INSTRUMENT AND UTILITY AIR SYSTEM (PRE-COMPRESSION FACILITIES)
The lnstrument and Utility Air System supplies up to 75 scfm of air for instrument
and utility usage. The system supplies air of two different qualities - utility air
which has not been dried and instrument air which has been filtered and dried. The
instrument air is dried to reduce its moisture content in order to minimize water
condensation which might foul the controllers when the air is let down from a
system pressure of 125 psig to 15 psig at the controllers. The Utility Air System
operates at 100 psig and provides starting air to the Diesel Generator, Diesel
Firewater Pump and air to the utility hose stations. The lnstrument Air System
provides air to all pneumatic controllers, transmitters and actuators as well as the
Sump Pump on ERP.
Atmospheric air is compressed in reciprocating compressors, cooled and sent to the
Utility Air Receiver where any water condensing from the compressed air is
removed. Part of the air from the receiver is sent under pressure control to the
utility air header to service the various users; the rest flows to a pair of heatless
dryers where the moisture is removed in desiccant beds. The dried instrument air is
filtered and sent to the lnstrument Air Receiver and subsequently distributed to the
instrument air header. All the equipment in the system is mounted on a single skid.
Figure 8.0-1 depicts the lnstrument and Utility Air System.
A skid mounted diesel engine driven Black Start Air Compressor is provided to
supply starting air to the Diesel Generator.
SYSTEM DESCRIPTION
8.1 .I Air Com~ressorsa nd Aftercoolers
Two reciprocating type compressors of 75 scfm capacity at 175 psig have been
provided. The Air Compressors, C-5350 and C-5355, draw ambient air through a
dry type inlet filterlsilencer into the low pressure cylinder of the compressor,
compress the air and route it through a forced draft intercooler to remove the heat
of compression. The air is further compressed to the desired operating pressure in
the high pressure cylinder. The compressor discharge line is protected by a high
temperature shutdown, TISHH-535015355 set at 400°F, that will shutdown the
affected Air Compressor. The first stage of the compressor is protected by a safety
valve, PSV-5350Al5355A set at 60 psig, while the second stage is protected by
PSV-53508153558 set at 200 psig. The compressed air passes through the
Aftercooler to remove the heat of compression. The discharge of Aftercoolers is
combined and piped to the Utility Air Receiver. The Aftercooler discharge line of
each compressor is provided with a high temperature shutdown, TISH-535115356.
The Air Compressors may be started and stopped either manually or automatically.
Selection of operation mode is made at the unit control panel, UCP-5350, located at
the skid. In the automatic mode, each compressor is controlled by pressure
switches provided at the discharge of Utility Air Receiver. The lead unit operates in
a loadlunload fashion between a range of 150 and 175 psig. The lag unit starts
when the Utility Air Receiver pressure drops to 140 psig and unloads when it
increases to 170 psig. The lag unit has a variable time delay (0-30 minutes) to
control the unloading cycle before the compressor is shutdown. The control system
is arranged so that either machine may be selected as the lead unit and the other
unit as the lag. For details of the control system, refer to manufacturer's drawings
in the Dossier Manual. The compressor is protected against low lube oil levels by
shutdown level switches, LSLL-535015355.
Page