Industrial Instrumentation

Industrial Instrumentation
Dr. –Ing. Naveed Ramzan PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan aaaa

Pressure Sensors “In any given plant, the number of pressure gauges used is probably larger than all other instruments put together” Most liquid and all gaseous materials in the process industries are contained within closed vessels. For the safety of plant personnel and protection of the vessel, pressure in the vessel is controlled. In addition, pressured is controlled because it influences key process operations like vapor-liquid equilibrium, chemical reaction rate, and fluid flow. PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Practical Considerations in Pressure Measurements
In industrial applications, good repeatability often is more important then absolute accuracy. If process pressures vary over a wide range, transducers with good linearity and low hysteresis are the preferred choice. Ambient and process temperature variations also cause errors in pressure measurements, particularly in detecting low pressures and small differential pressures. In such applications, temperature compensators must be used PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Selection Criteria Pressure transducers usually generate output signals in the millivolt range (spans of 100 mV to 250 mV). When used in transmitters, these are often amplified to the voltage level (1 to 5 V) and converted to current loops, usually 4-20 mA dc. The transducer housing should be selected to meet both the electrical area classification and the corrosion requirements of the particular installation. If the installation is in an area where explosive vapors may be present, the transducer or transmitter and its power supply must be suitable for these environments. This is usually achieved either by placing them inside purged or explosion proof housings, or by using intrinsically safe designs. The single most important decision in selecting a pressure transducer is the range When high process temperatures are present, one can consider the use of various methods of isolating the pressure instrument from the process. These include loop seals, siphons, chemical seals with capillary tubing for remote mounting, and purging. PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Maintenance Without exception, pressure sensors require scheduled, periodic maintenance and/or recalibration. PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Calibration Practical Considerations in Pressure Measurements
Pressure transducers can be recalibrated 1. on-line or 2. in a calibration laboratory. Laboratory recalibration typically is preferred, but often is not possible or necessary. In the laboratory, there usually are two types of calibration devices: deadweight testers that provide primary, base-line standards, and laboratory or field standard calibration devices PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Self Assessment Test October 2008
PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Question No. 1 The difference between gauge and absolute pressure is
a Vacuum 0.433 psia atmospheric pressure zero PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Question No. 2 Pressure is defined as Force per unit area F. A A/F
None of the above PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Question No. 3 Metals used in the bourdon tubes should not undergo
Fatigue hysteresis creep All of the above PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Question No. 4 A capsule diaphragm is made by welding two diaphragms
to a solid base together at the center together around the edges To two other diaphragms PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Question No. 5 Dead weight tester is used for testing dead weights
measuring process pressures accurately producing high pressure calibrating pressure instruments PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Question No. 6 One torr is defined as One mm Hg One inch Hg
One atmosphere One kilopascal PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Question No. 7 Which gauge measures pressure by sensing changes in the thermal conductivity of the gas Pirani gauge Slack diaphragm gauge Mcleod gauge None of them PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Question No. 8 A thermocouple gauge is one type of Ionization gauge
Thermal conductivity gauge Mcleod gauge None of these PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Question No. 9 Diaphragms used in pressure applications are light
Small in size slack bimetallic PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Question No. 10 A Mcleod gauge can measure pressure as low as
0.05 torr 0.005 torr torr torr PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Question No. 10 What type of manometer is best for measuring low pressures Well Inclined U-Tube Multiple tube PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Question No. 11 A capacitance pressure transducer indicates changes in pressure by changing the Voltage output of an ac circuit frequency capacitance alternating current PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Temperature and Pressure Sensors.
Home Work Make similar 10 MCQs from Temperature and Pressure Sensors. Solve Them and attached the sheet with the test on coming Tuesday to get 5 marks. Additional 5 marks for the student who prepare the best MCQs . Most liquid and all gaseous materials in the process industries are contained within closed vessels. For the safety of plant personnel and protection of the vessel, pressure in the vessel is controlled. In addition, pressured is controlled because it influences key process operations like vapor-liquid equilibrium, chemical reaction rate, and fluid flow. PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Total Lectures held 8 Assignment due on 10-10-2009
A Quick View! What we have covered? Total Lectures held 8 Fundamentals of Electrical Technology and digital logic employed in the measurement Review of Scientific principles employed in instruments Parts of Instrument Performance Characteristics of Instruments Selection and Calibration of Instruments Instruments Identification and Line Symbols Principle measurements desired in industry (a) Temperature (b) Pressure, Load (c) Level (d) Flow (e) Others ( Weight, Composition, pH etc.) (f) Transducers Installation and Installation Costs Case Studies Quiz Held = 1 : Next Test due on = (B) (A) Assignment due on . Engineers want to design and operate processes that remain in safe conditions, produce the desired amounts of high quality products and are profitable. Therefore, engineers must provide measuring devices for key variables and valves (or other devices, such as variable speed electric motors) to influence of “steer” the process aaaa

