Mnemonics for Piping & Instrumentation Diagrams

Mnemonics have been based generally on the philosophy described in the ISA S5.1 standard, and used in Piping & Instrumentation Diagrams by the EP-TA1-GS.

Mnemonic Meaning Notes
AV Analysis Valve Multi-way analysis valve
CAH Control/Regulation Alarm High Control/regulation limits for the process variable have been exceeded (beyond the defined limits)
CAH Control/Regulation Alarm High Control/regulation limits for the process variable have been exceeded (beyond the defined limits)
FC Flow Controller That which has a control output + flow feedback, e.g. the controller in a Flow controlling PID loop
FI Flow Indicator Can be gauge or digital readout
FIC Flow Indicating Controller As above but with flow indication provided with device
FIG Flow Indicating Glass/viewing device  
FIS Flow Indicating Switch
FIT Flow Indicating Transmitter
FQI Integrated/Totalized Flow Indication  
FQR Integrated/Totalized Flow Recorder  
FS Flow Switch
FSH Flow Switch High Switch is configured specifically to register High Pressure
FSHH Flow Switch High High Switch is configured specifically to register very high  Pressure
FT Flow Transmitter
FV Flow Valve Analogue valve in a PID loop controlling flow
GF Gas Filter G - users choice and F - undefined as passive function - defined by EP-TA1/Gas to represent Gas Filter
HS Hand Switch Hardwired switch or SCADA control
HV Hand Valve On-Off Valve
ISS Current Safety Switch Residual current detector (e.g. earth leakage fault detector)
KA Timer/timeout Alarm
NV Non Return Valve N - users choice -  defined by EP-TA1/Gas to represent Non return
PCV Pressure Control Valve Valve with Pressure Controlling/regulating/reducing action, self contained
PDCV Differential Pressure Control Valve Valve with Differential Pressure Controlling/regulating action, self contained
PDT Differential Pressure Transmitter
PSV Pressure Safety Valve
PT Pressure Transmitter  
PTY Pressure Transmitter Converter Electronic interface between Pressure Transmitter and Field Bus (E.g. Wago I/P module/bus coupler)
PUMP Pump
PV Pressure Valve Analogue valve in a PID loop controlling pressure
SCY Speed/frequency control relay/computer/converter  
TAH Temperature Alarm High
TAHH Temperature Alarm High High
TCY Temperature Control Relay/Converter Interface between Temperature Control Function in PLC and temperature driver/actuator
TE Temperature Element E.g. PT100
TEY Temperature Element Converter Electronic interface between Temperature Element and Field Bus (E.g. Wago I/P module/bus coupler)
TKA Temperature Rate of Change Alarm E.g. heating too slow
TSS Temperature Safety Switch Hardwired temperature activated switch
TT Temperature Transmitter
TZ Temperature Driver/Actuator E.g. Heating Wire
UC Multivariable/multifunction Controller Represents a logical section of a complex control program (usually within a PLC)
XBUB Bubbler The letter X, being unclassified, denotes that the mnemonic does not follow the normal code
XMFC Mass Flow Controller The letter X is unclassified, denoting that the mnemonic does not follow the normal code
XMFM Mass Flow Meter As for XMFC above
YC State Controller A control function which initiates a change of state
YIC State Indicating Controller State control is provided as above, but additionally the controller receives state feedback from the device being controlled.
YV State Valve An on/off valve controlled by a state, event or presence of a condition (derived from PLC program
YY State Relay/Converter Normally indicates the electro valve which is used as a pilot for a pneumatic valve
ZA Position Alarm Due to a fault in the position contacts or an intermediate position, the valve position cannot be determined.
ZAH Position Alarm High Valve still open when instructed to close
ZAL Position Alarm Low Valve still closed when instructed to open
ZC Position Controller Specific Device to Control Valve Position
ZCY Position Control relay/Converter Electronic interface between a software controller and the valve
ZSH Level Switch High A linit switch fitted to a device to indicate it is in the high/open position
ZSL Level Switch Low A linit switch fitted to a device to indicate it is in the low/closed position

the ISA S5.1 standard 

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
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