level sensors 101

2007-Chem-101 Level Sensors

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Assignment Report

Report Title: Level Sensors

Subject: Industrial Instrumentation

Submitted By:

2007-Chem-101

Submitted To:

Dr. Naveed Ramzan

University of Engineering & Technology,

Lahore

www.engineering-resource.com


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Abstract

A wide variety of level measurement systems are available to address the broad spectrum of applications, accuracy needs, installation requirements, and practices. Measurement technologies are made available in different versions to address a wide range of measurement needs or sometimes to address just one specific application. This subsection will attempt to define some of the general selection considerations of many available technologies, the general forms of these technologies, and some of their general advantages and disadvantages. As always, one must consult the specifications from the various manufacturers for specific products and users experiences in different installations to truly determine their applicability to measurement situations. The family of level measurement systems can be divided into many categories: liquids or solids level measurement, point or continuous level measurement, electromechanical or electrical/electromagnetic level measurement, or contacting or noncontacting /nonintrusive level measurement.



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Table of Contents Introduction ............................................................................................................................................ 4

Classification ........................................................................................................................................... 4

Types of Sensors ..................................................................................................................................... 5

Differential Pressure ........................................................................................................................... 5

Displacement ...................................................................................................................................... 6

Float Level Sensors .............................................................................................................................. 7

Ultrasonic / Sonic ................................................................................................................................ 8

Weight and Cable ................................................................................................................................ 9

Sight Glass ........................................................................................................................................... 9

Radioactive (Nuclear) ........................................................................................................................ 10

Bubbler .............................................................................................................................................. 11

Vibration ........................................................................................................................................... 12

Rotating Paddle Level Sensors .......................................................................................................... 13

Diaphragm ......................................................................................................................................... 13

Resistance Tape ................................................................................................................................ 14

Hook- Type Level Sensor ................................................................................................................... 16

Level Measurement Sensor Selection ................................................................................................... 16

References ............................................................................................................................................ 17



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Introduction Level measurement is defined as the measurement of the position of an interface between two media. These media are typically gas and liquid, but they also could be two liquids. Level sensors detect the level of substances that flow, including liquids, slurries, granular materials, and powders. All such substances flow to become essentially level in their containers (or other physical boundaries) because of gravity. The substance to be measured can be inside a container or can be in its natural form (e.g. a river or a lake). The level measurement can be either continuous or point values. Continuous level sensors measure level within a specified range and determine the exact amount of substance in a certain place, while point-level sensors only indicate whether the substance is above or below the sensing point. Generally the latter detect levels that are excessively high or low. There are many physical and application variables that affect the selection of the optimal level monitoring method for industrial and commercial processes. The selection criteria include the physical: phase (liquid, solid or slurry), temperature, pressure or vacuum, chemistry, dielectric constant of medium, density (specific gravity) of medium, agitation, acoustical or electrical noise, vibration, mechanical shock, tank or bin size and shape. Also important are the application constraints: price, accuracy, appearance, response rate, ease of calibration or programming, physical size and mounting of the instrument, monitoring or control of continuous or discrete (point) levels.

Classification Level devices operate under different principles. They can be classified into three main categories that measure the position (height) of the surface. the pressure head. the weight of the material through load cells.



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Types of Sensors Electromechanical level measurement and detection systems Floats for level detection and measurement of liquids Displacers for level detection and measurement of liquids Level detection of solids using rotating paddles Level measurement of liquids and solids using plumb bob Electronic/electromagnetic energy level measurement and detection systems Level detection of liquids by use of conductivity Level detection of liquids by use of vibrating forks resonance or rod attenuation Level detection of solids by use of vibrating fork or rod attenuation Level detection of liquids by use of ultrasonic gap Level detection of liquids by use of thermodispersion Level measurement of liquids by use of bubblers Level measurement of liquids by use of hydrostatic pressure Ultrasonic level detection and measurement of liquids and solids Capacitance level detection and measurement of liquids and solids Radar level detection and measurement of liquids and solids Level detection and measurement of liquids and solids by use of time-domain reflectometry Level measurement of liquids by use of magnetostrictive Level measurement of liquids by use of laser Level detection and measurement of liquids and solids by use of radiometric Level measurement of liquids and solids by use of weighing Level detection by use of optics Level detection in liquids by use of ultrasonic tank resonance [7]

