21-04-2012, 03:29 PM
CONTROL ENGINEERING
P&IDs.pdf (Size: 190.32 KB / Downloads: 52)
How to read P&IDs
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.