Sunday, June 28, 2015

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Level Measurement - Ultrasonic Type Working Principle Application Advantages and Disadvantages

  • Sunday, June 28, 2015
  • denizen robo
  • Ultrasonic Level Detectors

    Ultrasonic sound waves with frequencies of 1 to 5 megahertz can be used to detect liquid or solid levels.

    Ultrasonic are sound waves but are at higher frequencies that cannot be detected by the human ear.

    ultrasonic level measurement working principle



















    The most common kind of ultrasonic transducer consist of a piezoelectric crystal.When a voltage is applied to the plates the piezoelectric crystal expands or contracts.If the voltage is alternating at an ultrasonic frequency,the crystal expands and contracts at same the same ultrasonic frequency.The crystal vibrates and these vibrations can be transferred to a diaphragm to produce ultrasonic sound waves.

    Piezoelectric device can be mounted in the bottom or in the top of a vessel.The liquid surface acts as a reflector and the transducer receives the reflection of its transmitted pulses.The transducer is connected to a transmitter and to a receiver,into which the echo is fed.The transmitter and receiver are both connected to an echo timer which measures the amount of time between the emission of the sound wave and the reception of echo.The elapsed time can be converted into units of level of liquid.

    Application of Ultrasonic Level detector

    For situation where it is impractical or undesirable to install an instrument inside a tank ,non penetrating ultrasonic sensors are available.

    Advantages of ultrasonic level detectors

    1.Can be used in any tank size
    2.Can be used in a vacuum
    3.Can be used under high pressure
    4.Relatively easy to maintain because they don't have no moving parts.

    Disadvantages of Ultrasonic level detectors

    1.Expensive
    2.sensitive to wide range of temperature variations.

    Selection consideration - Ultrasonic sensor

    1.Choose a sensor range that at least as tall as the tank,doubling sensor margin to add margin against higher temperatures,condensation and turbulence.
    2.Tank height is defined as the distance from the installed face of the transducer down to the bottom of the tank.
    3.Riser height is the distance from the face of the transducer to top of the tank.
    4.Fill height is defined as the distance from the bottom of the tank,upto the maximum desired liquid height.
    5. Deadband is defined as the minimum distance from the face of the transducer from which the sensor can measure.

    how to select an ultrasonic level sensor



















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    Pressure Measurement - Manometers Working Principle - Vaious types of Manometers

  • denizen robo
  • Manometers

    Manometers are one of the oldest type of pressure measurement.

    Manometers can be used to measure gauge pressure,differential pressure and absolute pressure.

    Various types of Manometers
    1.U tube Manometer
    2.Well type or reservoir manometer
    3.Inclined manometer
    4.Float type manometer

    various types of manometers













    U tube Manometer.

    U tube is made of glass.The tube is filled with a fluid known as Manometer fluid.

    Manometer fluid may be  mercury , water etc...

    If the manometer is connected to same pressure source (P1 = P2) the level of the manometer will be same.
    U tube manometer














    If the manometer is connected to two different pressure source there will be a difference in level of the manometer fluid in both the limps.

    U tube manometer working principle















    If P1>P2 the differential pressure P2-P1=@h
    @=density of the fluid

    h=height difference

    While choosing the manometer fluid for a particular application we need to remember following things.
    Manometer fluid should not wet the wall
    Manometer fluid should not absorb gas
    Manometer fluid should not react chemically
    Manometer fluid should have low vapor pressure
    Move freely

    Mercury is one of the most commonly used manometer fluid.

    Well type Manometer

    In a well type manometer,one leg is replaced by a large diameter well.Since the cross sectional area of the well is much larger than the other leg,when pressure is applied to the well,the manometer liquid in the well lowers only slightly compared to the liquid rise in the other leg.As a result of this,the pressure difference can be indicated only by the height of the liquid column in single leg.
    For static balance,
    P2-P1= @(1+A1/A2)h
     where 
    A1= area of smaller-diameter leg
    A2= area of well

    If A1/A2 << 1 then P2-P1 = @h

    If the area of well is 500 or more times larger than the area of vertical leg,the error involved in neglecting the area is negligible. 

