Chemical Engineering December 2010 - 40

Engineering Practice
does not exceed 80% of the allowable
rod load.
As shown on the y-axis in Figure 2,
the rod load shall change sign from
negative to positive and then negative
again during one revolution of
the crankshaft in order to provide
proper lubrication for the mechanism
(especially for the cross-head pin). The
duration of rod sign reversal (the period
during which load has the opposite
sign) should not be less than 15
degrees of crank angle. The rod-load
reversal peak (maximum amount of
load in the reversed direction) should
not be less than 3% of the actual combined
load in the opposite direction.
These minimum requirements should
be satisfied under all possible operating
conditions (especially in the face of
suction-pressure variation and partload
operation, such as when an unloader
device is used to decrease flow
through the compressor).
In many cases, higher values of
rod-reversal duration and peak are
considered during compressor design
to increase reliability. In Figure 2,
minimum load-reversal duration corresponds
to 50% capacity, and the reversal
duration is more than 70 deg.
In general, the optimum speed for
the reliable operation of reciprocating
compressors is around 350 rpm.
For compressors operating below 400
kW, speed on the order of 450 rpm is
suitable. However, for those operating
below 100 kW, higher speeds (even as
high as 700 rpm) are acceptable.
Lubricated cylinders and packing
may be preferred to extend service
life. However, non-lubricated cylinder
compressors should be used when
the possibility of oil contamination
cannot be tolerated in downstream
operations (for instance, when trace
amounts of lubricating oils could
cause catalyst problems in downstream
reactors).
For the optimum operation of reciprocating
compressors, sufficient inertia
- provided by a properly sized
flywheel - is mandatory to regulate
the variable reciprocating torque.
Figure 3 shows brake torque versus
crank angle for one revolution of the
crankshaft for a reciprocating compressor
used in a petroleum refinery
setting. The red and blue curves rep0.2
0.4
0.6
0.8
100%
Capacity
75% Capacity
50% Capacity
93% Suction pressure
-0.8
-0.6
-0.4
-0.2
50
100
150
200
250
One rotation of the crankshaft angle, deg.
FIGURE 2. These rod-load curves are for a reciprocating compressor in refinery
service. This figure shows variation in the ratio of rod load to allowable rod load in
one revolution of the compressor crankshaft (from 0 to 360 deg.). Individual curves
show the rod load for various operating conditions, including 50%, 75% and 100%
compressor capacity, as well as full flow with 7% suction reduction (that is, operation
at 93% of rated suction pressure)
resent compressor torque for normal
full-load capacity and half-load (50%)
capacity, respectively.
A step-less capacity-control system
uses a hydraulically actuated, fingertype
unloader. This device unloads the
suction valve for only a portion of compression
cycle to achieve the desired
adjusted capacity.
A finger-type unloader has fingershaped
parts that act on the cylinder
valve elements and keep them open
for a defined duration during the compression
cycle. Users should note that
these finger-type unloaders have the
potential to damage the valve-sealing
elements and thus may have greater
maintenance requirements.
A step-less capacity control system
is recommended for larger machines
(units rated above 2 MW, when large
operation variation is expected). In
these cases, step-less capacity control
(working in the range of 20-100% capacity)
is extensively used due to process
requirements.
In general, valves and unloaders are
responsible for nearly half (roughly
45%) of unscheduled reciprocatingcompressor
shutdowns, so valve and
unloader selection can have a strong
impact on reliability. And many consider
the automatic cylinder valves
to be the most critical components of
such machines, as they are responsible
for many unscheduled mainte40
ChemiCal engineering www.Che.Com DeCember 2010
nance events. For large compressors
(that is, those that operate at relatively
low speeds with high pressure
ratios), relatively large-bore ringtype
valves (above 100 mm, or 4 in.)
combined with plug-type unloaders
should be considered first, to avoid reliability
issues associated with fingertype
unloaders. Since ring-type valves
and plug unloaders are not available
for smaller-sized compressors (those
that operate at relatively higher
speeds), such units typically use platetype
valves.
During operation, the rotating parts
of the compressor, power transmission
and driver will act like springs
connected in series. This torsional dynamic
system may create resonance
(where one natural frequency of system
coincides with one of excitation
torque). In reciprocating compressor
trains, there is always a risk of torsional
resonance and torsional fatigue
failure (that is, damage to component
resulting from excessive cyclic loads).
Couplings that connect the driver
to the compressor can be modified to
tune the system to avoid torsional resonance.
Several coupling options are
available as follows:
1. Direct, forged-flange rigid connection
(no coupling) between driver
and compressor
2. High-torsional-stiffness coupling is
allowed by torsional analysis. Since
300
350
Calculated rod load, kN / allowable rod load, kN
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Chemical Engineering December 2010

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