24 – 6
Section 24
•
Twin Screw Control
Vission 20/20 • Operation and Service Manual • Emerson • 35391SC 2.7
Twin Screw Oil Pressure
The twin screw compressor has two separate oil pres-
sure settings. They are named “Prelube Oil Pressure”
and “Run Oil Pressure” in the Alarm and Trips Menu.
Both of these oil pressures are calculated in the same
way - defined as “Filter Outlet Pressure minus Discharge
Pressure”.
As shown in Figure 24-6, the alarm and trip setpoints for
both of these oil pressures are set to the same values and
any adjustments to these oil pressures is usually done so
that the setpoints are the same.
OIL PRESSURE MONITORING BEFORE
COMPRESSOR STARTS
Pressing the “Auto” or “Manual” options from the “Unit
Start“ pop-up window will start the oil pump. The de-
crease solenoid will be energized as well if the capac-
ity slide is greater than 5%. A prelub oil pressure timer
called “Minimum Compressor Prelub Time” begins
timing, see Figure 24-7. This timer is adjustable, while
the default time is 5 seconds. This timer allows oil to be
pushed into the oil injection lines to fill the lines with oil
BEFORE the system starts looking for prelub oil pressure.
After the Minimum Comp Prelub Timer times out, then
prelub oil pressure monitoring begins. The oil pump will
run for the time setting of “Prelub Oil Pressure Monitor
Time” (typically 20 seconds) trying to achieve prelub oil
pressure. If it fails to establish prelub oil pressure, the
oil pump shuts down for 10 seconds, and then starts
and tries again. The cycle is repeated for the “Prelube
Oil Pressure Monitor Trials” setting, typically set at “3”
tries. After the third unsuccessful try, a failure message
“Prelub Oil Pump Inhibit” is generated. This indicates a
failure to establish Prelub Oil Pressure. When the Prelub
Oil Pressure is established, then the compressor is com-
manded to start.
LOW OIL PRESSURE SAFETY BYPASS
When the compressor starts, then the Low Oil Pressure
Safety Bypass timer is started (set at 60 seconds by de-
fault, but it is adjustable).
Figure 24-6. Prelube Oil Pressure and Run Oil Pressure Settings
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