A control valve symbol in a process P&ID is a composite mark. The valve body appears in the process line, while additional marks identify the actuator or operator, normal or fail position, instrument tag and control-signal connection.
These elements must be read separately. The same basic valve-body symbol may be combined with a pneumatic diaphragm, piston, electric motor, electro-hydraulic operator or solenoid. Even the complete symbol does not normally specify valve size, pressure class, body material, trim design, flow coefficient or required actuator output. Those details belong in the drawing legend, tag notes and valve datasheet.
Hydraulic and pneumatic schematics use a different visual system based on positions, ports, flow arrows and square envelopes. This article focuses on process P&ID control valve symbols and shows how to identify what each part communicates without assuming that a generic symbol overrides the conventions used on a specific project.
How a control valve symbol is built
A process P&ID often shows the control valve as one assembled symbol, but each part answers a different question. Start at the process line and read upwards through the valve body and actuator before checking the surrounding tag, signal lines and position notes.
Valve body symbol
The mark placed directly in the process line represents the valve body and its relationship to the connected piping. A generic control valve symbol may be used when the drawing does not need to show a specific body construction. Other P&IDs use distinct marks for globe, ball, butterfly, angle or three-way bodies.
The body symbol alone does not identify the valve size, pressure class, end connection, material or internal trim. These details must be traced through the valve tag to the equipment list, line specification and valve datasheet.
Actuator or operator symbol
The mark above the valve body identifies the intended operating method. Its shape may indicate a pneumatic diaphragm, piston, electric motor, electro-hydraulic unit, solenoid or manual operator. The connecting line represents the mechanical link that moves the valve stem or shaft.
An actuator symbol does not establish available thrust or torque, travel time, air-supply pressure, electrical duty or control accuracy. It also does not confirm the valve’s response to loss of air, power or control signal unless the drawing includes additional notation.
Normal state and fail information
Letters, arrows or supplementary marks may describe whether the valve is normally open or normally closed and where it should move after a defined failure. These indications must be read according to the project legend because normal operating state and loss-of-energy fail position are separate conditions.
Loop tag and signal connection
The nearby instrument tag connects the physical valve to its control-loop function. Tags such as FCV, PCV, TCV and LCV commonly identify flow, pressure, temperature and level control valves. The tag number then links the symbol to the instrument index, control narrative and valve datasheet.
Signal-line style can indicate pneumatic, electrical, hydraulic or data communication, depending on the drawing convention. A complete reading therefore combines the valve symbol with its tag, signal path and project documentation rather than treating the body icon as a complete specification.
Common control valve body symbols in P&ID
The valve-body mark identifies the basic flow-path arrangement before actuator and loop information are added. The uploaded PIP and ISO control valve images use the same visible graphic, so the generic control valve symbol is shown once rather than presented as two different designs.
| Symbol | Common reading | What it shows | What to check next |
|---|---|---|---|
Generic control valve | A two-port valve used as a final control element. | A controllable restriction installed in the process line. | Body type, actuator, fail action, tag, size, class, trim and flow direction. |
 Continuously operated valve | A valve intended to assume intermediate positions rather than only open or closed states. | Modulating or continuous positioning intent within the control scheme. | Control signal, positioner, travel range, inherent characteristic and actual valve construction. |
 Three-way valve | A three-port valve arranged to combine or divide process flow. | Three process connections and a controllable relationship between them. | Mixing or diverting duty, common port, flow arrows, port arrangement and fail position. |
The generic symbol is adequate when the P&ID only needs to show the control function. A more specific body symbol becomes useful when the valve construction changes the process arrangement, as with an angle body or a three-way mixing or diverting valve.
A three-way symbol confirms that three ports are involved, but it does not establish which port is common or whether the valve mixes two inlet streams or diverts one inlet stream. Follow the connected lines, flow arrows, valve tag and project legend before assigning that function.
