Level MEASUREMENT 1/2016

Transcription

1 Level MEASUREMENT 1/2016

2 AGENDA 2 A. Introduction B. Float method C. Displacer method D. Hydrostatic pressure method E. Capacitance method G. Ultrasonic method H. Radar method I. Laser method J. Level detections

3 A. Introduction 3 Level measurement technologies are made available in different versions to address a wide range of measurement needs or sometimes to address just one specific application. The family of level measurement systems can be divided into 3 main groups as follows: 1. Continuous Liquid Level Measurement and Control

4 A. Introduction 4 2. Point Liquid Level Detection 3. Solids & Dry Products Level Capability

5 A. Introduction 5 2. Point Liquid Level Detection 3. Solids & Dry Products Level Capability

6 A. Introduction 6 General considerations in level measurement technology selection Density and viscosity Chemical composition Ambient temperature Process temperature Process pressure Vapor, mist, and dust Process conductivity and dielectric constant Vibration Humidity/moisture Repeatability and accuracy requirement Cost

7 B. Float Methods 7 The level gauge consists of a float chamber, a float, and an external indication device. The float chamber is basically a column with process connections to match those of the storage tank, reactor, drum, column or other equipment where level is to be measured. These connections may be side couplings or flanges, or top and bottom flanges.

8 C. Displacers 8 Displacer level instruments exploit Archimedes Principle to detect liquid level by continuously measuring the weight of a rod immersed in the process liquid. As liquid level increases, the displacer rod experiences a greater buoyant force, making it appear lighter to the sensing instrument, which interprets the loss of weight as an increase in level and transmits a proportional output signal. In physics, buoyancy is an upward force exerted by a liquid, gas or other fluid, that opposes the weight of an immersed object.

9 C. Displacers 9 The displacement method is based on the difference between the weight of the displacement body and the upward force exerted by the medium on this body (buoyancy force). The upward force depends on the volume of displacement body, the relative density of medium, and the level of medium. For a given volumes and relative density, the upward force will depend on only the level of the medium. Displacers 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. Mounting may be either directly into a vessel or externally mounted in a chamber.

10 D. Hydrostatic pressure methods 10 D.1 Bubbler The bubbler system supplies a constant rate of air flow through a small diameter tube anchored near the bottom of the tank. The amount of pressure required to force the air bubble out of the bottom of the tube is equal to the hydrostatic pressure at that point (i.e. the deepest point in the tank). This is calculated using the formula Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity usually means relative density with respect to water.

11 D. Hydrostatic pressure methods 11 The higher level, the higher pressure. Simplicity of design and low initial purchase cost are frequently given as advantages of bubblers. The system consists of a pipe, an air supply, a pressure transmitter and a differential pressure regulator. The regulator produces the constant gas flow required to prevent calibration changes.

12 D. Hydrostatic pressure methods 12 d/p cell Close tank Among the level measurement methods, the measure based on differential pressure(dp) has become the most popular type. A DP is used to transmit the head pressure that the diaphragm senses due to the height of the material in the vessel multiplied by a density variable. The primary benefit of DP s is that it can be externally installed or retrofitted to an existing vessel. It can also be isolated safely from the process using block valves for maintenance and testing. There are certain measurements such as total level in separator vessels that due to wide variations in material composition of the upper phase DP is the only viable if not ideal option.

13 D. Hydrostatic pressure methods 13 DP transmitters are subject to errors due to changes in liquid density. Density variations are caused by temperature changes or change of product. Fluid density must be stable if readings are to be accurate. If liquid density is subject to change a second d/p transmitter is required to measure density and then used to. It should be noted that the change in specific gravity will affect the accuracy of the measurement using d/p cell.

14 D. Hydrostatic pressure methods 14

15 D. Hydrostatic pressure methods 15

16 D. Hydrostatic pressure methods 16

17 D. Hydrostatic pressure methods Example: If P = 0 20 psi, P = 0 => O/P 4 ma and P = 20 => O/P 20 ma 17 At the minimum level=> O/P 0 % and at maximum level=> O/P 100% Assume liquid in the tank has a specific gravity of G L and liquid in the tube has a specific gravity of G s. Max level P H = G s Z+G L (Y+X)+P a, P L = P a X Y Z P max = G s Z+G L (Y+X) Span = XG L, Elevation = YG L +ZG S d/p cell Min level P H = G s Z+G L Y+P a, P min = G s Z+G L Y P L = P a

18 D. Hydrostatic pressure methods 18

19 E. Capacitance method 19 Capacitance is the ratio of the electric charge on one of a pair of conductors to the potential difference between the conductors. A capacitance level probe determines the level of liquid in a column or receiver by measuring the combined capacitance of the liquid and gas (vapor) in the column. RF level measurement

20 Non-conductive material For non-metallic tank or horizontal cylindrical tank 20

21 Conductive material 21

22 E. Capacitance method 22 A. Introduction As the liquid level rises in the column, the total capacitance value increases. (The capacitance of vapor is very small compared to the capacitance of the liquid.) This increase is measured by the controlling electronic system and an output control signal is created.

