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Instrumentation Control Process


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Instrumentation Control Process

  1. 1. Instrumentation & Control IMRAN KHAN Registration No: - 10D2-113031 Program: - B-Tech (Pass) Electronics
  2. 2. What is instrument ? Instrument is a devices which is used to measure, monitor, display etc. of a process variable. • What are the process Variable ? The process Variable are : 1. Flow 2. Pressure 3. Temperature 4. Level
  3. 3.  A process of ( liquid, gas, or electricity) move steadily and continuously in a current or stream. What is Flow ?  go from one place to another in a steady stream, typically in large numbers. .  Flow can be measured in a variety of ways. Positive- displacement flow meters accumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow.
  4. 4. What is pressure ?  Pressure is force per unit area applied in a direction perpendicular to the surface of an object .  Pressure is the ratio of force applied per area covered …  P = F/A he unit of pressure is the Pascal  Pa = N = kg m/s2 =kg  The Pascal is also a unit of stress and the topics of pressure and stress are connected.  Bed of nails (not really pressure but shear strain?)  Finger bones are flat on the gripping side to increase surface area in contact and thus reduce compress ional stresses .
  5. 5. What is Temperature ?  In a qualitative manner, we can describe the temperature of an object as that which determines the sensation of warmth or coldness felt from contact with it.  Temperature is a degree of hotness or coldness the can be measured using a thermometer. It's also a measure of how fast the atoms and molecules of a substance are moving. Temperature is measured in degrees on the Fahrenheit, Celsius, and Kelvin scales.  A temperature is a numerical measure of hot or cold. Its measurement is by detection of heat radiation or particle velocity or kinetic energy, or by the bulk behavior of a thermometric material. It may be calibrated in any of various temperature , Celsius, Fahrenheit, Kelvin, etc. The fundamental physical definition of temperature is provided by thermodynamic .
  6. 6. What is Level ?  A device for establishing a horizontal line or plane by means of a bubble in a liquid that shows adjustment to the horizontal by movement to the center of a slightly bowed glass tube  A measurement of the difference of altitude of two points by means of a level .  Horizontal condition; especially : equilibrium of a fluid marked by a horizontal surface of even altitude <water seeks its own level>  The magnitude of a quantity considered in relation to an arbitrary reference value; broadly : magnitude, intensity <a high level of hostility>
  7. 7. What is meaning of Loop in instrumentation ?  In computer programming, a loop is a sequence of instructions that is continually repeated until a certain condition is reached. Typically, a certain process is done, such as getting an item of data and changing it, and then some condition is checked such as whether a counter has reached a prescribed number. If it hasn't, the next instruction in the sequence is an instruction to return to the first instruction in the sequence and repeat the sequence. If the condition has been reached, the next instruction "falls through" to the next sequential instruction or branches outside the loop. A loop is a fundamental programming idea that is commonly used in writing programs. An infinite loop is one that lacks a functioning exit routine . The result is that the loop repeats continually until the operating system senses it and terminates the program with an error or until some other event occurs (such as having the program automatically terminate after a certain duration of time).
  8. 8. Define the types of loop control ?  Open loop control system .  Close loop control system .  Cascade loop control system .
  9. 9. What is open loop system ?  An open-loop controller, also called a non-feedback controller, is a type of controller that computes its input into a system using only the current state and its model of the system.  A characteristic of the open-loop controller is that it does not use feedback to determine if its output has achieved the desired goal of the input. This means that the system does not observe the output of the processes that it is controlling. Consequently, a true open-loop system can not engage in machine learning and also cannot correct any errors that it could make. It also may not compensate for disturbances in the system.  An open-loop controller is often used in simple processes because of its simplicity and low cost, especially in systems where feedback is not critical. A typical example would be a conventional washing machine .
