Desuperheaters are used to cool steam in power plants. Desuperheating or steam conditioning is the process of restoring superheated steam to its saturated state through temperature reduction. The desuperheater unit has a welded-closed housing which includes a concentrically mounted valve guide structure. The lower portion of the desuperheater valve housing structure is arranged so that, after the unit is welded closed with the valve guide structure contained therein, the valve element may be inserted into the housing from the bottom or inlet opening, followed by a shouldered sleeve which functions as the valve seat. The sleeve is seated against a facing shoulder in the inlet opening of the valve housing and is secured against such shoulder by appropriate means such as welding. The housing surrounding the valve and valve guide is of minimum height, sufficient to accommodate the flow of steam and injected water around the valve and valve guide, and quickly converges the flow of materials back to the diameter of the main piping systems.