    Well-Type manometer





















    Inclined-Tube Manometers

    The inclined  leg expands the scale so that lower pressure differentials may be read easily.

    Sensitivity of the manometer increases.

    The scale of the manometer can be extended greatly by decreasing the angle of inclined leg to a small value.










    Float-Type Manometers

    This is a variation of well-type manometer

    Recording type manometer

    Span of the measurement can be changed by changing the diameter of the leg

    A large float can be placed to generate enough force 

    Float type manometers
















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    Sunday, June 21, 2015

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    Level Measurement - Magnetic Gauges

  • Sunday, June 21, 2015
  • denizen robo
  • Magnetic Gauges

    Magnetic gauges can often be used in applications where a sight glass is not practical.
    A Magnetic gauge uses the attraction between two magnets to follow the level of a liquid.One magnet float on the surface of the liquid.The other magnet is inside a nonmagnetic tube passing through the liquid.

    The passing through the liquid is made of a nonmagnetic metal.The magnet inside the tube is balanced by a counterweight so that it can rise or fall easily.The ring-shaped floating magnet fits around the outside of the tube and follows any change in liquid level.The magnet inside the tube follows the floating magnet and moves the indicator along a calibrated scale.

    Magnetic Gauges should not be used to measure liquids that can cause the floating magnet to stick to the tube.
    The material for the tube itself must be able to withstand the temperature, pressure and chemical action of the liquid that is being measured.

    magnetic gauge level measurement working principle

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    Saturday, June 20, 2015

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    Instrumentation : Basics of pressure measurement

  • Saturday, June 20, 2015
  • denizen robo
  • Pressure measurement 
    Pressure is an important process variable.
    Pressure is defined as force per unit area.

    Pressure = Force / Area
    Force can be represented as Pound force,Kilogram force,Newton,dyne
    Area can be represented as Square inches,Square feet,Suare Centimeter, Square Meters

    Common units of Pressure include:

    1.Pounds per square inch (psi)
    2.Newton/m2 (=Pa) : SI unit
    3.KiloPascals(kPa)

    Other units:
    Inches(or mm) of water(or Hg)
    Bar
    Atmosphere(1 atm = 760 mm Hg)
    Torr (1 mm of Hg)

    Gauge Absolute and Atmospheric pressure

    Gauge pressure : Pressure above atmospheric is called gauge pressure.
    Vacuum Pressure: Pressure below atmospheric is a vacuum (negative gauge pressure)
    Absolute pressure: Absolute pressure is measured from a perfect vacuum
    what is absolute pressure,atmospheric pressure,Gauge pressure










    Classification of Pressure measuring Instruments
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    Basics of Pressure Regulator (Pressure Reducing Valve - PRV)

  • denizen robo
  • Pressure regulator
    Self-Operated devices which work off the medium alone
    Pressure reducing valve - PRV : Lowers inlet pressure to desired point

    How PRV works?
    working principle of pressure reducing valve
















    Internal architecture of a Pressure regulator
    internal architecture of a regulator















    Basic Characteristics of a pressure regulator is standalone self actuating controllers.

