Understanding Valve Symbols in P&ID

Common P&ID Valve Symbols and How to Read Them

A gate valve symbol usually points to isolation service; it does not mean the valve is suitable for throttling. Read each P&ID valve symbol as a function clue first, then confirm the valve tag, line list and project legend before choosing the real valve.

Gate Valves: Symbols for Wedge, Slab and Knife Gates

Symbol	Valve Type	Meaning/ What to check before selection
	Gate Valve	On/off isolation with low restriction when fully open. Do not use the symbol alone for throttling duty; confirm pressure class, stem type and end connection. See the gate valve symbol in P&ID guide for a narrower reading.

Gate valves are primarily isolating devices used to start or stop flow with minimal pressure loss when fully open. In P&IDs, their symbol typically shows two opposing triangles with a vertical line between them, indicating the gate mechanism that moves up and down to block flow. Variants such as wedge, slab and knife gate all perform this basic function, but are used in different service conditions (e.g. slurries for knife gates). The shape and notation in the symbol can differentiate these types, ensuring that the diagram conveys whether a straight-through wedge or special knife gate is specified.

Globe Valves: Standard vs. Angle Patterns

Symbol	Valve Type	Meaning/ What to check before selection
	Globe Valve	Manual throttling, regulation or isolation where pressure drop is acceptable. Confirm flow direction, seat material, leakage requirement and whether the valve is manual or actuated.
	Angle Valve	Angle valves are used for controlling flow at a 90-degree angle. They are symbolized with a representation of the flow path turning at a right angle, typically with an indication of manual or other actuation methods.

Globe valves are a go-to choice when you need flow regulation as well as shut-off. They are throttling valves by design, due to their internal plug and seat arrangement, so their P&ID symbol often includes an internal shape that suggests the flow path through the body. An angle pattern globe valve — where inlet and outlet ports meet at a right angle — will be shown with the same core symbol adapted to reflect that geometry. Understanding this visual clue helps teams distinguish a straightforward straight-through globe from its angle counterpart without having to read extensive notes.

Ball & Butterfly Valves: Floating/Trunnion and High-Performance Offsets

Symbol	Valve Type	Meaning/ What to check before selection
	Ball Valve	Quarter-turn shut-off, often used where quick opening and tight shut-off are needed. Check full port or reduced port, seat material, fire-safe need, pressure class and actuator torque if powered.
	Butterfly Valve	Compact quarter-turn isolation or control, often on larger pipelines. Confirm wafer, lug or flanged body, seat type, disc material, pressure rating and whether it is only for isolation or also for control.

Ball and butterfly valves are quarter-turn devices used for rapid isolation or modulation. In P&IDs, ball valves are usually shown as a circle with a central bore inside the basic valve representation, symbolising the rotating ball. Variants such as floating and trunnion-mounted balls are often indicated with tag details or specific centre markings to show different support and operational characteristics. Butterfly valves, on the other hand, are depicted with a centred disc across the pipeline, implying the pivoting mechanism that regulates flow. High-performance butterfly valves — which offer better sealing and control than basic wafer types — might be annotated differently or referenced in the legend to clarify intent.

Plug & Needle Valves: Symbols for Precision Flow Control

Symbol	Valve Type	Meaning/ What to check before selection
	Plug Valve	Quarter-turn shut-off or diverting duty, depending on port design. Check lubricated or sleeved design, port pattern, media compatibility and operating torque.
	Needle Valve	Fine manual adjustment on small-flow or instrument lines. Confirm pressure rating, thread type, material and whether the flow is clean enough for the small orifice.

For precision control at lower flow rates, plug and needle valves are common. In diagrams, a plug valve may be shown as a diamond shape inside the valve symbol, distinguishing it from spheres or discs. Needle valves — designed for fine throttling — often include a tapered or needle-like element in their symbol, indicating their capability for small, incremental flow adjustments. These graphical cues help designers and operators see not just that there’s a valve, but what level of control it offers in context.

Check Valves: Swing, Lift and Dual-Plate Symbols

Symbol	Valve Type	Meaning/ What to check before selection
	Check Valve	Automatic reverse-flow prevention without manual operation. Confirm flow direction, installation position, cracking pressure and valve style. For more detail, use the check valve symbol guide.
	Check Stop Valve	Check stop valves combine the function of a stop valve and a check valve, allowing for flow control and preventing backflow. They are represented by a combination of symbols, indicating both the stop and check functions.

Check valves, often called non-return valves, allow fluid to flow in one direction only, preventing backflow that can damage pumps, upset control loops or create unsafe conditions. The basic check valve symbol on a P&ID typically shows a closure element within the line that points in the permitted flow direction.

