Jacksonville, AL-Ball valves have long been a component of choice in scores of instrument designs, and their use could be even more widespread, were it not for a single limitation: temperature. Extreme temperature fluctuations can cause even the best ball valve seals to leak.
Now, however, that's changing. Thanks to a new design that employs an innovative one-piece packing, a ball valve used in instrumentation can increase its temperature range by a staggering 129C (265F). Engineers from Parker Hannifin who designed the new valve say that it could open up applications in analyzers, gas panels, offshore oil panels, and other types of research equipment that operate at high internal temperatures.
The key to the new design is the one-piece packing. It is molded over the valve's ball-and-stem as a single entity. In that sense, it departs dramatically from conventional designs that employ two-piece packings, which allows them to be assembled around the ball-and-stem.
By changing from two pieces to one, Parker Hannifin engineers enhanced the valve's performance under large temperature fluctuations. How? By eliminating the most common mode of failure in conventional designs: The parting line between the packing's two pieces-once a likely site for leakage-is gone. "The major difference between this and all other designs is that we have eliminated those leak paths," notes John Thomas, staff engineer for Parker's Instrumentation Valve Div.
The secret? Materials. Unlike conventional ball valves, which typically employ a polytetrafluoroethylene (PTFE) seal member, the new valve seat uses Teflon perfluoroalkoxy (PFA). To design the new packing, Thomas and fellow engineer John Tow teamed with engineers from Parker's Partek facility in Tucson, AZ. Knowing that Teflon PFA could be injection molded more easily than PTFE, the team set out to develop a product that could be manufactured by this process.
Because the PFA part is injection-molded around the ball, it eliminates some of the tolerance issues associated with previous designs. As a result, it removes gaps that could lead to leakage. PFA also performs better during temperature fluctuations, Thomas says. "As Teflon gets warm, it wants to expand," he says. "If it's constrained from expanding, it flows into cracks and crevices. Then, when it cools down, those cracks and crevices open up again, and you have a potential leak path."
Although the miniature ball valve has been used in a host of new applications, Parker engineers say that its most prominent use may be in chemical process analyzers. "Analyzers often are internally heated, so they need a valve that can stand up to higher temperatures," Thomas says. "That's the advantage of this valve-it won't leak at higher temperatures and during big temperature fluctuations."
Additional details...Contact John Thomas, Parker Hannifin Instrumentation Valve Div., 2651 AL Highway 21 N., Jacksonville, AL 36265; (256) 435-2130.