Basics Of Industrial Instrumentation and Process Control.

This blog aims at providing the Aspiring Minds or Professionals to have answers to their questions relating Instrumentation and Process controls.All questions relating the topics would be answered. Bring it ON......

Saturday 30 September 2017

How does a Pneumatic valve positioner work?

How does a Pneumatic valve positioner work?

September 30, 2017 2 Comments

Pneumatic Valve Positioner Working Principle

The Pneumatic Valve Positioner is an instrument working on force balance principle to position the Control Valve stem in accordance to a pneumatic signal received from a controller.
The Valve Positioner is force balance device which, ensure the position of the plug, which is directly proportional to the controller output pressure. The Positioner compares the forces generated by the control signal and the control valve stem through the motion connector and the feed back cam, and accordingly it feeds or bleeds the air going to the valve actuator.
The instrument air signal is applied to the signal diaphragm. An increase in signal will drive the diaphragm and flapper-connecting stem to the right. The flapper-connecting stem will then open the supply flapper admitting supply pressure into the output which is connected to the actuator diaphragm. The exhaust flapper remains closed when the flapper connecting stem is deflected to right. The effect of increasing signal is to increase the pressure in the actuator. This increased pressure in the actuator drives the valve stem downward and rotates the positioner lever clockwise. This clockwise rotation of the lever results in a compression of range spring through cam. When the valve stem reaches the position called for by the controller, the compression in the range spring will give a balance force resulting the closure of both the flapper.
If the control signal is decreased, the force exerted by the signal diaphragm will also decrease and the force from the range spring will push the flapper-connecting stem to the left, opening the exhaust flapper. This causes a decrease actuator diaphragm pressure and allows the valve stem to move upward until a new force balance is established.

Direct Acting Control Valve Positioner

Reverse Acting Control Valve Positioner






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What is Flashing And Cavitation and how can it be cured?

What is Flashing And Cavitation and how can it be cured?

September 30, 2017 0 Comments
What’s the difference between flashing and cavitation?
The two words flashing and cavitation are interchangebly used but they arent the same.
 According the Bernoulli equation - when a fluid pass a valve seat and the fluid velocity increases - the fluid pressure decreases.

Cavitation

If the fluid speed through the valve increases enough, the liquid pressure drops to a level where the fluid may start to boil, bubble or flash. And when the pressure recovers sufficiently downstream of the control valve the bubbles will collapse upon themselves. This collapse causes cavitation.

Cavitation may be noisy but is usually of low intensity and low frequency. This situation is extremely destructive and may wear out the trim and body parts of a valve in short time.

What are the effects of flashing and cavitation on valves and processes?

If not controlled the flashing and cavitation can cause the trim to damage.
In an industrial process, the bubbles created by flashing get in the way of the liquid, reducing the flow while increasing the flow rate. The reduced capacity is often referred to as “choked flow.”
Flashing can also cause severe damage to your valves, mostly in the form of erosion of the valve plug. It’s important to note that this damage occurs irrespective of the liquid media flowing through the valve — even valves used for clean water applications can be damaged by flashing.
One way you can determine if you might have a problem is just to listen. Flashing makes a hissing sound, and cavitation makes a popping sound.

How can you prevent flashing and cavitation?

  • Put the valve in a high-pressure area. This will increase the differential between the fluid pressure and the vapor pressure, making it less likely the fluid pressure will fall low enough for flashing to occur. You can do this by putting the valve as far upstream as possible.
  • Use a downstream restriction device, like an orifice plate or a second valve, to increase the backpressure. This will increase fluid pressure and reduce velocity. 


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    Friday 29 September 2017

    Capacitance Level Transmitters.

    Capacitance Level Transmitters.

    September 29, 2017 0 Comments
    Capacitance level detectors are also referred to as radio frequency (RF) or admittance level sensors. They operate in the low MHz radio frequency range, measuring admittance of an alternating current (ac) circuit that varies with level. Admittance is a measure of the conductivity in an ac circuit, and is the reciprocal of impedance. Admittance and impedance in an ac circuit are similar to conductance and resistance in a direct current (dc) circuit. In this chapter, the term capacitance level sensor will be used instead of RF or admittance. 
    Theory of Operation

     A capacitor consists of two conductors (plates) that are electrically isolated from one another by a nonconductor (dielectric). When the two conductors are at different potentials (voltages), the system is capable of storing an electric charge. The storage capability of a capacitor is measured in farads. The capacitor plates have an area (A) and are separated by a gap (D) filled with a nonconducting material (dielectric) of dielectric constant (K).

