Environmental Engineering Reference
In-Depth Information
Pipe tap (high)
roat tap (low)
d
Flow
roat
D
D
figure 2.25 Venturi tube.
2.13.4.2.2 venturi
A
venturi
is a restriction with a relatively long passage with smooth
entry and exit (see Figure 2.25). It has a long life expectancy, simplic-
ity of construction, and relatively high pressure recovery (i.e., produces
less permanent pressure loss than a similar sized orifice), but it is more
expensive, is not linear with flow rate, and is the largest and heaviest
differential pressure flow meter. It is often used in wastewater flows, as
the smooth entry allows foreign material to be swept through instead of
building up as it would in front of an orifice. The accuracy of this type
flow meter is ±1% for a flow range of 10:1. The head loss across a Venturi
flow meter is relatively small, ranging from 3 to 10% of the differential,
depending on the ratio of the throat diameter to the inlet diameter (i.e.,
beta ratio).
2.13.4.2.3 nozzle
flow nozzles
(flow tubes) have a smooth entry and sharp exit (see
Figure 2.26). For the same differential pressure, the permanent pres-
sure loss of a nozzle is of the same order as that of an orifice, but it can
handle wastewater and abrasive fluids better than an orifice can. Note
that for the same line size and flow rate, the differential pressure at the
nozzle is lower (head loss ranges from 10 to 20% of the differential) than
the differential pressure for an orifice; hence, the total pressure loss is
lower than that of an orifice. Nozzles are primarily used in steam service
because of their rigidity, which makes them dimensionally more stable
at high temperatures and velocities than orifices.
Note:
A useful characteristic of nozzles is that they reach a critical flow
condition—that is, a point at which further reduction in downstream
pressure does not produce a greater velocity through the nozzle. When
operated in this mode, nozzles are very predictable and repeatable.
D
Flow
d
figure 2.26 Long-radius flow nozzle.
