What is the symbol for a butterfly valve? On a P&ID, a butterfly valve is drawn as the standard two-triangle valve body (two triangles meeting point-to-point on the pipe line) with a solid or curved line through the centre representing the disc, and often the letters BFV or a butterfly-shaped disc marker. The actuator and fail-action symbols above the body tell you how it is operated and where it moves on loss of power.
Why reading the symbol correctly matters
A P&ID is the shared language of a process plant. Piping engineers, instrument engineers, commissioning teams, and maintenance crews all rely on the same drawing, so a misread valve symbol can lead to the wrong actuator being specified, an incorrect fail position during a trip, or a valve installed backwards on aggressive media. In European process plants, where drawings frequently follow ISO 10628 and IEC 62424 alongside the widely used ISA-5.1 convention, small notation differences carry real cost consequences.
Butterfly valves are common in large-diameter, low-pressure-drop duties: cooling water, process water, low-pressure gas, and slurry lines. Because they are quarter-turn valves, their symbol and their actuator markings differ from the rising-stem gate and globe valves that share the same basic body outline.
What does the basic butterfly valve symbol look like?
Answer: The base symbol is two triangles meeting at their points on the process line, identical to the generic valve body. What identifies it as a butterfly valve is the disc marker: a line, lens, or curved segment drawn across the centre, and frequently a text tag such as BFV.
Because ISA-5.1 does not assign every valve type a unique unmistakable shape, many drawings distinguish valve types through a combination of the body outline, an internal marker, and a legend on the drawing. Always read the drawing legend first. On a well-maintained P&ID the legend defines exactly which internal marker the project uses for butterfly, ball, gate, globe, plug, and check valves.
How is the actuator shown above the valve?
Answer: The actuator sits directly above the valve body, connected by a short vertical stem line. The actuator shape tells you the drive type: a simple T or lever means manual (hand) operation, a domed or rectangular block means a pneumatic diaphragm or piston actuator, and an M in a circle means an electric motor actuator.
Common actuator notations you will see attached to a butterfly valve body:
- Manual / hand lever or gear: a simple line, T, or hand-wheel style marker.
- Pneumatic diaphragm: a rounded or dome shape on a rectangular base.
- Pneumatic piston (scotch-yoke or rack-and-pinion): a rectangular cylinder block.
- Electric motor operated (MOV): the letter M enclosed in a circle.
- Solenoid pilot: an S box, usually shown with the pneumatic actuator it controls.
For quarter-turn valves such as butterfly and ball, the actuator is almost always a quarter-turn type (rack-and-pinion, scotch-yoke, or part-turn electric), which is a useful cross-check: a rising-stem diaphragm marker on a valve tagged BFV is often a drawing error worth querying.
How do you read the fail-action of a butterfly valve?
Answer: Fail-action is shown by a letter code near the actuator: FC means fail closed, FO means fail open, FL means fail last (stays in position), and FI means fail indeterminate. This tells you where the valve moves if instrument air or power is lost.
Fail-action is one of the most safety-critical pieces of information on the whole drawing, and it is the detail most reference sites omit. On aggressive-media and safety-related loops, the fail position is chosen to drive the process to a safe state. A cooling-water isolation valve is frequently fail open to protect equipment, while a chemical dosing valve is frequently fail closed to stop addition. When you specify or replace an actuator, the fail-action on the P&ID is a requirement, not a preference.
Numbered guide: reading a butterfly valve symbol step by step
- Find the body. Locate the two-triangle valve symbol on the process line.
- Confirm the type. Check the internal disc marker and the tag (BFV) against the drawing legend.
- Read the actuator. Identify the shape above the body: manual, diaphragm, piston, or electric.
- Read the fail-action. Note FC, FO, FL, or FI near the actuator.
- Check the tag number. The valve tag links to the valve schedule and datasheet for size, rating, body, and disc material.
- Follow the signal lines. Dashed or specialised lines to the actuator show whether it is controlled by a positioner, solenoid, or hand switch.
- Cross-reference the legend and notes. Project-specific symbols always override generic assumptions.
Butterfly valve symbol vs other valve symbols
| Valve type | Body outline | Distinguishing marker | Typical motion |
|---|---|---|---|
| Butterfly | Two triangles | Disc line or lens across centre, tag BFV | Quarter turn |
| Ball | Two triangles | Circle (ball) in centre | Quarter turn |
| Gate | Two triangles | Plain body, rising-stem actuator | Rising stem (multi-turn) |
| Globe | Two triangles | Filled or shaded centre | Rising stem (multi-turn) |
| Plug | Two triangles | Rectangle or plug marker in centre | Quarter turn |
| Check | Two triangles | Arrow, ball, or flapper marker | Non-return |
Use this table as a quick decode, then always confirm against the specific drawing legend, because internal markers vary between project standards.
Checklist: confirming a butterfly valve symbol before you act
- ☐ The drawing legend defines the butterfly marker used on this project.
- ☐ The valve tag matches the valve schedule and datasheet.
- ☐ The actuator type is a quarter-turn type consistent with a butterfly valve.
- ☐ The fail-action (FC / FO / FL / FI) is stated and matches the safety intent.
- ☐ Line size and pressure class suit a butterfly valve duty.
- ☐ Disc and seat materials on the datasheet suit the process media.
