Eccentric vs Concentric Butterfly Valves

The primary difference between concentric and eccentric butterfly valves is the position of the shaft relative to the valve seat.

• A concentric butterfly valve has a shaft centered in the middle of the disc and seat. It is commonly used with resilient (soft) seats and is typically selected for general-purpose isolation and light throttling applications.
• An eccentric butterfly valve offsets the shaft from the seat centerline to reduce seat contact during operation. This design is commonly used for higher-pressure, higher-temperature, throttling, and severe-service applications, especially when paired with metal seats.

While most concentric butterfly valves use soft seats, metal-seated concentric designs do exist. However, metal-seated eccentric designs are generally preferred when demanding operating conditions, fire-safe performance, or extended seat life are required.

‍

Why It Matters

Understanding the difference between concentric and eccentric butterfly valves helps prevent common specification mistakes.

The valve design directly affects:

• Seat life
• Shutoff performance
• Throttling capability
• Temperature limitations
• Pressure capability
• Fire-safe performance
• Long-term maintenance costs

A concentric valve may be an economical and reliable choice for water service, while an eccentric valve may be necessary for steam, hydrocarbons, refinery processes, or high-cycle control applications.

What Is a Concentric Butterfly Valve?

A concentric butterfly valve (sometimes called a centerline butterfly valve) positions the:

• Shaft on the centerline of the pipe
• Shaft on the centerline of the disc
• Shaft on the centerline of the seat

When the valve opens and closes, the disc remains in contact with the resilient seat throughout much of its travel.

Typical Characteristics

• Simple design
• Lower cost
• Excellent bubble-tight shutoff with soft seats
• Compact and lightweight
• Suitable for on/off service and moderate throttling

Common Seat Options

Soft-Seated Concentric Valves

The overwhelming majority of concentric butterfly valves use:

• EPDM
• Buna-N (NBR)
• Viton® (FKM)
• PTFE-lined seat designs

These valves are common in:

• HVAC systems
• Water treatment
• Wastewater treatment
• General industrial utility services
• Cooling water systems

Metal-Seated Concentric Valves

Although less common, metal-seated concentric butterfly valves are available.

These designs are typically used when:

• Higher temperatures exceed elastomer limits
• Abrasion would rapidly damage a soft seat
• Chemical compatibility limits elastomer choices

However, because the disc continues to rub against the seat during operation, metal-seated concentric designs generally experience more seating friction and wear than eccentric designs.

What Is an Eccentric Butterfly Valve?

An eccentric butterfly valve offsets the shaft from the seat centerline. The purpose of this offset is to reduce friction between the disc and seat during operation.

As the valve begins to open, the disc "cams" away from the seat rather than dragging across it.

Types of Eccentric Designs

Single Offset

The shaft is offset from the disc centerline.

Double Offset (High-Performance Butterfly Valve)

The shaft is offset both:

• Behind the seat centerline
• Away from the pipe centerline

This design significantly reduces seat wear and improves throttling performance.

Triple Offset

A third offset creates a conical seating geometry that eliminates rubbing between the sealing surfaces.

Triple-offset valves are commonly used in:

• Steam systems
• Refining
• Petrochemical plants
• Power generation
• High-temperature process applications

Comparison Table

Preferred Applications for Each Design

When/Where Concentric Butterfly Valves Are Most Often Selected

Concentric valves are often selected when:

• Cost is a major consideration
• "Bubble-tight" shutoff is a high priority
• Temperatures are relatively moderate
• Service is clean and non-abrasive
• Frequent throttling is not expected

Common applications include:

• Chilled water
• Cooling water
• Potable water
• Wastewater
• HVAC systems
• General industrial utility services

When/Where Eccentric Butterfly Valves Are Commonly Used

Eccentric valves are often selected when:

• Higher temperatures are present
• Higher pressures are present
• Throttling performance is important
• Cycle frequency is high
• Longer seat life is desired
• Fire-safe design is required

Common applications include:

• Steam systems
• Hydrocarbon service
• Refining
• Petrochemical processing
• Power generation
• Process control applications

Metal Seats and Fire-Safe Design

One of the major advantages of eccentric butterfly valves is their compatibility with robust metal-seated designs.

In many hydrocarbon and refinery applications, fire-safe performance is a critical consideration.

If a fire destroys a resilient seat, the valve may lose its sealing capability. Metal-seated eccentric designs can continue providing a degree of shutoff capability even after exposure to extreme temperatures.

This is one reason high-performance double-offset and triple-offset butterfly valves are frequently selected for:

• Oil and gas facilities
• Refineries
• Chemical plants
• High-temperature process systems

It is important to note that fire-safe performance depends on the specific valve design and applicable certifications. Users should always verify fire-safe ratings and standards compliance from the valve manufacturer before specification.

Common Misconception: Concentric Does Not Always Mean Soft-Seated

A common misconception is that:

Concentric butterfly valve = soft-seated butterfly valve

While most concentric butterfly valves do use resilient seats, the valve geometry and seat material are separate design characteristics.

You can find:

• Concentric soft-seated valves
• Concentric metal-seated valves
• Eccentric soft-seated valves
• Eccentric metal-seated valves

The valve offset design determines how the disc engages the seat, while the seat material determines sealing characteristics, temperature limits, chemical compatibility, and fire performance.