How to read a P&ID diagram?


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P&ID stands for Piping and Instrumentation Diagram or Drawing. PID play a crucial role in the design and engineering of process plants and piping systems. P&IDs are schematic diagrams that contain engineering and design details of the process plants.

In the process industry, a standard set of symbols are used to prepare piping and instrumentation diagrams (P&IDs). Most of the P&ID drawings you may come across have instrument symbols based on ISA standard S5.1.

Tag Number and symbol:

The instruction symbol consists of a tag number and identification letter. Numbers on the P&ID symbols in instrumentation diagrams represent instrument tag numbers.

On P&ID drawings, individual instruments are indicated by circular symbols or circle. Shared control/display elements are circles surrounded by a square. Computer functions are indicated by a hexagon and programmable logic controller function are shown as a diamond inside a square.

Adding a single horizontal bar through any of the four graphic elements indicates that the function resides in the main location category. A double line indicates an auxiliary location, and no line places the device or instrument in the field. Devices located behind a panel in some other inaccessible location are shown with a dotted line.

The first letter in the identification letter defines the measured or initiating variables such as:

T - Temperature
P - Pressure
C - Conductivity
I - Current
L - Level
F - Flow rate
A - Analyzer
S - Speed
J - Power
Z - Position
V - Vibration
H - Hand

The succeeding letters defining functions such as:

E - Element
A - Alarm
Y - Computation
T - Transmitter
I - Indicator
S - Switch
H - High
L - Low
C - Controller
V - Valve
F - Fraction

Example P&ID diagram analysis:

The figure shows, FT 501 is a field mounted flow transmitter connected via electrical signals (dotted
lines) to a flow indicator and controller, FIC 501 located in the control room. Please note that a square root extraction of the input signal is applied as part of the functionality of FIC 501.

The output of the FIC 501 is an electrical signal shown in dotted signals is given to an I/P converter TY 501 mounted in the field. The output of TY 501 is a pneumatic signal which acts on the control valve connected to it. The pneumatic signal is shown as a line with pairs of cross lines.

TT 501 and TIC 501 are respectively temperature transmitter and temperature indicator and controller measuring, indicating and controlling temperature. The output of TIC 501 is connected through an internal software or data link (lines with bubbles) to the set point of FIC 501.

The YIC 501 arrangement is typical of most on/off valves. Here, the YIC is an on/off valve being controlled by a solenoid valve and is fitted with limit switches ZSH and ZSL. ZSH indicates that the valve is open while ZSL indicates that the valve is in the closed position or closed. All inputs and outputs are wired to a PLC that’s accessible to the operator