Page 13 - Parker - The Handbook of Hydraulic Filtration

Contamination Types and Sources

Air Contamination

Types Damage Sources
Air in a liquid system can exist in ▼ Loss of transmitted power ▼ System leaks
either a dissolved or entrained (undis- ▼ Reduced pump output ▼ Pump aeration
solved, or free) state. Dissolved air may
▼ Loss of lubrication ▼ Reservoir fluid turbulence
not pose a problem, providing it stays
▼ Increased operating temperature
in solution. When a liquid contains Prevention
undissolved air, problems can occur as ▼ Reservoir fluid foaming
▼ System air bleeds
it passes through system components. ▼ Chemical reactions
There can be pressure changes that ▼ Flooded suction pump
compress the air and produce a large Air in any form is a potential source of ▼ Proper reservoir design
amount of heat in small air bubbles. oxidation in liquids. This accelerates ▼ Return line diffusers
This heat can destroy additives, and corrosion of metal parts, particularly
the base fluid itself. when water is also present. Oxidation
of additives also may occur. Both
If the amount of dissolved air becomes processes produce oxides which pro-
high enough, it will have a negative mote the formation of particulates, or
effect on the amount of work form a sludge in the liquid. Wear and
performed by the system. The work interference increases if oxidation
performed in a hydraulic system relies debris is not prevented or removed.
on the fluid being relatively incom-
pressible, but air reduces the bulk
modulus of the fluid. This is due to the
fact that air is up to 20,000 times more
compressible than a liquid in which it
is dissolved. When air is present, a
pump ends up doing more work to
compress the air, and less useful work
on the system. In this situation, the
system is said to be ‘spongy’.

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