Page 41 - Fluid mechanics, heat transfer, and mass transfer

FLUID MECHANICS BASICS
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➢ While ultrasonic devices use high-frequency me- ➢ If the dust particle size increases, or if the foam or
chanical waves that require a carrier medium dust gets thick, the sensors will no longer detect
(typically air), radar devices use an electromag- the liquid level, and instead, the level of the dust or
netic wave that does not require a carrier medium. foam will be measured.
This means radar is virtually unaffected by ex- ➢ Radar is also very effective in applications with
treme environmental factors such as temperature, turbulence or extreme levels of condensation
turbulence, humidity, pressure, vacuum, vapor, found in chemical process applications.
steam, or dust. ➢ The first disadvantage is the high price of the radar
➢ Radar devices are installed at the top of vessels, technology.
allowing for easy installation and, in most situa- ➢ The second disadvantage is the need to know the
tions, no disturbance to the process. empty level, the full level, and application para-
& Guided wave radar (GWR) uses time domain reflec- meters in order to program the sensor.
tometry (TDR) principles to measure level by guid- ➢ The third disadvantage is the interference of ech-
ing an electromagnetic pulse down a probe (solid oes from multiple reflections, the tank walls, and
steel rod, steel cable, or coaxial cable) toward the improper orientation of the radar sensor.
material being measured.
➢ Another disadvantage is the loss of accuracydue to
➢ When the pulse reaches the surface of material, the gas layering in the tank that varies the speed of the
change in dielectric value between air and the radar signal.
material causes a portion of the pulse to reflect ➢ In some applications, the antenna can become
back toward the transmitter.
contaminated, which can hinder the emission and
➢ The signal strength of the reflected pulse is pro- reception of microwaves.
portional to the dielectric constant of the material. ➢ It requires application-specific antennas and
The higher the dielectric value of the material, the mounting conditions.
stronger the signal will be.
. What are bubbler sensors for level measurement? How
➢ Once the return pulse reaches the transmitter, the do they work? What are their applications?
device uses time-of-flight principles to calculate
& The air bubbler system works by introducing air into
the distance to the material surface.
a pipe. The pressure created in the pipe returns to the
➢ Guided wave radar is unaffected by vapor, density,
sensor where it can be displayed visually or con-
foam, dielectric, temperature, and pressure, and
verted into an electrical signal.
thus tends to work well for short- and medium-
& The pressure in the pipe is equal to the pressure
range measurements.
exerted by the water and effectively measures the
➢ Guided wave radar is not suited for agitated ves-
same as that by the hydrostatic pressure sensors. If
sels or vessels containing abrasive materials.
the tank is not vented, then another line needs to be
➢ The technology is commonly used to measure added so that the bubbler does not build pressure
liquid, plastic pellets, slurries, and displacer inside the tank.
replacement. & The pressure in the pipe has to be considered in the
& Other methods include radiation, neutron backscat-
level calculation depending on the air pressure above
ter technique, and X-ray methods. the liquid, the size and shape of the tank, and the
. What are the advantages and disadvantages of radar distance from the bottom to the pipe.
technology for level measurement? & Air bubbler systems are a good choice for open
& Because radar technology is noncontacting, it is ideal tanks at atmospheric pressure and can be built so that
for applications where deposits or encrustation may high-pressure air is routed through a bypass valve to
be a problem. dislodge solids that may clog the bubble tube.
& The other advantages include use of a microproces- & The technique is inherently self-cleaning.It is
sor to process the signal providing numerous mon- highly recommended for liquid level measurement
itoring, control, communication, setup, and diagnos- applications where ultrasonic, float, or microwave
tic capabilities. It can detect the level under a layer of techniques have proved undependable.
light dust or airy foam. Radar is effective in solids & Air bubbler systems contain no moving parts, making
applications with extreme levels of dust such as them suitable for measuring the level of sewage,
plastic powders, washing powders, fly ash, and lime drainage water, night soil, or water with large quan-
powder. tities of suspended solids.