    The dielectric constant of a substance is proportional to its admittance. The lower the dielectric constant, the lower the admittance of the material (that is, the less conductive it is). Capacitance (C) is calculated as: 
     
    In the case of a horizontally mounted level switch,a conductive probe forms one of the plates of the capacitor, and the vessel wall (assuming it is made from a conductive material) forms the other. An insulator with a low dielectric constant is used to isolate the conductive probe from the housing,which is connected to the vessel wall. 
    The probe is connected to the level sensor via the conductive threads of the housing. Measurement is made by applying an RF signal between the conductive probe and the vessel wall. 
    The RF signal results in a minute current flow through the dielectric process material in the tank from the probe to the vessel wall. When the level in the tank drops and the probe is exposed to the even less conductive vapors, the dielectric constant drops. This causes a drop in the capacitance reading and a minute drop in current flow. 
    The sensitivity of a capacitance sensor is expressed in pico-farads (pF). 

    In most level-sensing applications, the reference material is air (K1 = 1.0). 

    Probe Designs

    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. The probe is insulated from the housing and bin wall by an low-dielectric insulator, such as Nylon or Ryton. These polymers have maximum operating temperatures of 175-230°C (350-450°F). Ceramics can be used for higher temperature applications or if abrasion resistance is required. For applications where the process material is conductive and corrosive, the probe must be coated with PFA or Kynar. 
    Capacitance probes typically are coated with PFA (shown), Kynar, or polyethylene does not sense material build-up between the probe and vessel wall. 
    Typical insertion lengths of standard capacitance probes range from 7 to 16 in. These probes typically are side-mounted. Vertical probes can be extended by solid rods up to a length of 1.2 to 1.5 m (4 to 5 ft), or a steel cable with a weight can be used to suspend the probe up to 15 m (50 ft). Most capacitance level sensors are provided with 3/4 to 1- 1/2 in NPT mounting connectors. The matching female coupling is usually welded to the vessel wall and the capacitance probe is screwed into the mating connector. Low profile capacitance sensors also are available and are flange-mounted. 

     In applications where the vessel is non-conductive and unable to form the return path for the RF signal, a second probe placed parallel to the active one or a conductive strip can be installed. 

     Electronics & Housings

     The electronic circuitry of the probe performs the functions of: 1) rectifying and filtering the incoming power, 2) generating the radio frequency signal, 3) measuring the changes in current flow, and 4) driving and controlling interface devices such as relays, analog signal generators and display meters. The circuitry is usually of solid state design and provided with potentiometer adjustments for setting sensitivity and time delays. 

    The more advanced designs are also two-wire, intrinsically safe, and supply your choice of standard 4-20 mA or digitally enhanced output using the HART.

     The Dielectric Constant
    The dielectric constant of the process material is the most important aspect of the process data. The higher the difference between the dielectric constants (of the process material and the vapor space or between the two layers in the case of an interface measurement), the easier the measurement. If the difference is low (K2-K1 < 1.0 , a high sensitivity design (0.5 pF) must be used. 

     Each sensor has a capacitance     threshold, defined as the amount of capacitance change required to cause a change in the sensor output. The dielectric constant of a material can change due to variations in temperature, moisture, humidity, material bulk density, and particle size. If the change in dielectric constant results in a greater capacitance change than the calibrated capacitance threshold of the sensor, a false reading will result. This condition can usually be corrected by reducing the sensitivity (increasing the capacitance threshold) of the sensor. 

    Sensitivity can be increased by increasing the probe length (A) or by decreasing the size of the gap (D). Either or both changes will minimize the effect of dielectric constant fluctuations or increase sensitivity to low dielectrics. It is usually more practical to specify a longer probe than to decrease the distance (D) from the vessel wall. When the probe is installed from the side, D is fixed, whereas if the probe is inserted from the top of the tank, D can be changed (if other considerations permit) by moving the probe closer to the wall of the vessel.

    If the same vessel will hold different materials at different times, the capacitance sensor must be equipped with local or remote recalibration capability. 

     Light density materials under 20 lb/ft3 and materials with particle sizes exceeding 1/2 in. in diameter can be a problem due to their very low dielectric constants (caused by the large amount of air space between particles). These applications might not be suited for capacitance-type level measurement.

