of oil varies from one location to another. These factors
can make it challenging for a Corken compressor user to
select a suitable crankcase oil.
It is safe to say that purchasing a quality crankcase oil,
and changing it regularly, is significantly less costly than
the repair bill and downtime associated with a lubrication
failure in any gas compressor. Given the relatively
small volume of oil used in the crankcase of a Corken
compressor, and the critical nature of the services
where these compressors are typically used, selecting
the appropriate high-quality oil is the most economical
choice. It will help ensure the dependability and longevity
of the compressor.
Oils to Avoid
Selecting a crankcase oil based on low price or easy
availability is seldom the most economic decision.
Following are oils to avoid.
• Do not use engine/motor oil with an API Service SA
through SH.
• Do not use any oil with a viscosity index below 95.
• Do not use any oil with a pour point less than 15ºF
(8ºC) lower than the anticipated minimum ambient
temperature (unless a crankcase oil heater is used).
See below for additional detail on each of these parameters.
Industrial Oils
Corken recommends using industrial oils (rather than
engine oil or “motor oil”). Industrial oils have additives
specifically selected and blended for specific purposes.
Many are designed specifically for the conditions
inherent in compressor crankcases. Such industrial
oils are required for Corken compressors operating in
continuous duty or heavily loaded applications.
Industrial oils do not receive an API service designation
like an engine oil does.
Critical Oil Characteristics
Viscosity
The viscosity of a crankcase oil is a measure of its
resistance to flow. Viscosity is the most important
physical property of lubricating oil. Oils with higher
viscosity (ISO 100 and ISO 150) are thicker and are
used for higher ambient temperatures. Oils with lower
viscosity (ISO 68, ISO 46, and ISO 32) are thinner and
are used at lower ambient temperatures. However, oils
with a high viscosity index (see below) can be used at
wider ambient temperature range compared to oils with
a lower viscosity index.
Viscosity Index
Viscosity Index (VI) is a measure of how much the
oil’s viscosity changes as its temperature changes. A
low viscosity index is an indication that the viscosity
changes more as the temperature changes. A high
viscosity reflects a more stable viscosity, and is generally
preferred for Corken compressors.
Oil with a low viscosity index tends to thin out as the oil
temperature increases. This can cause lubrication failure
as well as unstable oil pressure. The minimum Viscosity
Index for oils used in Corken compressors is 95 (VI is
a unit-less number). This is particularly important when
operating at high or low temperature extremes, or at a
variety of ambient temperatures (seasonal changes).
Pour Point
The pour point of an oil is the lowest temperature at
which the oil flows. At temperatures below the pour
point, the oil is very thick and can’t freely flow to the
compressor’s bearings and other wear surfaces, or even
to the compressor’s oil pump.
In low ambient temperature operation, the oil’s pour point
is critical. An oil should have a pour point at least 15ºF
(8ºC) below the lowest expected ambient temperature.
For example, if the minimum ambient temperature is
expected to be 0ºF (-18ºC), the pour point must be no
higher than -15ºF (-26ºC).
Do not assume the pour point of an oil is low enough.
Consult the oil’s technical data sheet. Many oils have a
pour point around 0 to 10ºF (-18 to -12ºC) which is too
high for low ambient temperatures. Synthetic oils often
have a lower pour point than conventional oils.
Engine Oils (Motor Oils)
Engine oils are formulated for use in internal combustion
engines and contain additives that specifically counter
the contaminants created by the combustion of fuel
(soot, CO
2
, water, etc.). A gas compressor crankcase
is a different environment than an engine crankcase.
Thus, engine oils are not necessarily the best oils to use
in a gas compressor. They are by far the most readily
available oils.
If a suitable industrial oil is not available, engine oils can be
used in Corken compressors used in intermittent service.
Heavily loaded compressors or those in continuous duty
service should always use high quality industrial oil. If
engine oil is used, it is critical that the engine oil have an
adequate API Service Grade.
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