creates local water hammers of high intensity (shock), which grow as the front progresses toward the center of the bubble.

Audible and Visual Detection

Equipment users can detect cavitation audibly, visually, by acoustical instrumentation, by machine vibration sensors, through sonoluminescence measurement or by a decrease or change in performance from that produced under single-phase flow conditions (for example, loss of flow, rigidity and response).

Under cavitating flow conditions, the wear rate can be many times greater than that caused by erosion and corrosion alone. Cavitation wear can destroy the strongest of materials - tool steels, stellites, etc. Such damage can occur rapidly and extensively.

The amount of damage that cavitation causes depends on how much pressure and velocity the collapsed bubbles create. As a result of this pressure and velocity, the exposed surface undergoes a variety of widely varying intensities.

Each imposition lasts only a short time; the impulse magnitudes and collapse times are greater for larger bubbles at given collapsing pressure differentials. Thus, the greater the tensile stress on the fluid (the lower the static pressure), the larger the bubbles, the more intense the cavitation and the more serious the damage.

The impulses that result when vapor bubbles form and collapse cause individual symmetrical craters and permanent material deformations when the collapse occurs next to the surface. Consequently, cavitation damage, like fatigue failure, has several periods of activity:

• Incubation period - microcracks nucleate around grain boundaries and inclusions due to both elastic and plastic deformation of the surface.
• Accumulation period - crack growth proceeds in relation to the degree of splitting, shearing and tearing action on the material.
• Steady-state period - the rate of crack nucleation and propagation becomes constant for the remainder of the exposure time.

In a fluid flow system (unlike an ultrasonic tank), vapor bubbles form where fluid tensile stresses (low pressures) occur, and vapor bubbles collapse in higher-pressure regions where compressive stresses can be imposed on the fluid.

So the region where damage occurs is often quite separate from the region in which cavities are created - often leading to an incorrect diagnosis of the problem. Cavitation wear is mechanical in nature and cannot occur without the application of the tensile and compressive stresses.

Cavitation Hot Spots

Many areas in hydraulic systems are prone to cavitation wear, such as:

• Downstream of control valves that have high pressure differentials,
• In the suction chambers of pumps where starved inlet conditions exist,
• In rapidly moving actuators (both linear and rotary types) where negative load conditions occur,
• In leakage paths (across seals, valve seats and spool lands) where high velocities cause pressure levels to drop below the vapor pressure of the fluid (a cavitation condition often referred to as wire-drawing) and
• In all devices where fluid flow is subjected to sharp turns, reduction in cross-sections with subsequent expansions (in cocks, flaps, valves, diaphragms) and other deformations.

Cavitation disturbs the normal operating conditions of fluid-type mechanical systems and destroys the surfaces of components. The process consists of cavities forming when pressures are low, the growth of subsequent bubbles as pressure stabilizes and finally the collapse of the bubbles when the cavities (gaseous or vaporous bubbles) are exposed to high-pressure.

Note that the pressure drop across the component is the driving force for cavitation wear. Figure 2 depicts the cavitation process that occurs in a gear pump and in a spool valve showing how cavities generate, grow and collapse in fluid-type components.

Figure 2. Cavitation Process in Hydraulic Components

Reducing Cavitation Wear

In cavitation wear, microcracks propagate to the point where the material can no longer withstand the impulse load that the imploding vapor bubbles impose. Therefore, particles finally break off and enter the system.

As with any fatigue failure, microcracks first form at stress risers (notches, tears, undercuts, welding defects, etc.) or at heterogeneous areas of the material (such as at the directionality of metal flow, inclusions, and decarburized sections).

Therefore, a rough surface is prone to cavitation wear and because pittings and a rough profile characterize the cavitation damage, the damage increases as the surface becomes rougher.

The most basic means of combating cavitation wear is to minimize the tensile stress on the fluid. In other words, the equipment users must lower the level of refraction or vacuum conditions in zones of possible cavitation. In particular, the following steps may be appropriate:

• Increase the pressure level at the outlet of throttling valves.
• Increase the inlet pressure at the pump suction port by supercharging the pump inlet.
• Use anticavitation checks on negative load actuator applications.
• Reduce the water content of the fluid to eliminate the possibility of wire-drawing (water has a higher vapor pressure than oil) across valve seats and dynamic seals.
• Use a fluid with a low vapor pressure.
• Select a pump with good filling characteristics as opposed to a starved inlet configuration.
• Use a fluid with low viscosity or increase the fluid temperature.

In many cases, design engineers can minimize cavitation damage by properly selecting fabrication materials. For example, stainless steel may be selected instead of aluminum (Figure 3) and use hard facing with a cavitation-resistant alloy on the exposed surface. Rubber and other elastomeric coatings have also helped minimize cavitation wear. Despite their low resistance to cavitation, these surfaces reflect the shock wave without causing intense damage.

