Trim & Internals
Trim
Trim is the collective name for the internal parts of the valve that are in contact with the flowing fluid and perform the throttling: plug, seat ring, cage, stem, guide bushings and (on some designs) soft seats or characterizing components. The body contains; the trim controls.
Trim selection is where severe-service engineering actually happens: the characteristic is set by plug contour or cage windows; the pressure-recovery and noise/cavitation behaviour by the cage drilling; erosion resistance by materials (17-4PH, chrome carbide, Stellite facings, tungsten carbide); and tight shutoff by the seat design. The same body often accepts several interchangeable trim sets — one reason cage-globe construction dominates throttling service.
Spec sheets therefore always ask for trim number/material separately from the body: a "316 body" tells you little until you know what the trim is. Maintenance-wise, trim is the wear set — planned overhauls replace plug, seat and cage as a kit.