Page 187 - Mechanical Engineers' Handbook (Volume 2)

176 Temperature and Flow Transducers

The third class of flowmeters, the dynamic meters, contains those whose signal depends
on some dynamic aspect of a flow field. This class of meters depends upon the repeatability
of certain unstable behaviors of flow fields, for example, vortex precession in a swirling flow
in an adverse pressure gradient and vortex shedding from a bluff body.
In the remaining sections the more common types of flowmeters will be discussed in
terms of the following items: physical appearance, output data and peripheral equipment,
equations computed for off-design operation, and sources for more information.

9.3 Orifice, Nozzle, and Venturi Meters
Orifice, nozzle, and Venturi meters are momentum-based, fixed-area meters. The flow-related
signal reveals the pressure difference between two points on the meter body, as shown in
Fig. 26. These meters accelerate the fluid stream by imposing a contraction on the flow area
and then decelerate the flow by expanding back to the initial pipe diameter. As the fluid
accelerates, its static pressure goes down. As it decelerates, the pressure rises again but not
without loss. Losses are related to irreversibilities in the flow field, such as eddy structures
and turbulence. Mechanical energy dissipated by these mechanisms comes from flow work
done by the main stream. As a result, there is a loss in total pressure.
A principal determinant of ‘‘Which meter to choose?’’ is the irrecoverable loss in pres-
sure associated with the meter. As shown in Fig. 26, orifice meters have the largest loss for
a given signal among these three candidates and Venturi meters have the smallest. As might
be expected, Venturi meters are more expensive than orifice meters. Fluid Meters—Their
68
Theory and Application provides extensive data on the losses for different styles of orifice,
nozzle, and Venturi meters and data on the precautions that must be observed in installing
them.
Measurement of flow rate with these meters requires two pressure sensors and one
temperature sensor. The accuracy of the flow rate measurement will depend upon the ac-
curacy of these instruments.

Figure 26 Orifice, nozzle, and Venturi meter pressure distributions. (From Ref. 67, with permission.)