This article describes the symbolism of the instrumentation for measuring, actuating and regulating industrial processes.

 

The symbology for the identification of the measurement and control instrumentation on the flow and process diagrams and on the P&ID (Piping & Instrument Diagram), commonly called P&I (Piping & Instrumentation), is generally compliant with the Standard ISA (Instrumentation Society of Automation) identified as S.5, that is composed of identification codes and graphic symbols.

The symbology of the instrument it is placed in a circle of 7/16 “or 12 mm (see below), was in the top there is the functional identification (of the instrument or device) composed with max 5 letters, and in the bottom there is identification of the loop (measurement and/or control) consists of a progressive number of 3 or 4 numbers, followed by a literal suffix to identifies two or more identical instruments inserted in the same chain and therefore having the same functional identification;

The circle that identifies the instrument or device has:

  • Continue Line in the center: for an instrument suitable for control room mounting
  • The dotted line in the center: for an instrument suitable for technical room mounting
  • No line in the center: for an instrument suitable for field application

If an instrument has a circumscribed square means that it is an integrated instrument as a function in a DCS (Distributed Control System) or PLC (Programmable Logic Controller).

T
First Letter
(Measured variable)
RC
Later letters
(Functions)
101
Loop number
A
Suffix
Functional identification Loop identification

Instrumentation symbology

Graphical representation of a temperature recorder controller of the 101A loop, suitable for control room mounting

The following tables show:

– Table 1 identifying the function of the instruments or devices
– Table 2 the graphic identification of the valves
– Table 3 the graphic identification of the actuators
– Table 4 identifying the safety conditions of the valves and actuators

At the end will also be reported the literal and graphic symbols of the power supplies and signals of the instrumentation.

TABLE 1 – FUNCTIONS OF INSTRUMENTS

 

LETTER

FIRST LETTER  FOLLOWING LETTERS
Measured variable Modified Passive function Active function Modifier
A Analysis Alarm
B Burner

or combustion

C User (1) choice Control
D User (1) choise Differential
E Voltage Element (primary)
F Flow Fraction
G User (1) choice Glass (2)
H Hand High (3)
I Current Indicate
J Power Scan
K Time schedule Time rate              of change Control station
L Level Light Low (4)
M User (1) choise Momentary Meddle

or intermediate

N User (1)

choice

O User (1) choice Orifice

or restriction

P Pressure or vacuum Point test
Q Quantity Totalize or integrate
R Radioactivity Record
S Speed

or

frequency

Safety Switch
T Temperature Transmit
U Multivariable Multifunction Multifunction Multifunction
V Vibration Valve
W Weight or force Well
X Danger Axis X Unclassified Unclassified Fault
Y Event Axis Y Calculation module (5)
Z Position                   or dimension Axis Z Unclassified final element
(1) It must be chosen uniquely for each project. For example C – Conductivity; D – Density; etc.
(2) Glass instruments that allow the visualization of process data
(3) Very high or maximum is indicated with HH
(4) Very low or minimum is indicated with LL
(5) To define the Y function on a diagram, add the specific symbol of the relay or convert to a square of 6 mm on the side of the graphical sign of the instrument (in the north-east position).

 

TABLE 2 – INSTRUMENTATION VALVES SYMBOLS

Instrumentation valves symbols

 

TABLE 3 – ACTUATORS FOR INSTRUMENTATION VALVE SYMBOLS

 

 

Actuators for instrumentation valves symbols
Actuators for instrumentation valves symbols

 

TABLE 4 – IDENTIFICATION OF SAFETY CONDITION FOR VALVES AND ACTUATORS (WITHOUT SUPPLY)

IDENTIFICATION OF SAFETY CONDITION FOR VALVES AND ACTUATORS

 

SYMBOLOGY FOR POWER SUPPLY

AS –  Ais Supply:
IA – Instrument Air
PA- Plant Air
ES –  Electrical Supply
GS –  Gas Supply
HS –  Hydraulic Supply
NS –  Nitrogen Supply
SS  – Steam Supply
WS – Water Supply

 

SYMBOLOGY FOR INSTRUMENTATION SIGNALS

The table shows the symbology of control signals for instrumentation

Symbology for instrumentation signals
Symbology for instrumentation signals

 


Author: Eng. Alessandro Brunelli

Book: Manuale di Strumentazione