Valve diagrams use a consistent set of symbols to show what type of valve is installed, which direction flow moves, how the valve connects to piping, and what happens if it loses power. Once you learn the visual logic behind these symbols, you can read any piping and instrumentation diagram (P&ID) without memorizing hundreds of individual icons. The core system is straightforward: a handful of basic shapes represent valve types, surrounding markings show connection details, and letter codes tell you what the valve controls.
The Five Core Valve Symbols
Nearly every valve on a diagram starts with the same foundation: two triangles meeting at a point, forming a bowtie shape. The differences between valve types come from small modifications to that basic shape. ISO 14617, the international standard for diagram symbols, assigns each type a distinct visual shorthand.
- Gate valve: A plain bowtie, two triangles tip to tip with no fill. This is the most common symbol you’ll see on a P&ID.
- Globe valve: The same bowtie shape with a small circle or dot at the center where the triangles meet, representing the globe-shaped internal disc.
- Ball valve: The bowtie with a filled (darkened) circle at the center, indicating the ball element inside.
- Plug valve: A filled (solid black) bowtie, distinguishing it from the open triangles of a gate valve.
- Butterfly valve: A bowtie with a vertical line drawn through the center point, representing the disc that rotates inside the pipe.
If you can spot the bowtie and then look at what’s happening at the center, you can identify the valve type in seconds. Every other detail on the symbol builds outward from there.
Reading Check Valve Symbols
Check valves allow flow in only one direction, and their symbol reflects that. Instead of a symmetrical bowtie, a check valve uses a triangle or arrowhead pointing in the permitted flow direction, paired with a line or semicircle that represents the closure element. The arrow always points the way fluid is allowed to move. If flow tries to reverse, the valve shuts.
When you encounter a check valve on a diagram, start by finding the triangle and noting which direction it points. That’s your anchor. Compare it to the overall flow direction of the system to confirm it’s oriented correctly. In P&IDs, you’ll often see a small perpendicular line on the downstream side of the triangle, showing where the flap or disc seats when the valve closes.
Connection Types at a Glance
The way a valve connects to its piping is shown by small marks at the ends of the valve symbol, right where it meets the pipeline. Four connection styles appear regularly, each with its own visual shorthand:
- Flanged: Short perpendicular lines at each end of the valve symbol. These indicate the valve bolts between flanges and can be removed without cutting the pipe.
- Threaded: Small unfilled (open) circles at the connection points. These represent temporary, screwed-in connections.
- Welded: Small filled-in (solid) squares. The filled square signals a permanent connection that requires cutting to remove.
- Socket welded: Small unfilled (open) squares. The pipe slides into a socket before being welded, which is slightly different from a standard butt weld.
These markings are easy to overlook, but they carry important information about how the valve is installed and whether it can be swapped out during maintenance.
Letter Codes and Tag Numbers
Every control valve on a P&ID has a tag number next to it, built from a letter code defined by the ISA-5.1 standard (most recently updated as ANSI/ISA-5.1-2024). The tag tells you two things: what variable the valve controls, and what role it plays in the control loop.
The first letter identifies the measured variable:
- F = Flow
- L = Level
- P = Pressure
- T = Temperature
The succeeding letter “V” means valve. So when you see “FV” on a diagram, that’s a flow control valve. “PV” is a pressure valve. “LV” controls level, and “TV” controls temperature. A number follows the letters, identifying which specific loop the valve belongs to. For example, FV-101 is the flow valve in loop 101.
You’ll also see other letter combinations on instruments connected to the valve. “FIC” means flow indicating controller, the device that sends a signal to the valve. “TI” is a temperature indicator, used only for display. “ZT” is a position transmitter, which reports how far open a valve actually is. The first letter is always the process variable. The succeeding letters describe the function.
Multi-Port and Multi-Position Valves
Three-way and four-way valves show up frequently in hydraulic and pneumatic diagrams, and they use a different visual system than simple on/off valves. Instead of a bowtie, these valves are drawn as a row of boxes called flow envelopes. Each box represents one position the valve can be shifted to.
Inside each box, lines and arrows show which ports are connected in that position. An arrow between two ports means flow passes through. A blocked line (often a T-shape) means the port is closed. A four-way, three-position valve, for example, has three boxes side by side, each showing a different routing of fluid between four ports.
The ports are drawn as lines entering the center box (the default or neutral position). To read the valve’s behavior, you mentally slide each box into the center position and trace which ports connect. The operator type, whether it’s a lever, solenoid, or spring, is shown as a small symbol on each end of the envelope row. A zigzag line means a spring return. A rectangle with a diagonal line typically means a solenoid (electrically actuated). These tell you how the valve shifts and what forces it back to its resting state.
Fail-State Indicators
Control valves on a P&ID include markings that show what happens if the valve loses its air or electrical signal. This is the fail state, and it’s critical safety information. You’ll see it shown in one of two ways: an arrow on the valve symbol pointing in the direction the stem moves on failure, or a simple text abbreviation.
- FC = Fail closed. The valve shuts completely on loss of signal.
- FO = Fail open. The valve opens fully on loss of signal.
- FL = Fail locked (or fail in place). The valve freezes in its last position.
For pneumatic valves, the fail direction depends on whether the actuator uses a spring to push the stem closed or open when air pressure drops. A fail-locked valve requires a special device that traps pressure inside the actuator chamber, holding the stem in position even after the supply is lost. This is less common and typically reserved for situations where either fully open or fully closed would be dangerous.
Putting It All Together
When you look at a valve on a P&ID for the first time, read it in layers. Start with the bowtie shape to identify the valve type. Check the center of the symbol for fills, circles, or lines that narrow down whether it’s a gate, globe, ball, butterfly, or plug valve. Then look at the ends for connection marks: perpendicular lines for flanges, circles for threaded, filled squares for welded. Read the tag number to learn what process variable the valve controls and which loop it belongs to. Finally, check for a fail-state arrow or abbreviation to understand its safety behavior.
Each layer adds information, but none of them are complicated on their own. The real skill in reading valve diagrams is knowing what to look for and in what order. Once you’ve identified a dozen valves this way, the symbols become second nature, and you can scan a full P&ID and pick out valve types, flow directions, and control functions without slowing down.