• Swing check valves are illustrated with a hinged disc that swings open with forward flow and closes under reverse pressure. The P&ID symbol usually includes a diagonal line or arc that represents this swinging motion.

• Lift check valves use a disc that lifts directly off its seat; their symbols reflect a straight-line lift design and often include a lifted element inside the body.

• Dual-plate (or double-door) checks are compact designs with two spring-loaded plates that pivot to allow flow. Their P&ID symbols commonly show two opposing leaf-shaped elements within the valve body, signalling the paired plates.

Clear representation of these variations matters. Even though all are check valves, their dynamics and installation requirements differ – an oversight here can lead to incorrect specification or performance shortfalls in the field.

Pressure Safety Valves (PSV) & Relief Valves (PRV)

Symbol	Valve Type	Meaning/ What to check before selection
	Relief Valve	Relief valves are safety devices designed to protect piping systems from overpressure. The symbol usually includes an arrow indicating the direction of flow and a spring mechanism that represents the pressure relief function.
	Safety Valve	Safety valves, similar to relief valves, are designed to automatically release pressure when it exceeds a certain limit. These valves are represented with a similar symbol to relief valves, often including an additional actuator symbol.

Pressure safety devices are essential where overpressure could harm equipment or personnel. On a P&ID, these are shown as distinct symbols so that design intent and safety strategies are unmistakable:

• Pressure Safety Valves (PSVs) are typically spring-loaded devices that open automatically at a set pressure to protect a system from overpressure. Their symbol often includes a spring element above the valve body, indicating the spring mechanism.

• Pressure Relief Valves (PRVs) may be similar in appearance but can have pilot-operated mechanisms where a secondary line and pilot control element feed pressure information back to the main valve. This is reflected in the P&ID with additional linework or a distinct pilot control graphic connected to the relief element.

Other Common Valve Symbols in P&ID

Beyond the core isolation and throttling valves, P&IDs also include a wide range of specialised valve symbols used for safety, control, direction change and specific process functions. These symbols represent valves designed for particular operating conditions, such as backflow prevention, pressure protection, fine flow adjustment or level control.

Understanding these additional valve symbols helps ensure accurate interpretation of process intent and reduces the risk of misapplication during design, procurement and operation.

Symbol	Valve Type	Meaning
	Solenoid Valve	A solenoid valve is an electromechanically operated valve used to control the flow of liquids or gases. The operation of a solenoid valve is driven by an electric current through a coil, which generates a magnetic field to actuate the valve mechanism, either opening or closing it.
	Diaphragm Valve	Diaphragm valves are used for throttling and shut-off of fluids, particularly in corrosive applications. The symbol often includes a line representing the diaphragm, along with arcs showing its movement.
	Bleed Valve	Bleed valves are used to release small amounts of fluid from a system. The symbol often includes a small arrow or vent indication to represent the bleeding function.
	Block & Bleed Valve	Block & bleed valves are used to isolate a section of piping and bleed off any residual fluid. They are symbolized by a combination of a block valve and a smaller bleed valve symbol.
	Knife Gate Valve	Knife gate valves are used to control flow of thick or slurry-like fluids. They are represented with a gate symbol that includes a sharp blade, indicating their function for cutting through solid-laden flows.
	Piston-Operated Valve	Piston-operated valves use a piston mechanism to control flow. The symbol often includes a representation of the piston, indicating its movement to regulate flow.
	Float-Operated Valve	Float-operated valves are used in systems where the fluid level is controlled by a float mechanism. The symbol typically includes a float element, indicating its operation based on fluid level.
	Pinch Valve	Pinch valves are used to control flow by pinching a flexible tube. The symbol usually includes a depiction of the pinching mechanism, indicating its function to restrict flow by compressing the tubing.

Valve state symbols

Valve state symbols in P&IDs show the position a valve is in under normal conditions, when no power, pressure or manual input is applied. Put simply, they tell you what the valve is doing by default. Whether it’s normally open, normally closed, or designed for a specific duty, this information is vital for understanding how the process behaves, how safety is managed, and how control logic is intended to work.

The symbols below are commonly used to describe valve state and function. When they’re read correctly and used consistently, they help turn a P&ID from a static drawing into a practical reference for real-world operation.

