Page 106 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT 83

sensors are simply held in place to take a measure- . “Normally Doppler flow meters are calibrated for water
ment or, for a more permanent installation, they are flow measurements.” Do they require recalibration for
strapped around the pipe. Other advantages include other fluids?
the following: & Yes. Because the flow rate depends on the velocity of
➢ Easy installation and removal. No process down- sound through the fluid, which is dependent on fluid
time during installation. properties.
➢ No moving parts to wear out. . Give examples of typical applications of Doppler flow
➢ Zero pressure drop. meters.
➢ No process contamination. & Waste water.
➢ Works well with slurries or corrosive fluids. & Cooling water.
➢ Works with a wide range of pipe sizes. & Clarifier monitoring.
➢ No leakage potential. & Digester feed control.
➢ Meters are available that work with laminar, tur- & Hot and chilled water.
bulent, or transitional flow characteristics. & Crude oil flow.
➢ Portable. Battery powered units are available for & Mining slurries.
remote or field applications. & Acids.
➢ Sensors are available for pulsating flows.
& Alkalis.
➢ Insensitive to liquid temperature, viscosity, den-
& Liquefied gases.
sity, or pressure variations.
. What type of flow measuring devices are best for
Disadvantages
slurries?
& The main disadvantage is the fact that the liquid
& Doppler flow meter.
stream must have particulates, bubbles, or other types
& Venturi meters are also suitable for some slurries.
of solids in order to reflect the ultrasonic signal. This
. How does a laser velocity meter work?
means that the Doppler meter is not a good choice for
distilled water or very clean fluids. A good rule of & Small particles in a flowing fluid will follow the flow
thumb is to have a bare minimum of 25 ppm at and can be detected by observing the scattered light
roughly 30 mm in order for the ultrasonic signal to from a focused laser beam.
be reflected efficiently. & Seed particles are naturally present in the fluids.
& If the solids content is too high (around 50% and Some times, seeding is necessary.
higher by weight), the ultrasonic signal may atten-
uate beyond the limits of measurability. Another
4.1.5.2 Magnetic Flow Meters
disadvantage is that the accuracy can depend on
particle size distribution and concentration and also
. What is the principle of operation of an electromagnetic
on any relative velocity that may exist between the
flow meter?
particulates and the fluid. If there are not enough
& Magnetic flow meters operate on the principle of
particulates available, the repeatability will also
Faraday’s law of electromagnetic induction; an elec-
degrade.
trical voltage is induced in a conductor that is moving
& It can give rise to troubles when operated at very low
through a magnetic field and at right angles to the
flow velocities. Applications:
field. The faster the conductor moves through the
& Doppler meters, being nonintrusive, have a wide
magnetic field, the greater the voltage induced in
variety of applications in water, waste water, heating,
the conductor in proportion to speed of movement of
ventilation and air conditioning, and petroleum and the conductor.
general process markets. Possible applications are as
& The magnetic flow meter consists of a nonmagnetic
follows: Waste water, potable water, cooling water,
pipe lined with an insulating material. As current
makeup water, hot and chilled water, crude oil flow,
passes through the coils, generally mounted outside
mining slurries, acids, caustic, liquefied gases, and
of the pipe through which fluid flows, a magnetic field
so on.
is generated inside the pipe. As conductive fluid
. What is the important limitation of using a Doppler flow
passes through the pipe, a voltage is generated and
meter?
detected by electrodes that are mounted on either side
& Not suitable for clean liquids.
of the pipe. The flow meter uses this voltage value to
& Requires straight tube run upstream of the meter. calculate flow rate.