Figure 3. Order of Relative Cavitation Resistance of Materials

Cavitation Particles

The size of the particles generated by cavitation wear is a function of the Brinell hardness of the exposed material. The largest particles occur during the accumulation period. The slopes of the cumulative particle size distribution curves increase as the strain energy of the material increases. The average size of the particles produced by cavitation decreases as the cavitation intensity increases.

Precursors of Cavitation

When investigating a cavitation problem in a fluid system, you must identify all possible sources of low pressure (vacuum), high temperature (heat), and locations where air might be ingressing. The following list should serve as a guideline for identifying low pressure areas in a fluid system:

• Pump suction - improper suction line hydraulics (flow limiting conditions).
• Valve orifice effect - vortexes from high velocity jet in control valve flow passages.
• Submerged jet - a jet extending into unbounded flow areas where regions of low pressure are created.
• Negative loads on motors and cylinders - externally driven actuator loads create low pressure in actuator.
• Pressure surges and water hammer - the rarefaction portion of pressure waves are capable of creating negative pressure regions in the line.
• High altitude effect - low atmospheric pressure subjects the suction line to pressure that may prove inadequate in filling the pumping chambers.

Sources of Heat that Lead to Cavitation

The sources of heat that contribute to excessively high temperatures and cavitation in system fluid include the following:

• High ambient temperature
• Poor mechanical efficiency of pumps and motors
• Turbulent flow conditions in conduits
• Heat of vaporization in cavitating flow
• Heat of compression in aerated flow
• High pressure drops across control orifices
• Severe operational duty cycle
• Major flow restrictions in all parts of the fluid circulating system
• Poor cooling or lack of heat transfer
• High friction from rough surfaces and abrasive action

Possible Air Ingression Locations to Check

As far as air ingression points of a system are concerned, you should carefully scrutinize these locations when serious cavitation is occurring:

Reservoirs - sites where mechanical (agitation) type air entrainment occurs, swirling fluid exists, fluid impingement on liquid or solid surfaces, pressurized reservoir conditions, cyclonic flow at pump suction port, critical altitude (angled reservoir) occurring during operation that exposes the pump suction port to the atmosphere, jostling of the fluid due to movement over rough terrain and/or low reservoir fluid level that expose the pump suction port to the atmosphere.

Pump - small diameter conduits and/or ports, restrictive flow passages, flow diversions, and/or long suction line conditions, poor pump filling characteristics (restrictive internal flow passages, high pumping speed, overly large flow displacement); altitude too high to provide sufficient reservoir pressure to supply the pump at rated flow conditions; inadequate suction head to lift fluid to pump inlet level (that is, elevation between fluid level and pump intake too great), insufficient suction head to accelerate reservoir fluid to the rated flow conditions of the pump (non-responsive to the pump displacement demands).

Valves - jets discharging from orifices into limited flow space, streamline flow through channels terminating in chambers where low pressure is at the downstream walls of the valve, and/or throttle valves discharging into a low pressure (return line) conduit.

Actuators (extended seals) - air passing rod seals, air desorption existing, and/or vaporous cavities forming when negative loading occurs due to external inertial loads.

Motors (shaft seals) - air passing seals and gaseous/vaporous cavitation occurring when negative loading exists due to a flywheel effect.

Accumulator - air/gas leaking past worn piston seal, ruptured diaphragm, or torn bladder.

Filter - air passing external seals in suction line filters or internal flow restrictions causing air desorption.

Conduit connectors (hose couplings, tube fittings, and manifold seals) - air passing connector sealing surfaces that vibration and thermal expansion and contraction effects have loosened.

Conduit - rough walls, pinched down flow sections, or protrusions in flow stream.

Detecting and Controlling Water in Oil

Marianne Duncanson, 

Moisture is considered a chemical contaminant when suspended or mixed with lubricating oils. It presents a combination of chemical and physical problems for the lubricant and machinery, respectively. The potential problems, states of existence and methods for measuring moisture are discussed here.

Effects of Water on Equipment and Lubricants

The effects of water are insidious. Failure due to water contamination may be catastrophic, but it may not be immediate. Many failures blamed on lubricants are truly caused by excess water. The following are some of the effects of water on equipment:

• Shorter component life due to rust and corrosion
• Water etching/erosion and vaporous cavitation
• Hydrogen embrittlement
• Oxidation of bearing babbitt
• Wear caused by loss of oil film or hard water deposits

Rust and Corrosion

Water attacks iron and steel surfaces to produce iron oxides. Water teams up with acid in the oil and corrodes ferrous and nonferrous metals. Rust particles are abrasive. Abrasion exposes fresh metal which corrodes more easily in the presence of water and acid.

Water Etching

Water etching can be found on bearing surfaces and raceways. It is primarily caused by generation of hydrogen sulfide and sulfuric acid from water-induced lubricant degradation.

Erosion

Erosion occurs when free water flashes onto hot metal surfaces and causes pitting.