Differential Pressure Differential-pressure level measurement, also known as hydrostatic, is based on the height of the liquid head. Level measurement in open tanks is based on the formula that the pressure head is equal to the liquid height above the tap multiplied by the specific gravity of the fluid being measured. In closed tanks, the true level is equal to the pressure measured at the tank bottom minus the static pressure above the liquid surface. To compensate for that static pressure, a leg is connected from the tank top to the low side of the differential pressure transmitter . Two options are available: dry leg and wet leg. In dry leg applications, it is expected that the low side will remain empty (i.e., no condensation). [1] If condensation takes place, an error will occur because a pressure head will be created on the low side. This error is avoided by intentionally filling the low side with a liquidhence the term wet leg. Where filled systems (with diaphragm seals) are used between the transmitter and the tank, calibration of the transmitter should allow for the specific gravity of the fill fluid. The user should refer to the vendors instructions when setting the zero and span values. [1] Advantages/Disadvantages Differential-pressure level measuring devices are easy to install and have a wide range of applications. With proper modifications, such as extended diaphragm seals and flange connections, these instruments will handle hard-to-measure fluids (e.g., viscous, slurries, corrosive, hot). They are simple and accurate. Calibration of differential-pressure measuring



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devices is simple. Adjustments to zero, elevation/suppression, and span are easy, and no special tools are required. Differential-pressure measuring devices are affected by changes in density. They can only be used for liquids with fixed specific gravity. Changes in liquid density due to changes in temperature will introduce errors. Differential-pressure devices are susceptible to dirt or scale entering the tubing (in small process connections), which can easily plug them. Parts of the instrument are exposed to the process fluid, while the outside leg is susceptible to freezing. [1]

Figure 1: Differential Pressure

Displacement A displacer , which can be either partially or totally immersed, is restricted from moving freely with the liquid level. It transmits its change in buoyancy (mechanical force) to a transducer through a torque-tube unit. Sometimes the term float is used instead of displacer. [1] Advantages/Disadvantages Displacers are simple, dependable, and accurate. They may be mounted internally or externally. These level measurement can only be used for liquids with fixed specific gravity, where accuracy is not required. A suitable drain is provided at the low point and a vent valve at the highest point. Displacers are difficult to calibrate and have many mechanical components. Therefore, displacer, linkages, or levers should be free to move. Boiling liquid may cause violent agitation at the liquid surface, so stilling wells may be required where turbulence exists. the accuracy is also affected by coating, buildup, or dirt. [1]



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Figure 2: Displacer

Float Level Sensors The principle behind magnetic, mechanical, cable, and other float level sensors involves the opening or closing of a mechanical switch, either through direct contact with the switch, or magnetic operation of a reed. With magnetically actuated float sensors, switching occurs when a permanent magnet sealed inside a float rises or falls to the actuation level. With a mechanically actuated float, switching occurs as a result of the movement of a float against a miniature (micro) switch. For both magnetic and mechanical float level sensors, chemical compatibility, temperature, specific gravity (density), buoyancy, and viscosity affect the selection of the stem and the float. For example, larger floats may be used with liquids with specific gravities as low as 0.5 while still maintaining buoyancy. The choice of float material is also influenced by temperature-induced changes in specific gravity and viscosity - changes that directly affect buoyancy. [2] Float-type sensors can be designed so that a shield protects the float itself from turbulence and wave motion. Float sensors operate well in a wide variety of liquids, including corrosives. When used for organic solvents, however, one will need to verify that these liquids are chemically compatible with the materials used to construct the sensor. Float-style sensors should not be used with high viscosity (thick) liquids, sludge or liquids that adhere to the stem or floats, or materials that contain contaminants such as metal chips; other sensing technologies are better suited for these applications. [2] A special application of float type sensors is the determination of interface level in oil-water separation systems. Two floats can be used with each float sized to match the specific gravity of the oil on one hand, and the water on the other. Another special application of a stem type float switch is the installation of temperature or pressure sensors to create a multi-parameter sensor. Magnetic float switches are popular for simplicity, dependability and low cost. [2] Advantages/Disadvantages Floats work well with clean liquids and are accurate and adaptable to wide variations in fluid densities. Once commissioned, however, the process fluid measured must maintain its density if repeatability is required. Float Switches are available and are capable of fail safe operation in extreme process conditions. [2]



