Page 68 - Fluid Power Engineering

Hydraulic Oils and Theor etical Backgr ound 45

being introduced by mistake into the system during servicing. The
effects of such contamination depend on the contaminant, its amount
in the system, and how long it has been present.

2.2.12 Thermal Properties
The pour point of a fluid is the temperature 3°C above the temperature
at which the fluid ceases to flow. As a general rule, the minimum
temperature at which a fluid operates should be at least 10°C above
the pour point.
The flash point is the temperature at which a liquid gives off vapor
of sufficient quantity to ignite momentarily or flash when a flame is
applied. A high flash point is desirable for hydraulic liquids because
it provides good resistance to combustion and a low degree of evapo-
ration at normal temperatures. The required flash point varies from
150°C for the lightest oils to 265°C for the heaviest oils.
The fire point is the temperature at which a substance gives off
vapor of a sufficient quantity to ignite and continue to burn when
exposed to a spark or a flame. Like the flash point, a high fire point is
a basic requirement.
Specific heat capacity is the quantity of heat required to raise the
temperature of one unit of mass of a substance by one degree. It is
usually expressed in kJ/kg per K or kcal/kg per °C. The specific heat
capacity at constant pressure (C ) is temperature-related, but it is in-
P
significantly affected by the pressure level. For mineral oils and syn-
thetic hydrocarbons, C = 2060, J/kg per K at 50°C temperature.
P
Thermal conductivity is the ability to transmit heat, normally
expressed in units of W/m per K. For mineral oils and synthetic
hydrocarbons, the thermal conductivity is about 0.12 W/m per K.

2.2.13 Acidity
An ideal hydraulic liquid should be free from acids that cause corrosion
of the metals in the system. Most liquids cannot be expected to remain
completely noncorrosive under severe operating conditions. The degree
of acidity of a liquid, when new, may be satisfactory, but after use, the
liquid may tend to become corrosive as it begins to deteriorate.

2.2.14 Toxicity
Toxicity is defined as the quality, state, or degree of being toxic or poi-
sonous. Some liquids contain chemicals that are a serious toxic haz-
ard. These toxic or poisonous chemicals may enter the body through
inhalation, by absorption through the skin, or through the eyes or the
mouth. The result is sickness and, in some cases, death. Manufacturers
of hydraulic liquids try to produce suitable liquids that contain no
toxic chemicals and, as a result, most hydraulic liquids are free of
harmful chemicals. Some fire-resistant liquids are toxic, and suitable
protection and care in handling them must be provided.