  10. 10. Open loop control system Diagram  Open loop
  11. 11. What is close loop system ?  Closed loop control systems are those that provide feedback of the actual state of the system and compare it to the desired state of the system in order to adjust the system.  The closed loop control system is a system where the actual behavior of the system is sensed and then fed back to the controller and mixed with the reference or desired state of the system to adjust the system to its desired state. The objective of the control system is to calculate solutions for the proper corrective action to the system so that it can hold the set point (reference) and not oscillate around it.  When a scale out triggering event occurs, the input parameter that triggers the event is monitored around its set point. The system increases and decreases capacity on demand to stay as close as possible to the set point for the triggering parameter.  With closed loop systems, you can evaluate the system near the set point using a PID control algorithm or similar control scheme. A simpler approach, such as hysteretic, can be very effective and can be implemented with less complexity and tuning.
  12. 12. Close loop system diagram .  Close loop Block diagram
  13. 13. What is Cascade loop control system ?  A cascade control system is a multiple-loop system where the primary variable is controlled by adjusting the set point of a related secondary variable controller. The secondary variable then affects the primary variable through the process.  The primary objective in cascade control is to divide an otherwise difficult to control process into two portions, whereby a secondary control loop is formed around a major disturbances thus leaving only minor disturbances to be controlled by the primary controller.  The use of cascade control is described in many texts on process control applications.  Cascade control is most advantageous on applications where the secondary closed loop can include the major disturbance and second order lag and the major lag is included in only the primary loop. The secondary loop should be established in an area where the major disturbance occurs.
  14. 14. Cascade loop control system Diagram.  Cascade loop
  15. 15. Pressure measurements .  Pressure is the force exerted per unit area  Pressure is the action of one force against another force. Pressure is force applied to, or distributed over, a surface. The pressure P of a force F distributed over an area A is defined as P = F/A TOTAL VACUUM - 0 PSIA PRESSURE ABSOLUTE GAUGE COMPOUND BAROMETRIC RANGE ATMOSPHERIC PRESSURE NOM. 14.7 PSIA
  16. 16. Pressure Measurement Terms.  Absolute Pressure Measured above total vacuum or zero absolute. Zero absolute represents total lack of pressure.  Atmospheric Pressure The pressure exerted by the earth’s atmosphere. Atmospheric pressure at sea level is 14.696 PSI. The value of atmospheric pressure decreases with increasing altitude.  Barometric Pressure Same as atmospheric pressure.  Gauge Pressure The pressure above atmospheric pressure. Represents positive difference between measured pressure and existing atmospheric pressure. Can be converted to absolute by adding actual atmospheric pressure value.  Differential Pressure The difference in magnitude between some pressure value and some reference pressure. In a sense, absolute pressure could be considered as a differential pressure with total vacuum or zero absolute as the reference. Likewise, gauge pressure (defined above) could be considered as Differential Pressure with atmospheric pressure as the reference.
  17. 17. Pressure Units. psi 100 bar 6.895 mbar 6895 mm of Hg 5171 mm of WC 70358 in of WC 2770 Kg/cm2 7.032 Pascal 689476 kPa 689.5 atm 6.805
  18. 18. Types of Pressure Instruments  Pressure Gauges (Vacuum, Compound, Absolute, Gauge)  Differential Pressure Gauge  Pressure Switch (Vacuum, Absolute, Gauge)  Differential Pressure Switch  Pressure Transmitter (Vacuum, Absolute, Gauge)  Differential Pressure Transmitter PRESSURE GAUGE PRESSURE SWITCH DIFFERENTIALPRESSURE TRANSMITTER
  19. 19. PRESSURE GAUGES  A Pressure Gauge is used for measuring the pressure of a gas or liquid.  A Vacuum Gauge is used to measure the pressure in a vacuum.  A Compound Gauge is used for measuring both Vacuum and Pressure.  Pressure Gauges are used for Indication only.  Measuring Principle  Bourdon tube measuring element is made of a thin-walled C-shape tube or spirally wound helical or coiled tube. When pressure is applied to the measuring system through the pressure port (socket), the pressure causes the Bourdon tube to straighten itself, thus causing the tip to move. The motion of the tip is transmitted via the link to the movement which converts the linear motion of the bourdon tube to a rotational motion that in turn causes the pointer to indicate the measured pressure.
