2015 Contamination Control Lubrication Management & Technology

Keep Hydraulic Fluids Contaminant Free

Ken Bannister | August 6, 2015

Hydraulic systems rely on vital fluids to transfer and amplify power and lubricate critical components. Protecting those fluids from contamination should be a top priority.

Despite their complexity, hydraulic systems are very forgiving beasts—almost too forgiving for their own good. They’ll tend to perform inefficiently for a long time before catastrophic failure occurs. Unfortunately, this forgiving nature can foster a widespread, apathetic approach toward failure prevention, efficiency optimization, and service life-cycle management. Fluid is the most important part of any hydraulic system and when systems fail, the cause is most often related to fluid/fluid contamination. Those failures usually take other critical equipment and processes with them.

Exorcise the “big three”

Hydraulic-fluid contamination comes in three major forms: solid particulate, water, and air. All of these can seriously affect the fluid and the equipment components it serves. Maintaining hydraulic fluids in optimum condition requires measuring, controlling, and preventing the introduction of these contaminants. Since contamination in any form can be inherent and induced, understanding how the three types enter a system is important in developing effective preventive-maintenance (PM) strategies. 


Understanding how contaminants enter a hydraulic system is key to development of an effective maintenance strategy.

Solid-particulate contamination. Hydraulic system components are designed to operate with tolerances that can be as close as 1.5 microns. Solid particulate most often manifests itself as grit or dirt that, if allowed into a system, can be extremely damaging to bearing surfaces and hydraulic seals. The solid particles, which can be more than 100 microns in size, will set up in a three-body abrasion state and easily score the mated machined surfaces, creating rapid bearing and component-surface wear, leading to unwanted fluid bypass that reduces operating efficiency. Solids contamination will also cause valve stiction, increased fluid viscosity, and unwanted fluid leakage through nicked and scored cylinder seals.

If your equipment is new or rebuilt, solids contamination in the form of leftover dirt or swarf from the manufacturing/rebuild process could be present in the hydraulic lines. Prior to initial startup, lines should be wad cleaned, existing oil flushed from the system, and new, correct-viscosity fluid added.

Many end-users are unaware that solids-contamination levels can be excessive in new oil supplied from the manufacturer or introduced by the supplier if bulk-transferred through dirty transfer hoses and equipment. When receiving new—especially bulk—oil, always perform an oil analysis to detect solids and water contamination.

While new oil can often be found at a 19/17/14 cleanliness level on the ISO solid-contamination code, that’s not clean enough for high-pressure hydraulic systems.  These systems require fluids with a minimum 16/14/11 cleanliness level. To protect your site’s hydraulic systems, establish a cleanliness contract with your oil supplier who will then be bound to provide proof of cleanliness upon delivery.

Other sources of solids contamination, all of which are preventable, include:

• improperly stored oil
• dirty equipment used to transfer the oil into the equipment reservoir
• reuse of dirty “leaked” oil
• “open to air” reservoirs due to missing fill ports or reservoir breathers
• lack of filter maintenance, causing dirty oil to bypass into the system
• poor housekeeping practices. 

Water contamination. Water, a universal contaminant, will saturate hydraulic fluid at a mere 300 ppm or 0.04% concentration level. It can be present in:

• a free state, separated from the fluid in an unstable form
• an emulsified state, in a stable form that appears cloudy
• a saturated, dissolved form that appears invisible.

Water depletes vital oil additives or, even worse, reacts with additives to create corrosive acids that attack system components. It can also reduce a lubricant’s film strength and its ability to release air, which can increase wear, corrosion, and cavitation.

Some hydraulic fluids—such as brake fluids—are designed to be hygroscopic and entrain moisture in the fluid until its saturation point is reached. In high-heat applications, water can boil off and create great inefficiency in the hydraulic power-transfer motion. 

Typical water-contamination sources include:   

• incorrect outdoor lubricant storage practices that cause hot-cold cycle condensation
• “open to air” reservoirs into which washdown and/or process water can splash/spill.

In most instances, water can be detected visually in its free and emulsified state. To remove water contaminants use:

• polymeric-style filtration media designed to absorb the water as it passes through a filter
• vacuum distillation to boil off the oil
• dehumidification in the reservoir headspace.

Air contamination. This type of contamination presents in a number of forms, of which entrained air can be the most problematic. In this form, air bubbles (<1 mm dia.), dispersed throughout the fluid, reduce viscosity and, thus, film strength. This situation, in turn, can cause premature component wear; a reduction in the oil’s bulk modulus, causing a lack of efficiency and control due to the sponginess of the oil condition; an increased heat load, leading to fluid deterioration; and system erosion, due to cavitation.

Air bubbles greater than 1-mm dia. create foam, which can quickly deplete any antifoam additive and cause fluid oxidization.

Typical causes of air contamination include:

• over-/under-filled lubricant reservoirs
• clogged inlet/suction filters
• clogged reservoir breathers
• restricted inlet lines
• loosely clamped inlet lines
• pump-shaft seal failures.

Prevention control

Fortunately, the contamination problems discussed here are easily preventable—in most cases at minimal cost. While the following advice is not all-inclusive, this list provides an excellent starting point for a successful hydraulic-fluid PM strategy:

• Implement a fluid cleanliness standard.
• Store all lubricants in a cool, dry place and practice FIFO (first in-first out) stock rotation.
• Practice good housekeeping, ensuring that reservoirs are clean of dirt and debris.
• Cap all system hoses and manifolds during fluid-handling and system maintenance.
• Wad-clean all lines prior to initial system startup.
• Flush all lube systems and change oil, prior to startup.
• Use a dedicated filter cart for each hydraulic-fluid type, with quick-connect fittings to transfer/clean hydraulic fluid before it enters a reservoir.
• Install external sight gauges marked with high- and low-level marks in reservoirs to check for fluid levels and the presence of water.
• Use polymeric oil filters and desiccant reservoir air breathers.
• Specify rod wipers and replace all worn actuator seals.

With a little care and common sense, your plant’s hydraulic systems—and the vital fluids that keep them up and running—will be efficient, reliable, and long-lived.


— National Fluid Power Association
International Organization for Standardization
Society of Tribologists and Lubrication Engineers


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Ken Bannister

Ken Bannister

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