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