  20. 20. Gauge Construction types . “C” Type Bourdon Helical Bourdon Coiled Bourdon
  21. 21. Pressure Switch  The device contains a micro switch, connected to a mechanical lever and set pressure spring. The contacts get actuated when process pressure reaches the set pressure of the spring.  It can be used for alarming or interlocking purposes, on actuation.  It can be used for high / high-high or low / low-low actuation of pressure in the process . The set range can be adjusted within the switch range.  The sensing element may be a Diaphragm or a piston
  22. 22. Working principle  Pressure/Vacuum Switch - A device that senses a change in pressure/vacuum and opens or closes an electrical circuit when the set point is reached.  Pressure switches serve to energize or de-energize electrical circuits as a function of whether the process pressure is normal or abnormal.  The electric contacts can be configured as single pole double throw (SPDT), in which case the switch is provided with one normally closed (NC) and one normally open (NO) contact.  Alternately, the switch can be configured as double pole double throw (DPDT), in which case two SPDT switches are furnished, each of which can operate a separate electric circuit.
  23. 23. Pressure Transmitter
  24. 24. Pressure Transmitter Advantages  A Pressure Transmitter is used where indication and/or record of pressure is required at a location not adjacent to the primary element.  A Pressure Transmitter is used for both indication and control of a process.  A Pressure Transmitter is used where overall high performance is mandatory.  Both Electronic and Pneumatic Transmitters are used.  These can be either Gauge, Absolute or Differential Pressure Transmitters.
  25. 25. Transmitter Measuring Principle  The diagram shows an electronic differential pressure sensor. This particular type utilizes a two-wire capacitance technique.  Another common measuring technique is a strain gauge.  Process pressure is transmitted through isolating diaphragms and silicone oil fill fluid to a sensing diaphragm.  The sensing diaphragm is a stretched spring element that deflects in response to the differential pressure across it.  The displacement of the sensing diaphragm is proportional to the differential pressure.  The position of the sensing diaphragm is detected by capacitor plates on both sides of the sensing diaphragm.  The differential capacitance between the sensing diaphragm and the capacitor plates is converted electronically to a 4–20 mA or 1-5 VDC signal.  For a gauge pressure transmitter, the low pressure side is referenced to atmospheric pressure.
  26. 26. Flow Measurements.  The Orifice Plate  The orifice plate is the simplest and cheapest. It is simply a plate with a hole of specified size and position cut in it, which can then clamped between flanges in a pipeline  The increase that occurs in the velocity of a fluid as it passes through the hole in the plate results in a pressure drop being developed across the plate.  After passing through this restriction, the fluid flow jet continues to contract until a minimum diameter known as the vena contracta is reached.
  27. 27. Orifice Plate
  28. 28. Working principle & Advantages  The orifice plate is the simplest and cheapest.  The increase that occurs in the velocity of a fluid as it passes through the hole in the plate results in a pressure drop being developed across the plate. After passing through this restriction, the fluid flow jet continues to contract until a minimum diameter known as the vena contracta is reached.  The equation to calculate the flow must be modified
  29. 29. The Venturi Tube The classical or Herschel Venturi tube is the oldest type of differential pressure flow meter (1887). The restriction is introduced into the flow in a more gradual way The resulting flow through a Venturi tube is closer to that predicted in theory so the discharge coefficient C is much nearer unity (0.95). The pressure loss caused by the Venturi tube is lower, but the differential pressure is also lower than for an orifice plate of the same diameter ratio.
