Page 103 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT
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& This compensation is obtained by very large rotor
hubs, very light rotor assemblies and larger numbers
of rotor blades.
. What are the installation guidelines for turbine flow
meters?
& Turbine meters are sensitive to upstream piping
geometry that can cause vortices and swirling flow.
& Specifications require 10–15 diameters of straight
pipe run upstream and 5 diameters of straight run
downstream of the meter.
& However, the presence of any of the following ob-
structions upstream would necessitate that there be
more than 15 diameters of upstream straight pipe
runs:
➢ 20 diameters for 90 elbow, tee, filter, strainer, or

thermowell. FIGURE 4.22 Paddle wheel flow meter.
➢ 25 diameters for a partially open valve.
➢ 50 or more diameters if there are two elbows in
& Paddle wheel flow meters use a rotor whose axis of
different planes or if the flow is spiraling or
rotation is parallel to the direction of flow.
corkscrewing.
& Most paddlewheel meters have flat-bladed rotors and
& In order to reduce this straight run requirement,
are generally bidirectional.
straightening vanes are installed. Tube bundles or
& Figure 4.22 illustrates a paddle wheel type flow
radial vane elements are used as external flow
meter.
straighteners located at least 5 diameters upstream
. Are the paddle meters suitable for turbulent flows or
of the meter.
foamy liquids?
& Under certain conditions, the pressure drop across the
& Because the sensors use laminar flow characteristics,
turbinecancauseflashingorcavitation.Thefirstcauses
turbulent or foamy liquids will not be read accurately.
the meter to read high, the second results in rotor
The sensors must also be installed in a full flowing,
damage. In order to protect against this, the down-
straight section of pipe.
stream pressure must be held at a value equaling 1.25
times the vapor pressure plus twice the pressure drop. . What are straight pipe requirements upstream and
downstream of the meter?
& Small amounts of air entrainment (100 mg/L or less)
& For systems with no bends or restrictions, a minimum
will make the meter to read only a bit high, while
large quantities can destroy the rotor. of 15 pipe diameters upstream and 5 pipe diameters
downstream can be provided.
& Turbine meters also can be damaged by solids en-
. What are the advantages of using a paddle wheel flow
trained in the fluid.
meter?
& If the amount of suspended solids exceeds 100 mg/L
& Good repeatability.
of þ 75 mm size, a flushing Y-strainer or a motorized
cartridge filter must be installed at least 20 diameters & Low pressure drop.
of straight run upstream of the flow meter. & Easy maintenance.
. What is a shunt flow meter? . What is a propeller flow meter?
& Shunt flow meters consist of an orifice in the main & Propeller meters are commonly used in large diam-
line and a rotor assembly in the bypass and are used in eter over 10 cm (4 in.) irrigation and water distribu-
gas and steam service. tion systems.
. What is a paddle wheel flow meter and how does it & They are low cost and low accuracy flow meters.
work? Accuracy is generally is 2% of reading.
& A paddle flow meter consists of rotating paddles with & Propeller meters have a rangeability of about 4:1 and
magnets installed on each paddle of the sensor, which exhibit very poor performance if the velocity drops
is inserted into the liquid. As the paddle turns, an below 0.5 m/s (1.5 ft/s).
electrical frequency output proportional to the flow & Most propeller meters are equipped with mechanical
velocity is generated. registers.