Page 90 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT 67

FIGURE 4.10 Venturi meter.

& Discharge coefficient signifies the quantum of energy . What is the maximum flow velocity attainable in a
recovery or permanent energy loss in a flow meter. venturi meter?
. Why C d of venturi is higher than C d for orifice? & Sonic velocity at the throat, which corresponds to
& In a venturi meter, gradual change in cross-sectional minimum cross-sectional area.
area of the convergent and divergent cone ensures . What are the characteristic features and applications of
smooth flow profiles without formation of eddies that a venturi meter?
represent large energy losses. Because of the non- & High recovery of pressure and energy recovery
turbulent streamline conditions, discharge coeffi- makes a venturi meter particularly suitable where
cient—a measure of energy losses—will be high. only small pressure heads are available.
. Why the angle of discharge cone (diffuser) is smaller in & Suitable for clean, unclean, slurry, and viscous flow
a venturi meter? services. Plugging problems are less with slurries.
& The purpose of the diffuser is to ensure steady and & The rangeability is 4:1.
gradual deceleration of the fluid, with pressure rise to & Typical accuracy is 1% of full range.
nearly to that of original stream pressure, after the
& Required upstream pipe length is 5–20 diameters.
throat. The angle of the diffuser is normally in the
& Both meter and installation costs are comparatively

range of 6–8 . Wider angles cause separation of flow
higher for the venturi meter and generally used for
from the walls, creating eddies that increase friction-
line sizes less than 15 cm.
al losses between the walls and the fluid. If the angle
is less, the meter becomes very long with pressure & Venturi tubes have the advantage of being able to
losses increasing. The objective is to convert back the handle large flow volumes at low pressure drops.
increased kinetic energy due to increased velocity & Have no moving parts and can be used for fluids with
back to pressure energy. high solids content.
& Four or more pressure taps are usually installed with
the unit to average the measured pressure.
& The initial cost of venturi tubes is high, so they are
primarily used on larger flows or on more difficult or
demanding flow applications.
& Venturi meters are insensitive to velocity profile
effects and therefore require less straight pipe run
than orifice meters.
& Their contoured nature, combined with the self-
scouring action of the flow through the tube, makes
them immune to corrosion, erosion, and internal
scale buildup.
& In spite of their high initial cost, the total cost can still
be favorable because of savings in installation, op-
erating, and maintenance costs.
Effect of Reynolds numbers on different flow . What are the sources of error for pressure measurement
FIGURE 4.11
meters. in a differential pressure meter?