Page 39 - Fluid mechanics, heat transfer, and mass transfer

FLUID MECHANICS BASICS
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& If the difference in densities between the fluids is
significant, which is certainly true for a vapor and
liquid and can be true for two different liquids, the
difference in pressure can be used to determine the
interface level between the fluids.
& Usually, a seal liquid is used in the two connecting
pipes (legs) to prevent plugging at the sensing
points.
FIGURE 1.16 Float-type level measurement. & Differential pressure devices are a common means of
continuous level measurement in industry because of
their ease of use. The high-pressure side of a differ-
determined to measure the level. It cannot be used ential pressure instrument is connected to the vapor
with sticky fluids that coat the float. space at the top of the vessel. The measured pressure
& Figure 1.16 illustrates float-type level measurement. differential is the pressure of the liquid column in the
& Displacement. By Archimedes’ principle, a body tank. This provides a true level reading if the fluid
immersed in a liquid is buoyed by a force equal to density is constant. If not, changes in liquid compo-
theweight of the liquid displaced by the body. Thus, a sition or temperature will change the specific gravity
body that is denser than the liquid can be placed in and create a false reading.
the vessel, and the amount of liquid displaced by the & Figure 1.17 shows a typical open tank level mea-
body, measured by theweight of the body when in the surement installation using a pressure capsule level
liquid, can be used to determine the level. transmitter (LT).
& Mechanical floats and displacers measure on the & Any change in density, such as that caused by a
simple principle that the buoyant force on an im- change in temperature, necessitates recalibration.
mersed object equals the weight of fluid displaced. & Differential pressure devices offer the advantage of
Therefore, when liquid level rises, the weight of a easy installation in liquid applications that are rela-
displacer decreases. A low-density float is attached to tively clean and free of suspended solids.
a horizontal rod that is mounted to a tank wall and
& Drawbacks include the requirement of seal fluid in
linked to a switch. As the level rises and falls, the
pressurized vessels, difficulty in calibration, and
switch opens and closes.
technical difficulties related to density and
& Mechanical floats and displacers are inexpensive,
temperature.
easy to install, and work well in a variety of fluid
& Electrical Methods: Two electrical characteristics of
densities.
fluids, namely, conductivity and dielectric constant,
& However, the float or displacer is calibrated to the
are frequently used to distinguish between two
density of the liquid it measures, so if the density is phases for level measurement purposes.
changed, the float must be recalibrated.
➢ Electromechanical devices have a motor operated
& Mechanical floats operate at temperatures up to
paddle that is submerged into a vessel. The paddle
650 C and mechanical displacers can handle tem- stops rotating when its sensor is covered by liquid

peratures up to 370 C. or solid material. The stalled motor activates a

& Another limiting factor for mechanical technology switch, signaling the level of material to a control
is buildup of material on the float or displacer.
Buildup causes changes in weight displacement
and the displacer will require cleaning or recali-
bration.
& Other disadvantages include suitability only for non-
freezing liquids, difficulty of calibration, and sus-
ceptibility to changes in specific gravity.
& Mechanical floats and displacers can work well for
backup level measurement.
& Differential Pressure: The difference in pressures
between two points in a vessel depends on the fluids
between these two points. FIGURE 1.17 Liquid level measurement in an open tank.