control. Therefore, defoaming agents are often
added to fluids to prevent foaming. Minimizing
air in fluid systems is discussed later in this
chapter.
CLEANLINESS
Cleanliness in hydraulic systems has received
considerable attention recently. Some hydraulic
systems, such as aerospace hydraulic systems, are
extremely sensitive to contamination. Fluid
cleanliness is of primary importance because
contaminants can cause component malfunction,
prevent proper valve seating, cause wear in
components, and may increase the response time
of servo valves. Fluid contaminants are discussed
later in this chapter.
The inside of a hydraulic system can only be
kept as clean as the fluid added to it. Initial fluid
cleanliness can be achieved by observing stringent
cleanliness requirements (discussed later in this
chapter) or by filtering all fluid added to the
system.
TYPES OF HYDRAULIC FLUIDS
There have been many liquids tested for use
in hydraulic systems. Currently, liquids being used
include mineral oil, water, phosphate ester,
water-based ethylene glycol compounds, and
silicone fluids. The three most common types of
hydraulic liquids are petroleum-based, synthetic
fire-resistant, and water-based fire-resistant.
PETROLEUM-BASED FLUIDS
The most common hydraulic fluids used in
shipboard systems are the petroleum-based oils.
These fluids contain additives to protect the fluid
from oxidation (antioxidant), to protect system
metals from corrosion (anticorrosion), to reduce
tendency of the fluid to foam (foam suppressant),
and to improve viscosity.
Petroleum-based fluids are used in surface
ships electrohydraulic steering and deck
machinery systems, submarines hydraulic
systems, and aircraft automatic pilots, shock
absorbers, brakes, control mechanisms, and other
hydraulic systems using seal materials compatible
with petroleum-based fluids.
SYNTHETIC FIRE-RESISTANT FLUIDS
Petroleum-based oils contain most of the
desired properties of a hydraulic liquid. However,
they are flammable under normal conditions and
can become explosive when subjected to high
pressures and a source of flame or high tempera-
tures. Nonflammable synthetic liquids have been
developed for use in hydraulic systems where fire
hazards exist.
Phosphate Ester Fire-Resistant Fluid
Phosphate ester fire-resistant fluid for
shipboard use is covered by specification MIL-
H-19457. There are certain trade names closely
associated with these fluids. However, the only
acceptable fluids conforming to MIL-H-19457 are
the ones listed on the current Qualified Products
List (QPL) 19457. These fluids will be delivered
in containers marked MIL-H-19457C or a later
specification revision. Phosphate ester in
containers marked by a brand name without a
specification identification must not be used in
shipboard systems, as they may contain toxic
chemicals.
These fluids will burn if sufficient heat and
flame are applied, but they do not support
combustion. Drawbacks of phosphate ester fluids
are that they will attack and loosen commonly
used paints and adhesives, deteriorate many types
of insulations used in electrical cables, and
deteriorate many gasket and seal materials.
Therefore, gaskets and seals for systems in which
phosphate ester fluids are used are manufactured
of specific materials. Naval Ships Technical
Manual, chapter 262, specifies paints to be used
on exterior surfaces of hydraulic systems and
components in which phosphate ester fluid is used
and on ship structure and decks in the immediate
vicinity of this equipment. Naval Ships Technical
Manual, chapter 078, specifies gasket and seal
materials used. NAVAIR 01-1A-17 also contains
a list of materials resistant to phosphate ester
fluids.
Trade names for phosphate ester fluids, which
do not conform to MIL-H-19457 include Pydraul,
Skydrol, and Fyre Safe.
PHOSPHATE ESTER FLUID SAFETY.
As a maintenance person, operator, supervisor,
or crew member of a ship, squadron, or naval
shore installation, you must understand the
hazards associated with hydraulic fluids to which
you may be exposed.
3-5