Instrumentation of Control System

Monday, December 4, 2017

Instrumentation of Control System

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source: http://www.psi-repair.com

Process automation refers to processes involving batch and continuous flow of liquids, gases, and bulk solids, such as:

  • Chemical reactions
  • Heating, cooling, mixing, and separating
  • Measuring properties
One implementation of this process automation is factory automation. Factory automation involving in,
  • Moving, aligning, and transporting parts
  • Mechanical processing
  • Measuring dimensions
A process control system uses four elements, those are Primary element, Measurement element, Control element, and Final element.
source: Instrumentation and Process Control Sixth Edition

Four elements above forming a loop that transferring an information called Control Loop. There are two types of control loop, Open loop control and Closed loop control. Open loop control is a system that the output not affect the input. Otherwise Closed loop control is a system that the output affect the input, so the controller input is summing from set point and feedback from the output. 

A controller which control the final elements have about three strategies,
  1. ON/OFF control is a control strategy that activates or deactivates final element depending the measured value is above or below the set point. 
  2. Proportional control is depending on the differential value between set point and the process variable to determine the control output.
  3. Time proportional ON/OFF control is a control strategy that has predetermined output period during which the output contact is held closed for a variable portion of the output period.
In a control system, there is Piping and Instrumentation Diagrams (P&ID). Piping and Instrumentation Diagrams are used to drawing a detailed process industry which shows the piping and vessels in the process flow together with the instrumentation and control devices.
source: www.instrumentationtoolbox.com
TEMPERATURE
Heat transfer is movement of thermal energy from one place to another. There are three kinds of heat transfer,
  1. Conduction is occurs when molecule in a material heated and that heat is passed through a material.
  2. Convection is occurs by movement of gas or liquid caused by pressure difference.
  3. Radiation is occurs by electromagnetic waves emitted by a higher temperature object and absorbed by a lower temperature object.
Thermometer is one of device that utilize heat transfer. Types of thermometer are liquid-in-glass thermometer, bimetallic thermometer, pressure-spring thermometer, liquid-filled pressure-spring thermometer, vapor-pressure pressure-spring thermometer.

Thermometer that running electrically is called Electrical Thermometer. Thermocouple is one of electrical thermometer that consisting of two dissimilar metal wires joined at one end and a voltmeter to measure the voltage at the other end of the two wires. In electrical thermometer there are,
  • Seebeck effect is thermoelectric effect where continuous current is generated in a circuit where the junctions of two dissimilar conductive materials are kept at different temperatures.
  • Peltier effect is thermoelectric effect where heating and cooling occurs at the junctions of two dissimilar conductive materials when a current flows through the junctions.
  • Thomson effect is thermoelectric effect where heat is generated or absorbed when an electric current passes through a conductor in which there is a temperature gradient.
There are two laws about an electrical thermometer. 
  1. Law of Intermediate Temperatures is a law stating that in a thermocouple circuit, if a voltage is developed between two temperatures, T1 and T2, and another voltage is developed between two temperatures, T2 and T3, it will generate a voltage that is the sum of those two voltage when operating between T1 and T3.
  2. Law of Intermediate Metals is a law stating that the use of a third metal in a thermocouple circuit does not affect the voltage, as long as the temperature of the three metals at the point of junction is the same.
A difference thermocouple is a pair of thermocouples connected together to measure a temperature difference between two objects.
source: Instrumentation and Process Control Sixth Edition

When a difference thermocouple consist of a thermocouples pair, a Thermophile is consisting of several thermocouples connected in series to provide a higher voltage input. On the other hand when it is consisting of a set of parallel connected thermocouples it is called Averaging thermocouple. Averaging thermocouple is commonly used to measure an average temperature of an object or area. Next, a thermocouple pyrometer is an electrical thermometer consisting of a plain electrical meter with measurement range of 20mV to 50mV, a thermocouple, and a balancing resistor. Next, Resistance Temperature Detector (RTD) is a device that increase its resistance when exposed to heat. It is consist of heat sensitive wound that carefully made electrical resistor manufactured in the form of a bulb. A thermistor is also temperature-sensitive resistor consist of solidstate semiconductor made from sintered metal oxides and lead wires, hermetically sealed in glass. There are two kinds of thermistor, NTC (Negative Temperature Coefficient)  and PTC (Positive Temperature Coefficient).

