Deep Inside the Flow: Key Butterfly Valve Parts Explained

November 28, 2025
0 comments

Butterfly valves may look simple at first glance, but their real performance depends on the engineering behind every internal component. The way the body handles pressure, how the disc reduces flow resistance, how the stem transfers torque, and how the seat achieves a reliable seal—all of these details directly shape how the valve behaves in real operating conditions.

In this article, we take a practical, engineering-focused look at the key parts inside a butterfly valve. The goal is to help you better understand how each component affects sealing performance, durability, and overall reliability—so you can make more informed decisions when selecting, purchasing, or maintaining a butterfly valve.

Basic Structure of a Butterfly Valve

Valve Body
Valve Disc
Valve Stem
Valve Seat

Butterfly Valve Body

The valve body is the outer shell of a butterfly valve. It houses and supports all internal components, connects directly to the pipeline, and bears the pressure of the flowing medium. Its design plays a key role in overall strength, sealing performance, and long-term reliability.

The structure of the valve body determines how the valve is installed, how much load it can handle, and whether it can be used at the end of a pipeline. In practice, butterfly valves typically come in several connection styles, each suited for different applications,Below are some common valve body types:

Wafer Butterfly Valve

This is the most common and cost-effective design.
The valve body has no flanges of its own; instead, it is clamped between two pipe flanges using long through-bolts. Its slim profile makes it ideal for compact installations or budget-sensitive systems.
However, because it relies on the clamping force from both sides, it cannot be used as an end-of-line valve and is less convenient for maintenance.

Lug Butterfly Valve

Lug-style bodies feature threaded lugs on both sides, allowing the valve to be bolted independently to the upstream and downstream flanges using short bolts.
This structure enables the valve to carry pressure on its own, making it suitable for end-of-line service. It also offers easier isolation during maintenance.
The additional machining and material requirements increase the cost compared to wafer types.

Double-Flanged Butterfly Valve

In this design, full flanges are integrated into both ends of the body. The valve connects to the pipeline using short bolts directly through the flanges.
This gives the body much higher rigidity and stability, making it the best choice for large diameters, higher pressures, vibration-prone systems, pump outlets, long-distance water transmission, and other critical applications.
It is more expensive than wafer and lug styles and requires additional installation space.

wafer-lug-flange

Valve Body Materials

The choice of body material depends on the medium, temperature, pressure, and corrosion level. Common materials include:

Cast Iron – Cost-effective with good strength; suitable for water, air, and other non-corrosive media.
Ductile Iron – Higher strength and toughness than cast iron; widely used in industrial and municipal systems.
Carbon Steel – Ideal for high-temperature or high-pressure service.
Stainless Steel – Excellent corrosion resistance; commonly used in chemical, marine, and food-grade applications.
Plastics (UPVC, CPVC, PP) – Highly resistant to aggressive chemicals; used in chemical transfer and corrosive fluid systems.

Butterfly Valve Disc

The disc is the heart of a butterfly valve. Typically shaped like a circular plate, it rotates 0–90° inside the valve body through the stem. This rotation controls whether the flow is shut off, fully open, or regulated somewhere in between.

buterfly valve disc

Key Functions

Shut-off and On/Off Control:
A 90° turn moves the valve from fully closed to fully open.
Flow Regulation:
By holding the disc at any angle between 0° and 90°, the valve can throttle or fine-tune the flow rate.

Common Disc Types and Design Concepts

The design of the disc has a major impact on flow resistance, sealing performance, operating torque, and long-term durability. The most common designs include:

1. Centered (Concentric) Disc

This is the most traditional and widely used design. The disc rotates around a centerline that aligns with the valve body.

Advantages:

Simple structure
Lower cost
Reliable soft sealing

Limitations:
Because the disc stays in contact with the seat throughout the opening and closing cycle, it creates continuous friction. This increases operating torque and accelerates wear, making it less suitable for frequent throttling.

Typical Applications:
General water supply, air service, HVAC, and low-pressure ambient-temperature systems.

2. Double-Eccentric Disc (High-Performance Butterfly Valve)

To reduce friction found in concentric designs, the disc and stem are offset in two directions.

Advantages:
When the valve opens, the disc quickly lifts away from the seat, greatly reducing friction and wear. Contact with the seat occurs only in the final degrees of closing.

Typical Applications:
Municipal water networks, medium-pressure systems, and applications with more frequent operation.

3. Triple-Eccentric Disc (Triple Offset Butterfly Valve)

Building on the double-eccentric design, a third offset is introduced: the sealing surface forms a conical angle rather than aligning with the valve body centerline.

