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BULLETIN 101APRIL 2002

Cascade and Super Cascade Trims - 34699 CASCADE ANDSUPER CASCADE TRIMS

Application

Copes-Vulcan’s Cascade Trim solves theproblems of short life and high replacement costs for valve trim in high pressure drop service. This single-seated trim is designed primarily forhigh pressure modulating drop liquid applications such as pump recirculation or bypass; high pressure desuperheater-cooling water control; supercritical boiler start-up reheat; super-heat spray control; gas; fuel oil; continuous and intermittent blowdown and chemical injection. Due to the single seat design, it is equally appli- cable for on-off service, as is often the case in pump recirculation. This trim is not suited for non-deaerated liquidapplications. In these cases, multiple pressure breakdown HUSH
® or RAVEN
® Trim is recom-mended. Designed for differential pressures up to and including 5000 psi (34,450 kPa). Consult Copes-Vulcan for special applications.
Cascade TrimSuperCascade Trim

Function

amount of inner valve pressure recovery andsubsequent vapor formation. Cascade Trim therefore is ideally suited for applications where cavitation can exist. The plug and cage are designed so that metal-to-metal contact is made on beveled seating surfaces and the clearance between plug and cage tapers in the labyrinth groove section is held to .001" (.0254mm) on Super Cascade and .003" (.0762mm) on “inch” size for proper cascading effect. For those applications where valve pressure decreases with increased flow, Cascade Trim is ideal. At light loads when pressure drop is maximum, the plug is close to the seat and cascading has its optimum effect. As load increases and pressure drop de-creases, the need for cascading diminishes. In those cases where valve pressure drop is high throughout the load range, Super Cascade Trim utilizes cascading through the full travel of the plug. Because the total pressure drop is broken down into stages and because the pressure is prevented from reaching the vapor pressure, vapor bubbles are virtually elimi- nated. Erosion is held to a minimum, assuring this type of trim to be most economical in this type service. The most difficult service for a valve generally occurs at low flow when the plug is just off the seat. At these positions, seating surfaces are subjected to the greatest forces of erosion and cavitation. Cavitation will occur when the liquid pressure drops to or below the vapor pressure; at which point small bubbles are formed. As recovery progresses, the external pressure acting on the bubbles increases to a point where the bubbles’ internal pressure can‘t support theincrease. The bubbles then collapse or implode,causing pressure shock waves which, if directed against valve trim or body, can cause erosion damage. Therefore, it is essential that a trim design which minimizes these problems is selected. Cascade Trim is one such design. Cascade Trim features a series of labyrinthgrooves, machined into the plug along its throt- tling surface. This series of grooves provides a cascading effect, so that pressure drop is divided equally among the grooves, thus reducing the
©SPX Valves & Controls, April 2002