 Infrared Radiation Thermometers is a thermometer that measure infrared radiation. There are one color IR Thermometer and two color IR Thermometer. One color IR Thermometer using one IR detector, otherwise two color IR Thermometer use two IR detector. When it is not possible to place an IR Thermometer in a suitable location, a small lens assembly connected to a fiber-optic cable may be used to transmit infrared radiation. Dissapearing filament pyrometer is an optical pyrometer which comparing the glowing incandescent object with the light of a heated filament, usually used between 600°C and 3000°C temperature. A thermal imager is a non-contact temperature measurement device that detect the infrared energy emitted, transmitted or reflected by all materials and converts the energy factor into a temperature reading or thermogram. A thermogram is the thermal image displayed by the camera of the object which is emitting, transmitting or reflecting the infrared energy [1]. In temperature measurement, there is time constant that the time required for a process to change 63.2% of its total change when an input to the process is changed.

 Practical Measurements and Calibrations

 In industrial processes, a Thermowell is used to protect temperature sensors and to allow instrument maintenance to be performed without draining the process fluid. Thermowell consist of a tube closed at one end and mounted in the process stream. A temperature sensor is inserted in the open end of the tube, which is usually in the open air outside the process piping or vessel and any thermal insulation [2]. Thermowell has three shapes, Straight, Tapered, and Stepped. The most common thermowell is stepped shape. To ensure that the thermowell has the best chance of accurately sensing the process temperature, the sensitive portion of the insertion length must be in the actively flowing stream.

 A ground loop is current flow from one grounded point to a second grounded point in the same powered loop due to differences in the actual ground potential. Thermocouple break protection is a circuit where an electronic device sends a low-level current across the thermocouple in order to detect faults in the circuit. Shunt impedance is an unintended circuit caused by damaged thermocouple insulation. A resistance bridge is a circuit used to precisely measure an unknown resistance. Because of the very significant nonlinearity of the thermistor response, the bridge is usually modified by placing a resistor in parallel with the thermistor.

A dry well calibrator is a temperature-controlled well or box where a thermometer can be inserted and the output compared to the known dry well temperature. Different from Infrared Thermometer, to calibrate it there is a device called blackbody calibrator.

 PRESSURE
Pressure is force divided by the area over which that force is applied. Head is the actual height of a column of liquid. Hydrostatic pressure is the pressure due to the head of a liquid column. A hydraulic press shows the application of Pascal’s law. There are four common pressure scales, absolute, gauge, vacuum, and differential pressure. Differential pressure is the difference in pressure between two measurement points in a process.
source: Instrumentation and Process Control Sixth Edition

Mechanical Pressure Instruments
  • A U-tube manometer is a clear tube bent into the shape of an elongated letter U usually labeled with a graduated scale placed in the center between the vertical columns, or legs, with the scale markings increasing above and below the zero point. 
  • An inclined-tube manometer is a manometer with a reservoir serving as one end and the measuring column at an angle to the horizontal to reduce the vertical height. 
  • A well-type manometer is partly same as U-tube manometer with a vertical glass tube connected to a metal well, with the measuring liquid in the well at the same level as the zero point on the tube scale. 
  • A barometer is a manometer used to measure atmospheric pressure. 
  • A diaphragm is a mechanical pressure sensor consisting of a thin, flexible disc that flexes in response to a change in pressure.
  • A capsule is a mechanical pressure sensor consisting of two convoluted metal diaphragms with their outer edges welded, brazed, or soldered together to provide an empty chamber between them. 
  • The Bourdon tube is the original pressure spring and is a C-shaped tube that is flattened into an elliptical cross section.
  • A bellows is a mechanical pressure sensor consisting of a one-piece, collapsible, seamless metallic unit with deep folds formed from thin-wall tubing with an enclosed spring to provide stability, or with an assembled unit of welded sections.
  • A double-ended piston is a mechanical pressure sensor consisting of a differential pressure gauge with a piston that admits pressurized fluid at each end.
source: Instrumentation and Process Control Sixth Edition