  30. 30. Advantages of Venturi Tube  The smooth design of the Venturi tube means that it is less sensitive to erosion than the orifice plate, and thus more suitable for use with dirty gases or liquids.  The Venturi tube is also less sensitive to upstream disturbances, and therefore needs shorter lengths of straight pipe work upstream of the meter than the equivalent orifice plate or nozzle.  Like the orifice plate and nozzle, the design, installation, and use of the Venturi tube is covered by a number of international standards.  The disadvantages of the Venturi tube flow meter are its size and cost.
  31. 31. The Nozzle  The nozzle combines some of the best features of the orifice plate and Venturi tube.  It is compact and yet, because of its curved inlet, has a discharge coefficient close to unity.  There are a number of designs of nozzle, but one of the most commonly used in Europe is the ISA-1932 nozzle, while in the U.S., the ASME long radius nozzle is more popular. Both of these nozzles are covered by international standards
  32. 32. Rota meter  Rota meter consists of a conical transparent vertical glass tube containing a “bob”.  The flow rate is proportional to the height of the bob.  The Rota meter is characterized by:  Simple and robust construction  High reliability  Low pressure drop
  33. 33. Axial Turbine Flow meters  The modern axial turbine flow meter is a reliable device capable of providing the highest accuracies attainable by any currently available flow sensor for both liquid and gas volumetric flow measurement. It is the product of decades of intensive innovation and refinements to the original axial vaned flow meter principle first credited to Wolman in 1790, and at that time applied to measuring water flow.  The initial impetus for the modern development activity was largely the increasing needs of the U.S. natural gas industry in the late 1940s and 1950s for a means to accurately measure the flow in large-diameter, high- pressure, interstate natural gas lines.  Today, due to the tremendous success of this principle, axial turbine flow meters of different and often proprietary designs are used for a variety of applications where accuracy, reliability, and range ability are required in numerous major industries besides water and natural gas, including oil, petrochemical, chemical process, cryogenics, milk and beverage, aerospace, biomedical, and others.
  34. 34. Turbine Flow meter
  35. 35. Turbine Flow meter & working principle  The meter is a single turbine rotor, concentrically mounted on a shaft within a cylindrical housing through which the flow passes.  The shaft or shaft bearings are located by end supports inside suspended upstream and downstream aerodynamic structures called diffusers, stators, or simply cones.  The flow passes through an annular region occupied by the rotor blades. The blades, which are usually flat but can be slightly twisted, are inclined at an angle to the incident flow velocity and hence experience a torque that drives the rotor.  The rate of rotation, which can be up to several ×104 rpm  A magnetic pick up coil detect the rotation
  36. 36. Level Measurement Types  Level Gauges  Guided Wave Radar  Radar  Differential Pressure  Float / Displacer  Ultrasonic  Capacitance  Nuclear
  37. 37. Level Gauges  Tubular  Glass Tube with Option of Graduations  Not Popular for Process Applications  Typically Used for Calibrating Metering Pumps (Calibration Tubes)
  38. 38. Flat Glass Gauges  Glass Sections on Opposite Sides of the Chamber  View the Liquid Level through the Gauge  Used on Interface Applications and Dirty or Viscous Liquids  Illuminators Can be Used to Diffuse Light Evenly on the Back of the Gauge
  39. 39. Reflex Flat Glass Gauge  Single Glass Section with Prisms Cut in the Glass on the Process Side  Light Striking the Vapor Phase is Refracted to the Viewer which Appears Silvery White  Light Striking the Liquid Phase is Refracted into the Liquid which Appears Black  Used on Clean, Clear, No corrosive Liquids
