A Piping and Instrumentation Diagram (P&ID) is a detailed diagram in the process industry which shows process equipment together with the instrumentation and control devices. It is also called as mechanical flow diagram (MFD).[1]
Superordinate to the P&ID is the process flow diagram (PFD) which indicates the more general flow of plant processes and the relationship between major equipment of a plant facility.
Contents and function
Example of a single industrial control loop; showing continuously modulated control of process flow.Piping and instrumentation diagram of pump with storage tank. Symbols according to EN ISO 10628 and EN 62424.A more complex example of a P&ID.
A piping and instrumentation diagram (P&ID) is defined as follows:
A diagram which shows the interconnection of process equipment and the instrumentation used to control the process. In the process industry, a standard set of symbols is used to prepare drawings of processes. The instrument symbols used in these drawings are generally based on International Society of Automation (ISA) Standard S5.1
The primary schematic drawing used for laying out a process control installation.
Process control instrumentation and designation (names, numbers, unique tag identifiers), including:
Valves and their types and identifications (e.g. isolation, shutoff, relief and safety valves, valve interlocks)
Control inputs and outputs (sensors and final elements, interlocks)
Miscellaneous - vents, drains, flanges, special fittings, sampling lines, reducers and swages
Interfaces for class changes
Computer control system
Identification of components and subsystems delivered by others
P&IDs are originally drawn up at the design stage from a combination of process flow sheet data, the mechanical process equipment design, and the instrumentation engineering design. During the design stage, the diagram also provides the basis for the development of system control schemes, allowing for further safety and operational investigations, such as a Hazard and operability study (HAZOP). To do this, it is critical to demonstrate the physical sequence of equipment and systems, as well as how these systems connect.
P&IDs also play a significant role in the maintenance and modification of the process after initial build. Modifications are red-penned onto the diagrams and are vital records of the current plant design.
Based on STANDARD ANSI/ISA S5.1 and ISO 14617-6, the P&ID is used for the identification of measurements within the process. The identifications consist of up to 5 letters. The first identification letter is for the measured value, the second is a modifier, 3rd indicates passive/readout function, 4th - active/output function, and the 5th is the function modifier. This is followed by loop number, which is unique to that loop. For instance FIC045 means it is the Flow Indicating Controller in control loop 045. This is also known as the "tag" identifier of the field device, which is normally given to the location and function of the instrument. The same loop may have FT045 - which is the flow transmitter in the same loop.
Letter
Column 1 (Measured value)
Column 2 (Modifier)
Column 3 (Readout/passive function)
Column 4 (Output/active function)
Column 5 (Function modifier)
A
Analysis
Alarm
B
Burner, combustion
User choice
User choice
User choice
C
User's choice (usually conductivity)
Control
Close
D
User's choice (usually density)
Difference
Deviation
E
Voltage
Sensor
F
Flow rate
Ratio
G
User's choice (usually gaging/gauging)
Gas
Glass/gauge/viewing
H
Hand
High
I
Current
Indicate
J
Power
Scan
K
Time, time schedule
Time rate of change
Control station
L
Level
Light
Low
M
User's choice
Middle / intermediate
N
User's choice (usually torque)
User choice
User choice
User choice
O
User's choice
Orifice
Open
P
Pressure
Point/test connection
Q
Quantity
Totalize/integrate
Totalize/integrate
R
Radiation
Record
Run
S
Speed, frequency
Safety (Non SIS (S5.1))
Switch
Stop
T
Temperature
Transmit
U
Multivariable
Multifunction
Multifunction
V
Vibration, mechanical analysis
Valve or damper
W
Weight, force
Well or probe
X
User's choice (usually on-off valve as XV)
X-axis
Accessory devices, unclassified
Unclassified
Unclassified
Y
Event, state, presence
Y-axis
Auxiliary devices
Z
Position, dimension
Z-axis or Safety Instrumented System
Actuator, driver or unclassified final control element
For reference designation of any equipment in industrial systems the standard IEC 61346 (Industrial systems, installations and equipment and industrial products — Structuring principles and reference designations) can be applied. For the function Measurement the reference designator B is used, followed by the above listed letter for the measured variable.
Prior to the advent of computer-aided design (CAD) in the late 1980s, P&IDs were drawn by hand. The drawing template shown below, actual size 225.mm by 111 mm, is typical of those used to draw P&IDs.
Piping and instrumentation diagram manual drawing template (1980s). Symbol key:
^Turton, Richard; Bailie, Richard C.; Whiting, Wallace B.; Shaeiwitz, Joseph A.; Bhattacharyya, Debangsu (2012). Analysis, synthesis, and design of chemical processes. Prentice Hall international series in the physical and chemical engineering sciences (4th ed.). Upper Saddle River, N.J. Munich: Prentice Hall. ISBN978-0-13-261812-0.