Level Sensors October 2008 PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

-- a constant supply or storage of material Control:
Level Measurement Level is another common process variable that is measured in many industries. The method used will vary widely depending on the nature of the industry, the process, and the application. Inventory: -- a constant supply or storage of material Control: -- continuous, batch, blending, and mixing control -- stabilize flow to the next process Alarming: -- hi/lo limits, safety shut down Data Logging: -- material quantities for inventory and billing purposes and where regulatory requirements are necessary PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Units of level can be expressed in: feet (meters) gallons (liters)
What is measured? The measured medium can be liquid, gas or solid and stored in vessels (open/closed tanks), silos, bins and hoppers. Units of level can be expressed in: feet (meters) gallons (liters) pounds (kilograms) cubic volume (ft3, m3)

Methods ---- Direct or Indirect (inferential)
Hydrostatic Head Float Load Cells Magnetic Level Gauge Capacitance Transmitters Magnetostrictive Ultrasonic Microwave Laser Radar Guided Wave Radar Dip Stick Vibration Direct methods will measure level in terms the physical location of the surface of the material in relation to some reference point (e.g. bottom of the tank) whereby the indirect method will infer the measurement by measuring some other quality or characteristic of the material such as static head pressure, weight or mass. PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Direct Methods Direct methods sense the surface or interface of the liquid and is not affected by changes in material density (Specific Gravity) Examples: Dip Stick Resistance Tapes Sight Glass Floats Ultrasonic

Indirect Methods (Inferential)
Indirect methods “infer” liquid level by measuring some other physical parameter such as pressure, weight, or temperature. Changing materials means a corrective factor must be used or recalibrating the instrument. Examples: Hydrostatic head methods Load Cells Capacitance Conductivity

Open tank or closed tank?
Selection Criteria When determining the type of level sensor that should be used for a given application, there are a series of questions that must be answered: Open tank or closed tank? Can the level sensor be inserted into the tank or should it be completely external? Contact or non-contact? Continuous measurement or point measurement? Direct or Indirect measurement? What type of material is being measured? Liquid or Solid? Clean or Slurry? When determining what type of level sensor should be used for a given application, there are a series of questions that must be answered: Can the level sensor be inserted into the tank or should it be completely external? Should the sensor detect the level continuously or will a point sensor be adequate? Can the sensor come in contact with the process fluid or must it non-contact and be located in the vapor space? Is direct measurement of the level needed or is indirect detection of hydrostatic head (which responds to changes in both level and density) acceptable? Is tank depressurization or process shutdown acceptable when sensor removal or maintenance is required? What type of material is being measured? Liquid or Solid? Clean or Slurry? Once the type of sensor(s) that would suit the application has been narrowed down from the selection chart, a further refinement of the selection process is required in order to satisfy the following criteria: Type of measurement needed (point or continuous); Accuracy Reliability, maintenance and installation Compatibility with the temperature and pressure conditions Stability of the liquid surface (rough, smooth, vapourous) Mounting constraints on the vessel (fixed, free standing) Cost PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

