Modern machining methods reduce maintenance costs
The valve plug and seat ring are the parts that are exposed to the highest flow velocity and thus the highest mechanical stress. Depending on the process conditions, the plug and seat ring are therefore typical wear parts that sometimes need to be replaced. For valves with standard bonnets, especially when the valve size is DN 100 (4 in.) and smaller, the plug and valve stem are very often designed as one piece. In such cases, the plug, including the stem, will be replaced. For valves with bellows seal or with (cryogenic) extended bonnets, the plug and stem are normally separate parts. In contrast to on/off valves, modulating control valves require a well aligned and rigid connection between the valve plug and stem to prevent vibration damages and to ensure the control performance and tightness of the valve. A very common and proven design involves screwing the valve stem into the plug, usually with an additional cone or corresponding tolerance, to ensure perfect alignment of stem and plug. The main disadvantage of such a design becomes apparent as soon as either the valve plug or the bellows seal needs to be replaced. As the bore for the pin will normally never match the bore in the old stem or plug, spare parts are supplied either as a pre-assembled plug/stem unit or as individual parts, but without the bore. The first alternative is quite costly, the second alternative requires end users to drill the hole themselves. This company has taken up this challenge and developed special manufacturing equipment and precise CNC (computer numerical control) machining methods to eliminate this disadvantage. Since the middle of this year, all of this company’s valves with bellows seal or cryogenic extension bonnets have fully interchangeable plugs and spindles (photo). This means that for all valves built since that date, any replacement stem and plug is supplied fully machined, including the taper bore, and can be replaced without reworking, even on site. Hall 4, Stand A02 — ARCA Regler GmbH, Tönisvorst, Germany