In general, every conventional measuring or controlling instrument Installed in a process unit is identified by three separate codes as follows.
A location number code indicates the specific process unit in which the instrument is installed.
A function letter code indicates the property or process variable being measured or controlled.
A serial number code identifies the specific instrument and therefore prevents confusion when there are several Instruments In a single process unit, each having the same function letter code.
The combination of the three codes is known as the Instrument tag number, which has the basic format -
xx a - yyy
“xx” is a two-digit number used to identify the process unit.
'a’ is a letter code containing two or more capital letters and is used to identify the instrument function.
‘yyy’ is a three-digit number used to identify the particular instrument.
When the instrument code or tag number is written on a drawing or document, a dash is inserted between the ‘a’ and the ‘yyy’ sections of the format. For example, a pressure indicating controller installed in a process unit coded 10 and identified by serial number 101, is described in written form as 10 - PIC - 101.
In the case of the same tag numbers, the process pressure correcting element, usually a control valve, often has the same tag number as the control instrument.
However, when the controller operates two valves in a split range mode, the valves are tagged and numbered consecutively, for example,
10 - PIC - 101 >>>>>>>>>>>>>>>>>>>> 10 – PCV – 101-1 and 10 – PCV – 101-2