Flow Control Valve Symbol
Definition: The Flow Control Valve symbol represents a variable-restriction valve that regulates the volumetric flow rate of fluid or gas in a pipeline, shown in schematic and P&ID diagrams as a bow-tie or butterfly shape with an adjustable arrow, per ISA-5.1 and ISO 10628 conventions.
Also known as: needle valve, throttle valve, flow regulator valve, flow restrictor, metering valve, speed control valve.
What the Flow Control Valve symbol means
The Flow Control Valve symbol denotes a valve that reduces or precisely meters the flow of a liquid or gas by varying the size of the flow passage. Unlike an on/off valve, the flow control valve is set to a specific intermediate position to achieve a target flow rate, making it essential for process control and pneumatic actuator speed regulation.
In pneumatic circuit diagrams, the flow control valve symbol is commonly found controlling the extend and retract speeds of cylinders. In hydraulic and process piping drawings it appears wherever flow must be throttled or metered, with the terminals labelled In (in) and Out (out) representing the upstream and downstream connections.
How to identify the Flow Control Valve symbol
The flow control valve symbol is drawn as two triangles pointing toward each other (forming a bow-tie or butterfly shape), representing the valve body, with an angled arrow superimposed on the symbol to indicate variability or adjustability. In pneumatic P&ID diagrams per ISO 1219 and ISO 10628, the symbol may include an arrow through the center to emphasise the variable restriction. The In (in) and Out (out) terminals connect to the upstream and downstream piping.
Function in a circuit
The flow control valve restricts the cross-sectional area through which fluid passes, creating a pressure drop that limits the volumetric flow rate to a set value. Adjusting the valve needle or spindle changes the orifice size. In pneumatic systems, placing a flow control valve in meter-out configuration on a cylinder port controls the exhaust flow and thus the cylinder speed; in meter-in configuration it controls supply flow. In hydraulic and process systems, the valve maintains a steady flow rate independent of minor pressure fluctuations.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 does not define pneumatic/hydraulic valve symbols directly; fluid power circuit symbols are governed by ISO 1219-1 (hydraulics/pneumatics) and ISO 10628 (P&ID). IEC refers to ISO standards for these components. |
|---|---|
| ANSI/IEEE 315 | ANSI/ISA-5.1 (Instrumentation Symbols and Identification) uses a circle with valve body lines for control valves in P&ID; ANSI Y32.2 / IEEE 315 does not specifically address pneumatic valve symbols. |
| Key difference | ISO 1219-1 (used in hydraulic/pneumatic diagrams) and ISA-5.1 (used in P&ID) each have distinct symbol conventions; ISO 1219-1 shows the bow-tie valve body with variable arrow, while ISA-5.1 represents a control valve as a circle (instrument bubble) with connecting valve body lines. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Flow range: 0.1 L/min to several hundred L/min depending on valve size; pressure rating: 0.5 MPa to 70 MPa (hydraulic); typical pneumatic working pressure: 0.1–1.0 MPa (1–10 bar); adjustment: manual handwheel or screw, or actuated by electric/pneumatic positioner.
Where the Flow Control Valve symbol is used
- Pneumatic cylinder speed control: flow control valves on cylinder ports set the extend and retract speeds of double-acting pneumatic cylinders
- Hydraulic press stroke control: metering valves regulate the advance and return speed of hydraulic rams
- Chemical process plants: flow control valves maintain precise reagent flow rates in reactors and mixing vessels per ISA-5.1 P&ID
- Cooling water systems: needle valves throttle flow to individual heat exchangers to balance a distribution header
- Injection moulding machines: flow control valves set mould-filling speed to prevent flash or short-shots
- HVAC chilled-water circuits: balancing (flow control) valves equalise flow through parallel fan-coil units
Example
In a pneumatic double-acting cylinder circuit, a flow control valve is placed on each cylinder port in meter-out configuration. The In (in) port connects to the exhaust path and the Out (out) port to atmosphere. Adjusting the needle screw increases or decreases the restriction, slowing or speeding the cylinder stroke. This allows precise speed tuning without changing supply pressure.
Key facts
- The Flow Control Valve symbol represents a variable-restriction device that meters fluid or gas flow rate, shown as a bow-tie valve body with an adjustable arrow per ISO 1219-1.
- The two terminals are In (in) for upstream (supply or exhaust) and Out (out) for downstream (load or atmosphere).
- Flow control valves differ from pressure control valves: flow control valves regulate volumetric flow rate (L/min), while pressure control valves regulate pressure (bar or MPa).
- In pneumatic systems, meter-out configuration (restricting exhaust) is preferred over meter-in (restricting supply) because it provides smoother, more controlled cylinder motion.
- In P&ID diagrams, flow control valves carry the ISA-5.1 instrument tag FV (flow valve) or FCV (flow control valve) with a loop number.
- Typical pneumatic flow control valves work at 0.1–1.0 MPa (1–10 bar); hydraulic flow control valves may be rated to 35 MPa or higher.
- A check valve is often integrated into a flow control valve as a one-way bypass, allowing free flow in one direction and throttled flow in the reverse direction.
Frequently asked questions
What does the flow control valve symbol mean in a circuit diagram?
The flow control valve symbol represents a device that restricts and regulates the flow rate of fluid or gas passing through it. In pneumatic diagrams, it sets the speed of actuators; in process P&IDs, it meters reagent or utility flows to a specified rate.
What does the flow control valve symbol look like?
The flow control valve symbol looks like two triangles pointing toward each other (a bow-tie shape) representing the valve body, with an angled arrow across the symbol to indicate variability. The In (in) terminal connects to the upstream piping and the Out (out) terminal to the downstream load.
What is the difference between a flow control valve and a pressure control valve?
A flow control valve regulates volumetric flow rate (measured in litres per minute or cubic metres per hour), while a pressure control valve regulates the pressure at a port (measured in bar or MPa). Flow control valves create a variable restriction; pressure control valves open or close to maintain a set pressure.
Which standard defines the flow control valve symbol?
For hydraulic and pneumatic circuits, the flow control valve symbol is defined by ISO 1219-1 (Fluid power systems — Graphical symbols). For process P&IDs, ANSI/ISA-5.1 provides the control valve representation. IEC 60617 does not cover fluid power valve symbols.
What is meter-in vs meter-out for pneumatic flow control valves?
Meter-in configuration places the flow control valve on the supply port of a cylinder, restricting the air entering the working chamber. Meter-out configuration places it on the exhaust port, restricting the air leaving the chamber. Meter-out is generally preferred because it provides a back-pressure that keeps the piston motion smooth and controlled.
What does the arrow on the flow control valve symbol indicate?
The arrow superimposed on the flow control valve symbol indicates that the valve's restriction is adjustable (variable), distinguishing it from a fixed orifice or restrictor. If the arrow is diagonal across the valve body, it confirms variability; some symbols use a curved arrow to indicate continuous adjustment.
What is the instrument tag for a flow control valve in P&ID?
In ISA-5.1 P&ID notation, flow control valves are tagged FV (flow valve) or FCV (flow control valve) followed by a loop number, such as FCV-101. The associated flow transmitter and controller carry tags FT-101 and FIC-101 respectively in the same loop.
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