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Floats are affected by changes in product density since the displacement of the body is equal to the weight of the fluid displaced. If the specific gravity changes, then the weight of the displaced material changes, thus changing the calibration. This is especially problematic in interface measurements, where both liquids increase or decrease density, while the signal is proportional to the density difference.[2]

Figure 3: Float

Ultrasonic / Sonic Ultrasonic transmitters work on the principle of sending a sound wave from a peizo electric transducer to the contents of the vessel. The device measures the length of time it takes for the reflected sound wave to return to the transducer. A successful measurement depends on reflection from the process material in a straight line back to the transducer. [3] Advantages/ Disadvantages The main advantages of ultrasonic level instrumentation are that the transducer does not come into contact with the process material, they have no moving parts and a single top of vessel entry makes leaks less probable than fully wetted techniques.[3] There are various influences that affect the return signal. Things such as powders, heavy vapors, surface turbulence, foam and even ambient noise can affect the returning signal. Temperature can also be a limiting factor in many process applications. Ultrasonic devices will not operate on vacuum or high pressure applications. [3]

Figure 4 Sonic/Ultrasonic



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Weight and Cable With the weight and cable device , a cable or tape is attached to a weight that descends into the tank. This motion is activated by a timer. When the weight makes contact with the surface of the material, the motor automatically reverses direction and retrieves the weight at about 1 ft/s (0.3 m/s). During descent, pulses are generated and displayed on a counting unit, which indicates either material stored or available filling capacity. [1] Advantages/Disadvantages Weight and cables are accurate devices, and they are only momentarily in contact with the process material prevents product from building up on the weight. They can have mechanical problems, such as hang-up and friction. They must be activated in order to measure, and they have no signal transmission capability. In outdoor use, measures should be taken to protect the mechanical parts of the level measuring instruments from possible weather interference. Stilling wells are often used if the vessel is agitated. [1]

Figure 5: Weight and Cable

Sight Glass A sight glass or water gauge is a transparent tube through which the operator of a tank or boiler can observe the level of liquid contained within. Simple sight glasses may be just a plastic or glass tube connected to the bottom of the tank at one end and the top of the tank at the other. The level of liquid in the sight glass will be the same as the level of liquid in the tank. Today, however, sophisticated float switches have replaced sight glasses in many such applications.[4] Advantages/Disadvantages Gages are used as a local indicator for open or pressurized vessels. They must be accessible and located within visual range. Gages are cheap and provide direct-reading measurement. However, they are not suitable for dark liquids and dirty fluids will prevent the liquid level from being viewed. They can be easily damaged or broken. Glass gages should not be used to measure hazardous liquids. Reflex gages are used for low- and medium-pressure applications. For high-pressure applications, or where the fluid is toxic, magnetic-type armored gages are used. When installing such devices, good lighting is



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required. In installations where the gage is at a lower temperature than the process, condensation may occur on the walls, making the reading difficult. [1]

Figure 6: Sight Glass

Radioactive (Nuclear) With the radioactive (nuclear) device , a radioactive source radiates through the vessel. The gamma quantum is seen by the radiation detector (such as a Geiger counter) and is transformed into a signal. When the vessel is empty, the count rate is high. The radioactive source holder is designed to direct a collimated beam of radiation toward the tank and to be shielded in all other directions so as to reduce the radiation levels to below the legal limit. The strength of the sensed radiation depends on the thickness of the vessel wall, the distance between the source and detector, and the density and thickness of the measured material. The radiation source generally has a half-life of 30 years; therefore, corrections for source decay are rarely required. [1] Advantages/Disadvantages Radioactive level measurement is external device. It can be added or removed without disturbing the process. Radioactive (nuclear) devices are highly reliable, non-contacting devices with no moving parts. They are unaffected by temperature, pressure, and corrosion, and their mode of failure is limited and predictable. Radioactive (nuclear) devices require special engineering and licensing for the application they are used with, and extreme care is required when locating and installing the radioactive source. Operator exposure to radiation must be minimized, and therefore, plants may need shielding lead plates at the source or detector. Radioactive (nuclear) units are expensive to install. They are expensive and they are difficult to calibrate. On vessels larger than 30 ft (10 m) in diameter or on vessels with extremely thick walls, the source may have to be suspended vertically inside the vessel. [1]



