Plant Services - August 2008 - (Page 48) Flatness & Bore Alignment Safety Valves The ball’s not a ball Trunnion ball valve Trunnion ball LEVALIGN® Figure 4. The trunnions support the ball assembly. BORALIGN® h Watc O VIDEne Onli Laser tools for demanding measurement tasks. 305-591-8935 • www.ludeca.com In a floating ball design, the ball isn’t fixed inside the housing but, rather, floats between two seats. In the shutoff position (Figure 3), the ball seals against the seat on the low-pressure side, pushed downstream by a positive pressure differential. By contrast, the trunnion design uses a ball, but it’s not a discrete sphere. Rather, its geometry includes two cylinders – the trunnions – affixed to the ball at the top and bottom (Figure 4). The unit fits into a space in the valve body and can’t move along the flow axis. As the ball rotates from the open to closed positions, it glides on the trunnions, which can be fitted with bushings or bearings. In the case of high differential pressure across the seat, a free-floating ball can be pushed downstream – too far downstream. In the absence of an advanced seat design – such as a spring-energized seat with an O-ring and spring on each side – the ball might not return to the center position. As a result, the stem will tilt to one side, and, with time, uneven stem wear will occur. The trunnion design prevents excessive movement of the ball downstream. The trunnions keep the ball centered and the stem properly aligned. The different ball valve designs have appropriate applications, strengths and relative merits, and these have a direct effect on fugitive emissions. When choosing a ball valve, give due considwww.PLANTSERVICES.com eration to material compatibility, pressures, temperatures, desired frequency of inspection and adjustment, and frequency of actuation. Further, when cost becomes a leading valve selection determinant, be aware of the compromises you might be making. The real cost of a valve isn’t its purchase price, but the overall cost of ownership. With raw material and feedstock prices increasing, the frequency and severity of environmental noncompliance fines, and direct and indirect costs associated with frequent maintenance, you must account for valve upkeep, failure and replacement. Michael Adkins (michael.adkins@swagelok. com) is general industrial valve product manager and Peter Ehlers (peter.ehlers@ swagelok.com) is alternative fuels market manager for Swagelok Co., Solon, Ohio. Contact them at (440) 349-5934. More resources at www.PlantServices.com Rebuilding – “Rethinking the purchase of valves and valve repairs” Scrap versus repair – “To scrap or to fix: That is the question” Retrofitting – “Valves with a brain” Minimizing maintenance – “Curb your valve cost” Reliability – “10 tenets for reliable valves” For more, search www.PlantServices.com using the keywords emissions, leak and valve. 48 August 2008 http://www.PlantServices.com http://www.ludeca.com http://www.PlantServices.com http://www.PLANTSERVICES.com
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