For all liquids you will need:
Selection Criteria For all liquids you will need: The system operating temperature with max. and min. excursions? two wide range – expensive the sensor The system operating pressure? Check that system ‘T’ and ‘P’ do not conflict with the materials of construction? When determining what type of level sensor should be used for a given application, there are a series of questions that must be answered: Can the level sensor be inserted into the tank or should it be completely external? Should the sensor detect the level continuously or will a point sensor be adequate? Can the sensor come in contact with the process fluid or must it non-contact and be located in the vapor space? Is direct measurement of the level needed or is indirect detection of hydrostatic head (which responds to changes in both level and density) acceptable? Is tank depressurization or process shutdown acceptable when sensor removal or maintenance is required? What type of material is being measured? Liquid or Solid? Clean or Slurry? Once the type of sensor(s) that would suit the application has been narrowed down from the selection chart, a further refinement of the selection process is required in order to satisfy the following criteria: Type of measurement needed (point or continuous); Accuracy Reliability, maintenance and installation Compatibility with the temperature and pressure conditions Stability of the liquid surface (rough, smooth, vapourous) Mounting constraints on the vessel (fixed, free standing) Cost PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Expected particle size distribution?
Selection Criteria For Solids: Bulk density Be careful with very large silos as compaction at the bottom can greatly change assume bulk densities Flow characteristics? Expected particle size distribution? Is solid abrasive and/or corrosive and what is the moisture/solvent content? When determining what type of level sensor should be used for a given application, there are a series of questions that must be answered: Can the level sensor be inserted into the tank or should it be completely external? Should the sensor detect the level continuously or will a point sensor be adequate? Can the sensor come in contact with the process fluid or must it non-contact and be located in the vapor space? Is direct measurement of the level needed or is indirect detection of hydrostatic head (which responds to changes in both level and density) acceptable? Is tank depressurization or process shutdown acceptable when sensor removal or maintenance is required? What type of material is being measured? Liquid or Solid? Clean or Slurry? Once the type of sensor(s) that would suit the application has been narrowed down from the selection chart, a further refinement of the selection process is required in order to satisfy the following criteria: Type of measurement needed (point or continuous); Accuracy Reliability, maintenance and installation Compatibility with the temperature and pressure conditions Stability of the liquid surface (rough, smooth, vapourous) Mounting constraints on the vessel (fixed, free standing) Cost PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Can be used with any wet material and not affected by density.
For Liquids Dip Stick Simple and cheap Can be used with any wet material and not affected by density. Can not be used with pressurized tanks Visual indication only (electronic versions are available) RodGauge - similar to a dipstick found in a car, it has weighted line markings to indicate depth or volume 2. A rod gauge provides an inferred measurement for level. ANS: False, the level reading is obtained directly RodGauge - similar to a dipstick found in a car, it has weighted line markings to indicate depth or volume PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Another simple direct method of measuring liquids.
For Liquids Sight Glass Another simple direct method of measuring liquids. Can be used in pressurized tanks (as long as the glass or plastic tube can handle the pressure) 3. A sight glass level device is recommended for measuring the level of beer. ANS: T Good for applications where non-contact measurement is needed (like beverages) PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Liquid density does not affect measurement
For Liquids Floats Float rides the surface level to provide the measurement. Many different styles are available. Usually used for pump control, high/low level alarms and emergency shut-off All float operated liquid level controls operate on the basic buoyancy principle, which states, "a body (float) immersed in a liquid is buoyed upward by a force equal to the weight of the displaced liquid". As a result, floats ride on the liquid surface partially submerged and move the same distance the liquid level moves. Because of this, they are normally used for narrow level differential applications such as high-level alarm or low-level alarm (single point measurement). Float type level controls are available for top mounting, side mounting and external cage applications. A wide range of mercury, dry contact, hermetically sealed, pneumatic and vibration resistant switch mechanisms are available. The process temperature and pressure maximums for floats are +1000° F and 5000 psig with specific gravities as low as 0.3. Single, dual and three switch models are available. Standard models are also available for interface control (float rides on the interface between two liquids). One of the most simple, direct methods of float level measurement is the tape float gauge. A tape connects to a float on one end and to a counterweight on the other to keep the tape under constant tension. The float motion results in the counterweight riding up and down a direct-reading gauge board, thereby indicating the level in the tank. Standard floats are normally cylindrical for top-mounted designs and spherical or oblong for side-mounted designs. Small-diameter floats see use in higher density materials. You can use larger floats for liquid-liquid interface detection or for lower density materials. Liquid density does not affect measurement PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Point Level Measurement Continuous Level Measurement
For Liquids Conductivity Level Measurement Point Level Measurement Continuous Level Measurement Advantages and disadvantages Low Cost Conductive, non-coating liquids only Insulating coatings can cause problems The conductance method of liquid level measurement is based on the electrical conductance of the measured material, which is usually a liquid that can conduct a current with a low-voltage source (normally <20 V). Hence the method is also referred to as a conductivity system. Conductance is a relatively low-cost, simple method to detect and control level in a vessel. PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