Vaporous Cavitation

If the vapor pressure of water is reached in the low-pressure regions of a machine, such as the suction line of a pump or the pre-load region of a journal bearing, the vapor bubbles expand.

Should the vapor bubble be subsequently exposed to sudden high pressure, such as in a pump or the load zone of a journal bearing, the water vapor bubbles quickly contract (implode) and simultaneously condense back to the liquid phase.

The water droplet impacts a small area of the machine’s surface with great force in the form of a needle-like micro-jet, which causes localized surface fatigue and erosion. Water contamination also increases the oil’s ability to entrain air, thus increasing gaseous cavitation.

Hydrogen Embrittlement

Hydrogen embrittlement occurs when water invades microscopic cracks in metal surfaces. Under extreme pressure, water decomposes into its components and releases hydrogen. This explosive force forces the cracks to become wider and deeper, leading to spalling.

Film Strength Loss

Rolling element bearings and the pitch line of a gear tooth are protected because oil viscosity increases as pressure increases. Water does not possess this property. Its viscosity remains constant (or drops slightly) as pressure increases. As a result, water contamination increases the likelihood of contact fatigue (spalling failure).

The effects on lubricating oil can be equally harmful:

• Water accelerates oxidation of the oil
• Depletes oxidation inhibitors and demulsifiers
• May cause some additives to precipitate
• Causes ZDDP antiwear additive to destabilize over 180°F
• Competes with polar additives for metal surfaces

Maximum Recommended Water Concentrations

Oil, unless it is dried, contains some dissolved water. Figure 1 shows the amount of dissolved water that can be found in ISO 220 paper machine oil and ISO 32 turbine lubricant before it turns cloudy.

Figure 1. Dissolved Water as a Function of Temperature
in Paper Machine Oil and Turbine Oil

Table 1 helps determine the relative life of mechanical equipment versus the amount of water in the lubricant. To use the chart, estimate the current moisture level in the system along the y-axis, move toward the right to the target moisture level. The top of the chart gives the estimate of how much the life of the oil is extended. For example, by reducing moisture from 2,500 ppm to 156 ppm, machine life is extended by a factor of 5.

	Life Extension Factor
Current Moisture Level	ppm	2	3	4	5	6	7	8	9	10
	50,000	12,500	6,500	4,500	3,125	2,500	2,000	1,500	1,000	782
	25,000	6,250	3,250	2,250	1,563	1,250	1,000	750	500	391
	10,000	2,500	1,300	900	625	500	400	300	200	156
	5,000	1,250	650	450	313	250	200	150	100	78
	2,500	625	325	225	156	125	100	75	50	39
	1,000	250	130	90	63	50	40	30	20	16
	500	125	65	45	31	25	20	15	10	8
	250	63	33	23	16	13	10	8	5	4
	100	25	13	9	6	5	4	3	2	2
Table 1. Moisture Life Extension Method

Tests for Water in Oil

The guidelines in Table 1 help only if it is known how much water is in the oil. There are several qualitative and quantitative tests to determine water content. The easiest one to perform is a simple visual test. An ISO 68 turbine lubricant was observed at room temperature with controlled amounts of water. Table 2 shows the results of the test.

Amount of water, ppm	Appearance of oil
0	Bright and clear
100	Trace of translucent haze
200	Slight translucent haze
250	Translucent haze
500	Opaque haze
1000	Opaque haze with slight water drop out
Table 2. Visual Check of Water in Turbine Oil

Bear in mind that several factors can affect the cloudy or hazy appearance of the oil. First, as the oil sits, it will clear up and the oil may become supersaturated. Second, dye and dark-color oil can mask cloudiness.

Table 1. Comparison of Water-Removing Technologies

Options for Removing Water in Oil

Martin Williamson

Water, water, everywhere . . . Water is ever present in the environment. Unless you live in an arid region, it is a fundamental fact of life. Water co-exists in oil in essentially the same way it co-exists in the atmosphere. It starts off in the dissolved phase - dispersed molecule-by-molecule throughout the oil.

Just like water present in the air, it cannot be seen in oil, which may appear clear and bright. However, once the saturation point is exceeded, water is typically present in the emulsified phase creating a milkiness or fog in the oil, just like moist air on a cool day.

When sufficient water exists, or when the oil has adequate demulsibility, free water will collect. Because water is typically heavier than oil, it settles below the oil, at the bottom of sumps and reservoirs.

The point at which an oil contains the maximum amount of dissolved water is termed the saturation point. The saturation point is dependent on the oil’s temperature, age and additive composition. The higher the temperature, the higher the saturation point and hence more water held in solution, in the dissolved phase.

This is the same as being able to dissolve more sugar in hot water, than in cold water. Similarly, the older the oil, the higher the level of water that can be dissolved. This is due to polar by-products of oxidation in the oil, which act as “hooks” holding on to the water molecules and keeping them in solution. Likewise, highly additized oils, like crankcase oils, have a higher saturation point than lightly additized oils like turbine oils, because the additives - many of which are polar - also hold the water in solution.

