Chemical Engineering January 2012 - 46

Feature Report
Engineering Practice
Lubricating
Rotating
Machinery
Follow this guidance to improve
lubricant selection, process
operation and asset reliability
Amin Almasi
WorleyParsons Services Pty Ltd.
ubricants in rotating machines
reduce friction and wear, dissipate
heat, protect surfaces, keep
out foreign contaminants and remove
wear particles. Commonly used
liquid lubricants fall into two main
categories: mineral (petroleum-based)
oils and synthetic oils.
L
Mineral oils are produced by refining
crude petroleum. They usually contain
trace amounts of some unwanted substances.
By contrast, synthetic oils are
engineered, so their properties can be
more tightly controlled. Synthetic oil
lubricants include synthesized hydrocarbons
(such as polyalphaolefin, or
PAO), organic esters (such as diesters
and polyol-esters), polyglycols, polyalkylene
glycol (PAG), phosphate esters
and silicone lubricants.
Mineral oils are more frequently
used in chemical process industries
(CPI) applications, but the importance
and use of synthetic oils has
been steadily increasing in recent
years. In general, they offer superior
performance in terms of higher oxidation
stability, improved corrosion resistance,
and the ability to withstand
both higher and lower temperatures.
Compressors and pumps
Some compressors operate with gas
discharge temperatures exceeding
160°C. This calls for a lubricating oil
that has good oxidation properties and
thermal stability. Meanwhile, in some
FIGURE 1. In an oil-injected, twin-screw compressor, the lubricating
oil is injected into the gas stream to absorb the heat
of compression and act as lubricant and sealant. This enables
a much higher pressure ratio in a single stage and provides
significant protection against corrosive gases. In multi-stage
machines, inter-cooling is usually not required
compressors or pumps, the lubricant
is in contact with moisture (from the
handled fluid), so the lubricant must
have good demulsibility (that is, resistance
to emulsification, or good resistance
to mixing with water).
Today, the overwhelming majority
of compressors and pumps are best
served by premium-grade oils with
ISO VG 32 or 46 (sometimes ISO VG
68 or 100 grades). However, there are
many different types of compressors
and pumps, and each manufacturer is
likely to recommend only those lubricants
that have been used successfully
before. Occasionally, compressor lubricants
have to be formulated for exceptional
severe-service performance.
Reciprocating and screw compressors.
During operation, lubricants used
in screw compressors and in the cylinders
of reciprocating compressors are
in direct contact with compressed gas.
With conventional mineral oils, such
gas can become dissolved in the oil.
Additionally, any oil that becomes dissolved
in the gas can be carried away,
depleting the lubricating film. This can
result in machine component scoring
and higher wear rates. Specially formulated
synthetic lubricants generally
perform better in these instances.
When selecting a lubricant for reciprocating
or screw compressors, low
solubility in compressed gas should be
a key selection criteria. The selected
lubricants should also have relatively
high viscosity indexes, minimal viscosity
loss, good thermal stability over
an extended temperature range, excel44
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lent wear protection, excellent lubricity.
They should be non-poisoning to
catalysts, since some oil will be transferred
downstream by the gas.
For reciprocating or screw compressors,
the lubrication oils that meet all
of these criteria are mainly synthetic
lubricants formulated with PAG base
stock. Overall, these provide excellent
oxidative and thermal stability, which
are particularly important for hightemperature
applications.
Relatively
high viscosity indexes facilitate lowtemperature
startup and help to maintain
acceptable viscosity over a wide
temperature range.
PAG lubricants are highly stable,
even at sustained high temperatures,
and thus have very low deposit-forming
tendencies. And importantly, any
decomposition products that may form
tend to be soluble in the lubricant and
thus do not tend to separate as sludge
or contribute to the formation of varnish
or lacquer. Similarly, because of
their close contact, it is important to
select lubricants that are compatible
with the elastomer and coatings used
in the compressor's wetted parts.
Minimizing lubricant carryover to
downstream discharge streams is important
for any compressor, particularly
for screw compressors and reciprocating
compressors. In general, gas
solubility increases roughly linearly
with increasing pressure. As a rough
indication, PAG lubricants typically
have a gas solubility that is less than
half of that of mineral oils or PAO synthetic
oils. High-pressure reciprocat
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Chemical Engineering January 2012

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