    Application Considerations

    Materials that are conductive (water-based liquids with a conductivity of 100 micromhos/cm or more) can cause a short circuit between a bare stainless steel probe and the vessel wall. As the liquid level drops, the probe remains wetted, providing a conductive path between the probe and the vessel wall. The faster the level changes, the more likely this false indication is to occur. It is advisable to use PFA or Kynar insulator coating on the conductive probe surface when the process fluid is conductive. 

     Temperature affects both the sensor components inside the vessel (active probes and insulators) and the electronic components and housing outside. An active probe is typically made from stainless steel and, as such (unless it is coated), it is suitable for most applications. Probe insulators can be PFA, Kynar, or ceramic, and should be selected for the operating temperature of the application. The housing and the electronics are affected by both the internal and external vessel temperatures.

    Ambient temperature limits usually are specified by the manufacturer, but heat conduction from a high-temperature process is more difficult to evaluate. Heat conduction can be reduced by using an extended mounting coupling or one made of a low thermal conductivity material. If such methods are insufficient, the electronics may be mounted up to 20 ft away and connected via coaxial cable. The cable's inherent capacitance, however, reduces the overall sensitivity of the system. 

     Housings must also be compatible with the requirements for hazardous, wash-down, wet, and/or dusty environments. Explosion-proof environments may require the housing to be certified. In addition, the active probe might need to be intrinsically safe. 

    If the process material is corrosive to stainless steel, the probe should be coated with Kynar or PFA for protection. Ryton is a good choice for abrasive materials, and, for food grade or sanitary applications, stainless steel and PFA are a good probe-insulator combination.

    Installation Considerations

    The capacitance probe should be mounted in such a way that its operation is unaffected by incoming or outgoing material flow. Material impacts can cause false readings or damage to the probe and insulator. When measuring low-dielectric materials, it's important that the entire probe be covered, not just the tip. When rod or cable extensions are used, allow for 8-12 in. of active probe coverage. 

     Install the probe so that it does not contact the vessel wall or any structural elements of the vessel. If a cable extension is used, allow for swinging of the cable as the material level in the vessel rises, so that the plumb bob on the end of the cable does not touch the vessel wall. The probe should not be mounted where material can form a bridge between the active probe and the vessel wall. In addition, the probe should not be mounted at an upward angle, to avoid material build-up. 

    If more than one capacitance level sensor is mounted in the vessel, a minimum distance of 18 in. should be provided between the probes. Closer than that and their electromagnetic fields might interfere. If a capacitance probe is installed through the side wall of a vessel and the weight of the process material acting on the probe is sometimes excessive, a protective baffle should be installed above the sensor.
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    Thursday 28 September 2017

    Basic Operation of Mechanical Compression Chiller.

    September 28, 2017 0 Comments
    Basic operation of  Mechanical Chiller.
    Mechanical Compression Chiller.
    • Chillers Produce chilled water in the evaporator.In evaporator there are tubes filled with water and refrigerant surrounds the tube.The heat gets from the refrigerant to the water and hence the refrigerant evaporates.The Chilled water in the evaporator is used in the coils of air handling units to remove heat from the spaces,hot water is then returned to the evaporator and same cycle repeats.
    • The vaporized refrigerant leaves the evaporator and  enters the compressor.The compressor changes it into high temperature and high pressure gas which then travels to the water cooled condenser.
    • The condenser tube contains water.The condenser is connected to the cooling tower where the heat is transferred to the atmosphere. the heat is transferred from the hot refrigerant to the water.The condenser being connected to the water cooling tower helps in transferring heat to the atmosphere.The cooled refrigerant then travels to the expansion valves which controls the rate of cooling.
    • The further cooled refrigerant then travels to the evaporator and the whole cycle repeats.  
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    Ranging of a Radar Level Transmitter

    September 28, 2017 0 Comments
    Ranging of a Non-Contact Level Transmitter.

    When i was given the project of installing Level transmitter i thought it would be a cake-walk for me,instead it turned out to be a night mare.With hell of calculations boggling my mind then, i searched everywhere for help, even googled but came across nothing  that could bear fruit. This is what make me write this post so that many others could take at least some help.
    Dimensional Drawing of the tank.
    We begun ranging the transmitter simply from the top of the nozzle to the bottom of the tank with a span reading of 600+2100+280=2980 mm and thought it will as simple as this.The very next day we found our tank overflowing with solvent.Now that was dangerous.It should solved.After hundred of attempts we finally arrived at the final values.
    Zero reading or empty calibration was entered as 600+2100=2700
    Span reading was entered as  2100-280=1820 mm
    The dead level needs to be subtracted from the total length to be measured i.e the Full Calibration value.
    Also compare the Level Transmitter Value with the level indicator on tanks.
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    Working of Catalytic bead sensor in Gas Detection.

