Chemical Engineering June 2014 - 48

Engineering Practice
Small compressors are required in
many CPI units for different services.
Examples include flare-gas
recovery units, process-gas recovery
systems, small booster compressors,
small recycle machines, some
small-flow extraction services, tiny
refrigeration units, small gas-blowing
systems, tiny process-vacuum
machines, small nitrogen or air
compressors, small fuel-gas units
and many more.
The market for small and tiny
compressors is different from that
of medium and large compressors.
Two important characteristics of
the small and tiny compressor package
market are as follows:
* For many situations that require
tiny compressors, these units are
purchased in higher quantities
(for instance, a facility may purchase
several small, identical gasrecovery
compressor packages for
use in different units of the same
plant or different plants and facilities).
The specific aspects and capabilities
of tiny compressors tend
to be very vendor-dependent. The
specific vendor, competing designs
and specific optimization options
all play a critical role, beyond just
selection of the compressor type
and model
* Tiny compressors are highly dependent
on vendor design (manufacturing
details) and proper
size (or model) availability. For
instance, a given compressor vendor
may have an appropriate size
and model available that can be a
perfect match for applications that
require a tiny compressor. In this
situation, other compressor types,
sizes or models (and other vendors)
may not be able to compete with
that specific compressor package
Small compressor packages are
highly specialized machines that
are highly reliant on the application,
size, manufacturing details,
available models and vendors.
High-pressure compressors. Oldfashioned,
high-pressure reciprocating
compressors and oil-sealed,
high-pressure centrifugal compressors
with heavy bolted heads
have been used in CPI plants for
decades. They are no longer suitable
for modern CPI plants. Vertically
split, centrifugal compressors
with shear-ring heads and drygas
seals are advanced options for
high-pressure CPI service. Modern,
high-pressure turbocompressors
are designed with special focus on
critical areas and functions, such
as rotor-dynamics, thermodynamic
performance, potential excitations,
the risk of rotating stall, auxiliary
systems, fabrication processes, internal
seals, dry-gas seals and antisurge
systems.
High-pressure compressors generally
use closed-type, three-dimensional
impellers. Both efficiency and
pressure coefficients are higher for
three-dimensional impellers compared
to old-fashioned two-dimensional
ones. Impellers are usually
manufactured from low-alloy steels.
In-depth investigations of material
characteristics should be carried
out to examine the material integrity
with regard to potential failure
modes (such as sulfide stress cracking;
SSC). One major problem is
that even after the transition of the
gas phase to a dense phase (fluid
passing the critical point when it
becomes supercritical fluid), the
water could condense if the gas is
sufficiently cooled. SSC can occur
in minutes if a susceptible material
under stress is exposed to a sour
wet condition inside the compressor
in the event of any wet upset.
For high-pressure compressors,
the thrust force generated by an
impeller assembly may be much
greater than what the thrust bearing
can handle. Therefore, thrust
forces should be balanced by using
a back-to-back impeller arrangement
(double-entry impeller series)
and balance piston(s)) - usually
with two balance pistons. The remaining
axial force is the difference
between the forces, which are relatively
large in magnitude compared
to the load on the thrust bearing.
In various operating conditions, the
load on thrust bearing can vary considerably
- it might even change
the direction. In many cases, an advanced,
double-acting, tilting-pad
thrust bearing should be used.
Vibrations in high-pressure tur48
ChemiCal engineering www.Che.Com June 2014
bocompressors are usually controlled
by employing very stiff rotor
designs. Sufficient stability of the
rotor should be ensured under any
potential anticipated operating or
malfunction conditions. Maximizing
rotor stiffness is achieved by
limiting the number of impellers
per casing. In general, the best recommendations
involve the shortest
possible bearing span and the maximum
shaft diameter.
Modern high-pressure compressors
(which tend to be high-speed
machines) typically only use tiltingpad
bearings or magnetic bearings.
These bearings help to inhibit bearing
cross-coupling excitation forces
and other instabilities.
In high-pressure applications,
the fluid density can become high,
which presents a great challenge
for the design and dynamics of the
machine. Above 200 barg, an average
gas density of around 150-300
kg/m3 has been reported. This high
fluid density lowers the rotating assembly's
natural frequency, which
increases the risk of resonance (a
source of instability). For example,
in a case study for a high-pressure
centrifugal compressor handling
process gas, the natural frequency
at the operational pressure was decreased
to 70% of the natural frequency
at a low-pressure operation.
To prevent such instability, the operating
speed range should often be
limited. The ratio of outlet width to
passage width of an impeller is an
important factor in this regard.
An accurate calculation of gas
properties is very important for the
realistic modeling of a high-pressure
compressor. A " shaker test " is
usually required for a high-pressure
compressor to verify rotor-dynamics
behavior and stable operation. The
main aim of this test is to determine
the damping of a rotor system
at various operating conditions,
over the entire range of compressor
speeds and characteristics.
Generally, a suitable, broad-range
device for generating vibration (a
shaker) should be employed. One
modern procedure for performing
this test involves fitting the end
cover of the barrel carrying the rotor
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Chemical Engineering June 2014

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