Effective 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.
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.
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. Level-Measurement Devices: advantages and disadvantages|
|Mechanical floats are low-density floats mounted on a horizontal arm connected to a switch||Inexpensive Easy to install Work well for a variety of fluid densities||Float is calibrated to fluid it measures, so it must be recalibrated when density changes Only useful for point measurements|
|Differential pressure devices relate liquid level to the size of pressure difference between bottom of tank and vapor space at the top||Can monitor continuously Easy installation for liquid applications||Requires constant density Fluid needs to be sealed in pressurized vessels Calibration can be difficult|
|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||Cost-effective Low maintenance requirements Well-suited to solids, chips and pellets Independent of material dielectric properties||Cannot provide continuous measurement|
|Capacitance probes sense differences in capacitance when air or material is present in a tank or vessel||Produces highly accurate and repeatable results Easy to install (requires only one opening) No moving parts to wear out||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|
|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||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||Dense vapor, dust or foam can affect measurements Not for use in high (>300ËšF) temperatures and high (>8 bars) pressures|
|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||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||Cannot be used in open-air applications, because of regulatory requirements|
|Table 2. An overview of level-measurement technologies and suitable applications|
|Ultrasonic||Chemical storage tanks|
|Radar||Chemical bulk-storage vessels|
|Agitated process vessels|
|Guided wave radar||Liquid storage|
|Capacitance||Styrene and other aromatic compounds|
|Interface in agricultural chemical production|
|Electro- mechanical||Plastic pellets|
|Styrofoam beads and chips|
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.
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.