 Electric Pressure Instruments
  • A strain gauge is a transducer that measures the deformation, or strain, of a rigid body as a result of the force applied to the body. A dummy gauge, added to the bridge-conditioning circuit, is often used to provide temperature compensation.
  • A capacitance pressure transducer is a diaphragm pressure sensor with a capacitor as the electrical element.
  • A differential pressure (d/p) cell converts a differential pressure to an electrical output signal.
  • An inductance pressure transducer is a diaphragm or bellows pressure sensor with electrical coils and a movable ferrite core as the electrical element.
  • A linear-voltage differential transformer (LVDT) is an inductance transducer consisting of two coils wound on a single nonconductive tube.
  • A reluctance pressure transducer is a diaphragm pressure sensor with a metal diaphragm mounted between two stainless steel blocks.

 Common methods of protecting pressure instruments is the use of dry legs and wet legs. Adding snubbers (pulsation dampers) to inlet lines limits pulsations and surges is also one to protect pressure instrument. Adding enough inlet tubing to allow the process fluid to cool before entering the sensor may widen the useful temperature range of pressure sensors. Isolating sealing systems have been developed to protect pressure sensors from corrosive fluids. Dry air or nitrogen is commonly used to protect a pressure instrument from contact with the process gas. Liquid purges are usually done with water to prevent solids from entering or depositing at the process connection or in the connecting tubing.
source: Instrumentation and Process Control Sixth Edition

A deadweight tester is a hydraulic pressure-calibrating device that includes a manually operated screw press, a weight platform supported by a piston, a set of weights, and a fitting to connect the tester to a gauge. The portable one is Portable digital calibration test gauges which are available as hand-held models with manual pneumatic or hydraulic pressure generator and electronic transmission for certification.

 LEVEL
Level is A horizontal plane or line with respect to the distance above or below a given point [3].
source: Instrumentation and Process Control Sixth Edition

Mechanical Level Instruments
  • A gauge glass is a continuous level measuring instrument that consists of a glass tube connected above and below the liquid level in a tank and that allows the liquid level to be observed visually.
  • A reflex gauge glass is a flat gauge glass with a special vertical sawtooth surface that acts as a prism to improve readability.
  • A magnetically coupled level gauge is a gauge that consists of a stainless steel float containing a magnet riding in a stainless steel tube where the level indicator consists of horizontally pivoted magnetized vanes painted yellow or white on one side and black on the other in a housing bolted to the level tube.
  • A cable and weight system is an intermittent full-range level measuring assembly consisting of a manual or remotely operated switch, a relay and a servomotor, a plumb bob for a weight, and a cable.
  • A bubbler is a level measuring instrument consisting of a tube extending to the bottom of a vessel; a pressure gauge, single-leg manometer, transmitter, or recorder; a purge flowmeter or sight feed bubble; and a pressure regulator.
  • A float is a point level measuring instrument consisting of a hollow ball that floats on top of a liquid in a tank and is attached to the instrument.
  • A displacer is a liquid level measuring instrument consisting of a buoyant cylindrical object, heavier than the liquid, that is immersed in the liquid and connected to a spring or torsion device that measures the buoyancy of the cylinder.
  • A paddle wheel switch is a point level measuring device consisting of a drive motor and a rotating paddle wheel mounted inside a tank.
Electrical Level Instruments
  • A single-point level-control capacitance probe is used as a switch to signal an alarm or actuate a circuit when the level in a tank or vessel reaches a specified level. For capacitance continuous level measuring instruments, a vertically suspended capacitance probe can be used for measuring the level in a tank.
  • A conductivity probe is a point level measuring system consisting of a circuit of two or more probes or electrodes, or an electrode and the vessel wall where the material in the vessel completes the circuit as the level rises in the vessel.
  • An inductive probe is a point level measuring instrument consisting of a sealed probe containing a coil, an electrical source that generates an alternating magnetic field, and circuitry to detect changes in inductance.
  • A beam-breaking photometric sensor is a point level measuring instrument consisting of a light source and a detector that indicates a level of the contents of a vessel when the beam is broken.
  • An optical liquid-level sensor is a liquid point level measuring instrument where a light source and a light detector, shielded from each other, are mounted in a housing and the light source is directed against the inside of a glass or plastic cone-shaped prism.
  • A magnetostrictive sensor is the part of a continuous level measuring system consisting of an electronics module, a waveguide, and a float containing a magnet that is free to move up and down a pipe that is inserted into a vessel from the top.
  • A thermal dispersion sensor is a point level measuring instrument consisting of two probes that extend from the detector into the vessel, with one of the probe tips being heated.
Ultrasonic, Radar, and Laser Level Instruments
  • Gap Switches is one of ultrasonic sensor using air gap as sensor to the point level. Ultrasonic Sensors also can detect continuously level by transmit ultrasonic signal to travel from the ultrasonic level transmitter to the surface of the material to be measured and back to the receiver.
  • A tuning fork level detector is a point level measuring instrument consisting of a vibrating tuning fork that resonates at a particular sound frequency and the circuitry to measure that frequency.
  • A pulsed radar level sensor is a level measuring sensor consisting of a radar generator that directs an intermittent pulse with a constant frequency toward the surface of the material in a vessel.
  • A frequency modulated continuous wave (FMCW) radar is a level measuring sensor consisting of an oscillator that emits a continuous microwave signal that repeatedly varies its frequency between a minimum and maximum value, a receiver that detects the signal, and electronics that measure the frequency difference between the signal and the echo.
  • A guided wave radar is a level measuring detector consisting of a cable or rod as the wave carrier extending from the emitter down to the bottom of the vessel and electronics to measure the transit time.
  • Laser Sensor is take advantage from laser beam that is reflected back to the emitter where a very accurate timing device measures the out-and-back interval. The travel time will varies with the level.