Actuator symbols and what they mean
The operator symbol above a valve body identifies the mechanism intended to move the stem or shaft. It helps distinguish local manual operation from pneumatic, electric, hydraulic or electromagnetic actuation, but it does not provide enough information to size the actuator or determine its failure response.
| Operator symbol | Operating method | What the symbol suggests | What it does not prove |
|---|---|---|---|
 Manual operator | Handwheel, lever or another local manual device. | An operator must adjust or switch the valve locally. | Operator type, gear ratio, required force, locking device or remote-control capability. |
 Pneumatic actuator | Instrument air acting on a pneumatic actuator. | Air pressure supplies the force used to position the valve. | Diaphragm details, spring direction, air pressure, actuator size, positioner or fail-open/fail-closed action. |
 Piston operator | Fluid pressure acting on a piston. | A piston mechanism provides the valve movement. | Pneumatic or hydraulic supply, single- or double-acting construction, spring return, available thrust or torque. |
 Motor operator | Electric motor and mechanical drive. | Electrical power moves the valve through a geared or direct drive. | On-off or modulating duty, voltage, torque, operating time, control unit, emergency power or fail position. |
 Electro-hydraulic operator | Electrical command combined with hydraulic actuation. | Hydraulic force moves the valve in response to an electrical input. | Self-contained or external hydraulic supply, accumulator arrangement, operating speed, stored-energy capacity or failure response. |
 Solenoid operator | Electromagnetic operation from an electrical signal. | A solenoid directly operates the valve or controls a pilot stage. | Direct or pilot operation, coil voltage, hazardous-area rating, de-energised position or suitability for modulating control. |
The same actuator symbol can appear above different valve-body symbols. Actuator selection therefore cannot be made from the P&ID alone. Required stem thrust or shaft torque depends on valve construction, pressure differential, shut-off demand, packing friction and the defined failure position.
A solenoid mark also requires careful reading. It may represent an electrically operated on-off valve, or it may identify a separate solenoid pilot controlling air to a pneumatic actuator. Its position in the diagram, signal connections, tag and instrument index establish which function applies.
Normally open, normally closed and fail position are not the same thing
Normally open or normally closed describes the valve state under the normal condition defined by the project. Fail-open, fail-closed and fail-last describe the intended response to a specified loss of air, electrical power, hydraulic pressure or control signal. One condition cannot be derived safely from the other.
The meaning of “normal” must come from the drawing legend. Depending on the project convention, it may refer to the usual process state, the state before start-up or the position when an actuator or solenoid is de-energised. A normally open valve may therefore be open during ordinary operation while still being designed to close after loss of instrument air.
Fail-open, often marked FO, means the valve is intended to move towards the open position during the stated failure. Fail-closed, or FC, indicates movement towards the closed position. Fail-last, or FL, indicates an intention to retain the last commanded position, but the actual arrangement may require a lock-up valve, accumulator, stored electrical energy or control logic.
The failure being evaluated must also be named. Loss of the control signal does not always produce the same response as loss of air supply, electrical power or hydraulic pressure. Spring direction, actuator action, positioner configuration and any solenoid or lock-up device can change the result.
Confirm the final response in the project legend, control narrative, cause-and-effect document and valve datasheet. The NO or NC body symbol is useful for identifying the normal state, but it is not sufficient evidence for the valve’s safety position.
How to read FCV, PCV, TCV and LCV tags
The letters inside or beside a valve tag identify its control-loop function rather than its mechanical construction. In a common identification system, the first letter indicates the measured or initiating variable, C indicates a control function, and V identifies the valve as the final control element.
| Example tag | Common reading | Controlled variable | What the tag does not identify |
|---|---|---|---|
| FCV-101 | Flow control valve | Flow rate | Valve body type, size, Cv, trim or actuator |
| PCV-201 | Pressure control valve | Pressure or differential pressure, as defined by the loop | Upstream or downstream sensing point, regulator type or pressure class |
| TCV-301 | Temperature control valve | Process temperature | Heating or cooling medium, valve location, material or temperature rating |
| LCV-401 | Level control valve | Liquid or interface level | Inlet or outlet manipulation, vessel connection, flashing risk or fail position |
The numeric part normally identifies the control loop. For example, FCV-101 may share loop number 101 with FT-101, which transmits the flow measurement, and FIC-101, which indicates and controls that flow. Exact punctuation, prefixes, suffixes and loop-number rules vary between companies and projects.
ANSI/ISA-5.1-2024 provides a standard framework for instrumentation symbols and identification. A project may supplement that framework with its own tag conventions, so the drawing legend and instrument index remain the controlling references.
A tag identifies why the valve is in the control loop, but it does not specify how the valve performs that duty. Two valves tagged FCV may use different body styles, trim designs, actuators and fail actions because their flow rates, pressure drops and service conditions differ.
