Page 38 - Fluid mechanics, heat transfer, and mass transfer

FLUID STATICS 15

current flowing through it and cooled by the gas & Level can influence the performance of a process, the
surrounding it. If the gas pressure is reduced, the most common example being a liquid-phase chem-
cooling effect will decrease and hence the tempera- ical reactor.
ture of the wire will increase. The resistance of the & Reliability of level control systems should be of high
wire is a function of its temperature. By measuring order.
the voltage across the wire and the current flowing . What are the types of liquid level monitoring techniques
through it, the resistance and hence the gas pressure
used in industry?
can be determined.
& There are two techniques for liquid level monitoring
. Name other methods for measuring low pressures.
sensors, point and continuous level detection.
& At pressures below 1 Torr, forces exerted by gaseous
& Point level detection is mainly conducted by liquid
molecules are not sufficient to measure pressures
level switches. These liquid level switches consist of
directly with an absolute pressure gauge. Pressure
mechanical switches or liquid level gauges that can
must be inferred by assessing a pressure-dependent
detect the presence of only a liquid and not the level.
physical property of the gas, such as thermal con-
& The main application for point level detection con-
ductivity, ionizability, or viscosity.
sists of controlling the maximum or the minimum (or
& Ionization gauges (IGs) are one class of instruments
both) liquid level allowed in a container. Point level
for indirect measurement of gas density and pressure.
detection is less expensive because it uses mechan-
IGs generally work by ionizing neutral gas molecules
ical switches or simple gauges.
and then determining their number by measuring an
& The main drawbacks of point level detection systems
electric current.
are the need for an empty tank to add low cutoff
➢ One type of IG can be classified as emitting
sensors, the need to add mounting fixtures, and the
cathode IGs (ECGs) (also known as hot cathode
problems associated with contact sensing.
IGs). These are characterized by a heated filament
& Continuous level detection is used when it is neces-
that serves as a source of electrons.
sary to know the specific level at all times. The need
➢ Another type of ionization gauges can be classified
to know the actual level of the liquid is necessary
as crossed electromagnetic field (EMF) IGs (also
when transferring liquids, mixing liquids, and deter-
called cold cathode IGs). This IG type is charac-
mining production levels, and for many other com-
terized by the generation of a discharge between
pany- and industry-specific reasons.
cathode and anode that is maintained with a
& Monitoring of the liquid level is performed by con-
magnetic field.
tinuous liquid level gauges. Continuous monitoring
is typically achieved through entering existing open-
1.5.1 Liquid Level ings at the top of the tank and employing both contact
and noncontact sensors.
. What is the importance of control of liquid level in
& Continuous liquid level monitoring can also be used
process equipment?
as point detection.
& Liquid level in a vessel should be maintained above
& The main criterion in using point detection or con-
the exit nozzle as otherwise outflow from the vessel
tinuous monitoring is the users’ need to know the
will stop, starving downstream equipment such as
exact level of the liquid or the need for knowing when
pumps, process heaters, and numerous other equip-
the level is high or low.
ment, creating damaging situations for downstream
. How are level sensors located?
equipment. Loss of coolant is one of the worst
& Level sensors can be located in the vessel holding the
scenarios giving rise to catastrophic accidents in
nuclear power plants. liquid or in an external leg that acts as a manometer.
& When in the vessel, float and displacement sensors
& If liquid level increases beyond certain limits, over-
flow and spillages might occur, causing hazardous are usually placed in a chamber, free from flow
conditions such as toxic exposures or fires and ex- disturbances, which reduces the effects of flows in
plosions. Spillages have been the causes for major the vessel.
accidents in industry in the past and continue to be so . What are the methods used for liquid level
at present times. measurement?
& Also, high liquid levelsmight increase entrainment in & Float: The float material that is lighter than the fluid
a vapor–liquid separator and can lead to liquid flow- follows the movement of the liquid level. The posi-
ing into vapor lines leading to operational upsets. tion of the float, perhaps attached to a rod, can be