Hydrocarbon Processing - June 2022 - 13

Reliability
HEINZ P. BLOCH, RELIABILITY/EQUIPMENT EDITOR
Heinz.Bloch@HydrocarbonProcessing.com
Screw compressor volume control options
During their careers, most rotating
machinery experts have dealt with the
selection of advanced screw compressors.
These are often chosen for their overall
simplicity and efficiencies that generally
exceed those of centrifugal compressors.
Although control options can differ
for dry-running vs. liquid injection-type
screw compressors, volume control helps
control the wide possible operating ranges
of these machines.
Controlling dry-screw compressors.
The following options can be used to control
dry-screw compressors.1
Control by variable speed. Because
screw compressors displace the medium
positively, the most advantageous volume
control strategy is to vary the speed. This
may be done by using variable speed electric
motors, steam turbine drives, hydraulic
or hydro-mechanical torque converters,
and other means.
Speed may be reduced to about 50%
of the maximum permissible speed. Induced
flow volume and power transmitted
through the coupling are reduced in
roughly the same proportion.
Bypass. Using this method, the surplus
gas volume is made to flow back to
the intake side by way of a compressor
discharge pressure controller. An intermediate
cooler brings the surplus gas volume
down to intake temperature.
Full-load/idling-speed governor. As
soon as a predetermined final pressure is
attained, a pressure controller operates a
diaphragm valve that opens a bypass between
the discharge and suction sides of
the compressor. When this occurs, the
compressor idles until pressure in the system
drops to a predetermined minimum
value. The valve will close again on receiving
an impulse from a pressure sensor. This
brings the compressor back to full load.
Suction throttle control. Suction
throttling is suitable for only air compressors.
As in the case of the full-load/idlingspeed
control method, a predetermined
FIG. 1. Slide valve operation.
maximum pressure in the system (e.g., in
a compressed air receiver) causes pressure
on the discharge side to be relieved down
to atmospheric pressure. Simultaneously,
the suction side of the system is throttled
down to about 0.15 bar absolute pressure.
When pressure in the entire system
has dropped to the permissible minimum
value, full load is once again restored.
Controlling liquid-injected screw
compressors. The following are pathways
to control liquid-injected screw
compressors.
Suction throttle control. Since the
final compression temperature is governed
by the injected oil, a greater range
of compression ratios can be accommodated
by liquid-injected machines. This
permits the main flow volume to be varied
within wide limits.
Built-in volume governor. Large
compressors are frequently equipped
with an internal volume-regulating device.
FIG. 1 illustrates the principle. A slide
valve that is shaped to match the contours
of the housing is built into the lower part
of the housing. It is designed to move in
a direction parallel to the rotors, whereby
the effective length of the rotors can
be shortened. The range of this control
mode is typically between about 10% and
100%. Compared with suction throttling,
this type of control offers more efficient
operation and is often preferred.
REFERENCE
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 800 publications, among them 24 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.
Hydrocarbon Processing | JUNE 2022 13
Hydrocarbon Processing - June 2022

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