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Valve symbols are a compact way to show how fluid is isolated, regulated, redirected, or prevented from flowing backward in a piping system. On a piping and instrumentation diagram (P&ID), the valve body symbol identifies the general valve function, while additional marks may show the actuator, operating position, or control method.
A symbol is only the starting point. It normally does not provide enough information to select or purchase the physical valve. The valve tag, piping class, datasheet, and approved project legend must be read together with the symbol.
A valve symbol is a simplified graphical representation of a valve in a process line. It helps engineers, operators, maintenance teams, and suppliers identify the intended function of a valve without drawing its complete physical construction.
A P&ID may show:
the basic valve body type;
the connected process line;
a manual or powered actuator;
flow direction where relevant;
normal or failure position;
a valve tag linked to project specifications.
Symbol conventions are widely shared, but the exact drawing can vary between standards, engineering companies, owners, and individual projects. Always check the legend supplied with the drawing before making a technical or purchasing decision.
Start with the symbol placed directly in the process line. This indicates the general valve type, such as a gate, ball, globe, butterfly, plug, or check valve.
A symbol above the valve body may indicate how the valve is operated. Common examples include a handwheel, pneumatic diaphragm, electric motor, hydraulic piston, or solenoid.
The drawing may identify the normal operating position or the position the valve takes after loss of power or instrument air. Normal position and failure position are different concepts and should be checked separately.
The valve tag connects the symbol to the project documentation. Use the tag, line specification, or datasheet to confirm size, pressure class, material, end connection, trim, seat, and testing requirements.
A gate valve is mainly used for full-open or full-closed isolation. When fully open, it provides a relatively unobstructed flow path. Gate valves are normally selected for isolation rather than frequent throttling.
RST Valve offers cast steel and stainless steel gate valves in multiple pressure classes and connection forms.
A ball valve uses a rotating ball to provide quick quarter-turn shutoff. It is commonly used where tight isolation, rapid operation, and low flow restriction are important.
The symbol identifies the general ball valve function, but it does not show whether the physical valve is floating, trunnion-mounted, one-piece, two-piece, three-piece, soft-seated, or metal-seated. Those details must be confirmed separately.
View the RST Valve ball valve range for flanged, threaded, wafer, trunnion-mounted, and three-way configurations.
A globe valve is generally associated with throttling and flow regulation. Its internal flow path allows more controlled adjustment than a valve intended only for isolation.
The symbol alone does not identify the exact disc, stem, bonnet, or trim design. These features should be specified in the valve datasheet.
See available globe valves for cast steel and stainless steel applications.
A butterfly valve uses a rotating disc and is commonly selected for quarter-turn isolation or flow control. Its compact structure can be useful in larger pipe sizes and installations where space and weight are important.
The P&ID symbol does not normally distinguish between wafer, lug, flanged, concentric, double-offset, or triple-offset construction.
A plug valve uses a rotating plug to open, close, or redirect the flow path. It is commonly associated with quarter-turn operation and can be used for isolation or multiport flow routing.
The project legend and valve datasheet should be checked to determine the port arrangement and sealing design.
A check valve allows flow in the intended direction and closes automatically when flow reverses. It is used to protect pumps, pipelines, and process equipment against backflow.
Flow direction is particularly important when reading a check valve symbol. The symbol does not always identify whether the physical product is a swing, lift, wafer, dual-plate, axial, or vertical check valve.
RST Valve supplies cast steel and stainless steel check valves in several configurations.
A relief or safety valve protects equipment by opening when system pressure reaches a defined limit. A pressure regulator controls pressure during normal process operation.
These symbols should not be treated as interchangeable. The process function, set pressure, relieving capacity, and applicable code requirements must be confirmed in the project documentation.
A three-way valve has three process connections and can be used to redirect, combine, or divide flow. The symbol shows that multiple flow paths are involved, but the actual routing depends on the internal port configuration and valve position.
For ball valves, common internal passages include L-port and T-port arrangements. See the RST Valve three-way ball valve category for available configurations.
A handwheel or lever indicates local manual operation. Manual operation is common where automatic or remote movement is not required.
A pneumatic actuator uses instrument air to move the valve. Pneumatic systems are widely used for automated on/off and control service.
The actuator symbol should be read together with any positioner, solenoid, or failure-position information shown on the drawing.
An electric actuator uses a motor to open, close, or position the valve. It may be selected where electrical power is available and remote operation is required.
A hydraulic actuator uses pressurized liquid to produce operating force. It can be used where high thrust or torque is required.
A solenoid converts an electrical signal into mechanical movement. On many process systems, a solenoid valve controls the air supply to a pneumatic actuator rather than directly operating the main process valve.
| Valve type | Main function shown on the P&ID | Important specification to confirm |
|---|---|---|
| Gate valve | Full-open/full-closed isolation | Size, class, material, wedge and stem design |
| Ball valve | Quick quarter-turn isolation | Floating or trunnion design, seat and bore |
| Globe valve | Throttling and regulation | Flow direction, trim, disc and pressure drop |
| Butterfly valve | Compact isolation or regulation | Wafer/lug/flanged body and offset design |
| Plug valve | Isolation or flow routing | Lubricated/non-lubricated design and port pattern |
| Check valve | Backflow prevention | Flow direction and internal check mechanism |
| Three-way valve | Diverting, combining, or routing | L-port/T-port arrangement and operating position |
| Relief valve | Overpressure protection | Set pressure, capacity and applicable code |
A strainer is not a valve, but it frequently appears near valves, pumps, and meters on a P&ID. Its purpose is to remove solid particles from the process stream and protect downstream equipment.
The symbol does not identify the mesh size, screen material, pressure class, drain arrangement, or body construction. These must be confirmed in the specification. RST Valve supplies cast steel, stainless steel, and forged strainers.
Do not use the symbol alone to determine:
forged or cast body construction;
carbon steel, stainless steel, or alloy material;
pressure class or PN rating;
flanged, threaded, butt-weld, or socket-weld ends;
full bore or reduced bore;
soft seat or metal seat;
fire-safe or fugitive-emission qualification;
inspection, testing, or certification requirements.
These details belong in the valve tag, datasheet, piping material specification, purchase specification, or project notes.
“Control valve” describes a control function. The body may be globe, ball, butterfly, or another suitable design. Look for the complete symbol and project specification.
Normally open describes the intended normal operating state. Fail open describes the position after loss of the specified power source. They are not automatically the same.
The symbol is not a purchase specification. Before requesting a quotation, confirm the size, pressure class, material, end connection, operating method, process medium, temperature, and required standards.
Flow direction can affect check valves, globe valves, regulators, and other directional devices. Check arrows, line information, valve markings, and manufacturer instructions.
Valve symbols make P&IDs easier to read, but they are only one part of the valve specification. Begin with the valve body, identify the actuator and operating position, and then verify every purchasing detail through the tag, piping class, and datasheet.
For help matching a P&ID requirement to an industrial valve configuration, contact RST Valve with the valve tag, operating conditions, and available project documentation.
No. Many basic shapes are widely recognized, but details can vary by standard, owner, engineering company, and project. The approved drawing legend is the controlling reference.
Usually not. Forged or cast construction is specified in the valve tag, datasheet, material specification, or product description.
Not by itself. Pressure class may appear next to the tag or elsewhere in the project documentation.
Look for an actuator symbol above the valve body and for related signal lines, positioners, solenoids, or control tags.
Provide the valve type, nominal size, pressure class or PN rating, body and trim material, end connection, operating method, process medium, temperature, applicable standard, and quantity. A marked P&ID or datasheet is also helpful.
