Appendix F. Process Diagrams
The main reference for this appendix is from the International Society for Automation (ISA) and is found in ISA S5.1.
Two major types of process diagrams are used by the chemical industry to represent a chemical plant:
Process flow diagram (PFD): This shows how the process units are connected and how the process works. Various symbols are used for each unit operation. Pipes are indicated by lines connecting the units.
Process or piping and instrumentation diagram (P&ID): This shows the various sensor, control systems, and final control elements. It uses symbols and circles to represent how they are connected.
The symbols used are not universal and have changed over time. They are used in a flow diagram as follows:
Manual valve: 
Control valve: 
Solenoid valve: 
Check valve: 
Arrow points in direction of flow
Relief valve: 
Rupture disc: 
Pump: 
Flowmeter: 
Vessel: 
Heat exchanger: 
Furnace: 
Agitator: 
Packed column: 
Distillation column: 
Air blown cooler: 
Process instrumentation has tags that consist of letters and numbers placed within or near the instrument to identify the type and function. The following tag identifies this equipment as a TRC = temperature recording controller and is associated with control loop 100.
The tag identification lettering is shown in the following table:
First Letter |
Succeeding Letters |
||||
|---|---|---|---|---|---|
|
Measured or Initiating Variable |
Modifier |
Readout Function |
Output Function |
Modifier |
A |
Analysis |
|
Alarm |
|
|
C |
|
|
|
Control |
Closed |
D |
Density |
Differential |
|
|
|
F |
Flow |
Ratio |
|
|
|
H |
Hand |
|
|
|
High |
I |
Current |
|
Indicate |
|
|
L |
Level |
|
Light |
|
Low |
P |
Pressure, vacuum |
|
|
|
|
Q |
Quantity |
Totalize |
|
|
|
R |
Radiation |
|
Record |
|
|
S |
Speed |
Safety |
|
Switch |
|
T |
Temperature |
|
|
Transmit |
|
V |
Vibration |
|
|
Valve, damper |
|
Z |
Position |
|
|
Actuator |
|
Typical examples for chemical plants for these tags are shown here:
TI: temperature indicator
TR: temperature recorder
TT: temperature transmitter
TC: temperature controller
TRC: temperature recorder controller
LT: level transmitter
LIC: level indicator controller
FC: flow controller
PV: pressure valve
PSV: pressure safety valve
PAL: pressure alarm low
PAH: pressure alarm high
HV: hand valve or manual valve
FT: flow transmitter
FRC: flow recorder controller
PT: pressure transmitter
A line in the middle of the tag circle denotes the location:
No line: The instrument is mounted in the field near the process.
Solid line: The instrument is mounted in the control room and is accessible to the operator.
Dashed line: The instrument is mounted out of sight and is not accessible by the operator.
There are also symbols to identify how the instruments are connected to each other and which type of signal is being used (electrical, pneumatic).
Piping: thick line
Process connection: thin line
Electrical signal: dashed line
Pneumatic signal: 