Page 87 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT
64
& They are subject to blockages when highly viscous . Illustrate, by means of diagrams, some types of orifice
fluids or fluids with particulates are involved. Not plates.
suitable for slurry flow measurement. & Figure 4.8 illustrates different types of orifice plates.
& Measuring the differential pressure at a location close Concentric Orifice:
to the orifice plate minimizes the effect of pipe & The concentric orifice plate is recommended for
roughness, since friction has an effect on the fluid clean liquids, gases, and steam flows when Reynolds
and the pipe wall. numbers range from 20,000 to 10 in pipes under
7
& Installation requires a DP transmitter, manifold, val- 15 cm (6 in.) sizes.
ving, and impulse lines. The impulse lines leading to & The minimum recommended Reynolds number for
the DP transmitter can become plugged unless re- flow through an orifice varies with the b-ratio of the
mote seals and filled capillaries transfer the pressures orifice and with the pipe size. For larger size pipes
to the transmitter. The measured value varies with (more than 15 cm), the minimum Reynolds number
fluid density for both volumetric and mass flows. increases.
& Additionally, flow elements tend to be sensitive to & Concentric orifice plates can be provided with drain
flow profiles within the pipe, requiring long upstream holes to prevent buildup of entrained liquids in gas
pipe runs or flow straightening devices. streams, or with vent holes for venting entrained
. On what factors the recommended minimum distances gases from liquids. The unmeasured flow passing
upstream and downstream of a head meter depend? through the vent or drain hole is usually less than 1%
& b-ratio. of the total flow if the hole diameter is less than 10%
of the orifice bore.
& Type of fitting upstream, for example, 90 elbow, tee

or cross, short or long radius elbows, contraction/ & The effectiveness of vent/drain holes is, however,
enlargement, gate/globe/check valve, etc. limited because they often plug-up.
. “With an orifice meter, accuracy of flow measurement & Concentric orifice plates are not recommended for
depends on its range.” Comment. multiphase fluids in horizontal lines because the
secondary phase can buildup around the upstream
& Near top end of its range, indicated flow is likely to be
edge of the plate. In extreme cases, this can clog the
accurate.
opening, or it can change the flow pattern, creating
& Near the lower end, flow measurement cannot be
measurement error.
accurate.

Different types of orifice plates.
FIGURE 4.8