Butterfly valves owe much of their popularity to their economic cost and efficient designs. Among the butterfly valves available in today’s marketplace, the resilient-seated type (the most basic) is the design that’s commonly used in fluid-processing applications.
According to the fluid-handling experts at Crane Engineering (Kimberly, WI), the functionality of a butterfly valve is greatly dependent on its seat, which seals between the pipe flanges and the valve disc. In resilient-seated designs, the stem is centered in the middle of the valve disc that, in turn, is centered in the pipe bore. These valves typically feature a somewhat-flexible seat and rely on the disc for a high level of contact with the seat to ensure a proper seal.
Seat-Type Pros and Cons
Three basic seat styles are available for resilient-seated butterfly valves. A recent post on the Crane Engineering blog discussed the pros and cons of each, including their specific strengths and weaknesses. (Use Table I for quick reference.)
Booted (dovetail) seats. These seats have a dovetail shape that mates with the inner-diameter valve bodies. They’re easily removable and serviceable because the fit isn’t physically bonded. Unfortunately, they’re prone to movement when mounted between flanges, resulting in deformation that tends to bulge around the disc-contact points. This sensitivity to mounting conditions limits the versatility of booted-seat butterfly valves. Molded and cartridge seats were developed to address such weaknesses.
Molded seats. These seats are bonded to the bodies of valves through an injection-molding process. While this provides a direct bond, it makes the seat irreparable. Since the seat is integrated with the valve body, the entire valve must be replaced if the seat becomes damaged. Still, a molded seat’s permanent bond with a rigid valve body has advantages over a booted seat. Molded-style seats also resist deformation and dislocation during valve mounting and are capable of dead-end or vacuum service.
Cartridge seats. These seats are created by compression molding a layer of elastomer onto a rigid phenolic backing ring, which supports the elastomer in multiple directions. This process is much more consistent than the injection molding used to create molded-style seats. It provides constant pressure to form the seat shape and maintains tight control of its dimensions. Because of the tight tolerances, cartridge seats offer the best torque consistency and highest wear resistance. This type of seat also improves upon the molded style by making the seat replaceable. In highly abrasive applications, i.e., where valves need to be replaced on a regular basis, the cartridge seat could simply be replaced rather than the entire valve.
Cartridge seats offer advantages unmatched by other seat styles. When the valve body has an integrated retaining lip, a cartridge-seated valve is capable of dead-end service. Unlike booted or dovetail seats, cartridge seats can more efficiently operate in a system that requires vacuum service.
Update Your Valve-Speak
To learn more about general valve terminology, download Crane Engineering’s “Ultimate Glossary of Valve Terminology”. MT
Crane Engineering is a distributor of industrial-grade pumps, valves, filters, wastewater-treatment equipment, and other fluid-processing technology. Services include repair, corrosion-resistant coatings, and skid-system design and fabrication. For more information and instructional videos, visit craneengineering.net.