Centerline Butterfly Valve (Concentric Butterfly Valve)
Valves / Butterfly ValveA centerline butterfly valve (concentric butterfly valve) has the disc center, shaft center, and body center all on the same axis, with a resilient elastomer seat that the disc compresses against for shut-off, making it the most economical and widely used butterfly valve for low-pressure water and HVAC applications.
Working Principle
The disc rotates 90 degrees around a shaft that passes through the center of both the disc and the body. In the closed position, the disc compresses the elastomer seat lining the body bore, creating a tight seal. Because the shaft is centered, the disc scrapes across the seat during the closing stroke (about the last 30 degrees of rotation), which can accelerate seat wear. The resilient seat (EPDM, NBR, or Viton liner) deforms to accommodate the disc, providing bubble-tight shut-off at low pressures. The simple concentric design is the lightest, most compact, and least expensive butterfly valve type, but the scraping action and limited pressure rating confine it to Class 150 or 300 water and mild chemical service.
Key Components
Cast iron or ductile iron body, elastomer seat liner (EPDM, NBR, Viton), disc (cast iron, ductile iron, aluminum bronze, or SS), shaft (SS), packing, hand lever or gear operator.
Types & Variants
| Type | Key Feature | Typical Use |
|---|---|---|
| Wafer concentric | Wafer-style, between-flange | HVAC, water treatment, general |
| Lug concentric | Lug-style, dead-end capable | Isolation, one flange removal |
| U-section seat | U-shaped elastomer liner | Low-pressure water, zero leakage |
| Flanged concentric | Full flanged body ends | Water distribution, larger sizes |
Common Materials
Body: ASTM A126 Class 30 cast iron (economy) or ASTM A395 ductile iron (higher pressure). Seat liner: EPDM (water, steam), NBR (oil, fuel), Viton/FKM (chemical, high-temp). Disc: ASTM A351 CF8M (316SS) for corrosion, ASTM B148 C95800 (aluminum bronze) for seawater, ductile iron for economy. Shaft: 416 or 316 SS.
Applicable Standards
- API 609 (butterfly valves)
- MSS SP-67 (butterfly valves)
- AWWA C504 (rubber-seated butterfly valves for water)
- ASME B16.1 (flanged ends, cast iron)
Industrial Applications
- HVAC chilled and hot water systems
- Water treatment and distribution
- Cooling tower isolation
- Fire protection systems
- General industrial water service
- Low-pressure chemical and food processing
Advantages & Disadvantages
Advantages
- Lowest cost butterfly valve design
- Lightweight and compact — smallest face-to-face dimension
- Bubble-tight shut-off with resilient seat
- Easy installation — wafer or lug style fits between flanges
- Available in sizes from NPS 2 to 72+
Disadvantages
- Disc scrapes across seat during closing — accelerated seat wear compared to offset designs
- Limited to Class 150 or 300 pressure rating
- Not suitable for high-temperature service (elastomer seat limits)
- Not fire-safe — elastomer seat burns away
- Higher operating torque than double or triple offset designs at larger sizes
Selection Guide
- For frequent cycling service, double-offset designs last longer because the disc lifts off the seat before rotating, eliminating scraping wear
- EPDM seats are standard for water and HVAC. NBR for oil and fuel. Viton for chemicals and temperatures up to 200°C
- For dead-end service (one flange removed under pressure), specify lug-style body — wafer style is NOT rated for dead-end
- AWWA C504 is the governing standard for butterfly valves in municipal water applications
Practical Tips
- The concentric butterfly valve is the workhorse of HVAC systems worldwide — it is the most installed valve type by unit count in commercial buildings
- Seat replacement on a concentric butterfly valve typically requires removing the valve from the pipeline — for critical service, consider the total cost of ownership including maintenance downtime
FAQ
Concentric vs double-offset butterfly valve?
Concentric has the disc, shaft, and seat all on the same centerline — the disc scrapes across the seat. Double-offset moves the shaft behind and above the disc center, so the disc lifts off the seat before rotating. Double-offset lasts longer and handles higher pressure but costs more.
Can a concentric butterfly valve be used for throttling?
Yes, concentric butterfly valves are commonly used for balancing and throttling in HVAC water systems. However, they should not be used for continuous precise throttling (like a control valve) because the flow characteristic is not linear and seat wear accelerates in throttling positions.