A concentric AWWA C504 butterfly valve is a resilient-seated butterfly valve in which the shaft passes directly through the center of the disc and the center of the seat. The disc remains in continuous contact with the rubber seat through most of the quarter-turn stroke. Closing the valve compresses the resilient seat against the disc edge, producing bubble-tight shut-off.
This is the classic waterworks butterfly valve configuration. AWWA C504 covers rubber-seated butterfly valves for water supply and distribution, and the concentric design is the most common layout because it is simple, robust, and economical for moderate pressures and typical municipal water service.
A double eccentric butterfly valve, sometimes called a double offset valve, uses two shaft offsets:
These offsets create a cam-like disc motion. As the disc begins to open, it lifts away from the seat instead of sliding across it. After only a few degrees of rotation, the disc is completely clear of the seat, and friction drops to nearly zero for the rest of the stroke.
In waterworks service, a double eccentric valve may still use a resilient seat, but the offset geometry reduces rubbing and lowers operating torque. Some double eccentric valves also support more advanced seat systems, depending on the manufacturer and application.
| Design Feature | Concentric Butterfly Valve | Double Eccentric Butterfly Valve |
|---|---|---|
| Shaft position | Passes through the center of the disc | Offset behind the sealing plane and off the pipe centerline |
| Disc centerline | Aligned with the seat and pipe bore | Offset from the seat centerline |
| Disc motion | Rotates around a centered axis | Rotates with a cam-like lifting motion |
| Sealing mechanism | Continuous compression of the resilient seat | Final-position contact with reduced sliding |
In a concentric valve, the disc edge stays pressed against the rubber seat for most of the opening stroke. This continuous contact produces a reliable seal but also creates friction every time the valve operates.
In a double eccentric valve, the disc contacts the seat mainly near the fully closed position. During opening, the disc moves away from the seat quickly, so there is very little rubbing. This reduces seat wear and can help the valve maintain consistent sealing over many operating cycles.
Both designs can provide tight shut-off in resilient-seated configurations. The difference is primarily how much friction the seat experiences during operation.
| Item | Concentric Butterfly Valve | Double Eccentric Butterfly Valve |
|---|---|---|
| Breakaway torque | Generally higher because the disc must overcome full seat stiction | Generally lower because the disc lifts off the seat |
| Mid-stroke torque | Moderate and relatively uniform | Low through most of the stroke |
| Closing torque | Increases as the disc compresses the seat | Peaks near the final closed position |
| Actuator sizing | Must cover full resilient seat torque | Often allows a smaller actuator for the same size |
Because double eccentric valves generate less friction during operation, they can often use smaller and less expensive actuators at large diameters. This is one of the main reasons they are considered for larger pipelines and motorized service.
The continuous rubbing in a concentric valve accelerates seat wear when the valve is operated frequently. For isolation valves that cycle only occasionally, this wear is usually acceptable over a long service life. For valves that open and close often, or for throttling and modulating service, the continuous seat friction can shorten seat life.
A double eccentric valve reduces seat contact during operation, which tends to lower wear and support more frequent cycling. However, its geometry is more complex, and maintenance access and spare parts may differ from a standard concentric valve.
| Criterion | Concentric AWWA C504 | Double Eccentric AWWA C504-Style |
|---|---|---|
| Typical waterworks role | Standard isolation in water mains and treatment plants | Large pipelines, frequent operation, or lower-torque automation |
| Seat contact | Continuous during rotation | Mainly near the closed position |
| Seat wear | Higher with frequent cycling | Lower with frequent cycling |
| Operating torque | Higher | Lower |
| Design complexity | Simple | More complex |
| Cost | Generally lower for common sizes | Generally higher for the valve body |
| Best fit | Infrequent isolation, economical projects | Frequent operation, large sizes, or reduced actuator cost |
A concentric AWWA C504 butterfly valve is often the right choice when:
A double eccentric butterfly valve may be the better choice when:
Material selection for either valve should be based on the water quality, pressure, temperature, corrosion environment, and project specification. Typical considerations include:
Use this checklist to decide between a concentric and double eccentric configuration:
AWWA C504 is primarily written around rubber-seated butterfly valves, and the concentric design is the traditional interpretation. Double eccentric waterworks valves may be supplied to meet project requirements but are often built or referenced under additional or alternative standards. Always confirm with the manufacturer how the valve relates to the AWWA C504 specification and whether the required testing applies.
There is no single answer. A concentric valve can have a long service life when used for infrequent isolation. A double eccentric valve tends to experience less seat wear when operated frequently. The actual service life depends on operating cycles, water quality, temperature, and maintenance.
For common waterworks sizes, a concentric AWWA C504 valve generally has a lower valve body cost. However, a double eccentric valve can reduce actuator cost at larger sizes because of lower operating torque. Total installed cost should be compared, not just the valve price.
Yes. Both concentric and double eccentric butterfly valves can be supplied with gear operators, electric actuators, or pneumatic actuators. The actuator must be sized for the valve torque and duty cycle.
A double eccentric valve is often better suited to throttling and modulating service because reduced seat friction improves control and reduces wear. Concentric resilient-seated valves are generally recommended for isolation rather than continuous throttling.
In resilient-seated configurations, both designs can generally provide tight shut-off in both flow directions within their rated pressure. Confirm the specific bidirectional capability and pressure rating with the manufacturer.
The offset shaft geometry makes the disc lift away from the seat during opening. Because the disc does not drag continuously against the resilient seat, there is less friction to overcome, which lowers operating torque.
Choose a concentric AWWA C504 valve for standard waterworks isolation, moderate pressures, infrequent operation, and cost-sensitive projects. It is a proven, simple configuration for many municipal water applications.
Choose a double eccentric valve for large pipelines, frequent operation, modulating control, or where lower torque reduces actuator cost. It may cost more for the valve body but can offer lower operating and maintenance costs over time.
Confirm the size, pressure class, flange standard, face-to-face dimension, water quality, operating temperature, body and disc materials, shaft material, seat material, actuation method, coating, and testing requirements. Review these against the project specification with the manufacturer.
Contact LAUX Valve for technical drawings, torque calculations, material selection and project-specific waterworks butterfly valve support.