    Nuclear Level Instruments
    On nuclear level instrument, a radioactive source mounted at selected position, for point level, add just one receiver, and for continuous level, add more than one receiver to detect different level.
    source: Instrumentation and Process Control Sixth Edition

    Weigh System
    A load cell is a device used to weigh large items and typically consists of either piston-cylinder devices that produce hydraulic output pressure or strain gauge assemblies that provide electrical output proportional to the applied load.
    • Shear-type strain gauge load cells are used to measure the weight in vertical vessels.
    • Compression-type strain gauge load cells are used for long, horizontal vessels where one end of the vessel needs to be free floating to allow for dimensional changes with temperature changes.
    • Hydraulic load cells are part of a closed hydraulic pressure system in which the load cell transfers the pressure acting on the cell from the weight of the vessel and its contents to a piston.

    Practical Level Measurements and Calibration
    Mass flow is the flow of a bulk solid where all material in a silo flows down toward the bottom at the same rate. Mass flow is the most desired flow regime, but it rarely exists in practice.
    source: Instrumentation and Process Control Sixth Edition

    To perform a calibration, a simple way is using physical weights piled onto the vessel a little at a time, checking the indicated weight against the actual applied weight. Another calibration method uses known amounts of liquid added to the vessel as the calibration weight. The final method uses a portable load cell calibration system, a readout instrument, and hydraulic jacks.

     A water column is a boiler fitting that reduces the turbulence of boiler water to provide an accurate water level in the gauge glass. In water column, A try cock is located at valve, used to determine the boiler water level if the gauge glass is not functional. A low water fuel cutoff is a boiler fitting that shuts the burner OFF in the event of a low water condition. A transmitter can be protected from a corrosive process fluid by connecting capillary tubing, with diaphragm seals, that is filled with liquid of constant specific gravity. Some level measurement applications may require that the level transmitter have an elevation incorporated into its calibration.

