Hydrocarbon Processing - May 2022 - 15

Reliability
HEINZ P. BLOCH, RELIABILITY/EQUIPMENT EDITOR
Heinz.Bloch@HydrocarbonProcessing.com
Consider packaged twin-screw compressors
Compressors are widely used in the
process industries to move fluids and gases
within and outside the boundary of a
process. The fluids can be any compressible
fluid-either gas or vapor-and can
have a wide range of molecular weights.
The compression of gases and vapors is
an important operation in the chemical
and petrochemical industries.
Depending on the application, these
machines are constructed and configured
in various ways. They can be divided into
two primary groups: intermittent and
continuous. The intermittent mode of
compression is cyclical in nature, in that a
specific quantity of gas is ingested by the
compressor, acted upon and discharged
before the cycle is repeated. Compressors
operating in an intermittent compression
mode are referred to as positive displacement
compressors; the two distinct types
within this category are reciprocating and
rotary. Reciprocating and rotary screw
compressors are intermittent machines.
The continuous compression mode is
one in which the gas is continuously being
ingested and discharged without flow
interruption at any point in the process.
With very few exceptions, compressors
operating in a continuous mode are referred
to as dynamic compressors; the
two distinct types within this category
are centrifugal and axial.
In process
applications
of rotary
screw compressors, the user-purchaser
will find gases that vary from consumer
products, such as the home refrigerator,
to large complex petrochemical plant installations.
Recorded molecular weights
of compressed gases cover the range
from 2 (hydrogen) to 352 (uraniumhexafluoride).
Fields
of application overlap. The
overlap between the different types of
compressors is shown in FIG. 1. During
selection, the choice is usually based
on economic considerations. However,
a combination of initial cost, advanta1,000,000
Piston
Centrifugal
100,000
Axial
Screw
10,000
Liquid
ring
Lobe
1,000
100
10
100
1,000
10,000
Flow rate Q, m3/hr
FIG. 1. Selection chart for different types of industrial compressors.1
geous efficiencies of carefully engineered
asymmetrical rotor contours and projected
long-term reliability is indicative
of trends that will move today's packaged
twin-screw compressors into much closer
focus than a few years ago.
Experienced manufacturers of rotary
screw compressors offer
compressor
packages like the one shown in FIG. 2,
which depicts a two-stage, oil-free compressor
with intercooling. An oil refinery
in the UK uses this machine to compress
2,300 m3
/hr of flare gas from 1 bar to a
discharge pressure of 10.1 bar. Interestingly,
each stage is equipped with double,
self-acting dry gas seals.
What comes to mind are the lessons
one could learn from discussing details
with this manufacturer. The fact that
flare gas is not usually very clean would
be a sub-topic in such conversations.
Dry gas seals in this machine are a recent
choice for such services. However, what
prompted this choice? Chances are that
the owner-purchaser in the UK received
bids or offers for less expensive machines
but opted for the one with the best available
technology. Exploring the details
would be a value proposition.
Oil-free
vs.
liquid-flooded twinscrew
compressors. Rotary screw
compressors are available in oil-free or
oil-flooded construction. Oil-free refers
to not having oil in the compression
Hydrocarbon Processing | MAY 2022 15
FIG. 2. Fully preassembled two-stage
twin-screw flare gas recovery package.
Photo courtesy of Aerzener Maschinenfabrik.
100,000
1,000,000
Isentropic head, m
Hydrocarbon Processing - May 2022

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