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Figure 7: Radioactive

Bubbler In a bubbler , a small amount of air (or inert gas) purge flows through a dip tube in the vessel. Sometimes, to provide rigidity, a stand pipe is used instead of a dip tube. The dip tube (or pipe) generally extends to about 3 inches (75 mm) from the bottom of the tank and is notched to keep the size of the air bubble small. The pressure that is required to force air bubbles from the bottom of the tube is the liquid head above the end of the tube. A purge meter, which consists of a rota meter with a needle valve, is required to provide a constant airflow of about 0.2 to 2.0 scfh (0.005 to 0.05 m3/hr). A pressure regulator located upstream of the purge meter provides a smooth operation. In plants where remote level indication is required, the high-pressure side of the differential-pressure transmitter measures the tube pressure, and the low side measures the vessels top pressure, if it is not vented to the atmosphere. [1] Advantages/Disadvantages The bubbler offers low cost and easy maintenance. It can be operate without electrical power. It can be used on pressurized or unpressurized vessels. Variations in density will affect the bubblers reading. Bubblers can become coated or plugged by process fluid residue or dirt. The cost of purging fluid is ongoing, and the purge gas can introduce unwanted components into the process. The materials of construction for the bubbler must be compatible with the process it is used in, and the bubblers dip tube installation must be capable of withstanding the maximum air pressure that blockage causes. A tee piece at the top of the dip tube (or pipe) may be required to enable rodding. [1]



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Figure 8: Bubbler

Vibration Vibration devices consist of a tuning fork that vibrates at its natural resonant frequency by a piezoelectric crystal, which is located at the base of the probe. When the vibrating fork contacts a material, either dry or in suspension (20% minimum), the vibration frequency is altered, which switches a relay. The material needs to have a bulk density of 0.9 lb/ft (12.8 kg/m) or greater. When the level drops below the fork, the vibrating frequency is again in effect, and the relay is reversed. [1] Advantages/Disadvantages Vibration units have no moving parts, are rugged and reliable, are good for low-density materials, and require little maintenance. They cannot be used in vibrating bins, especially if the two frequencies are close. Product buildup will affect the performance of vibration units, the switch setting cannot be readily changed, and vibration units typically require protection from materials that are charged from the top. [1]

Figure 9: Vibration



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Rotating Paddle Level Sensors Rotating paddle level sensors are a very old and established technique for bulk solid point level indication. The technique requires a low speed gear motor that rotates a paddle wheel. When the paddle is stalled by solid materials, the motor is rotated on its shaft by its own torque until a flange mounted on the motor contacts a mechanical switch. The paddle can be constructed from a variety of materials, but tacky material must not be allowed to build up on the paddle. Build up may occur if the process material becomes tacky because of high moisture levels or high ambient humidity in the hopper. For materials with very low bulk densities (very low weight per unit volume) such as Pearlite, Bentonite or fly ash, the weight of the material is insufficient to stop the paddle. For such difficult applications, special paddle designs and the use of lower-torque motors can be employed. Fine particles or dust must be prevented from penetrating the shaft bearings and motor by proper placement of the paddle in the hopper or bin and using appropriate sealing technology.[3] Advantages/Disadvantages A paddle wheel is inexpensive, simple, and reliable. It is susceptible to shock, vibration, and damage by falling material. Paddle wheels generally require some protection(e.g., a protective baffle) from material charging from the top. Hang-ups or material buildup on the paddle will affect the device performance.

