What are the three types of butterfly valves?

Butterfly Valves are quarter-turn control devices renowned for their compact design, light weight, and low-pressure drop. They regulate the flow of fluid using a disk (the “butterfly”) that rotates 90 degrees around a central or offset axis.

Based on the relationship between the disc, the stem, and the valve body’s sealing surface, butterfly valves are primarily categorized into three types:

I. Concentric / Zero-Offset Butterfly Valve

Design & Principle

• Concentricity Defined: In this basic and most economical design, the three central points—the stem axis, the center of the disc, and the pipeline center—are all aligned on the same axis.
• Sealing Mechanism: It typically uses a resilient seat (soft seat) made of an elastomeric material (like EPDM or NBR) or PTFE liner. The disc’s edge continuously rubs against the soft seat during the entire opening and closing stroke. Sealing is achieved by the compression and elastic deformation of the soft seat against the disc.

Application Profile

• Pros: Simple construction, lowest cost, bubble-tight shutoff (Class VI) in low-pressure, low-temperature applications.
• Cons: High friction and wear on the seat, which limits its use in abrasive or high-cycle environments.
• Typical Applications: Water treatment, general service utilities, HVAC systems, and low-pressure applications requiring simple ON/OFF isolation.

II. Double-Offset Butterfly Valve (High-Performance)

Design & Principle

The Double-Offset design introduces two offsets to enhance performance and reduce friction compared to the zero-offset type:

1. First Offset (Axis Offset): The stem is offset from the center of the pipe/valve bore.
2. Second Offset (Plane Offset): The stem is offset from the center line of the disc sealing surface.

• Sealing Mechanism: This geometry causes the disc to lift off the seat immediately upon opening and only engage the seat during the final few degrees of closing. This dramatically reduces rubbing friction and seat wear. They use both soft seats (PTFE/RPTFE) and, commonly, metal seats.

Application Profile

• Pros: Significantly reduced operating torque and wear, handles higher pressure ratings (e.g., ANSI Class 150/300), excellent for throttling (modulating) service.
• Typical Applications: Chemical processing, oil and gas, refining, and power generation systems where medium-to-high pressures and temperatures are involved, and where a combination of shutoff and flow control is required.

III. Triple-Offset Butterfly Valve (TOV)

Design & Principle

The Triple-Offset Butterfly Valve is the most advanced design, introducing a third, geometric offset for superior sealing in critical, severe service conditions:

1. First Offset (Same as Double-Offset).
2. Second Offset (Same as Double-Offset).
3. Third Offset (Sealing Geometry): The valve seat and disc seal are machined into an eccentric cone profile.

• Sealing Mechanism: This geometric design ensures that the disc seal ring engages the body seat in a friction-free, camming action. The disc only makes line contact with the seat at the absolute point of closure.
• Material: TOVs almost exclusively feature a metal-to-metal seal (hard seal).

Application Profile

• Pros: Achieves true, bidirectional zero-leakage (bubble-tight) shutoff with metal seats, suitable for extreme high-temperature and high-pressure service, inherently fire-safe (per API 607/6FA standards).
• Typical Applications: High-pressure steam, thermal fluid, hydrocarbon service, abrasive media, and critical isolation points in industries like power generation, petrochemicals, metallurgy, and pulp & paper. They often replace bulkier, more expensive gate or globe valves.

Beyond Type: Essential Design Variations

In addition to the three functional types above, butterfly valves are also classified by their body connection style and method of operation.

IV. Body Connection Styles

The choice of connection style impacts installation, maintenance, and whether the valve can be used at the end of a pipeline (end-of-line service).

V. Actuation Methods

Butterfly valves are quarter-turn valves (90° operation) and can be operated by various means:

Would you like a specific deep dive into the materials of construction (body, disc, and seat) for these valve types, or perhaps a breakdown of flow characteristics?