Instrumentation & Data Acquisition Systems Section 3 -Level Robert W. Harrison, PE Bob@TheHarrisonHouse.com Made in USA 1
Level Section Question Which level measuring technology is the best solution when foam covers the top of the liquid being measured? 2
Key Liquid Level Sensor Questions What kind of material will be measured? Should the level sensor be external or can we insert it into the application in question? Do you need the sensor to detect the level continuously or will a point sensor be adequate? (point sensing involves taking measurements at certain instances) Can the liquid level sensor come in contact with the process fluid? Is direct measurement of the level needed? 3
Categories Level devices operate under different principles Non-Intrusive No contact with material No contamination problem Intrusive Contact with material Possible contamination problem Two types of level measurements Continuous level Point level (switch) Classified three main categories that measure The position (height) of the liquid surface The pressure head The weight of the material 4
Categories Material types Liquids Fill entire volume Have a flat surface Solids May have voids in volume Most do not have a flat surface 5
Liquids Solids Sensor Level Point Level Point Capacitance Y Y Y Conductance Y Y Differential Pressure Y Y Electromechanical Y Y Hydrostatic Y Mechanical Y Y Y Y Nuclear - Gamma Gauge Y Y Y Y Radar Free Space Y Y Y Y Radar Guided Wave Y Y Y Y Resistance Y Thermal Y Tuning Fork Vibration Y Y Ultrasonic Y Y Y Y Weight Y Y 6
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Sight Glass Gages Works on the same principle as a manometer or U-tube As the level in the tank rises or falls, the level in the sight tube will rise or fall at the same height Non-intrusive measurement There is no signal transmission 8
Dip Stick / Hand Dip Tape with paste applied is dipped into the tank An average of three measurements are taken Sounds simple, but is actually a complicated procedure 9
Dip Stick / Hand Dip Used in majority of custody transfer applications Filling stations Used on uninstrumented tanks Product contamination is a problem Not very reliable Subject to operator error Not safe Costly in terms of operator time Intrusive measurement so contamination is a problem Inexpensive upfront cost 10
Float Buoyant float rests on liquid surface Float is mechanically connected to a gage head by a perforated tape A spring motor in the gage head keeps the tape taut Tape perforations are counted and related to the liquid level 11
Float Most common method of tank gauging in the U.S. Used on a variety of products Used in conjunction with Hand Dip verification Provides reasonably accurate level information, spec ±1/8 Dependent on tape perforation resolution Installation dependencies can cause errors as great as ±2 Requires frequent maintenance to ensure that the tape moves freely Intrusive measurement so contamination is a problem Low upfront cost 12
Displacer Archimedes principle: a body immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced B = gv B = buoyancy force V = fluid volume displaced = fluid density g = gravitational constant g = weight density By measuring B the level can be determined Example Calculate B for an object in water that displaces 10 ft 3 B = g V = (63.2 lb/ft 3 )(10ft 3 ) B = (63.2 lb/ft 3 )(10ft 3 ) B = 623 lb 13
Displacer Not as commonly used as the float gage Used on a variety of products Important to seal the process from the displacer mechanism The seal must be frictionless and remain useful over a wide range of pressures, temperatures, and corrosion conditions Intrusive measurement so contamination is a problem Low upfront cost 14
Differential Pressure (DP) DP measurement, hydrostatic, is based on the height of the liquid head Pressure sensors determine the level by measuring the force exerted by the product in the vessel Pressure at the point of measurement equals height of liquid column above measurement point times the specific gravity of the liquid P = h( ) => h = P/ P = pressure h = liquid head height = liquid specific gravity The specific gravity must remain constant Non-intrusive measurement Low upfront cost 15
Differential Pressure (DP) Open Tank DP measurement uses atmospheric pressure as the reference The pressure sensor is mounted in contact with the process fluid in the bottom of the tank A pressure transmitter may be used 16
Differential Pressure (DP) Pressurized Tank DP measurement, hydrostatic, is based on the height of the liquid head A differential pressure transmitter must be used The head pressure is subtracted from the pressure at the tank bottom The square root of the resultant is proportional to the level 17
Bubbler Level Measurement The dip tube extends to approximately 3 above the bottom of the tank Air flow is set approximately 2ft 3 /hr The pressure required to force bubbles from the bottom of the tube is the liquid head above the end of the tube Easy to install Contamination problems result from intrusive measurement and introduction of plant air Maintenance required to prevent plugging Low upfront cost Commonly used to measure wastewater 18
Diaphragm Level Measurement Diaphragm detectors operate on the principle of detecting the pressure exerted by the process material against a diaphragm As the level rises above the diaphragm, the liquid head pressure compresses the captive air inside The static head exerts an upward pressure on the diaphragm The pressure is sensed by a differential pressure element May be intrusive or non-intrusive measurement 19
Capacitance Level Measurement A capacitance level transmitter as much like a capacitor The metal rod acts as one of the capacitor plates and the tank wall as the other The liquid product acts as a dielectric insulator between two capacitance plates The vessel is essentially turned into a variable capacitor Measured capacitance is proportional to liquid level Intrusive measurement so contamination is a problem 20
Capacitance Point and continuous level sensing Easy to install Compatible with a wide range of product types Very good for applications consisting of layers of conductive and non-conductive liquids Can be difficult to calibrate Some versions require temperature compensation Moderate upfront cost 21