For Liquids Resistance Tape The pressure of the fluid in the tank causes the tape to short-circuit, thus changing the total resistance of the measuring tape. An electronic circuit measures the resistance; it's directly related to the liquid level in the tank. The resistance tape spirally winds around a steel tape. This instrument mounts vertically from top to bottom on a process tank. The pressure of the fluid in the tank causes the tape to short-circuit, thus changing the total resistance of the measuring tape. An electronic circuit measures the resistance; it's directly related to the liquid level in the tank. PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Bubblers allow the indicator to be located anywhere.
For Liquids Bubblers Bottom of tube determines reference point P Regulated purge system (air or nitrogen) Instrument input does not matter Bubblers allow the indicator to be located anywhere. The air pressure in the tube varies with the head pressure of the height of the liquid. The air bubbler is another pressure-type level sensor where you install a dip tube in a tank with its open end a few inches from the bottom. A fluid forces itself through the tube; when the fluid bubbles escape from the open end, the pressure in the tube equals the hydrostatic head of the liquid. As liquid level (head) varies, the pressure in the dip tube changes correspondingly. For tanks that operate under pressure or vacuum, installing a bubbler system becomes slightly more complex, because the liquid level measurement is a function of the difference between the purge gas pressure and the vapor pressure above the liquid. Because differential pressure is now involved, the transducer used is normally a dP cell. One disadvantage of using a bubbler is limited accuracy. Another is bubblers will introduce foreign matter into the process. Liquid purges can also upset the material balance of the process, and gas purges can overload the vent system on vacuum processes. If the purge medium fails, not only do you lose the level indication on the tank, but you also expose the system to process material, which can cause plugging, corrosion, freezing, or safety hazards 4.The purge level measurement instrument is not used to measure pulp in a vat because the tube will become clogged with sludge. ANS: False, the purge keeps the tube clean Can’t be used in closed tanks or where purging a liquid is not allowed (soap). Very popular in the paper industry because the air purge keeps the tube from plugging. PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Advantages: For Liquids Bubblers -- Easy installation
Bottom of tube determines reference point P Regulated purge system (air or nitrogen) Instrument input does not matter Advantages: -- Easy installation -- Continuous reading providing analogue or digital signal -- No moving parts -- Good accuracy and repeatability The air bubbler is another pressure-type level sensor where you install a dip tube in a tank with its open end a few inches from the bottom. A fluid forces itself through the tube; when the fluid bubbles escape from the open end, the pressure in the tube equals the hydrostatic head of the liquid. As liquid level (head) varies, the pressure in the dip tube changes correspondingly. For tanks that operate under pressure or vacuum, installing a bubbler system becomes slightly more complex, because the liquid level measurement is a function of the difference between the purge gas pressure and the vapor pressure above the liquid. Because differential pressure is now involved, the transducer used is normally a dP cell. One disadvantage of using a bubbler is limited accuracy. Another is bubblers will introduce foreign matter into the process. Liquid purges can also upset the material balance of the process, and gas purges can overload the vent system on vacuum processes. If the purge medium fails, not only do you lose the level indication on the tank, but you also expose the system to process material, which can cause plugging, corrosion, freezing, or safety hazards 4.The purge level measurement instrument is not used to measure pulp in a vat because the tube will become clogged with sludge. ANS: False, the purge keeps the tube clean PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Limitations: For Liquids Bubblers
Bottom of tube determines reference point P Regulated purge system (air or nitrogen) Instrument input does not matter Limitations: -- Not suitable for pressurized tanks -- Sediments may block tube or probe -- Tanks must be freely vented The air bubbler is another pressure-type level sensor where you install a dip tube in a tank with its open end a few inches from the bottom. A fluid forces itself through the tube; when the fluid bubbles escape from the open end, the pressure in the tube equals the hydrostatic head of the liquid. As liquid level (head) varies, the pressure in the dip tube changes correspondingly. For tanks that operate under pressure or vacuum, installing a bubbler system becomes slightly more complex, because the liquid level measurement is a function of the difference between the purge gas pressure and the vapor pressure above the liquid. Because differential pressure is now involved, the transducer used is normally a dP cell. One disadvantage of using a bubbler is limited accuracy. Another is bubblers will introduce foreign matter into the process. Liquid purges can also upset the material balance of the process, and gas purges can overload the vent system on vacuum processes. If the purge medium fails, not only do you lose the level indication on the tank, but you also expose the system to process material, which can cause plugging, corrosion, freezing, or safety hazards 4.The purge level measurement instrument is not used to measure pulp in a vat because the tube will become clogged with sludge. ANS: False, the purge keeps the tube clean PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Hydrostatic Head Level Sensors
These methods infer level by measuring the hydrostatic head produced by the liquid column. A pressure sensing element is installed at the bottom of the tank and pressure is converted to level. Different liquid densities or closed tank applications must be accounted for. PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