The Effects of Water

Why is water considered an evil? Water will affect the oil’s base stock, encouraging oxidation, viscosity increase and foaming.

Water can also affect the additive package through water washing and hydrolysis, leading to acids and additive depletion. Water encourages rust and corrosion and will cause increased wear as a result of aeration, changes in viscosity resulting in film strength failure, hydrogen blistering and embrittlement, and vaporous cavitation.

Finally, water is a generator of other contaminants in the oil such as waxes, suspensions, carbon and oxide insolubles and even micro-organisms.

Water Prevention and Removal Strategies

Water ingression is either insidious as a result of atmospheric humidity levels or immediate as in water jet washing or sudden seal failure. Whatever the source, immediate attention is required to remove it.

If significant water ingress has occurred over a prolonged period, detailed oil analysis, such as rust and corrosion inhibition characteristics, remaining useful life measurements, demulsibility and foam suppression and tendency may also be necessary to determine the oil’s suitability for further use. Merely replacing the oil will not cure the ingress source. Root cause corrective measures are necessary to resolve or limit water ingression.

Basic measures to address water ingression include the use of desiccating breathers, improved seal technology and training maintenance and operations personnel to avoid direct contact with wash down water on shaft seals and breathers.

Measures to minimize water ingress should start in the oil store. Drums and tanks should be sheltered from the environment. Even indoors, this means they should be sheltered against process water sprays, fire sprinkler tests and general cleaning sprays. Open barrels should also be protected with desiccant-style filters, particularly in humid storage areas, to prevent water build up and oil degradation.

A number of methods or technologies, from inexpensive gravity separation to complex vacuum dehydration, exist to remove water. Which technology is most effective will depend on the target dryness level required, the volume of water that must be removed, the base oil (mineral, synthetic, etc.) and the required flow and processing rate. The following is an outline of technologies that can remove water from oil, together with their relative advantages and disadvantages.

Gravity Separation

As already mentioned, free water in the system will settle to the bottom of the tank (assuming the specific gravity of water is greater than the lubricant). The time it takes the water to separate will depend on the system’s temperature, as well as the additive formulation, age of the oil and the base oil type. Some oils are designed to hold water in suspension rather than to allow it to separate out, making gravity separation a less-than-effective strategy.

In basic systems, opening the drain valve and allowing the water to drain off may be sufficient. The effectiveness of this action, however, will depend upon how long the system was allowed to stand prior to draining the water, whether the temperature was low enough to lower the saturation point dramatically and the oil’s demulsibility characteristics.

Lowering the saturation point helps ensure that as much of the water as possible will exist in the free state. In larger volume systems, a separate settling tank may be employed to allow the oil to cool and demulsify prior to water removal (Figure 1).

Figure 1. Settling Tanks for Moisture and Solids Separation

The major downside to this method is that it removes only free water, so elements of emulsified and dissolved water will remain. The upside is the low cost of water removal.

Centrifuge - Spin Your Oil Clean

The principle of the centrifuge (Figure 2) is to separate the oil’s heavier elements by spinning the oil to create high G-forces - often in the tens of thousands of Gs.

Figure 2. Centrifugal Separation

The greater the difference in specific gravity between the contaminant and the oil, the more effective the process. For this reason, centrifuges often work better on low specific gravity and low viscosity oils, like turbine oils, rather than heavier gear type oils.

In a centrifuge, both free and emulsified water will be removed; this will depend to some extent on the type of additive package, as some water will be held in suspension in the oil. Just like gravity separation, the lower the oil’s temperature, the more effective the removal process will be, because much of the water will exist in the emulsified and free states.

As a tool, a centrifuge is relatively expensive. However, given that it is also a means of removing other heavier contaminants and has a comparatively high throughput compared to other technologies, centrifugal separators are relatively cost effective.

The downside of centrifuges is that only emulsified and free water will be removed - although this can be partially overcome by keeping temperatures low.

Absorption Removal

Typically, most filter media will absorb a small amount of moisture from the oil, resulting in swelling of the media. This is particularly true for cellulose-based media. In fact, examination of used filters will often indicate if the presence of water is a concern. Some filter cartridges with an additional wrap consisting of polymer and desiccants are available.

These filters are specifically designed to remove water by absorption and remove both emulsified and free water, as well as solids. However, the elements typically have a limited volume capacity and are best fitted to a portable filter cart for minor water ingression problems.

In fact, when a small gearbox is being fitted with an expansion chamber type breather, it is worthwhile to filter the gearbox with a water-removing element to remove any trace elements of moisture that may condense out on surfaces within the unit when it cools.

