How to Read Control Valve Symbols on a P&ID

Control valve symbols on a P&ID are read by starting with the basic bow-tie body shape, then checking the internal marker for valve type, the tag number for identification, and the actuator symbol for control method. Together, these elements tell you what the valve does, how it operates, and where it fits into the overall process.

Why Control Valve Symbols on a P&ID Matter for Process Understanding

A Piping and Instrumentation Diagram (P&ID) is the master reference drawing for an industrial process plant. Control valves shown on this diagram are the final mechanical elements that regulate pressure, flow, temperature, and liquid level.

Engineers use the P&ID to design the plant before construction begins. Once approved, the same diagram guides purchasing teams, equipment vendors, and the crews who install everything on site.

Misreading a single symbol can cause real problems. A line could be overpressurized, or flow could reverse unexpectedly when it shouldn’t. This is why PID valve symbols follow standardized conventions instead of varying by company or engineer.

Note: Plumbing plans and full P&ID drawings use overlapping but distinct symbol sets. This guide on plumbing valve symbols breaks down where the two diverge.

The Building Blocks of P&ID Valve Symbols and Tag Numbers

Every standard valve symbol starts with two triangles pointing toward each other, known as a “bow-tie.” This shape represents the physical valve body sitting in the pipeline.

Engineers identify each valve’s specific role using alphanumeric tags defined by ANSI/ISA-5.1, the main industry standard for instrumentation symbols. A few common examples:

• FV-101 designates Flow Valve number 101
• LV-202 indicates Level Valve number 202
• PV and TV prefixes mark Pressure and Temperature valves respectively
• TT, PT, and FT denote Temperature, Pressure, and Flow Transmitters, the instruments that send signals to these valves

Gate valves carry their own tagging quirks worth knowing, especially around sizing notes. We cover this in more depth in how to read the gate valve symbol in P&ID.

Notes near the symbol often include sizing and rating data. A note like 2″ 300 RF PB means the valve is 2 inches in diameter, rated to ANSI Class 300, has a raised-face connection, and uses a piston-balanced trim design. This kind of shorthand lets engineers pack a full spec sheet’s worth of detail into a few characters on the drawing.

Common Valve Symbols and What Each One Means

The internal design of a valve determines how it controls fluid movement, so P&IDs use distinct internal markers for each type:

• Globe Valves: A small solid dot at the intersection of the triangles, indicating a flow path built for precise throttling (fine control over how fast fluid moves through the line)
• Ball and Plug Valves: A small circle or line inside the bow-tie, representing quarter-turn, quick-acting designs
• Butterfly Valves: A diagonal line through the center of the symbol
• Gate Valves: A plain bow-tie with no internal marker, used for simple full-open or full-closed isolation
• Check Valves: An arrow showing the only direction flow is allowed to travel
• Diaphragm Valves: A curved line above the bow-tie body
• Needle Valves: A narrowed, tapered symbol used for very fine flow adjustment, often in instrument or sampling lines
• Angle Valves: A bow-tie drawn at a right angle to show the pipe changing direction at the valve
• Solenoid Valves: A bow-tie paired with a small rectangular coil symbol, indicating the valve opens or closes through an electrical signal rather than manual or pneumatic force
• Pressure Relief Valves: A triangle pointing away from the line with a spring symbol on top, used to vent excess pressure automatically

How Actuators and Accessories Appear on the Symbol

A valve only becomes a “control valve” once an actuator is attached to operate it automatically. On a P&ID, the actuator is drawn as a shape connected to the top of the valve stem:

• Pneumatic Diaphragm: A curved dome, the most common actuator for throttling control loops
• Piston/Cylinder: A rectangular box, used for high-pressure pneumatic or hydraulic operation
• Electric Motor: A small circle with the letter “M” inside

Two accessories often appear alongside the actuator symbol:

• A positioner is drawn as a small box beside the actuator. It fine-tunes the valve’s position based on the controller’s signal, similar to how cruise control in a car constantly adjusts the throttle to hold a set speed.
• A solenoid valve may sit between the air supply and the actuator. It acts as a fast on/off switch for the air signal, often used to force the valve into its fail-safe position during an emergency shutdown.

Fail-safe behavior is marked with an arrow or letter code. An upward arrow or “FO” means the valve fails open, while a downward arrow or “FC” means it fails closed. This tells operators exactly how the valve will behave if it loses power or air supply.

Reading a Complete Control Loop

To trace a full control loop, follow the signal path from the field sensor through the controller to the valve. A typical loop includes a transmitter, such as TT for temperature, a controller that processes the signal, and the actuated valve that responds to it. In many plants, this controller is part of a larger DCS (Distributed Control System) or PLC (Programmable Logic Controller) that manages dozens of loops at once.

Two line types show how these components communicate:

• A dashed line represents an electrical signal, most commonly the 4-20mA standard, sometimes layered with a digital HART signal
• A solid line crossed with diagonal hash marks represents a pneumatic air supply line

By following these lines from the transmitter to the final valve, you can tell whether a rising measurement will cause the valve to open further or close down. This single skill turns a dense, intimidating drawing into a clear story of cause and effect across the entire process.

Where Plug Valves Fit in P&ID Systems

Plug valves are quarter-turn, high-integrity isolation devices chosen over ball valves for high-pressure, slurry-heavy, or highly corrosive process streams, thanks to their cavity-free, self-cleaning seating surfaces. On a standard ANSI/ISA-5.1 P&ID drawing, a plug valve is distinguished from a ball valve by a rhombus (diamond) shape, or an explicit internal rectangle inside the bow-tie body, rather than an open circle.

B2B engineers should account for three technical layers when reviewing plug valves on a P&ID:

• Multi-port logic. Three-way and four-way plug valves use specialized T-port or L-port symbol markers. These indicate diversion or mixing logic built into a single valve body, removing the need for multiple isolation valves.
• Lubrication specification. P&ID notes will flag whether a line needs a lubricated plug valve (periodic sealant injection for high-pressure oil/gas lines) or a non-lubricated sleeved/lined plug valve (PTFE sleeve for zero-leakage chemical resistance). This affects maintenance infrastructure and valve data sheets.
• Actuator torque scaling. Plug valves rely on surface-to-surface contact rather than localized seat rings, which means higher breakaway torque than ball valves. When automated, the actuator symbol (e.g., a pneumatic piston) often corresponds to a larger physical footprint on layout drawings to handle this torque demand.