There are four analog pressure gauges installed on the pilot plant. A panel mounted gauge indicates the column overhead pressure. A second panel mounted gauge indicates the pressure downstream of the bottoms control valve. Note that following a piping revamp, the difference between these two gauges is not indicative of the column pressure drop and cannot, therefore, be used to detect the onset of flooding. A third pressure gauge indicates the pump discharge pressure (as described in Section 11.5.3). The fourth gauge is mounted on the top of the pressurised feed tank and provides a continuous indication of the tank pressure.
A pressure transmitter measures the column overhead pressure (duplicating the reading from the first analog gauge). The transmitter signal (4-20 mA) can be used as a controller input to provide closed loop pressure control via the distillate control valve.
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