Page 42 - Fluid mechanics, heat transfer, and mass transfer

FLUID STATICS 19

TABLE 1.3 Modern Level Measuring Methods and Their
Applications
Method Applications
Electromechanical Plastic pellets, Styrofoam beads and
chips, carbon black, fertilizers
Capacitance Acids, alkalis, styrene and other
aromatics, adhesives, and so on
Ultrasonic Chemical storage tanks, wastewater
effluents, plastic pellets
Radar Bulk storage vessels for chemicals,
agitated and reaction process vessels,
sulfur storage
Guided wave radar Liquids and slurries, plastic pellets,
displacer replacement

FIGURE 1.20 Inaccuracies in level measurements for foaming
liquids.
& The only part of the sensor that contacts the liquid is a
bubble tube, which is chemically compatible with the
& Figure 1.19 shows the discrepancy between internal
material level to be measured. Since the point of
measurement has no electrical components, the tech- and external levels for a vessel.
nique is a good choice for electrically classified & Ambient heat loss from external heat loss.
hazardous areas. & The density of the liquid in the glass tube (lower
& The control portion of the system can be safely temperature) is more than the density of the liquid
located, with the pneumatic plumbing isolating the inside the process vessel (higher temperature). Ex-
hazardous areas from the safe areas. ception is for the refrigeration process where refrig-
. Give an overview of modern level measurement meth- erated liquid inside the vessel is at lower temperature
ods with suitable examples of applications. than liquid in thegauge glass. The density differences
& Table 1.3 gives a summary of modern level measur- are due to ambient heat losses or heat gain from the
ing methods. gauge glass.
& Rule of thumb for hydrocarbon liquids: For every
. Illustrate the causes of discrepancy between external

level and internal level (inside an equipment). 40 C decrease in liquid temperature in the gauge
glass, density of the liquid increases by 5%.
& Example:
Height of the liquid in the gauge glass: 1.2 m.

Liquid temperature in the gauge glass: 15 C.
Liquid temperature inside the vessel: 290 C.

Level difference: 30 cm.
& Plugged taps give rise to false high level readings.
& Foaming liquids cause inaccurate level readings as
illustrated in Figure 1.20.
. What are differential pressure (DP) cells? How are they
used for liquid level measurement?
& DP cells measure pressure difference between two
points and send a differential pressure reading to the
control system.
& The control system converts the DP cell reading into
a liquid level based on an assumed specific gravity
inside the vessel. Levels based on DP cells can be in
error for different reasons. Some reasons are given in
FIGURE 1.19 Liquid level measurement. the above question.