There are four primary sources for contaminants and water to enter hydraulic oil in a system. They are:
1.Contaminated new oil
2.In-built contamination
3.External contamination
4.Internal contamination

When planning an effective maintenance programme all four sources of contamination need to be addressed. Otherwise poor quality hydraulic oil will damage the system at best and cause a catastrophic failure at worst. Either way to quote an old adage ‘prevention is better than cure’!

1.Contaminated new oil
Although hydraulic fluids are refined and blended under relatively clean conditions they can become polluted during manufacture and distribution. Contaminants can enter the fluid from pipes, hose, tanks and barrels. Few hydraulic oil suppliers give a categorical assurance as to the cleanliness level and water content of new hydraulic oil. Typically new oil out of a 20 litre plastic barrel from a reputable manufacturer will be ISO 17/16/14 with 100-200 ppm of water. These levels are marginal for modern equipment with servo controlled valves.
The solution is relatively simple and straightforward which is to use a ‘duplex’ filter rig to strain new oil before putting it in a system. In this set-up one filter removes particulate matter and the other water. Rigs can incorporate ‘real time’ particle counters that give a read-out of the cleanliness level being achieved.
2.In-built contamination
New machines, even from reputable manufacturers, contain contaminants in their hydraulic systems. Manufacturers set cleanliness standards for their own manufacturing processes and suppliers but tanks, hoses, pipes, hydraulic components and coolers are all potential sauces of contamination. In-built contaminants include burrs, chips, flash, dirt, dust, fibres, sand, pipe sealant, weld spatter, flushing fluid and moisture. Manufacturers are well aware of the issues and most use ‘end-of-line’ flushing rigs to clean hydraulic systems before new machines are even started. These rigs capture and remove the larger particles but the problem is with the process itself. Unless the flushing process can generate high internal velocity and turbulent flow not all the contaminants will be dislodged and removed. Your new ‘clean machine’ may not be as clean as you think!
The solution is to put new equipment through an off-load period of running in. Changing the filters after a few hours or using off-line filtration will also help reduce the risk of contamination doing damage early in a machine’s life.

3.External contamination
Most machines work in relatively arduous and dirty/dusty conditions. There are five ways that external dirt can enter a closed hydraulic circuit and all contribute in one way or another over the life of a machine.
a.Breathers and vent ports
These essential openings allow air in and out of the system to compensate for changes in fluid level due to the extension and retraction of hydraulic rams and thermal expansion. Open ports are one of the main sources of water entering the system as water vapour.
The solution is to fit all breathers and ports with hygroscopic breathers that absorb water in special hygroscopic agent that has the capability to extract water vapour from the air as it is drawn through the unit. Accompanying solid particles are then removed by a polyester fabric filter, allowing only clean, dry air to enter the system.
b.Access plates, fill caps and open components
‘Relaxed’ maintenance techniques cause untold problems during a machine’s life. A hydraulic system should only be opened when necessary and then for the minimum amount of time.
The solution is through training and education. Any technician working on machinery should be aware of the need to limit the time sealed systems are open to the bare necessity to get the job done. Any plates or cap that is removed should be replaced ASAP!
c.During routine maintenance and repair
Care should be taken during routine maintenance, replacing filters and oil for example, and during repair work to ensure that the risk of contamination is minimised. Lint free rags and proper absorbent materials should be used to minimise the risk of fibres entering the system. Care should be taken when hydraulic hoses are replaced. End caps should be sued to seal the system when a hose is removed and new hoses, especially if made ‘on-site’ should be internally cleaned at least twice. Hydraulic oil ‘top-ups’ should always be of the correct oil grade and ideally be pre-filtered or at the very least filled through a fine strainer.
d.Hydraulic cylinder seals
Rod wiper seals are there to minimise dirt ingression as cylinders open and close. Inevitably they wear and rods get damaged in use which results in dirt gradually being introduced into the system behind the seal as dirt that sticks to an extended rod is drawn back into the cylinder.
The solution is to regularly inspect rods and seals. ‘Wet’ rods and hydraulic oil drips are a sign of worn seals. Replacing worn seals and repairing damaged rods is sensible preventative maintenance as it could help save the catastrophic failure of an extremely expensive component like a main pump or large motor.
e.Fitment of hydraulically powered attachments

4.Internal contamination
As machines are operated and worked they inevitably wear. Hydraulic components are manufactured to extremely tight tolerances so any contamination in the oil, which is there to form an oil film to keep metal surfaces apart and to dissipate heat , soon starts to cause premature internal wear.
There are seven sources of internal contaminations:
a.Abrasive wear-hard particles trapped between two surfaces scraping one or both.
b.Adhesive wear-loss of the oil film allows metal to metal contact between hot moving surfaces.
c.Fatigue wear-particles bridging a clearance gap cause a surface stress that develops into a micro-crack that expands into a spall, a flake of a material that is broken off a larger solid body.
d.Erosive wear-fine particles in a high speed stream of fluid eat away a metering edge (in a valve spool) or a critical surface.
e.Cavitation wear- a restricted inlet flow to a pump, or high water content in the hydraulic oil, causes fluid voids that implode causing shock waves that damage critical surface material in the pump.
f.Aeration wear-air bubbles in the fluid implode breaking away surface material