P&ID control valve symbols vs hydraulic and pneumatic symbols
The term “control valve symbol” is used for two different drawing systems. A process P&ID shows how a valve interacts with process piping, instrumentation and a control loop. A hydraulic or pneumatic schematic shows how a fluid-power valve directs, blocks or meters fluid within a machine circuit.
| Comparison point | Process P&ID symbol | Hydraulic or pneumatic symbol |
|---|---|---|
| Primary purpose | Shows process flow, equipment, instrumentation and control relationships. | Shows the operating paths and states of a fluid-power circuit. |
| Visual structure | Valve body in the process line, with operator, tag and signal information added around it. | Adjacent squares represent valve positions; arrows, blocked paths and ports show the flow condition in each position. |
| Meaning of control valve | Usually a final control element that changes process flow in response to a control signal. | May be a directional, pressure or flow-control component that controls a cylinder, motor or another circuit function. |
| Actuation mark | Normally placed above the valve body as part of the assembled process-valve symbol. | Normally placed at the side of the position boxes to show solenoid, spring, pilot, lever or other actuation. |
| Document to check | P&ID legend, instrument index, control narrative, line list and valve datasheet. | Circuit legend, component data sheet, port designation and operating sequence. |
ISO 10628-2 covers graphical symbols used in diagrams for the chemical and petrochemical industries. These symbols support process-diagram reading alongside the instrumentation conventions defined by the applicable project standard.
ISO 1219-1 covers graphical symbols for fluid-power systems and components. Directional-valve symbols from this family are recognised by their position boxes, port connections and internal flow-path arrows.
The two systems should not be translated symbol for symbol. A square-envelope directional valve does not identify the body and actuator of a process control valve, while a P&ID control valve symbol does not show every internal flow path or spool position in a hydraulic circuit.
Check the drawing title and legend before interpreting the symbol. If the drawing describes plant piping and instrument loops, use the P&ID convention. If it describes pumps, cylinders, motors and fluid-power circuits, use the hydraulic or pneumatic convention.
A five-step method for reading a control valve symbol
Start with the drawing convention rather than trying to identify the valve from its outline alone. The following sequence separates the information shown on the P&ID from the details that must be verified in other project documents.
- Identify the drawing family and project legend. Check the title block, symbol key and project notes before interpreting individual marks. Confirm that the drawing is a process P&ID rather than a hydraulic, pneumatic or electrical schematic. For a wider reference to process-diagram conventions, see the guide to valve symbols in P&ID.
- Read the valve body and process connections. Count the connected ports and look for a generic, globe, ball, butterfly, angle or three-way body mark. Follow the process lines and flow arrows to determine how the valve is installed. A generic two-port symbol identifies a control point but may leave the actual body construction to the valve datasheet.
- Identify the actuator or operator. Read the mark attached above the valve body to distinguish manual, pneumatic, piston, motor, electro-hydraulic or solenoid operation. This identifies the intended actuation method, but actuator thrust, torque, travel, supply and control accessories still require separate verification.
- Check normal position, fail action and signal path. Look for NO, NC, FO, FC or FL notation, then identify the failure condition to which it applies. Trace the signal line to the controller, solenoid, positioner or other control device. Do not assume that normally open means fail-open or that loss of signal produces the same response as loss of actuator power.
- Follow the tag into the project documents. Use the valve tag to find the instrument index, control narrative, cause-and-effect document, line list and valve datasheet. These records confirm the controlled variable, loop function, valve size, pressure class, body and trim materials, Cv, actuator requirements and specified fail position.
A P&ID symbol may contain enough information to understand the control relationship while remaining incomplete for valve selection or purchasing. That is intentional: the drawing shows how the valve fits the process, while the datasheet defines the equipment required to perform the duty.
If the symbol, project legend, control narrative and valve datasheet appear to disagree, treat the difference as a document-coordination issue. Resolve the discrepancy before using the drawing to confirm operation, maintenance work or procurement.
Treat the symbol as an index, not the complete valve specification
A control valve symbol can identify the flow-path arrangement, operating method, loop function and intended position under defined conditions. It cannot replace the project legend, control narrative or valve datasheet. Those documents establish the exact construction, operating limits and response required from the installed valve.
Use the tag beside the symbol to connect the P&ID with the instrument index, line specification and valve datasheet. Confirm the body type, size, pressure class, materials, trim, Cv, actuator supply and failure response before selecting or replacing the valve.
Once those requirements agree with the drawing, compare them with the available MacoTango control valve series. The product decision should follow the documented service and control requirements rather than the appearance of the P&ID symbol alone.