Flow nozzle is used for high velocity flow measurement.
Flow nozzles are particularly suited for measurement of steam flow and other high-velocity fluids, fluids with some other solids,
wet gases and similar materials.
Because of the exact contour is not critical, the flow nozzle can be expected to retain good calibration for a long time
under corrosion or other hostile condition.
And the stream lined contour helps to tends to sweep solid or moisture through the through the throat and is far superior to orifice plates in these services.
Also, the straight length requirement is lesser than orifice plates.
A flow nozzle will pass about 60% more flow than an orifice plate of the same diameter and DP.
It also has the advantage of operating acceptably over a wide beta ratio range of 0.2 to 0.8. For the same flow and DP,
the flow nozzle has a similar but slightly lower pressure loss than an orifice plate.
Types of Flow Nozzles
The ISA 1932 nozzle is a European and it has a smooth rounded convergent inlet leading to a throat section with sharp outlet.
The length of the ISA Nozzle depends on the Beta-Ratio.
A special variation known as a Venturi nozzle and it is hybrid combination of a ISA 1932 nozzle inlet profile with the
divergent cone of a venture tube. It gives a high accuracy with low residual pressure loss.
The common nozzle used in the US is so-called Long radius or ASME flow nozzle.
These nozzles come in two versions, known as low-beta ratio ads high-beta ratio design.
This flow nozzle is a metering primary whose shape consists of a quarter ellipse convergence section and a cylindrical throat section.
The nozzle inaccuracy of ±1% is standard with ±0.25% flow calibrated.
ASME gives an uncertainty of ±2% for nozzles having a beta ratio between 0.2 - 0.8 and throat Reynolds number between 1x10^5 – 2.5 x 10^6