Hydrocarbon Processing - May 2022 - 16

Reliability
space, but bearings still require lubrication
by a clean medium. This lubricant is
typically a clean premium grade lubricating
oil, although clean pressurized water,
glycerin and several other fluids can also
be used.1
While pressurized water represents
less well-known advanced technology,
it has been applied for decades. In
the hands of truly competent compressor
manufacturers, water-flooded screw
compressors have been remarkably effective
and successful in some of the
dirtiest services.1
While oil-free twin-screw compressors
are widely called " dry-screw " machines,
at
least one prominent manufacturer
defines and designates as dry
screw any screw compressor equipped
with timing gears. Therefore, whether
the compression space is dry, oil-flooded
or water-injected makes no difference:
With timing gears keeping the two screws
synchronized, it should be labeled a dryscrew
machine. Without timing gears, it
cannot function as a dry-screw machine
because the resulting contact of mating
rotors would destroy it. If there are no
timing gears, a separating liquid must be
used. Any separating liquid introduced to
or circulating in the compression space
will make it a wet-screw machine.
Wet-screw compressors with singleoil
circuits are not suited for heavily
contaminated gas streams; such streams
have sometimes managed to ruin single
circuit machines within days. In one such
case, the problem was related to the use
of oil filters (25 micron nominal) that
plugged after just a few hours of operation.
At another installation, asphaltenecontaminated
vapors had been allowed
to " bubble through " crude oil in gathering
tanks before reaching the bearings in
several screw compressors.
Fields of application for oil-free machines
include all processes that cannot
tolerate contamination of the compressed
gas or where the lubricating oil
would be contaminated by the gas. Oilflooded
machines can achieve slightly
higher efficiencies than dry-screw machines
and can utilize the oil for cooling,
as well. The same statements could
be made for water-flooded machines. In
some instances, the bearing lube circuit
must be totally separate from the fluid
circuit used in the compression space.
Whenever this requirement is disregarded,
the purchaser-user may end up with
either high maintenance costs or low
equipment reliability.
REFERENCES
1
Bloch, H. and H. Elliott, Compressor Technology
Advances: Beyond 2020, DeGruyter, Berlin, Germany,
2021.
HEINZ P. BLOCH resides in
Montgomery, Texas. His
professional career commenced
in 1962 and included long-term
assignments as Exxon Chemical's
Regional Machinery Specialist
for the U.S. He has authored or
co-written more than 780 publications, among them
23 comprehensive books on practical machinery
management, failure analysis, failure avoidance,
compressors, steam turbines, pumps, oil mist
lubrication and optimized lubrication for industry.
Mr. Bloch holds a BS degree and an MS degree
(cum laude) in mechanical engineering from NCE,
Newark College of Engineering. He is an ASME Life
Fellow and was awarded life-time registration as
a Professional Engineer in New Jersey. He is one of
10 inaugural inductees into NCE's Hall of Fame,
which honors its most distinguished alumni.
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Hydrocarbon Processing - May 2022

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