About 75% of hydraulic system failures relate to
contamination, which can also increase wear and shorten
machinery and lubricant service life. Taking action to
minimise contamination is time well spent, as Kevin Lacey
of Brammer UK explains

Contamination can be gaseous, liquid
and solid (particulate). Gaseous
contamination, ie air, can impair the
hydraulic medium’s lubricating properties,
increasing metal-to-metal contact, creating
wear and a likely increase in other types of
contamination. Air can also cause cavitation
and impact on pump performance.

Liquid contamination (water) can affect
the hydraulic medium’s lubricating
properties, and cause rust. There are also
issues caused by cross-contamination –
mineral oil-based hydraulic fluids, which
interfere with the anti-wear properties of
water glycol hydraulic fluids, causing
‘varnishing’ of system components. Mineral
oil also reacts with fatty acids contained in
water glycol products, forming ‘white soap’
which can block filters and strainers.

There are three categories of particulate
contamination. Soft particles like fibres, gasket
or seal abrasion particles, rubber and paint,
may only have the potential for limited
damage. However, hard particles (iron, steel,
bronze, brass and aluminium) and extremely
hard particles, like corundum, scale, rust and
furnace dross, are highly abrasive and can
significantly degrade surfaces.

Particulate contamination can cause valve
blockages, pump damage, and seal and gasket
destruction. A chain reaction can result. Gaps
grow larger, leakage oil-flows increase,
component operating efficiency decreases,
blockages can occur, and metering edges
wear away, creating control inaccuracies.

The most common standards for measuring
particulate contamination levels are the NAS
class or ISO4406:99, which considers the
number of particles of three different sizes per
unit volume of fluid. These standards also help
understand how equipment performs at
different cleanliness levels, allowing evaluation
of the contamination protection needed in
each application, based on operating pressure.

A particle count analysis should be
undertaken, with corrective action required if
fluid cleanliness is below the desired target.

Filtration
New hydraulic oil is not always clean and can
contain around 50% more contaminants than
would be considered acceptable under
ISO4406:99. Furthermore, many people are
unaware how clean their hydraulic oil should
be. The correct level can be established by
considering duty, component sensitivity, life
expectancy, component replacement costs,
downtime costs, safety, and environmental
considerations. With each of these weighted
and the required cleanliness level established,
appropriate filtration can be implemented.

A dedicated off-line filtration system
operates at a constant flow, maximising filter
life and performance, while trolleys
connected to the power unit can offer a
secondary filtration system. Contaminant
introduction can also be reduced by using
flat face couplings with offline filter trolleys.

Filters have a nominal or absolute pore
size rating. The former describes the ability
to retain the majority of particulate at the
rated pore size; the latter refers to the
capability to retain all particulate of that size.

Good housekeeping
The filler breather life indicator must be
clearly visible, enabling easy checking for
when the unit should be changed. Off-line
filtration systems should be kept clean, with
QRC couplings wiped with a clean, lint-free
cloth before connection to minimise
contamination between QRC faces.

Oil drums should be stored in clean
conditions and clearly labelled stating
whether they contain clean or dirty oil, and
what type. Drum tops should be kept clean.

Taps should be fitted correctly, with the tap at
the bottom of the drum pointing downwards.

The bung should be slightly unscrewed when
filling from the drum and immediately
tightened to prevent contaminant entry. When
topping up hydraulic power units, the suction
pipe should be wiped with a clean, lint-free
cloth before lowering into the drum. Spare
parts should be stored in a clean, dry, dust-free
environment, with packaging checked for
intactness against contaminant ingress.

Each filter trolley should be marked stating
for which fluid or oil type it is suitable. Dust
caps should be fitted in all ports of hydraulic
cylinders and valves. Filters which appear
damaged should be replaced.

Regular sampling – at least monthly – will
establish any changes in the fluid’s physical or
chemical properties and excessive water or
particulate contamination. The latter will
indicate that filters are not keeping the system
clean – because they are inadequate for the
task, not well enough maintained, or the
system is subject to excessive ongoing
corrosion and wear.

Contamination management is just one
aspect of best practice and should be
combined with functions like hydraulic hose
inspection and power unit temperature testing.

A specialist maintenance, repair and
overhaul (MRO) service provider can advise
on hydraulic systems maintenance regimes
and provide all necessary consumables.