General Theory for Head Measurement
Hydrostatic Head Level Sensors General Theory for Head Measurement The Pressure exerted by the Height of the liquid is: P = H x Density* If the Density of the liquid is known then H = Pressure Density* Height (H) Pressure PSI Liquid Density (D) *Note: For liquids other than water, use the density of water lb/in3 as a reference and multiply by the SG of the liquid.

Example A dip stick measurement of the level of these 2 tanks indicates 30 feet of liquid in both tanks. Calculate the pressure that each gauge will read if tank 1 contains water (S.G. = 1) and tank 2 contains oil (S.G. = 0.85) Height (H) Tank 2 PSI Oil Density (D) Height (H) Tank 1 PSI Water Density (D) P = ? psi P = ? psi

Example A dip stick measurement of the level of these 2 tanks indicates 30 feet of liquid in both tanks. Calculate the pressure that each gauge will read if tank 1 contains water (S.G. = 1) and tank 2 contains oil (S.G. = 0.85) Height (H) Tank 2 PSI Oil Density (D) Height (H) Tank 1 PSI Water Density (D) Alternate questions 1. What is the maximum level (in feet) that can be measured if a 15 psi gauge is used? P = ? psi P = H x Density = 30 ft x lbs/in3 = (30 x 12) x = 13 psi

Example A dip stick measurement of the level of these 2 tanks indicates 30 feet of liquid in both tanks. Calculate the pressure that each gauge will read if tank 1 contains water (S.G. = 1) and tank 2 contains oil (S.G. = 0.85) Height (H) Tank 2 PSI Oil Density (D) Height (H) Tank 1 PSI Water Density (D) Alternate questions 1. What is the maximum level (in feet) that can be measured if a 15 psi gauge is used? P = H x Density x SG = 30 ft x lbs/in3 x 1 = (30 x 12) x = 13 psi P = H x Density x SG = 30 ft x lbs/in3 x 0.85 = (30 x 12) x x 0.85 = 11 psi

Practical Considerations when using head type instruments
Hydrostatic Head Level Sensors (Cont‘d) Practical Considerations when using head type instruments The reference point of the tank vs instrument input must be considered. Height (H) Pressure PSI Liquid Density (D) This may not be practical in some applications where the tank elevation is below grade or where a remote visual reading is required. PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Hydrostatic Head Level Sensors (Cont‘d)
Tank Elevations Vertical rises and drops contribute to the overall height and therefore head pressure. Horizontal runs have no effect. Height (H) P Water Density (D) P

