Page 93 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT
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TABLE 4.1 Comparison of Head Flow Meters
Minimum
Primary Element Recommended Service N Re Advantages Limitations
Square-edge Clean liquids, 2,000 Easy to install, easy to replace, Calming piping length required,
concentric orifice gases, and steam low cost high head loss, accuracy is
affected by conditions of
orifice and installation
Conical quadrant- Viscous liquids 500 Same as above Same as above
edge concentric
orifice plate
Eccentric/segmen- Two-phase fluids 10,000 Same as above Same as above, higher
tal orifice uncertainties in discharge
coefficient data
Venturi/flow tube Clean and unclean 75,000 Low head loss, two to nine times High initial cost
fluids, slurries, steam less calming section for same
DP, accuracy is less affected by
wear and installation
conditions than orifice
Flow nozzle Clean fluids, steam 50,000 Higher capacity than orifice, High head loss, harder to replace
accuracy less affected by wear
and installation, good for high
temperature and high velocity
Segmental-cost Unclean and viscous 500 40% less head loss than orifice, Requires calibration and a
wedge liquids, slurries, minimum calming piping differential transmitter
gases

. Give a comparison of head flow meters. & Relatively expensive but has high accuracy of about
& A comparison of head flow meters with respect to 0.5% of the measured flow rate.
their recommended service along with their plus and & The segmental wedge has a V-shaped restriction
minus points for their choice is presented in characterized by the H/D ratio, where H is the height
Table 4.1. of the opening below the restriction and D is the
. What is an elbow meter? diameter. The H/D ratio can be varied to match the
flow range.
& A differential pressure exists when a fluid changes
direction due to an elbow or pipe turn. The pressure & The unique flow restriction can be designed to match
difference results from the centrifugal force gener- the life of the installation without any deterioration.
ated by the flowing fluid.
& Since pipe elbows exist in plants, the cost for these
meters is very low, but the accuracy is very poor.
These are only applied when reproducibility is suf-
ficient and other flow measurements would be
expensive.
& Figure 4.14 illustrates an elbow flow meter.
. What is a V-element flow meter?
& A flow meter in which the flow is restricted by a
V-shaped indentation in the side of the pipe
(Figure 4.15).
& The pipe requires only 5 diameters upstream straight
pipe section.
& The segmental-wedge element is a proprietary de-
signed for use in services such as slurry, corrosive,
erosive, viscous, gas flow with liquid droplets or
liquid flow with gas bubbles. Elbow flow meter.
FIGURE 4.14