Figure 10: Paddle Wheel

Diaphragm The diaphragm is a point measurement device. The process materials (or hydrostatic pressure) apply pressure on a diaphragm, which in turn actuates a switch. [1] Advantages/Disadvantages The diaphragm is reliable, easy to maintain. Coating may affect the flexing of the diaphragm, and abrasive material may affect its performance. The accuracy of the unit is affected by changes in specific gravity. The diaphragm must be in contact with the material. It should be at least 2 to 3 in. (50 to 75mm.) above any sediment in the vessel bottom to prevent dirt from building up at the diaphragm. [1]



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Figure 11: Diaphragm

Resistance Tape The resistance-tape material level sensor with which the invention is employed is sold commercially and is known as a "Metritape" sensor, The Metritape sensor comprises an elongated metallic base strip having electrical insulation on the edges and back of the strip to define an un-insulated zone along the length of the base strip, and a resistance wire or ribbon helically wound around the insulated base strip, with the helical turns bridging the insulated edge portions being spaced from the underlying un-insulated zone of the base strip. This sensor structure is enclosed within a continuous polymeric or other protective sleeve to provide a clean and dry inner chamber for the sensor. The sensor is disposed within a tank or vessel containing the liquid or fluent material, the level of which is to be monitored. The pressure of the material surrounding the immersed sensor causes the deflection of the enclosing sleeve and helical turns in the immersed portion of the sensor into engagement and electrical contact with the underlying base strip, such that an electrical resistance proportional to material level is provided.[5] Advantages/Disadvantages A resistance tape will handle corrosive liquids and slurries. It must contact the material and is susceptible to moisture getting inside the tape. Resistance tape devices are affected by changes in specific gravity, are not suitable for flammable atmospheres, and are neither accurate nor rugged. They require careful engineering and careful installation. Plants may need to use stilling if turbulence exists. [1]



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Figure 12: Resistance tape

Laser There are two types of laser measurement pulsed and continuous wave (frequency modulated). In industrial applications, the pulsed-type is the most common because of its range and ability to penetrate through vapors and dust. The pulsed-type laser operates as follows: its transmitter emits a continuous series of pulses at a target. The time taken by each pulse to travel from the transmitter to the target (e.g., the liquid surface) and back is measured and converted into distance. The continuous wave laser consists of a transmitter that emits a continuous laser beam at the target. When the beam hits the target, phase-shifting occurs. Based on the degree of phase shift and on other constant parameters such as wave frequency, the device determines the distance of the target and therefore level. [1] Advantages/Disadvantages Laser transducers mounted outside a metal vessel can measure level through a process-rated sight glass. This means the laser unit can be accessed without having to interrupt the process. Laser-type level measurement uses an extremely short wavelength and produces a very narrow beam. These features provide very good accuracy and non-contact measurement for difficult applications. Lasers are relatively expensive, though still better then radioactive (nuclear) types.[1]

Figure 13: Laser



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Hook- Type Level Sensor Hook- Type level Indicator consists of a wire of corrosion resistance alloy bent into U-shaped with one arm longer than the other. The shorter arm is pointed with 60 degrees while the longer is attached to a slider on a Vernier scale, which moves over the main scale and shows the reading. [6] In this type of sensor the hook is pushed below the surface of the liquid whose level is to be measured and gradually raised until the point is just about to break through the surface. This principle is also utilized in measuring point manometer in which measuring point consists of a steel point fixed with the point upward underneath the water surface. [6]

Level Measurement Sensor Selection General considerations in level measurement technology selection

Density and viscosity Chemical composition Ambient temperature Process temperature Process pressure Regulated environments Process agitation Vapor, mist, and dust Interfaces and gradients Process conductivity and dielectric constants Vibration Material buildup or stickiness Static charge Humidity/moisture Repeatability, stability and accuracy requirements



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References 1. Battikha, N. E., The Condensed Handbook of Measurement and Control; Publisher

ISA: The Instrumentation, Systems, and Automation Society (2006); 3rd Ed., pg. 99-

121

2. http://www.iceweb.com.au/technical.html (retrieved on 05-11-2010)

3. http://www.iceweb.com.au/Level/LevelWeb.htm (retrieved on 05-11-2010)

4. http://en.wikipedia.org/wiki/Sight_glass#cite_note-bell (retrieved on 06-11-2010)

5. Edwin P. Dews, N.H. William.E.Pierce, D. Ehrenfried, (1990), United States Patent, p

6

6. Singh, S. K., Industrial Instrumentation and Control; McGraw Hill Publishing

Company Limited; 2nd edition, pg. 225; 226

7. McMillan, Gregory K. , Process/Industrial Instruments and Controls Handbook;

McGraw Hill Publishing Company Ltd; 5th Ed. , pg. 155 Section 4


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