UNIVERSIT À DEGLI STUDI DI SALERNO

1. Bachelor Degree in Chemical Engineering Course: Process Instrumentation and Control (Strumentazione e ControllodeiProcessiChimici) Measuring devices of the main process variables Level measurement Rev. 1.5 – April 10, 2019 UNIVERSITÀ DEGLI STUDI DI SALERNO

2. LEVEL SENSORS CLASSIFICATION • 1st CLASSIFICATION • Based on the separation surface: • Gas / liquid, • Vapor / liquid, • 2 unmixable liquids • Solid (granular or powder) / air • Fluidized solid / air • 2nd CLASSIFICATION • Based on the instrument function: • Continuous Level sensors • They continuously measure the height of a separation surface generated by gravity (for liquid and granular solid) from a reference plane. • Point-level sensors or multi-point sensors • They detect when a separation surface (for liquid and granular solid) is above or below a reference height. Process Instrumentation and Control - Prof M. Miccio

3. LEVEL SENSORS CLASSIFICATION • 3rd CLASSIFICATION • Depending on whether the sensor is in direct contact or not with the measuring medium: • contact sensors, • non-invasive or non-intrusive sensors • 4th CLASSIFICATION • Depending on whether the sensor is immersed or not in the measuring medium: • Immersion probes 2. Surface probes Process Instrumentation and Control - Prof M. Miccio

4. COMMON LEVEL SENSORS for LIQUIDS from Magnani, Ferretti and Rocco (2007) Hydrostatic sensor NOTE: Stevin’s principle is applied. The measurement is insensitive to foam formation and bottom deposit Process Instrumentation and Control - Prof M. Miccio

5. COMMON LEVEL SENSORS for LIQUIDS Float sensor float COUNTER- WEIGHT Visual sensor Float (or probe) sensor ↓ ↓ Level indicator LI Process Instrumentation and Control - Prof M. Miccio

6. CAPACITANCE LEVEL SENSORS Variable Capacitance: They measure the capacitance variation between the sensor and the wall of the vessel caused by level variation of the liquid or the solids (granular or powder) • They are sensitive to the dielectric constant variation of the vessel content • They are recommended also for liquids at high pressure and temperature (up to 500 bar and 500°C) and for powders and granular solids with a constant and much larger permittivity than vacuum (or air) permittivity • They have a good resistance to mechanical and chemical corrosion • Good precision • The calibration has to be performed in the measuring range • It works as a transducer Process Instrumentation and Control - Prof M. Miccio

7. SONAR LEVEL SENSOR SONAR (SOund Navigation And Ranging): It detects the transmission (or propagation) time of a sound or an ultrasound • tTR =2l/C • l = Level decrease from transmitter/receiver (T/R) • C = Sound (or ultrasound) velocity • They are recommended for liquid at atmospheric pressure or slightly higher and for granular solids having particle sizes larger than a tenth of the wave length of the transmitted sound. • There is no contact with the vessel content. • The measurement does not depend on the electric and physical properties of the content. • Good precision. • They cannot be used for vacuum or pressurized vessels and for acoustic absorbent vapors. • It works as a transducer Process Instrumentation and Control - Prof M. Miccio

8. RADIOISOTOPE-EMISSIONLEVEL SENSOR • It is used also for solids • It works as a transducer Process Instrumentation and Control - Prof M. Miccio

9. PROBE LEVEL SENSOR • For solids in high silos (tens of meters) • High precision • It requires programmed maintenance for moving components Process Instrumentation and Control - Prof M. Miccio

10. CONDUCTIVE LEVEL SENSORS Conductive: It detects the liquid level from the measurement of the conductivity betwen two or more specific points thanks to the probes installed into the vessel • three points for measuring three level states: • minimum • medium • maximum • They can be used only for conductive liquid having conductance between 1 S and 10-5 S (R from 1  to 105 ) • They are use as point-level for discrete-pointor continue measurement • Robustly built (low maintenance is required) Process Instrumentation and Control - Prof M. Miccio

11. VIBRATING LEVEL SENSORS Vibrating: a vibrating probe (piezoelectric element) operating at 400 Hz for liquids and 80-160 Hz for powders. The frequency changes if the material brushes the probe (e.g. 20%) To detect the point-level positions (max and min) in liquids and solids (powders) Process Instrumentation and Control - Prof M. Miccio

12. FINAL SUMMARY http://www.ktekcorp.com/dnn20/default.htm Process Instrumentation and Control - Prof M. Miccio