  40. 40. Magnetic Level Gauge  Consists of a Non-Magnetic Chamber, Internal Float with Magnet and Bi- Colored Indicator Wafers
  41. 41. Float / Displacer  The Visible Length Should Cover the Full Operating Range of Interest Including any Other Level Instrumentation on the Vessel  If More than One Gauge is Required, the Gauges Must Overlap Each Other  Level Chamber Needs to be Installed Vertically Level to Reduce any Possible Friction with the Float  Require Jig Set Connections  May Require a Magnetic Trap
  42. 42. Level Float / Displacer Advantages  Long Visible Lengths  Corrosive or Toxic Liquid Applications  Adaptable to Variations in Fluid Densities  High Pressure or Temperature Applications Limitations  Affected by changes in fluid density  Coating media may seize moving parts  Over Pressuring can Implode Float  Long ranges may require additional support
  43. 43. Bubbler  When Air Pressure Enters a Dip Pipe with a Pressure Greater Than the Hydrostatic Head of the Process Fluid, the Air will Bubble out the Bottom of the Dip Pipe  As the Liquid Level Changes, the Air Pressure in the Dip Pipe also Changes  Consists of Pressure Regulator, Rota meter and Pressure Gauge Along with a Stilling Well VENT D/P TRANSMITTER INSTRUMENT AIR
  44. 44. Types of Temperature Instrument  Thermocouple T/C  Resistance Temperature Detector (RTD)  Thermo well  Bi-metallic Thermometers  Filled Thermal Systems
  45. 45. Various Units of Temperature Measurement  °C – degrees Celsius (or Centigrade)  °F – degrees Fahrenheit  K – Kelvin  R – Rankin Relationship between different units  °C = (°F - 32)/1.8  °F = 1.8 x °C + 32  K = °C + 273.15  R = °F + 459.67 Conversion tables or software can be utilized to facilitate with converting between these units.
  46. 46. Thermocouples (TC’s)  Basic Theory  In 1821 a German physicist named See back discovered the thermoelectric effect which forms the basis of modern thermocouple technology. He observed that an electric current flows in a closed circuit of two dissimilar metals if their two junctions are at different temperatures.  The thermoelectric voltage produced depends on the metals used and on the temperature relationship between the junctions.  If the same temperature exists at the two junctions, the voltage produced at each junction cancel each other out and no current flows in the circuit.  With different temperatures at each junction, different voltages are produced and current flows in the circuit.  A thermocouple can therefore only measure temperature differences between the two junctions, a fact which dictates how a practical thermocouple can be utilized. Iron (Fe) Constantan (CuNi) 0ºC100ºC Thermocouple Circuit
  47. 47. Thermocouple measuring circuit 20ºC Copper (Cu) Copper (Cu) 0 mV 10 Hot Junction: In Process 100ºC Iron (Fe) Constantan (CuNi) Equivalent to 80ºC reading Cold Junction: Needs to be held constant to give a fixed reference. ( early methods held cold junction at 0ºC using ice or refrigeration unit).
  48. 48. Standard Thermocouple Alloy Conductor Combinations CODE CONDUCTOR COMBINATION TYPICAL OPERATING RANGE ºF B Platinum-30% Rhodium / Platinum-6% Rhodium +2500 to +3100 C Tungsten-5% Rhenium / Tungsten-26% Rhenium +3000 to +4200 D Tungsten-3% Rhenium / Tungsten-25% Rhenium +2800 to +3800 E Nickel Chromium / Constantan 0 to +1650 J Iron / Constantan +0 to +1400 K Nickel Chromium / Nickel Aluminium 0 to +2300 N Nickel-Chromium-Silicon / Nickel-Silicon-Magnesium 1200 to +2300 R Platinum-13% Rhodium / Platinum 1600 to +2600 S Platinum-10% Rhodium / Platinum 1800 to +2600 T Copper / Constantan -300 to +650
  49. 49. Thermocouple Construction • Normally element is in a thermowell • Commonly element is 1/4” outside Diameter • Sheath material, normally Stainless steel but can be special material such as Inconel, Incoloy, Hastelloy etc. • Duplex thermocouples have 2 elements inside one sheath.
  50. 50. RTDs  RTDs (Resistance Temperature Detectors) operate under the principle that the electrical resistance of certain metals increases and decreases in a repeatable and predictable manner with a temperature change.