    September 28, 2017 0 Comments


    Catalytic bead sensor

    • Under certain circumstances flammable gases and vapors can be oxidized by the means of air ‘s oxygen to release the heat of reaction.This is achieved through special and suitable heated catalyst material which slightly increases its temperature by heat of the reaction .this increase in temperature is the measure of gas concentration.
    • So called pellistors are very tiny porous ceramic beads embedding a small platinum wire coil. There is an electric current flowing through the platinum wire so that pellistor is heated to some hundred degree Celsius.
      Pellistor
    • If the ceramic bead contains some suitable catalytic material the pellistors temperature will rise in presence of flammable gas and the platinum wire coils resistance will increase accordingly. This change in resistance wrt resistance change in clean air is used for electronic evaluation.
    •  One pellistor is alone not suitable for detection of flammable gases and vapours. It needs a second one to compensate for environmental parameter and it needs to be explosion protected. By means of flameproof enclosure and sinter disk  a useful catalytic bead sensor results.
    • The compensator pellistor is built  very similar to active pellistor but does not contain the pellistor material so that gas cannot be oxidized. If the ambient temperature changes resistance of both pellistor changes and there is  no bridge signal.
    • However if gas is present only the resistance of active pellistor changes and the wheatstone bridge is balanced.
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    Electrochemical Sensors.

    September 28, 2017 0 Comments


    Electrochemical Sensors
    Principle of operation.  

    • Electrochemical sensors work by reacting with the gas of interest and producing an electric signal proportional to the gas concentration.
    • The gas first passes through the capillary diffusion barrier and diffuses through the hydrophobic membrane.This allows specific amount of gas  to react with the sensing electrode to produce sufficient amount of electrical signal and also preventing the leak of electrolyte.
    • The gas that passes through the barrier reacts at the surface of the sensing electrode involving either an oxidation or reduction reaction.
    • A current is generated in the process hence it is called as amperometric gas sensor or a microfuel cell.
    Electrochemical Sensor
    Importance of Reference Electrode

    • For a sensor requiring  an    electrical driving voltage it is important to have a stable & constant potential at the sensing electrode. In reality the potential at the sensing electrode does not remain constant due to continuous chemical reaction . It causes deterioration of the electrodes over the extended periods of time. To improve the performance of the sensor reference electrode is introduced. This reference electrode is placed in close proximity of the sensing electrode. This reference electrode maintains the value of fixed voltage at the sensing electrode.
    • No current flows through the reference electrode.
    • Current flows between sensing electrode and  counter electrode.
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    Friday 22 September 2017

    ATEX Standards

    ATEX Standards

    September 22, 2017 0 Comments
    Atex  standards:
    • Atex stands for atmosphere explosives .
    • As of july 2003 ,organizations in EU must follow the directives to protect employees from exploision risk in areas with an explosive atmosphere.
    • There are two atex  directives (one for the manufacturer and one for user of the equipments.
    1.The Atex  95 equipment directive 94/9/EC equipment and protective systems  intended for use in potentially explosive atmospheres.
    2.Atex 137 work place directive  99/92/EC ,minimum requirements for improving the safety  and health protection of the workers potentially at risk from explosive atmospheres.
    ATEX Coding
    EX
    European union explosives atmosphere symbol
    ii
    Equipment group

    1.Mining

    2.Surface
    2
    Equipment Category:

    M1. Energized

    M2. Deenergized.

    1. Very high protection.

    2. High Protection.

    3.Normal Protection
    G
    Gas

    0
    1
    2
    D
    20

    21

    22


    Hazardous area zones and equipment categories Hazardous places are classified in terms of zones on the basis of the frequency and duration of the occurrence of an explosive atmosphere.
    Gases, vapours and mists For gases, vapors and mists the zone classifications are:
    Zone 0:  A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is present continuously or for long periods or frequently.
    Zone 1:  A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is likely to occur in normal operation occasionally.
    Zone 2:  A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
    Dusts: For dusts the zone classifications are:
    Zone 20: A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently.
    Zone 21:  A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally.

    Zone 22:  A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
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