control. Therefore, defoaming agents are oftenadded to fluids to prevent foaming. Minimizingair in fluid systems is discussed later in thischapter.CLEANLINESSCleanliness in hydraulic systems has receivedconsiderable attention recently. Some hydraulicsystems, such as aerospace hydraulic systems, areextremely sensitive to contamination. Fluidcleanliness is of primary importance becausecontaminants can cause component malfunction,prevent proper valve seating, cause wear incomponents, and may increase the response timeof servo valves. Fluid contaminants are discussedlater in this chapter.The inside of a hydraulic system can only bekept as clean as the fluid added to it. Initial fluidcleanliness can be achieved by observing stringentcleanliness requirements (discussed later in thischapter) or by filtering all fluid added to thesystem.TYPES OF HYDRAULIC FLUIDSThere have been many liquids tested for usein hydraulic systems. Currently, liquids being usedinclude mineral oil, water, phosphate ester,water-based ethylene glycol compounds, andsilicone fluids. The three most common types ofhydraulic liquids are petroleum-based, syntheticfire-resistant, and water-based fire-resistant.PETROLEUM-BASED FLUIDSThe most common hydraulic fluids used inshipboard systems are the petroleum-based oils.These fluids contain additives to protect the fluidfrom oxidation (antioxidant), to protect systemmetals from corrosion (anticorrosion), to reducetendency of the fluid to foam (foam suppressant),and to improve viscosity.Petroleum-based fluids are used in surfaceships’ electrohydraulic steering and deckmachinery systems, submarines’ hydraulicsystems, and aircraft automatic pilots, shockabsorbers, brakes, control mechanisms, and otherhydraulic systems using seal materials compatiblewith petroleum-based fluids.SYNTHETIC FIRE-RESISTANT FLUIDSPetroleum-based oils contain most of thedesired properties of a hydraulic liquid. However,they are flammable under normal conditions andcan become explosive when subjected to highpressures and a source of flame or high tempera-tures. Nonflammable synthetic liquids have beendeveloped for use in hydraulic systems where firehazards exist.Phosphate Ester Fire-Resistant FluidPhosphate ester fire-resistant fluid forshipboard use is covered by specification MIL-H-19457. There are certain trade names closelyassociated with these fluids. However, the onlyacceptable fluids conforming to MIL-H-19457 arethe ones listed on the current Qualified ProductsList (QPL) 19457. These fluids will be deliveredin containers marked MIL-H-19457C or a laterspecification revision. Phosphate ester incontainers marked by a brand name without aspecification identification must not be used inshipboard systems, as they may contain toxicchemicals.These fluids will burn if sufficient heat andflame are applied, but they do not supportcombustion. Drawbacks of phosphate ester fluidsare that they will attack and loosen commonlyused paints and adhesives, deteriorate many typesof insulations used in electrical cables, anddeteriorate many gasket and seal materials.Therefore, gaskets and seals for systems in whichphosphate ester fluids are used are manufacturedof specific materials. Naval Ships’ TechnicalManual, chapter 262, specifies paints to be usedon exterior surfaces of hydraulic systems andcomponents in which phosphate ester fluid is usedand on ship structure and decks in the immediatevicinity of this equipment. Naval Ships’ TechnicalManual, chapter 078, specifies gasket and sealmaterials used. NAVAIR 01-1A-17 also containsa list of materials resistant to phosphate esterfluids.Trade names for phosphate ester fluids, whichdo 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 navalshore installation, you must understand thehazards associated with hydraulic fluids to whichyou may be exposed.3-5
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