Symbol	Valve Type	Meaning
	Special Purpose Valve	Special purpose valves are designed for unique or specialized applications, such as mixing, diverting, or controlling hazardous fluids. The symbol varies depending on the specific application but often includes additional features like multiple arrows or unique shapes to indicate the valve’s specialized use.
	Normally Open (NO) Valve	Normally open valves are valves that remain open when not actuated and are symbolized by an open path in the valve symbol. These valves are used in applications where flow must be maintained by default and only stopped during specific operational scenarios.
	Normally Closed (NC) Valve	Normally closed valves remain closed when not actuated and are symbolized by a closed path in the valve symbol. These valves are used in applications where flow should be stopped by default and only allowed during specific conditions.

Actuator Type Symbols

Actuation and automation symbols show whether a valve is moved by hand, powered remotely, or designed to respond automatically to changes in the process. This section explains the most common actuation methods and how fail-safe logic is represented, helping your readers decode diagrams with confidence.

Manual Operators: Handwheels, Levers and Chain-Wheels

Symbol	Actuator Type	Meaning
	Hand Operated (Manual)	Hand operated valves are manually controlled using a lever or handwheel. The symbol typically includes a handwheel or lever icon, indicating manual operation. These are commonly used in applications where automated control is not necessary.
	Hand Wheel Actuator	A hand wheel actuator allows a valve to be operated manually by turning a wheel, providing simple and reliable control without the need for power or automation.

Some valves are intended to be operated manually. In P&IDs, manual actuation is typically shown by a simple symbol above the valve body, such as a handwheel, a lever, or occasionally a chain-wheel for valves located at height. These symbols tell operators that a person must physically turn or pull the mechanism to open or close the valve. Manual operation is common for small isolating valves or equipment that doesn’t require frequent adjustment.

Using clear manual-operator symbols is important because it informs maintenance and operations staff what action is required at each valve, and where manual access must be planned during plant layout and installation.

Power Actuators: Diaphragm (Pneumatic), Piston, Motor (Electric) and Solenoid Symbols

When valves are part of an automated or remote-controlled system, power actuators come into play. In a P&ID, these are shown by distinct symbols on a line extending from the centre of the valve symbol, indicating the type of drive:

• Pneumatic diaphragm actuators use compressed air to flex a diaphragm and move the valve. These are common where rapid, reliable operation is needed and where air supply is available.

• Piston actuators use air or hydraulic pressure on a piston to give higher force or longer travel range than diaphragm types, suitable for larger valves or higher pressure applications.

• Electric motor actuators attach to the valve stem and use a motor to turn or slide the valve. They are ideal where electrical control and feedback are integrated into a control system.

• Solenoid actuators are compact electro-magnetic devices that switch quickly between open and closed states, useful for on/off control in smaller valves.

Above the valve symbol, simple identifiers such as letter codes or mini-symbols show which type of actuator is fitted. Knowing these at a glance helps process engineers and automation specialists understand how a valve will behave under control system commands.

Symbol	Acutator Type	Meaning
	Pneumatic (Diaphragm)	Pneumatic diaphragm actuators use compressed air to move a diaphragm, which in turn actuates the valve. The symbol usually includes a diaphragm icon or spring to indicate pneumatic actuation.
	Motor	Motor actuators are electrically driven, allowing for precise and remote control of valve positions. The symbol often includes a lightning bolt or motor icon to represent electric motor actuation.
	Hydraulic	Hydraulic actuators use pressurized fluid to drive a piston or diaphragm, controlling the valve. The symbol often includes a piston or fluid element, indicating the use of hydraulic force.
	Pneumatic (Rotary Piston)	Pneumatic actuators use compressed air to control valve movement. They are similar to diaphragm actuators but may not include a diaphragm specifically. The symbol often features an air pressure element.
	Balance Diaphragm	Balance diaphragm actuators are used to maintain balanced pressure within the valve. The symbol often includes a double diaphragm or balance element to indicate this functionality.

End Connections, Process Lines, Signal Lines and Tags

The line and tag around a valve symbol often decide what the symbol really means on a project drawing. A ball valve symbol on a small instrument drain line, a Class 300 flanged process line and a pneumatic shutdown line can point to very different products, even if the basic valve shape looks similar.

End Connection Symbols

End connection marks may show whether the valve is flanged, threaded, socket-welded, butt-welded, clamp-connected or joined by another project-specific connection. This matters because the same valve type can be built with different face-to-face dimensions, gasket surfaces, flange drilling, thread standards and pressure ratings.

A flanged butterfly valve symbol does not automatically confirm wafer, lug or double-flanged construction. A threaded ball valve symbol does not confirm NPT, BSPP or BSPT thread. Before ordering, match the symbol with the piping class, line specification and valve list, especially when replacing an old valve in an existing plant.

Symbol	End Connection Type	Meaning
	Flange Connection	Flange connections are used to join two pieces of pipe or equipment with bolted flanges and gaskets to create a seal. The symbol typically includes two parallel lines with bolts, indicating the flanged joint.

