Chemical Engineering March 2014 - 37

LevelDepartment
Editor: Scott Jenkins
E
ffective level measurement in the
chemical process industries (CPI) helps
maintain material inventory at economic
quantities, improves product quality
and maximizes plant output by avoiding
spills and process upsets. When they are
incorrectly matched to their application,
level-measurement devices can contribute
to lower quality and poorer process
consistency. No single level-measurement
technology is suitable for all applications.
This column provides information about
level measurement technologies and guidance
for choosing the most accurate and
effective device for an application.
Technology approaches
The technology used by level measurement
devices can broadly be divided in multiple
ways. One method is to think about them
as point-level versus continuous-monitoring
devices, and another way is to classify them
as either contact or non-contact devices.
Point versus continuous. Point-level detection
is mainly conducted by liquid-level
switches. These switches or liquid-level
gages are designed for controlling the
maximum or minimum liquid level allowed
in a container. Point-level detection is
inexpensive, because it uses mechanical
switches or simple gages. Continuous
level sensing is used when it is necessary
to know the specific level at all times.
This information can be important when
transferring liquids, mixing or determining
product levels.
Contact versus non-contact. Placing the
level sensor in contact with the material being
measured presents potential challenges
having to do with corrosion, pH level and
other environmental factors. Foaming, dust,
tank pressure or steam can limit the use of
non-contacting devices. For some applications,
both types can be used.
Technology
Key questions
The following application-related questions
should be considered when selecting a
level-measurement device:
* What types of material(s) will the sensor
be exposed to?
* Are solids or liquids being measured?
* Will the level sensor be placed on the
internal or external surface of the tank or
vessel?
* What temperatures and pressures will the
sensor experience?
* What is the density of the material to be
measured?
* Does the operation require multiple
sensors?
* Does the sensor need to comply with any
specific design codes?
* In liquid-level measurement, what is the
boiling or flash point?
* What level of precision is desired or
required for the measurement?
* Is there steam present in the tank or vessel
where the measurement is taken?
* What is the size of the tank or process
vessel?
* Is the material being measured corrosive
or highly viscous?
* Does the material being measured contain
suspended solids?
* Does the material change state with varying
temperature or pressure?
* Are any reactive or hazardous substances
present in the material to be measured?
* What level alerts need to be transmitted
from the sensor?
* What software and hardware are required
to integrate the level measurement
sensor into the operation?
Table 1 includes several technologies used
in level-measurement devices, while Table 2
outlines some of the specific CPI applications
for which the technologies are used.
Mechanical floats are low-density floats
mounted on a horizontal arm connected to a
switch
Differential pressure devices relate liquid
level to the size of pressure difference between
bottom of tank and vapor space at the top
Electromechanical devices have a
motor-operated paddle or vibrating fork that is
submersed into a vessel and stops rotating or
vibrating when covered with material
Capacitance probes sense differences in
capacitance when air or material is present in
a tank or vessel
Ultrasonic devices use a piezoelectric crystal
to create sound waves, which are directed at the
material. Sound waves are reflected back to the
receiving device. The distance can be calculated
from the echo, and level determined
Radar devices transmit electromagnetic
waves toward the material. Waves are reflected
off the material and back to the source. Transit
time is related to level of material
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
Inexpensive
Easy to install
* Work well for a variety of fluid densities
Can monitor continuously
Easy installation for liquid applications
Cost-effective
Low maintenance requirements
* Well-suited to solids, chips and pellets
Independent of material dielectric properties
Produces highly accurate and repeatable results
Easy to install (requires only one opening)
No moving parts to wear out
Can provide continuous monitoring of level
Few compatibility problems, since no contact
with material is made
Low maintenance requirements
Not affected by changing dielectric properties
Can provide continuous monitoring of level
Unaffected by environmental factors, including
pressure, temperature, vapor, steam and dust
Non-contacting (so no material deposits)
Relatively easy, top-of-tank installation
*
*
*
*
*
*
Chemical compatibility between device and material is important
Changes to chemical composition and temperature can affect
dielectric properties and alter results
Calibration can be time-consuming
Dense vapor, dust or foam can affect measurements
Not for use in high (>300 ˚F) temperatures and high (>8 bars)
pressures
Cannot be used in open-air applications, because of regulatory
requirements
Limitations
*
*
*
*
*
*
Guided
wave radar
Measurement
Device Selection
TabLe 2. an overview of LeveLMeaSureMenT
TechnoLogieS
anD SuiTabLe appLicaTionS
Technology Application
Ultrasonic
Chemical storage tanks
Wastewater effluent
Plastic pellets
Radar
Chemical bulk-storage vessels
Sulfur storage
Agitated process vessels
Reactor/process vessels
Liquid storage
Plastic pellets
Slurries
Displacer replacement
Capacitance Styrene and other aromatic compounds
Acids, caustics
Adhesives
PVC pellets
Interface in agricultural
chemical production
Electromechanical
Plastic
pellets
Carbon black
Fertilizer
Styrofoam beads and chips
References
1. Schmidt, K., Level Measurement Technologies for the
CPI. Chem. Eng., July 2008, pp. 34-37.
2. Schaffer, C., Vessel Sizing and Level Instrumentation,
Chem. Eng., February 2012, pp. 30-34.
3. Aiken, L., Liquid Level Measurement Options for
the CPI, Chem. Eng., July 2008, pp. 38-42.
Editor's note: Portions of this month's " Facts at your
Fingertips " column were contributed by Brian Sullivan,
sales manager at Valin Corp. (San Jose, Calif.;
www.valin.com). Other content was adapted from
articles noted in the references.
TabLe 1. LeveL-MeaSureMenT DeviceS: aDvanTageS anD DiSaDvanTageS
Advantages
Float is calibrated to fluid it measures, so it must be recalibrated
when density changes
Only useful for point measurements
Requires constant density
Fluid needs to be sealed in pressurized vessels
Calibration can be difficult
Cannot provide continuous measurement
http://www.valin.com
Chemical Engineering March 2014

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