Closed Tank Applications
Hydrostatic Head Level Sensors (Cont‘d) Closed Tank Applications P Head = h x D P (atmospheric) P Head = (h x D) + P vapour P vapour Open tanks are vented to atmosphere so the pressure at the bottom of the tank is only due to the head pressure of the liquid. Closed tanks are not vented to atmosphere so the pressure at the bottom of the tank is due to the head pressure of the liquid + the vapour pressure above the surface. 5. A hydrostatic head level detector can be used to measure level within a confined and pressurized container. ANS: False, the vapour pressure in the confined space will affect the reading. A dp cell must be used

Used where the sight glass level gauge can not be used.
Magnetic Level Sensor Used where the sight glass level gauge can not be used. Magneto-resistive types can provide an electrical output. These gauges are the preferred replacement for sight glasses. They are similar to float devices, but they communicate the liquid surface location magnetically. The float, carrying a set of strong permanent magnets, rides in an auxiliary column (float chamber) attached to the vessel by means of two process connections. This column confines the float laterally so that it is always close to the chamber's side wall. As the float rides up and down with the fluid level, a magnetized shuttle or bar graph indication moves with it, showing the position of the float and thereby providing the level indication. The system can work only if the auxiliary column and chamber walls are made of nonmagnetic material. Many manufacturers provide float designs optimized for the specific gravity of the fluid being measured, whether butane, propane, oil, acid, water, or interfaces between two fluids, as well as a large selection of float materials. This means the gauges can handle high temperatures, high pressures, and corrosive fluids. Oversized float chambers and high-buoyancy floats are available for applications where buildup is anticipated. Liquid/liquid interface (such as water and oil) can be measured by changing the buoyancy of the magnetic float PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Displacers Not the same as a float. The displacer is immersed in the tank and the buoyant force of the liquid produces a torque which is proportional the amount of liquid level. The output force can be converted to provide a proportional pneumatic or electrical continuous output of tank level. Displacer level sensors are force balance devices (restrained floats) whose apparent weight varies in accordance with Archimedes' principle which states that “the buoyant force acting on an object equals the weight of the fluid displaced”. As the level changes around the stationary (and constant diameter) displacer float, the buoyant force varies in proportion and can be detected as an indication of level. Regular and displacer floats are available as both continuous level transmitters and point-sensing level switches. In industrial applications, displacer floats are often favored because they do not require motion. Furthermore, force can often be detected more accurately than position. PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Displacer – side mounted unit
The displacer float size and diameter is selected according to the size of tank and the height of the level to be measured. The output force can be converted to provide a proportional pneumatic or electrical continuous output of level. Advantages and disadvantages: Very little movement of displacer float Affected by different liquid densities Limited to smaller tanks sizes