Flanged connections are commonly represented by small parallel lines at the pipe ends. They indicate a bolted joint, allowing easy removal for inspection or replacement. Seeing flanged ends on a P&ID immediately tells engineers and maintenance teams to expect gaskets, bolt loads and flange ratings to be part of the specification.

Symbol	End Connection Type	Meaning
	Weld Connection	Weld connections are used to permanently join two pieces of pipe or fittings by welding. The symbol often includes a weld line or notation indicating the type of weld used, such as butt weld or fillet weld.

Butt-weld ends are shown with a clean, uninterrupted transition between valve and pipe, sometimes annotated to clarify the weld type. This symbol implies a permanent joint, typically used in high-pressure or high-temperature services where leak integrity is critical.

Symbol	End Connection Type	Meaning
	Socket Weld Connection	Socket weld connections involve fitting one pipe into a recessed area of another pipe or fitting, then welding it. The symbol typically includes a socket representation, indicating the overlap and weld point of the connection.

Socket-weld connections are often identified with a stepped or recessed detail, reflecting how the pipe fits into the valve body before welding. These are common in smaller-bore, high-pressure systems and the symbol helps distinguish them from full butt-weld joints.

Symbol	End Connection Type	Meaning
	Threaded Connection	Threaded connections are used to connect pipes or fittings by screwing them together. The symbol usually includes a thread pattern or indicates the use of screw threads for joining.

Threaded ends are usually indicated by short angled or serrated markings. They signal a screwed connection, typical for small lines, utilities or temporary services. On a P&ID, this detail alerts designers to potential limitations around pressure, vibration and maintenance frequency.

Process Lines and Signal Lines

Process lines normally show the flow path for liquid, gas, steam or slurry. The line designation may include pipe size, service code, line number, piping class, insulation or heat tracing. These details can change the real valve choice more than the symbol shape itself, especially for steam, corrosive media, high-pressure gas or abrasive slurry.

Signal lines show control communication rather than process flow. A signal line between a controller, solenoid, positioner and actuator may show that the valve is controlled remotely, but it does not confirm the actual signal range, air supply pressure, voltage, hazardous-area rating or fail-safe action. Those details belong in the instrument index, actuator datasheet and control philosophy.

Symbol	Process Lines Type	Meaning
	Standard Pipe	Standard pipes are the basic type of process line used for fluid transfer in a system. The symbol typically consists of a simple, straight line representing the basic pipe connection without any additional insulation or protection.
	Insulated Pipe	Insulated pipes are used to maintain fluid temperature, either hot or cold, by adding insulation. The symbol often includes an additional line or zig-zag pattern to indicate the presence of insulation.
	Jacketed Pipe	Jacketed pipes have an outer jacket through which a heating or cooling medium flows to regulate the temperature of the fluid inside. The symbol typically includes two parallel lines, one indicating the primary pipe and one the jacket.
	Cooling or Heating Pipe	Cooling or heating pipes are used to either cool down or heat up the fluid flowing through the system. These pipes are often symbolized by an added element, such as arrows or temperature indicators, to denote their thermal function.
	Flexible Pipe	Flexible pipes are used where movement or vibration is expected, allowing the pipe to bend as needed. The symbol usually includes a wavy or curved line to represent flexibility.
	Pipes Cross But Are Not Connected	Pipes that cross but are not connected are depicted with two crossing lines, one of which has a gap or a bridge symbol to indicate that the pipes do not intersect or share flow at the crossing point.

Signal Lines

Symbol	Signal Lines Type	Meaning
	Pneumatic Signal	Pneumatic signal lines are used to transmit signals using compressed air. The symbol is often represented by a dashed line to indicate the presence of pneumatic control.
	Guided Electromagnetic, Sonic, or Fiberoptic Signal	These signals are transmitted through guided mediums like cables or fibers. The symbol typically includes a dashed line with markers indicating the guided nature of the signal.
	Unguided Electromagnetic, Sonic, or Fiberoptic Signal	Unguided signals are those that travel through open space without a physical guiding medium. The symbol usually features a wavy line or other distinguishing markers to indicate the unguided transmission.
	Electric or Electronic Signal	Electric or electronic signals are used to control or monitor systems. The symbol often includes a dashed line with small circles or markers, representing the flow of electric current or electronic communication.
	Hydraulic Signal	Hydraulic signal lines use pressurized fluid to transmit control signals. The symbol is represented by a line with specific markers indicating the hydraulic pressure used to control system components.
	Various Data Communication Signals	Data communication signals represent digital or analog information transmission, often between control systems. The symbol typically features a dashed line with markers such as arrows to indicate bidirectional or unidirectional data flow.

