Alarm And Shutdown Readings - Emerson VILTER VSS Installation, Operation And Service Manual

Section 2 • Theory of Operation

Alarm and Shutdown Readings

WARNING
Software programming credentials shall only be
made available by the supplier. The user will only
have access to operational features established by the
supplier. Failure to comply may result in serious injury
or death.
The control system for the Single Screw compressor
must protect the machine from damage caused by run-
ning outside of normal operating conditions by provid-
ing operators with alarms when operating parameters
have reached an abnormal condition, and by automati-
cally stopping the compressor before these conditions
can cause a unit failure. Pressures and temperatures of
the process gas and the oil, as well as motor amperage
and VFD frequency must all be continuously monitored
to ensure the compressor is operating properly.
Refer to Figure 2-2 for locations of RTD and PT:
1. Low Gas Suction Temperature - This point protects
the compressor from suction gas entering the compres-
sor at too low of a temperature, and is activated by a di-
rect reading from the suction temperature RTD located
at the suction tee.
2. High Gas Discharge Temperature - This point pro-
tects the compressor against high gas temperature at
the discharge of the unit, and is activated by a direct
reading from the RTD located at the compressor dis-
charge manifold.
3. Low Oil Separator Start Temperature - This point pro-
tects the compressor from starting with low oil tempera-
ture in the separator, and is activated by a direct reading
from the RTD located at the bottom of the oil separator.
4. Low Oil Separator Run Temperature - Similar to the
Low Oil Separator Start Temperature described above,
however this point only becomes active after a pre-
determined period of running time, and uses a higher
setpoint.
5. Low Oil Injection Temperature - This point protects
the compressor from running with cold oil being in-
jected into the screw housing, and is activated by a di-
rect reading from the RTD located at the oil injection
line. This point is bypassed for a predetermined period
of time after starting to allow the unit time to start and
warm up.
6. High Oil Injection Temperature - This point protects
the compressor from running with hot oil being injected
into the screw housing, and is activated by a direct read-
ing from the RTD located at the oil injection line.
VSS/VSR/VSM/VSH/VSSH Compressor • Installation, Operation and Service Manual • Emerson • 35391S
7. Low Suction Pressure - This point protects the com-
pressor from drawing low suction pressure and is ac-
tivated by a direct reading from the suction pressure
transducer, which reads the pressure from a tap located
at the suction stop/check valve housing.
8. High Discharge Pressure - This point protects the
compressor from developing high discharge pressure
and is activated by a direct reading from the discharge
pressure transducer, which reads the pressure from a
tap located at the oil separator. In addition to this alarm
and shutdown, the compressor package is ultimately
protected from damage due to over pressurization by at
least one discharge pressure relief valve located on the
oil separator. The purpose of this safety setpoint is to al-
low for a lower setpoint to conform to a process require-
ment, and to prevent the relief valve from opening.
9. Prelube Oil Pressure - This point acts as a permissive
to start the compressor, and protects against the com-
pressor starting with no oil lubrication. If, during a start
sequence, the prelube oil pressure fails to rise above
4.0 PSID, the compressor will fail to start. The prelube
oil pressure is a calculated value obtained by subtract-
ing the discharge pressure reading from the oil manifold
pressure (oil filter outlet pressure) reading.
10. Low Oil Pressure - This point protects the compres-
sor from running with insufficient lubrication pressure,
and becomes active after a predetermined period of
running, usually sixty seconds. The oil pressure is a cal-
culated value obtained by subtracting the suction pres-
sure from the oil manifold pressure (oil filter outlet pres-
sure) reading, which results in the actual pressure under
which the oil is entering the screw housing.
11. High Running Oil Filter Differential Pressure - This
point alerts operators to clogging oil filters. When the oil
filters develop a high differential pressure while running
at normal operating temperatures, it is an indication
that they are becoming dirty and must be changed. An
alarm initially warns of dirty filters; if the situation wors-
ens before the filters are changed a shutdown will stop
the compressor.
In most cases, the safety setpoints described above will
have settings which are dictated by process require-
ments, and not necessarily mechanical constraints of
the compressor. Process pressures and temperatures
may vary considerably depending on the application of
the compressor, and the Single Screw compressor is de-
signed to work well in a broad range of applications. For
this reason, it is impractical to suggest "initial" setpoints
to fit all applications. Instead, minimum and maximum
values for each safety setpoint are provided, while pre-
cise settings for the safety setpoints must be derived for
each installation.
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