Lubricating
remedies
Specialised
lubrication is the key to safe, efficient and reliable usage of air
compressors, says Vivek L Dev
Today,
compressed air is a vital utility that serves to power a growing number
of applications in diverse fields, from civil construction to aviation
and space exploration. As compressed air systems continue to evolve
in size and complexity, their proper maintenance and upkeep, which is
at the heart of compressed air systems, assumes a great amount of importance.
The
air compressor and associated equipment is available in a variety of
design types. Compressors ranges from the reciprocating-piston or the
reciprocating type to the rotary screw type. The associated equipment
like air receivers and after-coolers are available in various designs
to suit the compressor types. While the reciprocating technology is
well established, with millions of machines in operation, the rotary
screw is a recent development. Rotary screw compressors are now gaining
increasing acceptance due to a number of features that offer substantial
benefits over the reciprocating design. Compressed air is a vital resource
in most industries. Compressors whether reciprocating or rotary screw
or any other type, have therefore to be designed, built and maintained
to very high engineering standards.
No
shortcuts
Modern
production techniques, advanced manufacturing technologies and sophisticated
test equipment help ensure that design specifications and tolerances
are precisely met. They ensure that the product that leaves the factory
conforms to stringent safety and reliability standards. The quality
of maintenance of the machines, which starts from the point of installation
and commissioning, is an unquantifiable variable and is more often than
not unsatisfactory and perfunctory.
Attending
to the compressor’s lubrication needs and ensuring that the lubrication
system is always primed with the right oil, contributes most towards
the safe and reliable operation of the machine. This is because, unlike
in other industrial equipment, the lubricating oil in a compressor has
to perform multiple tasks. Some of these tasks directly impact the dynamics
of air compression. Besides, the environment in which the oil has to
perform is particularly hostile. When a gas is 
compressed
in a cylinder by a piston, work and energy are being put in and the
temperature of the gas rises. This temperature rise creates a hostile
environment for the lubricating oil. Water condensation, from the water
vapour in the compressed air, makes things worse. In such conditions,
unsuitable oils fail to separate out from the water. They form thick
sludgy deposits that become acidic with time, leading to the formation
of lacquers and coat-like layers. This causes a number of problems.
For instance, it causes the piston rings to stick in reciprocation compressors,
thereby affecting the action of the valves. It should be noted therefore,
that the use of lubricants that are not approved by the manufacturer
is a losing proposition in the long run, even though it might appear
to save money in the short term. Only specially developed high-stability
compressor oils can perform under the severe operating conditions because
they are capable of remaining cool even as they lubricate. Non-stable
oils on the other hand, breakdown under prolonged conditions of high
temperatures.
To
appreciate the lubrication needs of the most widely used compressor
types in industry, let us understand the internal dynamics of air compression
in the reciprocating and rotary compressors.
Reciprocating
compressors
The
reciprocating compressor has a long established record of industrial
service. It was once the most popular compressor type in service, and
millions of such compressors still continue to be in use today. Though
it is no longer the preferred type in many industrial applications,
reciprocating machines are still being manufactured in large quantities
across the world. They cover the widest pressure range, from huge machines
producing pressures of 850 bar to miniature portable units.
The
lubricating requirements of reciprocating compressors are particularly
demanding and unique in certain respects. The oil required to both lubricate
and protect the compressor is circulated via the oil pump to the cylinders
and the crank-shaft bearings. Discharge air temperatures in these machines
may often exceed 300ºC in some of the applications. On the whole the
reciprocating compressors run hotter than other types of compressors.
During
such circulation often there is a slight loss of oil. Such loss should
be made-up through topping up of the sump. Unless so made-up, oil starvation
may result in metal-to-metal-contact, leading to overheating and severe
damage. But that is not all. Even more serious, is the effect of oil
oxidation on the discharge valves and pipings; the hottest parts of
the machine. If the lubricants used are not of the specified type, they
will probably not contain special inhibitors and the right base oils.
The lubricants will rapidly oxidise in the high-temperature high-pressure
conditions and a variety of lacquers and carbonaceous deposits will
form in the piping. Dirt in the intake air will then settle on the film
of oil on the hot surfaces and make it more viscous. The deposits will
therefore stay longer and get still hotter.
Similar
problems may be encountered in the presence of finely divided rust in
the intake air, which accelerates oxidation by catalysis. The operation
of discharge valves will be seriously affected as deposits develop on
the piston crown. Under prolonged conditions the discharge valves start
to leak and get distorted leading to further overheating.
Even
more serious problems may be encountered in the discharge side.....
....CONTD
