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