Page 82 - Fluid mechanics, heat transfer, and mass transfer

4

FLOW MEASUREMENT

4.1 Flow Measurement 59 measurement is referred to as the fully developed
4.1.1 Differential Pressure Flow Meters 60 velocity profile.
4.1.1.1 Orifice and Venturi Meters 60 & ISO 5167 refers to a fully developed velocity flow
4.1.2 Other Differential Pressure Flow Meters 68
profile as a state of flow where the actual fluid
4.1.2.1 Pitot Tubes 71
velocity flow profile resembles the fully developed
4.1.3 Variable Area Flow Meters 73

state within 5%, and the swirl angle is within 2 of
4.1.3.1 Rotameters 73
zero. This requirement is deemed to be acceptable
4.1.4 Mechanical Flow Meters 78
for all types of metering applications, yet not impos-
4.1.4.1 Turbine and Paddle Wheel
Flow Meters 78 ing impossible restrictions on fluid flow conditions in
4.1.5 Electronic Flow Meters 81 the pipe.
4.1.5.1 Ultrasonic Flow Meters 81 & Ninety-nine percent of flow meter applications do not
4.1.5.2 Magnetic Flow Meters 83 exhibit the fully developed flow state. Piping lengths
4.1.6 Mass Flow Meters 85
of up to 150–200 internal pipe diameters are required
4.1.6.1 Coriolis Flow Meters 85
to create this essential pipe flow condition. The
4.1.7 Positive Displacement Flow Meters 88
alternative is to install flow conditioners into the pipe.
4.1.8 Miscellaneous Types of Flow Meters 94
This is the most economical and accurate method to
4.1.8.1 Target Flow Meters 94
4.1.8.2 Vortex Shedding Flow Meters 94 create a fully developed flow profile in the pipe with
4.1.8.3 Anemometers 96 meter run lengths as short as 13 pipe diameters.
4.1.8.4 Bubble Flow Meters 97 . How are pulsations eliminated in gas flow upstream of a
4.1.9 Weirs 98 metering device?
& For gas flow, a combination of a surge chamber
located close to pulsation source and a constriction
in the line between surge chamber and metering
element are used to eliminate pulsations.
. What is turndown ratio for a flow meter? Illustrate.
4.1 FLOW MEASUREMENT
& Ratio of the highest to lowest flow rates measurable
by the flow meter is called turndown ratio.
. What are the requirements for reliable flow measure-
ment in piping systems? & Turndown ratio is often used to compare the span (the
& The basic requirement for accurate flow measure- range) of flow measuring devices.
ment is to ensure that the flow is fully developed. & Turndown ratio can be expressed as
& Uncertainty values for metering devices are always
TR ¼ q max =q min ; ð4:1Þ
based on the steady-state flow of the single-phase
homogenous Newtonian fluids with a meter en-
where TR is the turndown ratio, q max is the maximum
trance velocity profile that resembles that obtained
flow, and q min is the minimum flow.
in a long straight run of pipe. Departure from these
flow conditions will result in metering errors due & Maximum and minimum flows are stated within a
to installation effects. The fluid flow condition at specified accuracy and repeatability for the device.
the flow meter entrance necessary for accurate & Turndown ratios are illustrated in Figure 4.1.

Fluid Mechanics, Heat Transfer, and Mass Transfer: Chemical Engineering Practice, By K. S. N. Raju
Copyright Ó 2011 John Wiley & Sons, Inc.

59