Advantages:
This geometry ensures zero rubbing during opening and closing.
The disc achieves sealing through a precise metal-to-metal wedging action, making it suitable for extreme conditions.

Typical Applications:
High temperature, high pressure, corrosive media, abrasive fluids, and other severe service environments where near-zero leakage is required.

Butterfly Valve Stem

The stem is one of the most critical transmission components in a butterfly valve. It connects the actuator or handwheel to the disc and delivers the torque that makes the disc rotate.

Depending on the design requirements, stems are typically made in two configurations: one-piece and two-piece.

butterfly valve stem

One-Piece Stem

In a one-piece design, the stem runs through the entire disc and forms an integrated structure with it.

Advantages:

High structural strength
More stable torque transmission
Better durability under high load

This design is commonly used in valves that require higher reliability and stronger torque capacity.

Two-Piece Stem

A two-piece stem is made of upper and lower sections that connect to the disc using pins, screws, or a clamping mechanism without passing completely through the disc.

Advantages:

Lower manufacturing cost
Simpler structure
Suitable for small to medium sizes and low-pressure applications

Limitations:
The connection interface can loosen or wear over time, making it unsuitable for high-torque or severe service conditions.

Butterfly Valve Seat

The seat is the key sealing element inside a butterfly valve. Installed as a circular sealing ring along the inner wall of the valve body, it forms the surface against which the disc presses when the valve is closed. This tight contact blocks the flow path and prevents leakage.

Butterfly valve seats generally fall into two main categories:

Soft Seats (EPDM, NBR, PTFE):
These rely on the elasticity and deformation of the material to achieve tight, zero-leakage sealing.
Metal Seats:
Used mainly in triple-offset designs, where a conical, wedge-type contact provides high-temperature and high-pressure sealing performance.

valve seat

Seat Installation Types

How the seat is mounted inside the body affects not only sealing performance but also maintainability. Common installation methods include:

1. Press-Fit / Inserted Seat

The seat is mechanically pressed into a groove inside the valve body.
This is the most widely used structure because it offers good sealing performance and makes replacement straightforward during maintenance.

2. Bonded Seat

The seat is attached to the body using adhesives.
While the structure is simpler and manufacturing costs can be lower, seat replacement becomes more difficult.

3. Fully Lined / Integral Seat

Common in fully lined PTFE or fluoroplastic butterfly valves.
A complete layer of PTFE (or similar polymer) is molded or lined along the inner surface of the valve body, serving as both the flow path lining and the sealing seat.
This design offers exceptional corrosion resistance for aggressive chemical media.

4. Replaceable Seat Design

The seat is built as a separate module that can be removed and replaced independently.
This makes maintenance much easier, reduces downtime, and helps extend the overall valve service life—especially valuable in systems with abrasive or corrosive media.

valve seat

Butterfly Valve Actuators

The actuator is the driving mechanism that opens, closes, or modulates the butterfly valve. Depending on the application and level of automation required, butterfly valves can be equipped with several types of actuators:

1.Manual Actuators

The most basic and economical option.
Manual operation is performed through a lever handle or a gear-operated gearbox.
Levers offer quick on/off control for small sizes, while gearboxes provide mechanical advantage, making it easier to operate larger valves without external power.
They are simple, reliable, and low-cost.

2.Pneumatic Actuators

Powered by compressed air, pneumatic actuators offer fast response and are ideal for frequent cycling.
They are available in spring-return (single-acting) and double-acting versions.
Because of their high speed and intrinsic safety, they are widely used in water treatment, industrial process systems, and emergency shut-off applications.

3.Electric Actuators

Driven by an electric motor, these actuators provide stable, controlled operation and are capable of precise modulation.
They are commonly used where no air supply is available or where automation and fine control are essential, such as HVAC, building systems, and clean environments.

4.Hydraulic Actuators

Hydraulic units generate very high torque using pressurized hydraulic oil, making them suitable for large-diameter, high-pressure, or heavy-duty service conditions.
They are often found in power plants, water distribution networks, and industrial systems requiring high force and rugged operation.

butterfly valve drive

Final Thoughts

The performance of a butterfly valve is defined by how well each component—body, disc, stem, and seat—works together. Understanding the structure and functional role of each part is essential for choosing the right valve for your project.

If you need assistance matching valve specifications to your operating conditions, feel free to reach out to Union Valve.We provide expert engineering support and a full range of high-quality butterfly valves to ensure your system operates safely and reliably.