Vessels

In P&IDs, vessels represent where fluids are stored, mixed, heated or held under pressure, making them central to how a process actually works.

Symbol	Type	Description	Application
	Jacketed Mixing Vessel (Autoclave)	This vessel has an outer jacket layer for heating or cooling, allowing temperature regulation of the contents inside. Includes a mixing mechanism for temperature-sensitive reactions.	Frequently used in pharmaceutical, chemical, and food industries for temperature-controlled reactions.
	Half Pipe Mixing Vessel	A vessel with half-pipe coils on its outer surface, allowing fluids to circulate for heating or cooling. Includes a mixing mechanism for thorough mixing.	Common in chemical production and process engineering, where temperature control and continuous mixing are needed.
	Pressurized Vessel (Vertical)	A vertically oriented pressurized container, designed to store materials under high pressure without a mixing mechanism.	Utilized in industries like chemical, petrochemical, and energy for storing or processing pressurized materials.
	Pressurized Vessel (Horizontal)	Similar to the vertical pressurized vessel but oriented horizontally, often chosen for space efficiency.	Used in storage facilities or transport applications in oil and gas for large volumes of pressurized materials.
	Gas Bottle	A small, portable pressurized container for storing compressed gases, such as oxygen or nitrogen.	Commonly found in laboratories, industrial settings, and healthcare for storing gases in a mobile format.
	Bag	A flexible, non-rigid container, often with a triangular opening, used for bulk materials or liquids.	Used in food, agricultural, and pharmaceutical industries for temporary storage or transport of bulk materials.

Pumps, Fans, & Compressors

In P&IDs, pumps, fans and compressors show how energy is added to a system to move liquids or gases. Their symbols don’t just indicate equipment presence; they communicate operating principles, flow behaviour and application intent at a glance. Just like valve symbols in P&ID, these symbols form part of a shared technical language that allows engineers, designers and operators to understand how the process actually works without lengthy notes. Recognising the differences between centrifugal, positive displacement, and air-handling equipment helps ensure the right assumptions are made during design, procurement and troubleshooting.

Symbol	Type	Description	Application
	General Pump	A basic symbol representing a pump without specifying the type.	Used when the specific pump type is not critical to the diagram.
	Centrifugal Pump	Depicted as a circle with an internal radial pattern, indicating a pump that uses rotational energy to move fluids.	Common in water distribution, HVAC systems, and chemical processing.
	Gear Pump	Shown as a circle with two interlocking smaller circles inside, representing a positive displacement pump using gears.	Suitable for handling viscous fluids in chemical dosing and lubrication systems.
	Positive Displacement Pump	Illustrated by a circle with a square inside, indicating a pump that moves a fixed volume of fluid per cycle.	Ideal for applications requiring consistent flow, such as in hydraulic systems.
	Helical Rotor Pump	Depicted as a circle with a wavy line inside, symbolizing a pump that uses a helical screw mechanism.	Used for pumping viscous fluids and slurries in wastewater treatment and food processing.
	Screw Pump	Represented by a circle containing a screw-like arrow, indicating a pump that uses a screw mechanism.	Applied in scenarios involving high-viscosity fluids or when a smooth, continuous flow is needed.
	Vacuum Pump or Compressor	A circle with a triangle and additional lines indicating compression, representing a device that creates negative pressure.	Common in air handling, vacuum generation, and gas compression applications.
	Fan	A general fan symbol with a simple propeller shape inside a circle, representing a generic fan.	Typically found in ventilation, cooling, and air circulation systems.
	Axial Fan	A circle with a propeller-like symbol that represents airflow parallel to the fan’s axis.	Ideal for applications requiring large volumes of air at low pressure, such as in HVAC systems.
	Radial Fan	A circle with a radial line inside, symbolizing a fan with a radial flow direction (perpendicular to the axis).	Used in high-pressure applications like dust collection and fume extraction.

Valve Tags and Drawing Standards

A valve tag such as XV, MOV, FV, FCV or PSV connects the symbol to the project records. For example, an FV tag usually points to a flow control valve, while a PSV tag points to a pressure safety or relief function that needs set pressure and certified capacity checks. If the tag and the symbol seem to conflict, use the project legend and valve list before making a selection decision.

Drawing conventions are often based on company standards and recognised references. ISA5.1 symbols and identification support instrumentation and control identification, while ISO 10628-2 graphical symbols are relevant to chemical and petrochemical process diagrams. The project legend still controls the actual symbols used on a specific drawing package.