5.3 Advanced Blueprint Reading: P&IDs, Instrument Tags, and Welding Symbols
Key Takeaways
- A P&ID shows every line, valve, instrument, and interlock in a system but is not drawn to scale and carries no dimensions.
- In an ISA instrument tag, the first letter identifies the measured variable and the succeeding letters identify the function, so FIC is a flow indicating controller.
- A plain instrument bubble is field mounted; a bubble with a single horizontal line through it is mounted on the front of a main control panel.
- On a welding symbol, a symbol below the reference line applies to the arrow side of the joint and a symbol above the line applies to the other side.
- A circle at the junction of the arrow and reference line means weld all around, and a filled flag at that junction means a field weld.
The piping and instrumentation diagram
A P&ID is the schematic that shows how a process actually works: every line with its number, size, and specification; every valve; every instrument; every interlock. It is the document a maintenance crew uses to plan an isolation, and it is where you find the block valves and bleeds you need for a lockout.
What a P&ID is not: it is not to scale, it carries no dimensions or elevations, and it does not show physical routing. Use the isometric and the arrangement drawing for those.
Line numbers on a P&ID typically encode size, service, sequence number, and specification — for example, a designation reading 6"-CWS-1204-A1 identifies a 6-inch cooling water supply line, line 1204, in material specification A1.
| Line symbol on a P&ID | Meaning |
|---|---|
| Heavy solid line | Major process piping |
| Light solid line | Secondary or utility piping |
| Dashed line | Electrical signal |
| Line with double slash marks | Pneumatic signal |
| Line with small circles | Software or data link |
| Line with crossed hatching | Capillary tubing (filled system) |
ISA instrument tags
An instrument tag is read in two parts. The first letter names the measured or initiating variable. The succeeding letters name the readout or function.
| First letter | Variable | Succeeding letter | Function |
|---|---|---|---|
| F | Flow | I | Indicate |
| L | Level | R | Record |
| P | Pressure | C | Control |
| T | Temperature | T | Transmit |
| A | Analysis | A | Alarm |
| S | Speed | S | Switch |
| V | Vibration | V | Valve |
| Z | Position | E | Element (primary sensor) |
Put them together:
- PI — pressure indicator (a gauge)
- FIC — flow indicating controller
- LSH — level switch, high
- TE — temperature element (the thermocouple or RTD itself)
- PSV — pressure safety valve
- FCV — flow control valve
The bubble around the tag tells you where the instrument physically lives:
| Bubble | Location |
|---|---|
| Plain circle | Field mounted, at the equipment |
| Circle with a single horizontal line | Front of a main control panel, accessible to the operator |
| Circle with a dashed horizontal line | Behind the panel, not normally accessible to the operator |
| Circle inside a square | Shared display or shared control, such as a distributed control system |
| Hexagon | Computer function |
For a mechanic, the practical payoff is knowing whether the pressure reading the operator is quoting comes from a local gauge at the pump or from a transmitter feeding the control room — two very different diagnostic starting points.
Valve and actuator symbols
P&IDs use consistent body symbols: a bowtie for a generic valve, with the internal detail identifying gate, globe, ball, or butterfly. The actuator is drawn above the body:
- A dome or diaphragm shape means a diaphragm actuator.
- A rectangle with a stem means a piston or cylinder actuator.
- An M in a circle means a motor operator.
- A solenoid is shown with an S in a box.
- A spring drawn with the actuator indicates fail-safe action, and the notation FC (fail closed) or FO (fail open) states what happens on loss of signal or air.
Fail action is a safety-critical detail: before an isolation, you need to know which control valves will slam shut and which will swing open when instrument air is cut.
Welding symbols
Welding symbols follow AWS A2.4 and appear on fabrication and repair drawings. The structure is a reference line with an arrow at one end and an optional tail at the other.
The single rule to memorize:
Below the reference line = arrow side of the joint. Above the reference line = other side of the joint.
Symbols on both sides mean weld both sides.
| Element | Meaning |
|---|---|
| Triangle (right triangle) | Fillet weld; the vertical leg is always drawn on the left |
| V, bevel, U, J shapes | Groove weld types |
| Number to the left of the symbol | Weld size (leg length for a fillet) |
| Number to the right, in parentheses or after a dash | Length of weld, and pitch for intermittent welds |
| Circle at the arrow/reference junction | Weld all around |
| Filled flag at the junction | Field weld (made at the site, not in the shop) |
| Tail | Specification, process, or procedure reference such as a WPS number |
Worked reading. A symbol with a triangle below the reference line, "1/4" to its left, "3-6" to its right, a circle at the junction, and a flag: a 1/4-inch fillet weld on the arrow side, 3 inches long on 6-inch centers, all around, made in the field.
Contour and finish symbols may sit on top of the weld symbol — flat, convex, or concave contour, with a letter for the finishing method such as G for grinding or M for machining.
An instrument on a P&ID is tagged FIC-204 inside a plain circle. What is it and where is it?
A welding symbol shows a fillet triangle above the reference line with a circle at the junction of the arrow and reference line. What does this specify?
Before isolating a control valve for maintenance, why does the crew check whether it is marked FC or FO on the P&ID?