    What to expect from a regulator?
    1.Accurate regulation
    2.Tight shutoff
    3.Fast response
    4.Minimum maintenance
    5.Low noise
    6.Low initial cost

    Advantages of regulator

    No external power is needed to position the valve
    No need for separate measuring elements or feedback controllers
    Design tend to be simple,providing low cost,high reliability and easy maintainability
    Absence of stem packing eliminates external leakage and source of high friction
    Regulators are in direct contact with the controlled variable and offer very fast response

    Disadvantages of Regulators

    The controlled media must be relatively clean and benign as material of constructions are limited
    Regulators lose controllability when the pressure drop across the valve becomes small because the media cannot supply enough operating power
    Operating points are not constant due to drops
    Regulators cannot accommodate anti-noise /cavitation trims
    Failure modes fixed

    Do not use a regulator when following conditions are there
    1.The desired pressure or temperature set point is beyond the range of regulator
    2.Process offset cannot be tolerated
    3.The pressure drop is extremely small or extremely great
    4.A "fail safe" feature is required
    5.The system requires control of a multi-variable process
    6.Feedback is required

    Understand what is Piloted regulator,Advantages and Disadvantages 
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    Sunday, June 14, 2015

    1

    Types of Valve

  • Sunday, June 14, 2015
  • denizen robo

  • Valves are used for

    1.Full flow of throttle :which is a wide range for valves use,because valves are designed in a way way to reduce pressure losses for fluids passing through theses valves,diaphragm valve is used with dangerous and toxic liquids.
      
    2,Flow regulating: Sometimes we need to regulate flow by changing calve positions between fully open and fully closed and throttling the valve reducing the flow,this is accomplished by using control valves.

    3.One-Way flow: Sometimes we need to prevent liquid in the reverse way,this type of  valves is installed at the pump or compressor discharge lines to prevent liquid and gas back flow when shut down these equipment.Check valve is a good example for this kind of valve.

    Types of Valves.
    1.Gate Valve
    2.Plug Valve
    3.Diaphragm Valve
    4.Ball valve
    5.Globe Valve.
    6.Butterfly Valve
    7.Check - Valve

    1.Gate Valve
    This type is designated to be fully open or fully closed,making a regular opening without any pressure losses and smooth flow.This type can be throttled,because it causes a corrosion in its internal parts,and can not be closed fully.
    Internal architecture of gate valve















    2.Plug Valve
    This type is a quick action valve when open or closed by rotating its lever,in addition to providing smooth flow and a very little pressure loss.This type is used in small sizes, and it has few failures.
    internal architecture of plug valve















    3.Diaphragm Valve
    In this kind of valves there is no contact between the medium and moving parts,that keeps it from oxidizing material,so this valve can be made of cheap metals.There is diaphragm between the valves and its cover.
    internal architecture of diaphragm valve















    4.Ball Valve  
    This types of valve consist of round cavity,it provides a full close position and usually used in petrochemical industries.This types used in small sizes.
    internal architecture of ball valve















    5.Globe Valve
    This type of valve is used to control liquid flow and when need to full close position,is cause a big pressure loss.There is no capability of seat corrosion because there is no contact between the seat and the disc which is conic to reduce the corrosion probability.
    internal architecture of globe valve












    6.Butterfly Valve 
    This types of valve contains a circular disc of a diameter same to the valve body.It is hardly used to for full open position because of the dirt between body and disc,in this case it needs to be opened and cleaned too much.Leak from packing happens too much in this valve.
    butterfly valve internal architecture















    7.Check Valve  
    All valves are used to control the flow,but this type is used prevent reverse flow,it is used at compressor or pump discharge lines.The common problems in this valve is the damage between the disc and body because of  continuous crash and the accumulation of dirt which leads to reverse flow.



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    Saturday, June 13, 2015

    0

    Working Principle of a Float operated Level Switch

  • Saturday, June 13, 2015
  • denizen robo
  • In a float operated level switch the float rides in the process liquid surface precisely tracking liquid surface motion.
    Rising liquid level lift the float sliding the attraction slip up inside the enclosing to and into the magnetic field to actuate electrical or pneumatic switch.
    Float type level switch working principle




















    Subsequently falling liquid level lowers the flow drawing the attraction slip out of the magnetic field to deactivate the switch.

    Float type level switch




















    Advantages of Float type level switches
    1.High pressure Capability
    2.Inexpensive
    3.Easy to install and maintain

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