10-08-2012, 01:52 PM
classification of fluid
classification of fluids.pdf (Size: 65.01 KB / Downloads: 35)
Environmentally Acceptable Hydraulic Oils
Recent years have been marked by efforts to combat serious threats
posed by industrial pollution of our environment. Hydraulic fluids are
naturally included in these threats. They are utilized in large amounts by
excavating machines, bulldozers, mobile cranes, and other outdoor
equipment. An oil leak may result in considerable pollution of the surroundings
and ground water. Mineral oils are composed of relatively
stable hydrocarbon compounds, and are only very slowly broken down
by microorganisms in the environment. Thus, pollution by conventional
mineral hydraulic oils can disturb the ecological balance for long periods.
This fact led to a growing interest in biodegradable products. Microbiological
degradation is the process where by microorganisms, with the
help of oxygen, break down organic material and extract nourishment
from the decomposed products. The following are the basic specifications
of commercially available and environmentally acceptable hydraulic
oil. It is a synthetic base oil designed to biodegrade to its natural
state when subjected to sunlight, water, and/or microbial activity.
Mineral Oils
Mineral-based oils are the most widely used hydraulic fluids. They
are relatively inexpensive, widely available, and can be offered in
suitable viscosity grades. They are of good lubricity, noncorrosive,
and are compatible with most sealing materials with the exception of
butyl rubber.
Mineral oils are chemically stable for reasonable operating temperatures.
At higher temperatures, however, they suffer chemical
breakdown. Premium grade mineral oils contain a package of additives
to combat the effects of wear, oxidation, and foam formation,
and to improve viscosity index and lubricity. There are, however,
certain disadvantages of mineral oils that cannot be remedied by incorporating
additives. The two most important are the flammability
and the increase in viscosity at high pressures. Fire risk excludes the
use of mineral oils in hazardous areas such as injection and plastic
molding machines, coal mines, and near furnaces. The viscositypressure
characteristics limit their use to pressures below 1000 bar
(see Fig. 2.3).
Oil-in-Water Emulsion
This hydraulic fluid consists of tiny droplets of oil dispersed in a continuous
water phase. The dilution is normally between 2% and 5% oil
in water, and the characteristics of the fluid are more similar to water
than oil. It is extremely fire-resistant, is highly incompressible, and has
good cooling properties. Its main disadvantages are poor lubricity
and low viscosity.
Water-in-Oil Emulsion
The water-in-oil emulsions are the most popular fire-resistant fluids.
They have a continuous oil phase in which tiny droplets of
water are dispersed. Their lubrication properties are very much
reduced. This is partially overcome by running pumps at reduced
speeds. Therefore, larger displacement pumps are necessary to
obtain the required flow rate. The usual dilution is 60% oil + 40%
water. For optimum life, the operating temperatures should not
exceed 25°C, but intermittent operation up to 50°C is permissible.
48 Chap t e r T w o
At the higher temperature, water content is affected owing to evaporation,
which decreases the emulsion’s fire-resistance properties.
When the system has been idle for long periods, there is a tendency
for the oil and water to separate. However, during running, the
pump will re-emulsify the fluid.
Water-Glycol Fluids
These fluids were developed primarily for use in aircraft because of
their very low flammability characteristics. However, their application
is limited since they cannot be used at high temperatures because
of their water content. Their lubricating ability is inferior to that of
mineral oils; they attack most paints; they are very stable with respect
to shear because of the low molecular weight of their constituents;
and their good anti-freeze properties make them particularly suitable
for low-temperature applications.
Synthetic Oils
Synthetic oils, such as phosphate esters, have remarkably good fireresistance
properties. They are used in industries such as plastic
molding and die-casting, where unusually great fire risks occur. Their
lubricating ability is similar to that of mineral oil.
Elastomers used in conjunction with phosphate esters must be
chosen carefully. Some silicone polymers and butyl rubber are suitable.
Certain metals, particularly aluminum, and most paints are
susceptible to attack.
Synthetic oils are superior when compared with mineral-oil–
based fluids in one or more of the following respects: