A high pressure letdown valve which maintains a smooth flow of hot, high-pressure fluid to a lower pressure without excessive noise or vibration comprises a valve throat (14) having a convergent inlet portion (13) for high pressure fluid to enter the valve, a divergent outlet portion (15) connected to a discharge outlet (16) substantially perpendicular to the inlet portion through which fluid at reduced pressure is discharged from the valve. A cylindrical plug (17) at one end of a shank (19) is coaxial with the inlet portion (13) and movable towards and away from the narrower end of the latter by means of elongated axially-movable stem (35) and spindle (36) movable within a valve bonnet (12) of the valve. The shank (19) comprises a plurality of channels (22) for the passage of quenching fluid from a quenching fluid inlet (23) in the bonnet (12) to recesses (24) in the shank adjacent to the plug (17). The recesses (24) have curved ends to direct quenching fluid radially outwardly into the flow path of the fluid passing around the shank to protect the latter and cool the former. The relative cross-sections of the inlet portion (13) and discharge outlet (16) are such that vapour flows subsonically at all design conditions of the valve.
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