Several embodiments are disclosed of various spool configurations for controlling the flow of an expandable fluid from a high pressure zone to a zone of lesser pressure. The spools structurally conform to a novel principle of controlling the expansion of the fluid from a point of initial throttling throughout the entire expansion profile provided by the valve in such a way that overexpansion of the fluid at any given point is substantially reduced or entirely eliminated.Balance Valves In a pressure reducing valve for an expandable fluid having a valve housing defining a valve chamber, said housing to be connected to a source of expandable fluid under high pressure for the transmission of said fluid at a reduced pressure to a valve outlet defined by said housing, a seat formed by said housing,Globe Valves a valve spool mounted for reciprocal movement in said housing between fully opened and closed positions, said spool having a mating annular seat sealing against the housing seat, REDUCING VALVESmovement of said spool away from said housing seat defining a gradually increasing entrance flow area for said fluid to be admitted to the interior of said housing, the improvement residing in that the valve chamber together with said valve spool defines three longitudinal sequentially related fluid flow zones, said first zone being an entrance zone beginning from said housing seat and extending for the distance of spool movement between fully opened and closed positions, said housing having the internal diameter Do of said housing seat at the beginning of the entrance zone, said valve spool converging evenly throughout said entrance zone toward the axis of said spool to have a diameter D1, the difference between diameters Do and D1 defining in combination with surrounding valve housing a maximum entrance cross-sectional flow area which can admit a maximum mass flow of fluid into said chamber; the second flow zone being defined by said spool and adjacent areas of said housing wherein said spool continues to converge smoothly toward its axis to a minimum valve spool diameter D2, the transverse cross sectional annular flow area at D2 being substantially equal to the minimum transverse cross sectional area of said valve outlet such that the converging surface of the spool together with said housing provides a continuous expansion of the fluid at a constantly decreasing rate of expansion but without over-expansion of said fluid; and a third flow zone is defined by said housing and said spool which diverges from its axis in an amount to re-direct the expanded fluid completely to fill the valve chamber adjacent to the diverging portion of the spool with a minimum of further expansion of fluid and optimum flow distribution to said valve outlet. The present invention comprises, in an expandable fluid pressure reducing valve, a valve housing of generally annular configuration adapted to connect respective zones of high and low pressure expandable fluid, the housing defining a valve seat and reciprocally mounted axially of the housing a valve spool of unique construction. In its simplest and most basic form, the spool consists of a generally hourglass shape having two bell-like sections of opposite curvature joined by a center section of neutral curvature. More specifically, the bell-shape section nearest the valve seat constitutes a throttle area together with the adjacent housing, and with respect to this initial curvature, known techniques governing such throttle control areas may be used. The second oppositely curved spool section constitutes a valve outlet control area together with the surrounding valve housing and the curvature of this spool section is regulated to cause the expanding fluid to be directed radially outward or away from the axis of the valve and to provide the expanding pressure flow at all points. The diameter of the valve spool at its mid-section or point of neutral curvature is further determined in accordance with the cross-sectional annular area as measured from the mid-point to the adjacent valve housing, such annular area being proportionally related to the minimum outlet area of the valve. In particular embodiments, the valve spool according to the present invention may have the simple hourglass shape defined above, or superimposed upon such shape may be vanes that frictionally dissipate energy while guiding and controlling the flow throughout its passage along the valve spool. In another form, a skirt may be provided around the throttle section of the valve which helps meter throttle flow, particularly under low flow conditions.

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