UNIVERSIT DEGLI STUDI DI SALERNO Bachelor Degree in

UNIVERSITÀ DEGLI STUDI DI SALERNO Bachelor Degree in Chemical Engineering Course: Process Instrumentation and Control (Strumentazione e Controllo dei Processi Chimici) Measuring devices of the main process variables Level measurement Rev. 1. 5 – April 10, 2019

LEVEL SENSORS CLASSIFICATION • • • 1 st CLASSIFICATION Based on the separation surface: Gas / liquid, Vapor / liquid, 2 unmixable liquids Solid (granular or powder) / air Fluidized solid / air 2 nd CLASSIFICATION Based on the instrument function: 1. Continuous Level sensors They continuously measure the height of a separation surface generated by gravity (for liquid and granular solid) from a reference plane. 2. 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. 21/02/2021 Process Instrumentation and Control - Prof M. Miccio 2

LEVEL SENSORS CLASSIFICATION 3 rd CLASSIFICATION Depending on whether the sensor is in direct contact or not with the measuring medium: • contact sensors, • non-invasive or non-intrusive sensors 4 th CLASSIFICATION Depending on whether the sensor is immersed or not in the measuring medium: 1. Immersion probes 2. Surface probes 21/02/2021 Process Instrumentation and Control - Prof M. Miccio 3

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 21/02/2021 Process Instrumentation and Control - Prof M. Miccio 4

COMMON LEVEL SENSORS for LIQUIDS Float sensor FLOAT COUNTERWEIGHT Visual sensor ↓ Float (or probe) sensor ↓ Level indicator 21/02/2021 LI Process Instrumentation and Control - Prof M. Miccio 5

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 21/02/2021 Process Instrumentation and Control - Prof M. Miccio 6

SONAR LEVEL SENSOR SONAR (SOund Navigation And Ranging): It detects the transmission (or propagation) time of a sound or an ultrasound t. TR =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 21/02/2021 Process Instrumentation and Control - Prof M. Miccio 7

RADIOISOTOPE-EMISSION LEVEL SENSOR • It is used also for solids • It works as a transducer 21/02/2021 Process Instrumentation and Control - Prof M. Miccio 8

PROBE LEVEL SENSOR • For solids in high silos (tens of meters) • High precision • It requires programmed maintenance for moving components 21/02/2021 Process Instrumentation and Control - Prof M. Miccio 9

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: Ø minimum Ø medium Ø 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) 21/02/2021 Process Instrumentation and Control - Prof M. Miccio 10

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) 21/02/2021 Process Instrumentation and Control - Prof M. Miccio 11

FINAL SUMMARY http: //www. ktekcorp. com/dnn 20/default. htm 21/02/2021 Process Instrumentation and Control - Prof M. Miccio 12