- ☐ Any positioner, solenoid, or limit switch is shown and tagged.
Is the butterfly valve symbol the same in every standard?
Is the butterfly valve symbol identical worldwide? No. The general two-triangle body is broadly consistent, but the internal disc marker, actuator symbols, and fail-action notation vary between ISA-5.1 (common globally and in US practice), ISO 10628 and IEC 62424 (common in European projects), and company or EPC in-house standards. The drawing legend is the authority for the sheet in front of you.
This is why experienced reviewers never assume. A symbol that means butterfly on one project may be reserved for another valve type on the next. On multi-language EU projects, GreyTec has seen the same physical valve represented with different internal markers across contractor drawing sets, which is exactly why legend-first reading is the professional habit.
More questions (FAQ)
Does the butterfly valve symbol show whether it is wafer, lug, or double-flanged? No. The P&ID symbol shows function, not body style. Wafer, lug, and double-flanged construction is specified on the valve datasheet and valve schedule, not the schematic.
How is a butterfly control valve different from an on/off butterfly valve on a P&ID? A control (throttling) butterfly valve is usually shown with a positioner and a control-loop tag, and it connects to a controller. An on/off (isolation) valve is usually shown with a solenoid and limit switches or a hand switch.
What does BFV mean on a drawing? BFV is a common abbreviation for butterfly valve used in tags and legends. Some projects use BV, but BV is also used for ball valve, so always confirm against the legend.
Where do I find the valve material and rating? On the valve schedule and datasheet referenced by the valve tag, never on the symbol itself. For aggressive or corrosive media, the body, disc, seat, and liner materials are the critical selection.
Why is the actuator sometimes drawn but not the fail-action? On simplified or early-issue drawings the fail-action may be listed in the valve schedule rather than on the symbol. If it is missing from both, raise it as a query, because it is safety-relevant.
How are signal and control lines shown to a butterfly valve?
Answer: Signal and control connections are drawn as distinct line types running to the actuator or valve. A solid thin line is process piping, a dashed line is an electrical signal, a line with small circles or slashes indicates a pneumatic or capillary signal, and a line to a positioner or solenoid shows how the valve is commanded. These line types tell you whether the valve is hand-operated, on/off, or modulating.
Reading the connections matters because two identical butterfly valve bodies can behave completely differently. One may be a simple hand-operated isolation valve, while the other is a modulating control valve driven by a controller through a positioner. The valve body looks the same; the signal lines and loop tags are what reveal the difference. On a control loop, expect to see a tag such as FV (flow valve) or FCV linked back to a controller bubble, whereas an on/off valve typically carries a hand switch or solenoid and open/closed limit switches.
A quick real-world reading example
Imagine a line tagged BFV on a cooling-water header, with a rectangular actuator block above it, an S box beside it, two small limit-switch circles, and the letters FO. Reading it in order: the body plus BFV confirms a butterfly valve, the rectangular block is a pneumatic piston actuator, the S box is a solenoid pilot, the two circles are open and closed position switches, and FO means the valve fails open. That single glance tells you it is an on/off cooling-water isolation valve that opens on loss of air to protect the equipment it serves. This is the habit worth building: decode body, actuator, pilot, feedback, and fail-action every time.
More expert questions
Can a butterfly valve be shown as a control valve and an isolation valve on the same drawing? Yes. Different butterfly valves on the same P&ID can serve different functions. Always read each valve’s own actuator, signal lines, and tag rather than assuming all butterfly valves on the sheet behave alike.
What does a diamond or bubble near the valve mean? A circle or bubble is an instrument or function tag (for example a controller or switch), and its letters follow the instrument identification convention. It links the valve to its control or indication function in the loop.
Why do some P&IDs show a handwheel on a powered actuator? A small handwheel on a powered actuator indicates a manual override or declutchable gear, allowing local hand operation if power or air is lost. It is a useful maintenance and safety feature worth noting.
How do I tell a butterfly valve from a damper? Dampers, used on ducts and gas flow, can look similar but appear on airflow or flue lines and carry damper tags rather than BFV. Context (duct versus process pipe) and the legend resolve it.
Key facts at a glance
- A butterfly valve uses the generic two-triangle valve body plus a disc marker and a BFV tag.
- The actuator symbol above the body shows manual, pneumatic diaphragm, pneumatic piston, or electric drive.
- Fail-action codes are FC (fail closed), FO (fail open), FL (fail last), FI (fail indeterminate).
- Butterfly valves are quarter-turn, so expect a quarter-turn actuator.
- The drawing legend always overrides generic symbol assumptions (Source: ISA-5.1 instrumentation symbols and identification; ISO 10628 diagrams for the chemical and petrochemical industry).
Specifying butterfly valves for aggressive media in the Netherlands
Reading the symbol is step one. Selecting the right valve for corrosive or abrasive service is where projects succeed or fail. GreyTec supplies and integrates valve and pump packages for aggressive media, including lined and corrosion-resistant butterfly valves for chemical and process duties across the Netherlands and the wider EU. If your P&ID calls for a butterfly valve on demanding service, our team can match the disc, seat, and liner materials to your media and pressure class.
Explore our valve and pump packages for aggressive media in Rotterdam, or contact GreyTec to review your valve schedule.
Related reading: Chemical engineering process flow diagrams explained and our guide to corrosion-resistant piping selection.