Page 219 - Chemical engineering design

196

Fails open CHEMICAL ENGINEERING Maintains position
Fails shut
Figure 5.2.

Instruments and controllers

Locally mounted Main panel mounted
Figure 5.3.

Locally mounted means that the controller and display is located out on the plant near
to the sensing instrument location. Main panel means that they are located on a panel
in the control room. Except on small plants, most controllers would be mounted in the
control room.

Type of instrument
This is indicated on the circle representing the instrument-controller by a letter code (see
Table 5.1).

Table 5.1. Letter Code for Instrument Symbols (Based on BS 1646: 1979)
Property First Indicating Recording Controlling Indicating Recording
measured letter only only only and controlling and controlling
Flow-rate F FI FR FC FIC FRC
Level L LI LR LC LIC LRC
Pressure P PI PR PC PIC PRC
Quality, analysis Q QI QR QC QIC QRC
Radiation R RI RR RC RIC RRC
Temperature T TI TR TC TIC TRC
Weight W WI WR WC WIC WRC
Any other
property (specified
in a note) X XI XR XC XIC XRC
Notes:
(1) The letter A may be added to indicate an alarm; with H or L placed next to the instrument circle to indicate
high or low.
(2) D is used to show difference or differential; eg. PD for pressure differential.
(3) F, as the second letter indicates ratio; eg. FFC indicates a flow ratio controller.
Consult the standard for the full letter code.