Mainly used for measurements on high-pressure and high-velocity steam, they guarantee better resistance to erosion caused by the fluid passing through the restriction compared to orifice plates. They are machined from bar stock to obtain a curved inlet and a cylindrical throat. Depending on the curvature of the inlet, they are called short-radius (ISA 1932) or long-radius. There is also a third type called a Venturi-nozzle, as it has an inlet section typical of a nozzle and a conical outlet section typical of Venturi tubes.
For performance tests of steam turbines, a particular type of nozzle called a “Throat-Tap” or ASME PTC-6 (from the reference standard) is used, in which the downstream pressure taps are made directly within the throat. This type of meter is generally supplied mounted inside a pipe section and is calibrated by accredited laboratories.