The main disadvantage of absorption removal is that it has a limited capability for water removal per element. The positive aspect is not just its ability to trap solids, but also that it is a relatively cost-effective means of dealing with small systems that require polishing to remove moisture.

Vacuum the Oil Dry

The vacuum dehydration process (Figure 3) lowers the partial pressure, which assists in removing the water from the oil. Just like boiling water on top of Mount Everest, lowering the pressure allows water (and other volatile materials) to boil at a much lower pressure.

Figure 3. Vacuum Dehydrator

At the typical pressures used by most vacuum dehydrators (25" to 28" of mercury), water boils at 120°F to 130°F. By heating the oil, typically to 150°F to 160°F, water is vaporized inside the dehydrator, without causing excessive oil degradation due to thermal and oxidative stress. In most dehydrators, the air is warmed and dried prior to being passed over the oil, encouraging the water to transfer from the oil into the air.

To maximize the process, the oil is thinned to obtain the greatest amount of surface area exposure possible. This is achieved by allowing the oil to pass over a number of surfaces internally in the vacuum chamber, or by creating an umbrella spray within the chamber through which the dry air passes.

The real benefit of this process is its ability to remove dissolved water and other low-boiling liquid impurities such as fuels and solvents. The removal of dissolved water makes it ideal for systems requiring low target levels of moisture. It is particularly useful in environments where large volumes of oil are at risk from the process or system, such as in steam turbines or paper mills.

In fact, for lightly additized oils such as turbine oils and transformer oils, a vacuum dehydrator can remove as much as 80 percent to 90 percent of dissolved water, achieving water levels as low as a few ppm.

The main disadvantage of vacuum dehydrators is their cost and comparatively low flow rate. Because of the cost, many companies chose to rent dehydrators on an “as-needed” basis rather than purchase them.

The main advantage of vacuum dehydrators is that they offer the ability to remove moisture to very low levels. The greater the volume of oil and water, and the lower the target moisture level, the more cost-effective vacuum separation becomes.

Dehydration by Air Stripping

An alternative technology to vacuum dehydration is dehydration by air stripping (Figure 4), a process that removes water as well as gaseous contaminants in the oil. Not only does it remove free and emulsified water, but also dissolved water down to less than 100 ppm.

Figure 4. Air Stripping Dehydrator

Because of its ability to degas, it is also suitable for removing hydrocarbons in seal oil systems. Air stripping works by drawing air or nitrogen gas into a stream of heated oil, which mixes in and absorbs the water and gasses within the oil. The oil/air is then expanded to release the air or nitrogen, which takes the impurities with it.

Generally, the water removed will be of a reasonable quality, sufficient to allow it to be drained off in the normal network without special disposal requirements. The exhaust air and gasses are also controlled to minimize the oil vapor released.

Just like vacuum dehydrators, cost is an issue with air stripping. However, its advantage is that it costs less to maintain than a typical vacuum dehydrator because it has fewer moving parts. The fact that it can also remove other gaseous impurities, as well as dissolved water, makes air stripping technology an effective alternative to vacuum dehydration.

Heat the Oil Dry

Some applications are self-cleansing because they run at elevated temperatures and consequently, water is evaporated. The combustion engine is a perfect example of a self-cleaning application. However, some settling tanks (see gravity separation) may also include heating elements to assist with water removal below the saturation point.

Whether it is best practice to deliberately heat the oil briefly to drive off moisture to maintain oil health is open to debate. Allowing the water to remain in the oil is usually far more damaging than briefly heating the oil. Therefore, heater units are available as portable water removal systems. In static systems, like reservoirs, it is important to ensure that the power density of such elements remains below 5W/in2 to minimize thermal stress to the oil.

The downside to heating oil is that it must be controlled, particularly with mineral oils, to avoid harm. However, the relative cost is less than the centrifugal or vacuum separation technologies, making this an effective water removing tool in certain circumstances. The decision about which main water removal technology is best will predominantly be based on the volume of oil and the water to be treated.

The decision will be further impacted by the need to reach a target moisture level. If target moisture levels are well below the saturation limit, then more complex and expensive methods will be required as necessary if large quantities of free and emulsified water are to be removed

How to Measure Water In Oil

Noria Corporation

Water is perhaps the most harmful of all contaminants with the exception of solid particles. While the presence of water is often overlooked as the primary root cause of machine problems, excess moisture contamination can lead to premature oil degradation, increased corrosion and increased wear.

Moisture, upon contaminating hydraulic and lubricating oils, has a degrading effect to both the lubricant and the machine itself. While some additives adsorb to the water and are removed when the water separates from the oil, others are destroyed by water-induced chemical reactions. Water also promotes oxidation of the oil's base stock, causes rust and corrosion of machine surfaces, and reduces critical, load-bearing film strength. Essentially, water represents a real risk to equipment and should be aggressively controlled.

