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Level Switches by : Muhammad Jahangir Iqbal khan 07-Chem-34.
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Muhammad Jahangir Iqbal khan
Level is measured at the position of the interface between phases, where the phases are liquid/gas, solid/gas, or immiscible liquid/liquid and after reaching certain level,switch turns the equipment ON & OFF. Level is simply a measure of height.
Direct methods employ physical properties such as fluid motion and buoyancy, as well as optical, thermal, and electrical properties. Direct level measurement does not require compensation for changes in level caused by changes in temperature. Direct level measurements show the actual level of the interface.
Indirect level measurement involves converting measurements of some other quantity, such as pressure to level by determining how much pressure is exerted over a given area at a specific measuring point, the height of the substance above that measuring point can also be determined.
A dipstick used to measure the amount of fuel remaining in a tank. Dipsticks can also be used to measure the quantity of liquid in an otherwise inaccessible space, by inserting and removing the stick and then checking the extent of it covered by the liquid.
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
Rotating Paddle Level Sensors
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.
Float Level Sensor
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
Ultrasonic / Sonic
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.
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.
Hook- Type Level
Displacer level detector works on the Archimedes Principle which states, “A body wholly or partially immersed in fluid is buoyed up a force which is equal to weight of the fluid displaced”. By detecting the apparent weight of immersed displacer the level of the fluid is measured
Displacer Level Sensor
High accuracyReliable for clean liquidsCan be mounted internally or externallyExternally mounted can be disconnect from main unit for maintenanceAdoptable to liquid interface measurement
These have limited ranges and devices exceeding 1.2m in length are difficult to measureCost of these increases appreciably for externally mounted units as pressure increasesExternal units may require heating to avoid freezingExternal units may be in error due to temperature difference between vessel fluid and level chamber fluid
Key questions to ask before selecting a level measurement sensor: 1. Are you measuring a liquid or solid? 2. What is the application’s temperature and pressure ranges? 3. Is point level or continuous measurement required? 4. What level measurement range do you need?
5. Is the measured material electrically conductive? 6. Will the material coat or build up on surfaces? 7. Does turbulence, foam, or vapor occur at the surface of the liquid? 8. Will you need contact or non-contact level measurement? 9. What kind of output do you need–analog, relay, digital display, etc.?
A switch is an electrical device usually electromechanical used to control continuity between two points
Hand switches: Hand Switches are actuated by human touch.
Process Switches: Process Switches are actuated by changes in some physical process(temperature ,level , flow etc.)
Limit Swiches:Limit Switches are actuated by machine motion.
A floating object can be used to actuate a switch mechanism when the liquid level in an tank rises past a certain point.
Both source and detector are outside of the vessel, with no intrusion at all except the radiation flux itself. The radioactive sources used are fairly weak and pose no immediate health threat to operations or maintenance personnel.
All float operated liquid level controls operate on the basic buoyancy principle which states "the buoyancy force action on an object is equal to the mass of liquid displaced by the object."
Sensor:Tell us the value
Switch: Turns something ON or OFF
Transducer: A sensor that can send a signal
The many level technologies available in the market place today all work. If they did not work, companies would not be able to stay in business manufacturing them, and other companies would not be buying them. The important thing to remember is that all technologies work when applied properly. Special consideration must always be given to the principle of operation, and where the limitations of each technology lie. The correct selection of a technology and then the subsequent correct application of the technology will make the level measurement a successful one.