Page 121 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT
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TABLE 4.2 Summary of Plus and Minus Points of Different Types of Flow Meters
Liquids
Flow Meter Type Low m High m Slurries Gases Turndown Ratio Accuracy ( %) Cost

Head type
Orifice Yes Limited No Yes 20:1 2–3 Low
Venturi Yes Limited Yes Yes 20:1 2–3 High
Rotometers Yes Limited Limited Yes 10:1 2–4 Low
Target Meters Yes Limited Limited No — — Medium
Velocity type
Magnetic Yes Yes Yes No 20:1 0.5–1 High
Vortex Yes Limited No Yes 20:1 1 Medium
Displacement
Turbine Yes Limited Limited Yes 10:1 0.25–1 Medium
Oval Gear Yes Yes (>10 cP) No No 25:1 0.1–0.5 Medium
Mass flow type
Coriolis Yes Yes — Yes 100:1 0.05–0.1 High
Gas mass flow — — — Yes 50:1 1.5–2 Medium

minor inconvenience, since it needs to be cleaned up . Name some best practices that aid in the selection of
after the measurement. flow measurements.
& Applications: Bubble meters are used in laboratory & Measurement of mass flow in gases and steam.
and low-flow research applications. Their use in & Elimination of impulse lines.
more industrial applications is extremely limited.
& Minimization of permanent pressure loss.
Some of the applications for a bubble flow meter
& Selection of in-line meters for small lines and inser-
include the following:
tion meters for large lines to reduce the overall
➢ Supercritical fluid extraction.
installation cost of the flow meter.
➢ Chromatography column, detector and carrier gas
& Elimination of mechanical meters to reduce mainte-
measurement.
nance costs. Moving parts in mechanical meters
➢ Monitoring post-detector flow volumes in HPLC
require regular maintenance.
systems.
. What are the common flow measurement problems and
➢ Calibration and flow verification for variable area
possible solutions?
and electronic flow meters.
& Table 4.3 gives a summary of flow measurement
➢ Accurate flow measurement of gas mixtures with-
problems.
out recalibration.
➢ Accurate flow measurement of changing gas
➢ concentrations. 4.1.9 Weirs
➢ Calibration of air sampling pumps.
. What are weirs? Name different types of weirs and give
. Give a brief summary of plus and minus points of their applications?
different types of flow meters. & A weir is a dam over which the liquid flows.
& Table 4.2 summarizes the plus and minus points of & Weirs operate on the principle that an obstruction in a
different types of flow meters. channel will cause water to back up, creating a high

TABLE 4.3 Summary of Flow Measurement Problems
Symptom Problem Source Solution
Low mass flow indicated Liquid droplets in gas Install demister upstream. Heat gas upstream of sensor.
Mass flow error Static pressure change in gas Add pressure recording pen
Transmitter zero shift Free water in fluid Mount transmitter above taps
Measurement is high Pulsation in flow Add process pulsation dampener
Measurement error Nonstandard pipe runs Estimate limits of error