Chemical Engineering December 2010 - 41

coupling options are limited, it may
not be possible to find an acceptable
coupling with the required torsional
characteristics and service factor,
especially for large compressors
(above 3 MW)
3. Flexible coupling (which provides
more elasticity and damping, but
may require greater maintenance
since elastic elements in such a coupling
may need frequent replacement)
The
most common reasons for problems
caused by torsional vibration
are lack of comprehensive torsionalvibration
analysis, improper application
and maintenance of couplings
(especially flexible ones) and lack of
appropriate monitoring. As a general
rule-of-thumb, the shaft diameter of
the electric motor should be equal
to or greater than the reciprocating
crankshaft diameter (because the
crankcase is generally forged from a
stronger steel grade compared to the
motor rotor).
Condition monitoring
Condition monitoring, when done
properly, can pay for itself by helping
operators identify potential systems
malfunctions at an early stage. A rigorous
program should include monitoring
of these important conditions:
Vibration (including continuous
vibration monitoring of the compressor
and motor casing, providing
both alarm and shutdown capabilities):
*
In general, velocity transducers
are preferred over accelerometers
(because interested frequencies for
monitoring better match with velocity-measurement
sensors). The
optimum configuration for using a
velocity transducer is to install one
on each end of the crankcase, about
halfway up from the base plate in
line with a main bearing, both for
compressor and motor
* Crosshead accelerometer (alarm)
Temperature:
* High gas-discharge temperature for
each cylinder (with both alarm and
shutdown capabilities)
* Pressure packing piston-rod temperature
(alarm)
* High crosshead pin temperature
(alarm), only for relatively large
compressors (around or above 3
MW)
* High compressor main, and motor
bearing, temperatures (alarm)
* Valve temperature (monitoring)
* Oil temperature, out of compressor
frame (alarm)
* High jacket-water temperature of
each cylinder (alarm)
In addition, proximity probes, typically
located under the piston rods, provide
alarm capabilities but are not used for
shutdown. These are used to measure
the rod position and determine wear
or malfunctions. Such probes can
quickly identify problems such as piston
or rider band malfunctions, cracks
in the piston rod attachment, a broken
crosshead shoe or even a liquid carryover
to a cylinder.
Improving maintenance
To support regular maintenance, the
installation of any reciprocating compressor
must ensure proper access to
the entire compressor system, especially
the non-drive end. In particular,
adequate space and work areas
must be provided to enable the complete
withdrawal of the piston, removal
of the cooler bundles or piping
spool and laydown area (to carry out
maintenance, dismantling of parts
and repairs).
Similarly, three crane capacities
must be properly identified: The total
capacity of the overhead crane (to lift
components for routine maintenance),
the maximum maintenance weight (to
ensure that the heaviest parts, usually
the motor, can be lifted during
overhauls), and the maximum installation
weight (maximum skid weight,
usually the compressor skid).
For a typical 7-MW API 618 compressor
train for petroleum-refinery
service, these crane capacities would
be roughly 11 tons, 55 tons and 100
tons, respectively, and the required
crane height would be roughly 12 m
(around 40 ft)
Any time a given compressor must
be stopped for an extended time, it
should be turned a quarter-turn every
week, using a barring device (this is a
device that slowly turns the compressor
to avoid locking and other problems
that often arise during long stoppages
of reciprocating compressors). A
manual barring device can be used for
relatively small compressors. A pneumatic
barring device must be used for
compressors rated above 750 kW (provided
there is no area classification or
power-availability problem).
For larger compressors (2 MW or
larger), these special tools are often
needed to carry out routine maintenance
on reciprocating compressors.
These tools cannot be easily purchased;
they must be specially designed
and fabricated based on the
actual machine:
* Bearing extractor
* Piston extractor
* Valve extractor
* Piston fit-up tool
* Hydraulic tightening system
* Crosshead assembling tool
* Special lifting tools
* Partition plate-assembling tools
* Mandrels for wear bands
During maintenance of compressor
mechanical components, the following
criteria are important:
* Cylinder clearance for the outboard
end should be around 4-6 mm
(0.2-0.3 in.), and for the inboard
end, clearance should be around 2-4
mm (0.1-0.2 in.)
* The allowable temperature of the
machine bearings, piston rod, connecting
rod bearing and crosshead
should be maintained around 85ºC,
and for the crosshead pin, it should
be maintained around 90ºC
* The vibration level of the crankcase
should not exceed 100 microns,
and the expected vibration level of
the cylinders should be around 150
microns (these vibration recommendations
are peak-to-peak vibration
readings for an installed, troublefree,
middle-range machine around
1 MW)
* Bearings, piston rings and piston
shoes should also be inspected regularly.
Auxiliaries
and accessories
For auxiliaries and accessories, the
optimum configuration is to install a
local panel near the compressor skid
(around 250 mm, or one foot away
from the compressor skid), and on a
standalone skid to minimize the potential
for vibration damage.
The oil system should include two
ChemiCal engineering www.Che.Com DeCember 2010 41
http://www.Che.Com
Chemical Engineering December 2010

Table of Contents for the Digital Edition of Chemical Engineering December 2010
