Double Eccentric Vs. Centric Design
The centric operating principle that most metal and plastic butterfly valves employ is often not enough for demanding applications. Let's examine a workable alternative: double eccentricity.
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Butterfly valves in pipe dimensions larger than 2-1/2 in. are the most common type of shut-off valves used in industrial water treatment and the chemical process industry. But, the centric operating principle they feature is often not adequate. There is another operating principle — double eccentricity — to consider. GF Piping Systems uses this technique in its Type 567 butterfly valves. The double eccentric design makes these thermoplastic butterfly valves (Figure 1, left) an attractive alternative to centric designed metal or plastic butterfly valves.
The valves seal when pressurized from both sides up to 150 psi as per ISO 9393-2. This bi-directional sealing is realized via the special design of the passage seal as shown in Figure 2. The double eccentricity permits using separate sealing elements for the exterior seal and the passage seal. Double O-rings seal to the outside on both sides. In the passage, a patented profile seal is implemented.
The standardized mounting flange, illustrated in Figure 3, has a master F07 gauge for holes according to ISO 5211. The valves close at 90 degrees (plus or minus 3 degrees) and require no subsequent adjustment, allowing easy actuator mounting or dismounting at a later time. Also noteworthy is the fact that the electric position indicator is integrated into the mounting flange, contributing to the compact valve design.
Of course, the requirements placed on actuating levers are diverse, and corrosive environments call for an all-plastic design. The ergonomic levers on these valves have five-degree indexing and six teeth that are always engaged as depicted in Figure 4. In addition, the sturdy design can be converted into an infinitely variable, lockable lever.
Type 567 valves, featuring a glass-filled polypropylene wafer body, are available for intermediate installations. For end installations, Type 568 valves with an epoxy-coated iron lug body are available. New types to look for in the near future include silicone-free versions as well as special sealing adaptations.