     FLOW
    Flow rate is the quantity of fluid passing a point at a particular moment. Kinematic viscosity is the ratio of absolute viscosity to fluid density and has units of centistokes (cS). A Reynolds number is the ratio between the inertial forces moving a fluid and viscous forces resisting that movement. The three gas laws, Boyle’s law, Charles’ law, and Gay- Lussac’s law, provide the basis for determining the volume of a gas at one set of pressure and temperature conditions when data from another set of conditions are known. That three law can be combined into one equation that shows the relationship between volume, pressure, and temperature.
    source: Instrumentation and Process Control Sixth Edition

     Differential Pressure Flowmeters
    • An orifice plate is a primary flow element consisting of a thin circular metal plate with a sharp-edged round hole in it and a tab that protrudes from the flanges.
    • A flow nozzle is a primary flow element consisting of a restriction shaped like a curved funnel that allows a little more flow than an orifice plate and reduces the straight run pipe requirements.
    source: Instrumentation and Process Control Sixth Edition
    • A pitot tube is a flow element consisting of a small bent tube with a nozzle opening facing into the flow.
    • The vena contracta is the point of lowest pressure and the highest velocity downstream from a primary flow element.
    Flow measurement is only accurate as long as the flowing conditions remain the same as when the system was designed.

    Mechanical Flowmeters
    • A rotameter is a variable-area flowmeter consisting of a tapered tube and a float with a fixed diameter.
    • A clear-tube rotameter is a rotameter consisting of a clear tube to allow visual observation of the flow rate.
    • A purge meter is a small metal or plastic rotameter with an adjustable valve at the inlet or outlet of the meter to control the flow rate of the purge fluid.
    • A bypass meter is a combination of a rotameter with an orifice plate used to measure flow rates through large pipes.
    • A metering-cone meter is a flowmeter consisting of a straight tube and a tapered cone, instead of a tapered tube, with an indicator that moves up and down the cone with changes in flow.
    • A shaped-float and orifice meter is a flowmeter consisting of an orifice as part of the float assembly that acts as a guide.
    • A positive-displacement flowmeter is a flowmeter that admits fluid into a chamber of known volume and then discharges it.
    • A turbine meter is a flowmeter consisting of turbine blades mounted on a wheel that measures the velocity of a liquid stream by counting the pulses produced by the blades as they pass an electromagnetic pickup.
    • A paddle wheel meter is a flowmeter consisting of a number of paddles mounted on a shaft fastened in a housing, which can be inserted into a straight section of pipe.
      source: Instrumentation and Process Control Sixth Edition

     Magnetic, Ultrasonic, and Mass Flowmeters
    • A magnetic meter, or magmeter, is a flowmeter consisting of a stainless steel tube lined with non- conductive material, with two electrical coils mounted on the tube like a saddle.
    • A Doppler ultrasonic meter is a flowmeter that transmits an ultrasonic pulse diagonally across a flow stream, which reflects off turbulence, bubbles, or suspended particles and is detected by a receiving crystal.
    • A transit time ultrasonic meter is a flowmeter consisting of two sets of transmitting and receiving crystals, one set aimed diagonally upstream and the other aimed diagonally downstream.
    • A vortex shedding meter is an electrical flowmeter consisting of a pipe section with a symmetrical vertical bluff body (a partial dam) across the flowing stream.
    • A Coriolis meter is a mass flowmeter consisting of specially formed tubing that is oscillated at a right angle to the flowing mass of fluid.
    • A thermal mass meter is a mass flowmeter consisting of two RTD temperature probes and a heating element that measure the heat loss to the fluid mass.
    • A belt weighing system is a mass flow measuring system consisting of a specially constructed belt conveyor and a section that is supported by electronic weigh cells.
    Practical Flow Measurements
    source: Instrumentation and Process Control Sixth Edition

     When liquid flow is being measured, the measuring instrument must be mounted below the elevation of the flow element and the impulse lines must be filled with the liquid. Equalizing the instrument is necessary so that the instrument can be periodically calibrated and zeroed. An integrator is a calculating device that totalizes the amount of flow during a specified time period. A flow switch is a device used to monitor flowing streams and to provide a discrete electrical or pneumatic output action at a predetermined flow rate.

      references:
      [All] Instrumentation and Process Control Sixth Edition
      [1] https://www.omega.com/prodinfo/thermal_imagers.html
      [2] https://en.wikipedia.org/wiki/Thermowell
      [3] https://en.oxforddictionaries.com/definition/level

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