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How to read P&IDs

Dave Harrold, senior editor

Discussion about the acceptability of combining safety and process control functions in a common system has heated up recently. Much of the public debate, however, isn't fueled by end-users, but by suppliers of traditional, multi-modular redundant systems who have a vested interest in their technologies. Some vendors have targeted the end-user community with fear tactics insinuating that your plant will not be "as safe" when combining control and safety onto a single, robust platform.

Instrumentation detail varies with the degree of design complexity. For example, simplified or conceptual designs, often called process flow diagrams, provide less detail than fully developed piping and instrumentation diagrams (P&IDs). Being able to understand instrumentation symbols appearing on diagrams means understanding ANSI/ISA's S5.1-1984 (R 1992) Instrumentation symbols and identification standard. S5.1 that defines how each symbol is constructed using graphical elements, alpha and numeric identification codes, abbreviations, function blocks, and connecting lines.

Deciphering symbols

ISA S5.1 defines four graphical elements-discrete instruments, shared control/display, computer function, and programmable logic controller-and groups them into three location categories (primary location, auxiliary location, and field mounted).

Discrete instruments are indicated by circular elements. Shared control/display elements are circles surrounded by a square. Computer functions are indicted by a hexagon and programmable logic controller (PLC) functions are shown as a triangle inside a square.

Adding a single horizontal bar across any of the four graphical elements indicates the function resides in the primary location category. A double line indicates an auxiliary location, and no line places the device or function in the field. Devices located behind a panel-board in some other inaccessible location are shown with a dashed horizontal line

Letter and number combinations appear inside each graphical element and letter combinations are defined by the ISA standard. Numbers are user assigned and schemes vary with some companies use of sequential numbering, others tie the instrument number to the process line number, and still others adopt unique and sometimes unusual numbering systems.

The first letter defines the measured or initiating variables such as Analysis (A), Flow (F), Temperature (T), etc. with succeeding letters defining readout, passive, or output functions such as Indicator (I), Record (R), Transmit (T), and so forth.

Example shows the story

Referring to the Example P&ID diagram, FT 101 represents a field-mounted flow transmitter connected via electrical signals (dotted line) to flow indicating controller FIC 101 located in a shared control/display device. A square root extraction of the input signal is applied as part of FIC 101's functionality. The output of FIC 101 is an electrical signal to TY 101 located in an inaccessible or behind-the-panel-board location. The output signal from TY 101 is a pneumatic signal (line with double forward slash marks) making TY 101 an I/P (current to pneumatic transducer). TT 101 and TIC 101 are similar to FT 101 and FIC 101 but are measuring, indicating, and controlling temperature. TIC 101's output is connected via an internal software or data link (line with bubbles) to the setpoint (SP) of FIC 101 to form a cascade control strategy.

Often P&ID's include a cover page where common and typical terms, symbols, numbering systems, etc., are defined. On the example, Typical YIC would likely appear on the cover page and the simplified form of YIC would appear throughout the P&IDs.

Typical YIC indicates an on/off valve is controlled by a solenoid valve and is fitted with limit switches to indicate open (ZSH) and closed (ZSL) positions. All inputs and outputs are wired to a PLC that's accessible to the operator (diamond in a square with a solid horizontal line). The letter 'Y' indicates an event, state, or presence. The letter 'I' depicts indication is provided, and the letter 'C' means control takes place in this device.

Adherence to ISA's S5.1 Instrumentation Symbols and Identification standard ensures a consistent, system independent means of communicating instrumentation, control, and automation intent is developed for everyone to understand.

General instrument or function symbols
  Primary location accessible to operator Field mounted Auxiliary location accessible to operator
Discrete instruments
Shared display, shared control
Computer function
Programmible logic control

1. Symbol size may vary according to the user's needs and the type of document.
2. Abbreviations of the user's choice may be used when necessary to specify location.
3. Inaccessible (behind the panel) devices may be depicted using the same symbol but with a dashed horizontal bar.

Source: Control Engineering with data from ISA S5.1 standard


Identification letters
  First letter Succeeding letters
  Measured or initiating variable Modifier Readout or passive function Output function Modifier
A Analysis   Alarm    
B Burner, combustion   User's choice User's choice User's choice
C User's choice     Control  
D User's choice Differential      
E Voltage   Sensor (primary element)    
F Flow rate Ration (fraction)      
G User's choice   Glass, viewing device    
H Hand       High
I Current (electrical)   Indication    
J Power Scan      
K Time, time schedule Time rate of change   Control station  
L Level   Light   Low
M User's choice Momentary     Middle, intermediate
N User's choice   User's choice User's choice User's choice
O User's choice   Orifice, restriction    
P Pressure, vacuum   Point (test connection)    
Q Quantity Integrate, totalizer      
R Radiation   Record    
S Speed, frequency Safety   Switch  
T Temperature     Transmit  
U Multivariable   Multifunction Multifunction Multifunction
V Vibration, mechanical analysis     Valve, damper, louver  
W Weight, force   Well    
X Unclassified X axis Unclassified Unclassified Unclassified
Y Event, state, or presence Y axis   Relay, compute, convert  
Z Position, dimension Z axis   Driver, actuator  
Source: Control Engineering with data from ISA S5.1 standard

Common connecting lines
Connection to process, or instrument supply:
Pneumatic signal:
Electric signal:
Capillary tubing (filled system):
Hydraulic signal:
Electromagnetic or sonic signal (guided):
Internal system link (software or data link):
Source: Control Engineering with data from ISA S5.1 standard


For more information on ISA standards, visit www.isa.org or call 919/549-8411.

Dave Harrold, senior editor
Comments? E-mail dharrold@cahners.com

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Updated January, 2010