23 G. Ultrasonic method Ultrasonic level measurement devices basically employ sound waves for detection of liquid level. They usually work over the frequency range between 20 khz to 200 khz. 23 Ultrasonic level measurement method is based on the fact that sound through a medium with a know propagation speed, depending on the density and the temperature of that medium. The pulse is generated and then travels through the medium (typically air). When the pulse hits the surface of material, it is reflected back to transducer to be measure. The distance to the level surface and level height can be calculated from the reflection time and the speed of sound wave.

24 G. Ultrasonic method 24

25 G. Ultrasonic method 25 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. 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.

26 H. Radar method The operation of all radar level detectors involves sending microwave beams emitted by a sensor to the surface of liquid in a tank. The electromagnetic waves after hitting the fluids surface returns back to the sensor which is mounted at the top of the tank or vessel. The time taken by the signal to return back i.e. time of flight (TOF) is then determined to measure the level of fluid in the tank. 26 This non-contact technology produces highly accurate measurements in storage tanks and some process vessels. Radar is an excellent, but fairly expensive technology for continuous level measurements. 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. It s primary disadvantage is cost, which can be justified for tank gauging and inventory control. The pressure ratings on radar antenna are limited and these devices cannot measure interfaces.

27 H. Radar method 27 Radar is becoming a rapidly important method for measuring the level of liquids and some case of solids. The two technologies on the market are frequency modulated continuous wave (FMCW) or pulsed wave time of flight. Pulsed Wave systems emit a microwave burst towards the process material, this burst is reflected by the surface of the material and detected by the same sensor which now acts as a receiver. Level is inferred from the time of flight (transmission to reception) of the microwave signal. FMCW systems, however, continuously emit a swept frequency signal and distance is inferred from the difference in frequency between the transmit and receive signals at any point in time.

28 H. Radar method 28

29 I. Laser method 29 Advances in optical technology have reduced the cost of laser engines to the point where it has become practical to use infrared lasers as level measurement devices. The LASERMETER measures the time it takes for a laser pulse to travel from the instrument to a target and back. The distance to the target is calculated from this time. In some cases, they have become competitive in the same range of applications as microwave level. They are still susceptible to attenuation of the beam by vapor, and other forms of beam scattering. They have been tried on granulars and solids, with wildly varying results. Disadvantages: cost, dust, dirt, maintenance

30 I. Laser method 30

31 I. Laser method 31

32 Magnetostrictive Level Sensor 32 Inside the probe tube there is a rigid wire made of magnetostrictive material. The sensor circuitry emits pulses of current through the wire, generating a circular magnetic field. The level transmitter is a magnet, which is integrated into the float. Its magnetic field magnetizes the wire axially. Since the two magnetic fields are superimposed, around the float magnet a torsion wave is generated which runs in both directions along the wire. One wave runs directly to the probe head while the other is reflected at the bottom of the probe tube. The time is measured between emission of the current pulse and arrival of the wave at the probe head. The position of the float is determined on the basis of the transit times.

33 Magnetostrictive Level Sensor 33 Automatic tank gauging(underground oil tank)

34 J. Level detections 34 J.1 Vibrating level switch The piezo electrically stimulated probe vibrates at its natural resonance frequency. If the bulk material covers the probe, the damping thus generated is registered electronically and a corresponding signal output is actuated.

35 J. Level detections 35 J.2 Level paddle switch Level is detected by the change in inertia of a rotating paddle depending on whether the paddle is in the air or in contact with the product. The location should be selected such that the product to be monitored is allowed to freely flow both into and away from the rotating paddle. However, the paddle should not be placed directly under the free-falling path of the product.

36 J. Level detections 36 J.4 Level conductivity switch This method is suitable only for level measurement in conductive liquids. The difference in the conductivity of partially insulated electrode is measured when the probe is covered and not covered with the conductive product. The advantages of this method are simple, inexpensive and suitable for dual or multiple point control. The disadvantages are probe can not become contaminated with grease or other deposits and has limited suitability for products of varying conductivity.

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