Chemical Engineering September 2012 - 55

When the compression gas is changed,
set point values are only adjusted for
the new condition with no change in
control procedure.
Maintenance mode. In this mode,
during which the compressor is at
standstill, all auxiliary drivers (such
as auxilliary oil pump, water pumps,
and bypass valves) can be manually
started by operators.
Off mode. In this mode, all drivers
are out of operation, and standby functions
are disabled. This mode will be
activated only when the compressor is
not running; otherwise, selecting the
" off " mode will have no effect on the
operation of the compressor.
Mode selection between auto, off
and maintenance is done within the
distributed control system (DCS). A
compressor is provided with several
local instrument boards (LIBs), with
gages for indicating pressure, temperature,
flowrate and level of process
gas, lubrication oil and other utilities
(such as water, air and nitrogen), during
compressor operation in all modes.
Additionally, the LIB shows whether
local motor-control stations are also
available for local starting and stopping
of auxiliary equipment in maintenance
mode. Auxiliary motors also
have local motor-control stations located
within a few meters from the associated
auxiliary-drive motor. Visualization
and control of the compressor
operation is normally done from a central
control room (CCR) station.
Compressor auxiliary systems
Hydro jacking device. For large reciprocating
compressors and motors,
manufacturers consider adding an additional
system to the equipment in
order to lessen the breakaway torque
in the bearings in transient conditions
(startup, shutdown or maintenance).
For this purpose, one oil pump is usually
added in series after the lubrication
pump. A jacking oil pump assures
lasting oil film existence between
bearing and journal (shaft) surfaces at
the instant a rotation commences.
Barring device. The compressor
should be fitted with a barring device
(barring gear), which can be manual,
electrical or pneumatic. This device is
used only for maintenance, while the
compressor is stopped. For compressors
with a rated power equal to or
greater than 750 kW (1,000 hp), and
torque requirement equal to or greater
than 1,600 Nm (1,200 ft-lb), either the
electrical or pneumatic type should be
considered. Manual barring may be
achieved by turning the flywheel. The
barring device should be designed so
that gas forces acting on the pistons
cannot cause the compressor either to
accelerate or to rotate in the reverse
direction. With the barring device
engaged, it should not be possible to
start the compressor [6].
Manual barring devices should have
a locking feature, whereas pneumatic
and electrical devices should incorporate
appropriate safety switches or
measures for interlocking with the
main driver. Moreover, a pneumatic
system should be equipped with an
air-supply inlet filter and oiler, and a
four-way control valve for operation in
forward or reverse directions.
The barring device should be designed
to enable barring over the
pressurized machine at suction pressure.
Before the barring device is enabled
to start, the hydro jacking of
compressor and main motor have to
be started, and the minimum pressure
must be obtained.
The barring device operates with a
local push button (forward and reverse)
on the local panel near the motor. The
barring device motor works only while
one of the push buttons is pressed; release
of the button stops the barring
device and locks the crank gear. When
maintenance is finished, the stopped
barring gear must be moved back in
the " turn wheel free " position, which is
displayed on the DCS system.
Compressor frame lubrication
system. A frame lubrication system
is pressurized through two electrically
driven oil pumps. One is used
for normal operation (main pump)
and the second is in standby mode
as an auxiliary pump for necessary
conditions. A crankshaft-driven main
pump is allowed per API 618, but for
large machines, each oil pump should
be individually equipped with an
electrical motor.
In auto mode, the main oil pump
has to be started before starting the
compressor main motor. Generally,
pre-lubrication time is set for approximately
one minute. When pre-lubrication
time has passed, the startup condition
" Pre-lubrication time passed " is
activated and displayed on the DCS.
When neither the low-lube-oil pressure
alarm nor the low low-lube-oil
pressure alarm is activated, the hydro
jacking pump and subsequently the
main motor are permitted to start.
During normal operation of the compressor,
the standby oil pump should
be in auto mode and start automatically
when a low-oil-pressure alarm is
triggered on the DCS.
If the low low-oil-pressure signal
lasts more than a specified time (usually
2 s), the compressor main motor
should be stopped immediately by a
shutdown (trip).
Since the compressor is stopped normally,
post lubrication is started; this
means that the lubrication oil pump
will continue to run for a minimum of
approximately 2 min. When post lubrication
time has passed and the normal
stop sequence is accomplished, the oil
pump can either be stopped by the operator
or continues running. Note that
in maintenance mode, the frame oil
pump can be started individually.
Moreover, the operator can enable
the oil heater by pushing a button in
the DCS. If the heater is enabled, the
temperature of the frame lubrication
oil is controlled by the switch controller
inside the heater. Note that the
crankcase oil temperature should not
exceed 70°C (160°F).
For safety reasons, the heater is
interlocked and must be switched off
when the low " low oil level " is activated.
It should be noted that during
compressor operation, the heater shall
always be stopped.
Compressor cylinder lubrication
system. The cylinder lubrication
system is pressurized by electrically
driven multiple-plunger-type oil
pumps (plunger per point) or by the
divider block method. However, using
a divider block system is not recommended
because of complexity and
low reliability.
The cylinder lubrication system will
be started locally if the cylinder oiltank
level is fulfilled (above low level).
When the pre-lubrication time (approximately
2 min) has elapsed, the
compressor start condition " cylinder
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