The Varying States of Water

Water coexists with oil in either a dissolved or a free state. When single water molecules are distributed throughout the oil due to the water's chemical attraction to the fluid, it is in a dissolved state. Numerous factors such as viscosity, base stock type and condition, impurities, and additive package determine the volume of water that will be dissolved by the oil.

Additionally, the dissolved volume is a function of the oil's temperature, thus the humidity is reported as relative humidity (depending on the temperature). If the oil has dissolved all of the water possible at a given temperature, it is saturated. Dissolved water is difficult to control but causes only minimal harm to the machine and oil.

When the saturated oil experiences a temperature decrease, it reaches a point where water will not condense into a free form. This is called the dew point temperature. Free water is the other state in which water coexists with the oil. Water is in a free state when undissolved globules of water are physically suspended in the oil.

Large globules tend to separate to the bottom of the reservoir or sump. However, in mechanical equipment, the shearing forces of gears, pumps, bearings, etc. tend to crush the water into such small globules that a stable emulsion exists.

An emulsion is the stable state of physical coexistence of chemically insoluble substances, like oil and water. Additives and impurities that lower the oil's surface tension can serve as agents to strengthen the emulsion. Free and emulsified water pose the greatest risk to the machine and the lubricant, and should be placed under strict control.

Visual Crackle Test

The simplest way to determine the presence of water in oil is to use the Visual Crackle test. While this is an effective test for identifying free and emulsified water down to say 500 ppm, its biggest limitation is that the test is nonquantitative and fairly subjective.

False positives are possible with entrained volatile solvents and gases. Nevertheless, as a screening tool in the lab and the field, the crackle test will always have a role to play where a quick yes or no answer is required for free and emulsified water.

FTIR Analysis

FTIR can be an effective method for screening samples containing in excess of 1,000 ppm of water, provided a correct new oil baseline is available for spectral subtraction. However, due to its limited precision and comparatively high detection limits, FTIR is not adequate in many situations where precise water concentrations below 1,000 ppm or 0.1 percent are required.

Dean and Stark Method

The classic method for determining water-in-oil is the Dean and Stark distillation method (ASTM D95). This test method is fairly cumbersome and requires a comparatively large sample to ensure accuracy, which is why it is rarely used in production-style oil analysis labs today.

The method involves the direct codistillation of the oil sample. As the oil is heated, any water present vaporizes. The water vapors are then condensed and collected in a graduated collection tube, such that the volume of water produced by distillation can be measured as a function of the total volume of oil used.

Dean and Stark Distillation Apparatus (ASTM D95)

Karl Fischer Moisture

The Karl Fischer Moisture test is the method of choice when accuracy and precision are required in determining the amount of free, dissolved and emulsified water in an oil sample. However, even within the scope of Karl Fischer testing, there are several methodologies that are used.

All Karl Fischer procedures work in essentially the same way. The oil sample is titrated with a standard Karl Fischer reagent until an end-point is reached. The difference in test methods is based on the amount of sample used for the test and the method used to determine the titration end-point.

The most frequently used Karl Fischer method follows ASTM D1744 and involves volumetric titration of the sample, using a potentiometric cell to determine the end-point. While this method is reliable and precise, there can be reproducibility problems at low water concentrations (200 ppm or less). In addition, the test can be subject to interferences from sulfurous additives (for instance, AW and EP-type additives) and ferric salts which may be present due to wear debris.

Both of these react with the Karl Fischer reagent as if they were water and can give a false positive, resulting in an overstating of the water concentration. In fact, a new, clean, dry AW or EP oil may give a reading of as much as 200 to 300 ppm, due to the reaction of the additives, rather than because of excess moisture.

More recently, labs have been switching to a coulometric titration method described in ASTM D6304. This method is more reliable than D1744 at low water concentrations and is less prone to interference effects, although again, AW and EP additized oils can show as much as 100 ppm of water as a result of the effects of the sulfurous additives.

The most reliable method is ASTM D6304, complete with codistillation. With the codistillation method, the oil sample is heated under a vacuum so that any water present in the sample evaporates. The water vapors are condensed and dissolved into toluene, which is then titrated using the D6304 procedure.

Because the additives and other interfering contaminants that may be present in a used oil sample remain dissolved or suspended in the oil, the condensed water in the toluene is free from interference effects and is a true count of water present in the sample.

Another less commonly used method is ASTM D1533, which is used for determining water concentrations down to 10 ppm or less in transformer oils using a coulometric Karl Fischer reagent.

Calcium Hydride Test Kits

One of the simplest and most convenient ways to determine water concentrations in the field is by using a calcium hydride test kit. This method employs the known reaction of water with solid calcium hydride to produce hydrogen gas. Because the reaction occurs stoiciometrically, the amount of hydrogen gas liberated is directly proportional to the amount of water present in the sample.

Therefore, the water content of the sample can be determined by measuring the rise in pressure in a sealed container due to the liberation of hydrogen gas as any water in the sample reacts with the calcium hydride. Used correctly, these test kits are reported to be accurate down to 50 ppm free or emulsified water.

