One basic requirement for any control valve design is easy adaptation to given process data. In almost all cases, this requires the replacement of the valve plug and/or seat ring. Furthermore, the plug and seat ring are the parts which 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.
For valves with standard bonnet, especially when the valve size is DN100 (4″) and smaller, the plug and valve stem are very often designed as one piece. This is quite cost effective both in production and
maintenance. In such cases the plug including the stem will be replaced. For valves with bellows seal or with (cryogenic) extended bonnet, the plug and stem are normally separate parts. This is necessary for valve design reasons and for lower production costs. 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. The reason for this is that control valves are normally operated in partially open position, with considerable pressure drops and high flow velocities across the valve trim. A very common and proven design is to screw the valve stem into the plug, usually with an additional cone or corresponding tolerance, to ensure perfect alignment of stem and plug. This connection is normally secured by drilling a hole (tapper bore) through both the plug and stem and, after assembly, inserting a pin or dowel pin into this hole (Fig. besides) – as already mentioned, a very safe and proven design used by many control valve manufacturers.
The main disadvantage of such a design becomes visible 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 the end user to drill the hole himself. This is certainly possible but requires some knowledge and adjustment to ensure that the hole in the new plug matches the existing hole in the stem or vice versa. Also, this work must be carried out in a well-equipped workshop and is not suitable for standard on-site service.
ARCA has taken up this challenge and developed a special manufacturing equipment and uses precise CNC machining methods to eliminate this disadvantage. Since mid-2020 all ARCA valves with bellows seal or cryogenic extension bonnets have fully interchangeable plugs and spindles (Fig. in the top). 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; a real advantage in terms of reducing downtimes and maintenance costs.