Resistance Level Measurement A resistance tape is spiral wound around a steel tape The pressure of the fluid in the tank causes the tape to be short circuited, thus changing the total resistance of the measuring tape The resistance is proportional to the fluid height Will handle corrosive liquids and slurries Susceptible to moisture getting inside tape Affected by specific gravity changes Not very accurate Not very rugged Not for flammable atmospheres Intrusive measurement Low upfront cost 22
Switching Resistance Level Measurement A set of resistors and magnetically actuated solid state switches are encased in a pipe The pipe is encircled by a magnetic collar that floats on a process liquid The collar activates the switches selecting the resistors that generate a signal proportional to the fluid level The signal is in discrete steps defined by the incremental value of the resistors resulting in limited accuracy Intrusive measurement so contamination is a problem Little maintenance Low upfront cost 23
Conductance Level Measurement Conductivity probes require a liquid that conducts electricity When the probe contacts the liquid, a conductivity path is established to ground The ground is usually the grounded tank For an insulated tank, another probe connected to ground is used These probes are for level switches only, no continuous measurements Intrusive measurement so contamination is a problem Low upfront cost 24
Thermal Level Measurement A heating element next to a temperature switch When the liquid rises above the switch, the liquid dissipates the heat and the temperature switch activates No moving parts, sensitive, simple and reliable design Sensitive to coating, fragile, and provides point measurement only Not used where heating will affect product quality Intrusive measurement so contamination is a problem Low upfront cost 25
Ultrasonic Level Measurement A transceiver generates an electrical signal that is converted into a sound wave that is directed toward the process surface The reflection (echo) from the surface is picked up by the receiver The time difference between the transmitted and received signal is directly proportional to the distance, thus the level This is commonly referred as the time of flight (ToF) method The sensors are temperature compensated to give high level repeatability for distances of 2 to 230 feet Ultrasonic has poor accuracy in vapor, high dust, surface foam, and high temperature atmospheres. Good accuracy for highly viscous fluids Ultrasonic will not work in a vacuum. 26
Ultrasonic No moving parts Non contacting measurements Easily calibrated Easy to install Subject to influence from product atmosphere Sensor surface coating by product contamination Non-intrusive measurement Moderate upfront cost 27
Free Space Radar Level Measurement A transceiver generates a radar signal that is directed toward the process surface The reflection (echo) from the surface is picked up by the receiving device The time difference (ToF) between the transmitted and received signal is directly proportional to the measured distance 28
Free Space Radar No moving parts Non contacting measurement Easily calibrated Easy to install Can measure level in a vacuum Not influenced by product vapors or dust Not sensitive to temperature or pressure variations Sensitive to sensor surface coating by product contamination Does not handle surface foam well Non-intrusive measurement Moderate upfront cost 29
Guided Wave Radar Level Measurement Guided Wave Radar, or time domain reflectometry (TDF), works very similar to a Free Space Radar The main difference is the addition of a cable or rod from the radar unit to guide and focus the radar signal The radar signal is very concentrated on the cable or rod The point at which the cable or rod contacts the surface gives the level When the cable or rod encounters foam, it does a better job of going through the foam to reflect off the liquid surface 30
Guided Wave Radar No moving parts Non contacting measurement Easily calibrated Easy to install Sensor surface coating by product contamination Intrusive measurement Part of measurement instrument no adverse effects Directed beam for precise measurements Good for solids level measurements Not influenced by product vapors, dust, or foam Moderate upfront cost 31
Free Space Radar / Guided Wave Radar Comparison 32
Nuclear Gamma Gauge Low level gamma energy is beamed in a precise shape through the walls of a vessel Process level blocks energy transmission The ionization chamber detector measures the energy received on the other side of the tank The higher the level, less radiation is received Wide range of products including liquids, bulk solids, and slurries 33
Nuclear Gamma Gauge No moving parts Non contacting measurements for difficult applications Easily calibrated Easy to install Nuclear source presents safety concerns The main disadvantages are the regulations and licensing requirements High installed cost due to nuclear certification Non-intrusive measurement High upfront cost 34
Solids Level Measurement Single point continuous measurement Solids may have voids Angle of repose Rounded surface Ratholing Bridging Buildup on vessel sides Clumping Impossible to eliminate voids Agitation or vibration (compaction) to fill voids Compaction - large clumping Agitation - difficult to measure level 35
Tuning Fork - Vibration Single point continuous measurement Tuning fork shaped sensing element with two tines, consisting of two Piezoelectric elements, are inserted into the target material. Detects the change in harmonic vibration frequency of the sensing element as the result of the presence of the target material An excellent solution for detecting the presence of solids Intrusive measurement Care must be taken as solid product can become lodged between the two probes 36
Rotary Paddle Switch Bindicator is invasive A low power synchronous motor keeps the paddle in motion When a solid reaches the paddle, the low torque motor stalls Detection of the torque determines the state of the switch Used for point detection, no continuous level detection Low upfront cost 37
Level Measurement Lab Continuous level measurement Point Switch 38