Saturation Meters

When the amount of water present in an oil sample is below the saturation point, saturation (dew-point) meters can be used to indirectly quantify water content. The saturation point of an oil is simply the point at which the oil contains as much water in the dissolved state as possible, at a given temperature.

At this point, the oil is saturated or has a relative humidity of 100 percent. Most saturation meters use a thin film capacitive device, whose capacitance changes depending on the relative humidity of the fluid in which it is submerged. Saturation meters have proven to be accurate and reliable at determining the percent saturation of used oils.

The biggest drawback with saturation meters is the fact that the saturation point is strongly dependent on temperature as well as the presence (or absence) of polar species, including additives, contaminants and wear particles. In addition, with water levels in excess of the saturation point, typically 200 to 600 ppm for most industrial oils, saturation meters are unable to quantify water content accurately.

Despite these limitations, saturation meters can be a useful trending tool to determine moisture onsite, provided they are used frequently and routinely.

Monitoring and controlling water levels in any lubricating system is important. Whether it is a large diesel engine, a steam turbine, a hydraulic system or an electrical transformer, water can have a significant impact on equipment reliability and longevity.

Regular water monitoring, whether it be a simple onsite crackle test or a lab-based Karl Fischer moisture test should become a standard condition-monitoring tool. But remember, like all tests, the methods used to detect water in oil have strengths and weaknesses, so be sure to select the one that meets your needs and desired detection limits.

Water In Oil Contamination

Noria Corporation

Much has been said about particle contamination and its effect on component longevity. It is well known that an improvement in particle contamination by one ISO Cleanliness Code can result in a 10 to 30 percent increase in the life of contamination-sensitive components such as hydraulic valves, pumps, and journal and rolling element bearings.

Industry spends millions of dollars each year on improved filtration technology in an attempt to reduce particle contamination, with some of the more advanced companies reducing failure rates by up to 90 percent simply by controlling fluid cleanliness. However, in some industries and environments, water is a far more insidious contaminant than solid particles, and is often overlooked as the primary cause of component failure.

Water In Oil States of Coexistence

Water can exist in oil in three states or phases. The first state, known as dissolved water, is characterized by individual water molecules dispersed throughout the oil. Dissolved water in a lubricating oil is comparable to moisture in the air on a humid day - we know the water is there, but because it is dispersed molecule-by-molecule, it is too small to see.

For this reason, an oil can contain a significant concentration of dissolved water with no visible indication of its presence. Most industrial oils such as hydraulic fluids, turbine oils, etc., can hold as much as 200 to 600 ppm of water (0.02 to 0.06 percent) in the dissolved state depending on the temperature and age of the oil, with aged oils capable of holding three to four times more water in the dissolved state than new oil.

Once the amount of water has exceeded the maximum level for it to remain dissolved, the oil is saturated. At this point, the water is suspended in the oil in microscopic droplets known as an emulsion. This is similar to the formation of fog on a cool, spring day. In this case, the amount of moisture in the air exceeds the saturation point, resulting in a suspension of small droplets of moisture or fog. In a lubricating oil, this “fog” is often referred to as haze with the oil said to be cloudy or hazy.

The addition of more water to an emulsified oil/water mixture will lead to a separation of the two phases producing a layer of free water as well as free and/or emulsified oil. This is like rain falling when the amount of moisture in the air becomes excessive. For mineral oils and PAO synthetics whose specific gravity is less than 1.0, this free water layer is found on the bottom of tanks and sumps.

The Effects of Water Contamination

In a lubricating system, the two most harmful phases are free and emulsified water. In journal bearings for example, the incompressibility of water relative to oil can result in a loss of the hydrodynamic oil film that in turn leads to excessive wear. As little as one percent water in oil can reduce the life expectancy of a journal bearing by as much as 90 percent.

For rolling element bearings, the situation is even worse. Not only will water destroy the oil film strength, but both free and emulsified water under the extreme temperatures and pressures generated in the load zone of a rolling element bearing can result in instantaneous flash-vaporization causing erosive wear to occur.

Under certain conditions, water molecules can be ripped up into their constituent oxygen and hydrogen atoms as a result of the high pressures generated in the load zone of a rolling element bearing. Due to their relatively small size, the hydrogen ions produced by this process can absorb onto the surface of the bearing raceway resulting in a phenomenon known as hydrogen embrittlement.

Hydrogen embrittlement is caused by a change in subsurface bearing metallurgy. This change causes the bearing material to become weak or brittle and prone to cracking beneath the surface of the raceway. When these subsurface cracks spread to the surface, the result can lead to pitting and spalls.

Because the effects of free and emulsified water are more harmful compared to dissolved water, a general rule of thumb is to ensure that moisture levels remain well below the saturation point. For most in-service oils this means 100 to 300 ppm or less depending on the oil type and temperature.