Non-Contact direct level sensor
UltraSonic Level Measurement Non-Contact direct level sensor Level is a function of the time it takes an ultrasonic pulse to hit the surface and return Non-Contact Ultrasonic level sensors transmit and receive ultrasonic waves in the range of 25 – 50 KHz. The transducer is mounted above the maximum level of the media to be measured and the ultrasonic pulse is directed at the surface. As the level of the media moves, the time interval for the signal return also changes. The transmitter converts this time interval to a direct reading of level. The transmitted wave can be either a continuous wave or a pulsed wave depending on the application. Generally speaking, pulsed waves are short bursts of energy 5 – 10 times stronger than the continuous wave and are used in applications to provide more accurate measurement in conditions of aeration, turbulence, suspended solids, and highly viscous liquids. Certain process conditions may interfere with the ultrasonic signal. These conditions include high pressure, slurries with a very high percentage of solids, extreme aeration of the liquid, and very aggressive turbulence and splashing. Vapour and condensate create false echo’s resulting in an incorrect level measurement. Limitations include: Surface foam absorbs signal, agitation create reflections High Pressure & High Temperatures affect the signal speed Vapour and condensate create false echo’s PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Various designs Radar Level Sensors (Microwave)
Similar to ultrasonic but at a much higher frequency (6.3 GHz) Various designs -- Frequency Modulated Continuous Wave -- Pulsed Wave -- Guided Wave Radar and ultrasonic level sensors use the same principle of time propagation of the transmitted and received signal to determine the location of the surface of the material. Radar sensors operate at a much higher frequency (6.3 GHz is typical), which gives them better performance in applications with vapour or uneven surfaces. The sensors operate according to the FMCW (Frequency Modulated Continuous Wave) method, pulsed wave or guided wave. Guided wave is a contact method of radar level measurement and functions according to the principle of Time Domain Reflectometry (TDR). Sophisticated circuitry captures these signals in real time (nanoseconds) and reconstructs them in equivalent time (milliseconds) to make level measurement a practical reality. Unlike conventional radar, which launches its signal into free air, Eclipse launches its signal within the sealed path of its probe (wave guide) which is in direct contact with the process media. This direct contact makes the signal less vulnerable to distortion brought on by process conditions that might thwart through-air technologies. A generated pulse of electromagnetic energy travels down the probe. Upon reaching the liquid surface the pulse is reflected. These sensors have better performance in applications where vapour, dust or uneven surfaces exist. PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Tank level is determined by the weight of the quantity of material
Load Cells Tank level is determined by the weight of the quantity of material Load Cells (strain gauge transducers) placed at the bottom of the tank measure the weight and then convert it to an electrical signal. Bonded strain gage load cells convert weight or force into electrical signals, which can be used to actuate or drive a variety of measuring or control apparatus. These rugged electrical mechanical devices utilize bonded foil strain gages to convert tension or compression forces into electrical signals that have an extremely high order of reliability and accuracy. Because of its simplicity, this type of load cell offers many advantages such as long-term reliability, low order of maintenance, simplicity of installations and precision of measurement. For level measurement in silos and tanks, load cells provide a wide range of products that satisfy the demands of different circumstances. 6. The level of a material can be determined by measuring the weight of the contents in a pressurized container. ANS: T Level is measured by locating the boundary between two media, called the interface Level can be measured directly or indirectly Noninvasive devices are preferred when the material is corrosive, hazardous, sterile, or at a high temperature or pressure PEC – UET Lahore Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Capacitance Level Detection (RF Level Probes)
RF Capacitance level sensors are similar in operation to conductivity sensors and are typically used when non-conductive liquids are measured. RF Capacitance level sensors are similar in operation to conductivity sensors and are typically used when non-conductive liquids are measured. The operation of RF Capacitance level sensors is based on the principles of the electronic component, the capacitor. The capacitance (in micro or picofarads) varies as a function of the area of the parallel plates, the distance between them, and the material (dielectric constant) that separates them. When a bare metal probe is installed in a metal tank, the probe and the side of the tank form a fixed capacitive plate area separated by a fixed distance. The only variable is the dielectric of the tank material that varies in proportion to the level. The most common probe design is a stainless steel rod of in. or 1/2 in. diameter, suitable for most non-conductive and non-corrosive materials, for applications where the process material is conductive and corrosive the probe must be coated with PFA or Kynar. 9. A capacitive probe is a recommended device to measure the level of mineral water in a tank. ANS: F The only variable is the dielectric of the tank material that varies in proportion to the level. Conductive liquids like mineral water will short out the probe to the tank wall. Good for solids

Can be used for non-liquids
RF capacitance probes are used in a wide range of applications and are subjected to certain limitations such as: Electrode Coating Low sensitivity Temperature Different materials Shielding & noise

Level can be measured directly or indirectly
Summary Level is measured by locating the boundary between two media, called the interface Level can be measured directly or indirectly Noninvasive devices are preferred when the material is corrosive, hazardous, sterile, or at a high temperature or pressure

Batch application using point level measurement
Creative Assignment! HAZOP????? Batch application using point level measurement Detect if the level is at a predetermined point Generally used to detect either a low-level condition or high-level condition The output of a point level sensor is on-off, or 1 or 0 state digital signal A batch process uses a continuous level measurement device. ANS: F A batch process does NOT use a continuous level measurement device.

Discussion & Questions?
Dr. Shahid Naveed März 2017 / Dr. –Ing Naveed Ramzan

Industrial Instrumentation

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Industrial Instrumentation

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