However, even at these levels, a significant amount of damage can still occur. Generally speaking, there is no such thing as too little water and every reasonable effort should be made to keep water contamination as low as possible.

Water-Related Bearing Damage

The Effects of Water on a Lubricant

Not only does water have a direct harmful affect on machine components, but it also plays a direct role in the aging rate of lubricating oils. The presence of water in a lubricating oil can cause the progress of oxidation to increase tenfold, resulting in premature aging of the oil, particularly in the presence of catalytic metals such as copper, lead and tin.

In addition, certain types of synthetic oils such as phosphate esters and dibasic esters are known to react with water, resulting in the destruction of the base stock and the formation of acids.

It is not just the base oil that can be affected by moisture contamination. Certain additives such as sulfurous AW and EP type additives and phenolic antioxidants are readily hydrolyzed by water, resulting in both additive mortality and the formation of acidic by-products.

These acidic by-products can then cause corrosive wear, particularly in components containing soft metals such as Babbitt used with journal bearings and bronze and brass components. Other additives such as demulsifying agents, dispersants, detergents and rust inhibitors can be washed away by excessive moisture. This results in sludge and sediment buildup, filter plugging and poor oil/water demulsibility.

Measuring Water

In order to control moisture levels, one must be able to detect its presence. There are five basic test methods used to determine the moisture content of a lubricating oil. These methods range from a simple apparatus to a more complex chemical test or slightly more expensive percent saturation probe test ideal for on-site screening purposes. It may also include more advanced technology typically used in laboratories for precise determination of the water level in ppm.

The most basic is the Crackle Test. In this test, a hot plate is held at 320°F (130°C) and a small drop of oil placed in the center. Any moisture present in the oil is reflected in the number of bubbles observed as the water vaporizes. Depending on the lubricant, relatively few small bubbles indicate approximately 500 to 1,000 ppm (0.05 to 0.1 percent) water.

Significantly more bubbles of a larger size may indicate around 1,000 to 2,000 ppm water, while an audible crackling sound indicates moisture levels in excess of 2,000 ppm. The Crackle Test is sensitive only to free and emulsified water.

Another simple on-site test is the use of a pressure cell where the sample is prepared with a chemical reagent (calcium hydride) and placed in a container and shaken vigorously. A change of pressure within the cell is monitored to determine if free water is present.

The cost of this type of product is relatively low, although the running costs must be considered with regard to the reagents, as well as the health and safety issues of these reagents. Suppliers include Kittiwake, Koehler and Dexsil.

A third type of on-site screening test for water is the use of a relative humidity sensor. The sensor uses a thin film capacitance grid that can determine the amount of moisture permeating through the film. Whether used in air or oil, the technology is the same and the output of data is normally in a percent RH value.

As discussed earlier, the percent RH is an indication of whether the oil has yet reached the saturation point, although as in the atmosphere, the lower the temperature, the lower the saturation point in terms of water concentration. While it is mathematically possible to derive a ppm value from the percent RH against the saturation curve for the oil at a known temperature, the thinking behind this type of sensor is to provide a proactive early warning of imminent problems as well as providing a screening capability prior to sending a sample to a commercial laboratory.

The water saturation article gives a clear description of the performance and applicability of this tool. The advantage of this method is its relatively low running costs and that it can be permanently mounted on critical plant equipment to provide real-time monitoring. Suppliers include Pall Corporation and Rockwell Automation - Entek.

Aside from the on-site screening methods, another commonly used method to screen for water is Fourier Transform Infrared Spectroscopy (FTIR). This test is sensitive to free, emulsified and dissolved water, however it is limited in precision to a lower detection limit of about 1,000 ppm.

This is adequate for some applications, but insufficient for typical industrial applications. Commercial laboratories that use this method often report that less than 0.1 percent volume of water is present in the sample. Suppliers include Bio-Rad, MIDAC, Nicolet and Thermolube.

The most precise method for determining the amount of free, emulsified and dissolved water in a lubricating oil is the Karl Fischer moisture test. When used correctly, the Karl Fischer test is capable of quantifying water levels as low as 10 ppm or 0.001 percent and should be the method of choice when more exact water concentrations need to be known. Care should be exercised when using the Karl Fischer moisture test to avoid interference effects caused by sulfurous EP and AW additives. Suppliers include Mettler and Metrohm.

Whichever method is used to determine water levels, one thing is certain: Water is a major cause of lubricant failure, component failure and poor machine reliability. Like all contaminants, it is important not only to recognize its presence, but also to take steps to control or eliminate the source of water ingression. If possible, water levels in all equipment should be kept below the saturation limit, with every effort made to keep moisture levels as low as possible.

Whether you choose to install desiccant style breathers, improve seals, or to use a centrifugal filter or a large vacuum dehydration unit, reducing the level of water in all types of equipment can dramatically extend the life of the lubricant and the machine.