  51. 51. RTD Elements  Wire Wound Element Precise lengths of wire are wrapped around a ceramic mandrel, then inserted inside a ceramic shell which acts to support and protect the wire windings.  Inner Coil Element Wires are coiled then slid into the holes of a ceramic insulator. Some manufacturers backfill the bores with ceramic powder after the coils are inserted. This keeps the coils from shorting against each other.  Thin Film Element Metallic ink is deposited onto a ceramic substrate. Lasers then etch the ink to provide a resistance path. The entire assembly is encapsulated in ceramic to support and protect.
  52. 52. RTD Lead wire Configuration  2-wire: Should only be used with very short runs of leadwire. No compensation for leadwire resistance.  3-wire: Most commonly used for industrial applications. Leadwire compensation.  4-wire: Laboratory use historically, moving more into industrial applications. Full compensation for leadwire resistance.
  53. 53. Wheatstone Bridge  The most common method for measuring the resistance of an RTD is to use a Wheatstone bridge circuit. In a Wheatstone bridge, electrical excitation current is passed through the bridge, and the bridge output current is an indication of the RTD resistance. R 1 R 2 R 3 AMMETER RTD
  54. 54. Temperature Element Assembly Head Nipple-Union-Nipple Thermowell
  55. 55. Thermo wells Straight Shank Flanged Van Stone Step Shank Tapered Shank Threaded Weld-in Plug Plug with Chain Accessories
  56. 56. Thermo wells
  58. 58. Temperature Transmitters  Signal Conditioner  Low level inputs mV from thermocouples  from RTD’s  High level outputs 4-20mA current Digital (i.e. Fieldbus)
  59. 59. Thermistors  Thermistors are temperature sensing devices that are similar to RTD’s in that their resistance changes as temperature changes.  The major difference is that for most thermistors the resistance decreases as temperature increases.  Thermistors are an inexpensive alternative to RTD’s when temperature ranges are below 150°C. Thermistors can be used from temperatures of –80°C to 300°C.  Most thermistors have base resistances, which are much higher than RTD’s.  One of the greatest advantages of using a thermistor sensor is the large change in resistance to a relatively small change in temperature. This makes them very sensitive to small changes in temperature.
  60. 60. Bimetallic Thermometers A Bimetallic Thermometer consists of an indicating or recording device, a sensing element and a means for connecting the two. Basic example: Two metal strips expand at different rates as the temperature changes. A pointer is attached to the rotating coil which indicates the temperature on the dial. Coil rotation is caused by the difference in thermal expansions of the two metals. Bimetal Coil
  61. 61. Gas chromatography & analyzer  Gas chromatography - specifically gas-liquid chromatography - involves a sample being vaporized and injected onto the head of the chromatographic column. The sample is transported through the column by the flow of inert, gaseous mobile phase. The column itself contains a liquid stationary phase which is adsorbed onto the surface of an inert solid.  Carrier gas  The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, argon, and carbon dioxide. The choice of carrier gas is often dependant upon the type of detector which is used. The carrier gas system also contains a molecular sieve to remove water and other impurities.
  62. 62. schematic diagram of a gas chromatograph
  63. 63. Sample injection port  For optimum column efficiency, the sample should not be too large, and should be introduced onto the column as a "plug" of vapor - slow injection of large samples causes band broadening and loss of resolution. The most common injection method is where a micro syringe is used to inject sample through a rubber septum into a flash vaporizer port at the head of the column. The temperature of the sample port is usually about 50°C higher than the boiling point of the least volatile component of the sample. For packed columns, sample size ranges from tenths of a micro liter up to 20 micro liters. Capillary columns, on the other hand, need much less sample, typically around 10-3 mL. For capillary GC, split/split less injection is used. Have a look at this diagram of a split/split less injector;
  64. 64. The split /injector
  65. 65. Detectors
  66. 66. Columns There are two general types of column, packed and capillary (also known as open tubular). Packed columns contain a finely divided, inert, solid support material (commonly based on diatomaceous earth) coated with liquid stationary phase. Most packed columns are 1.5 - 10m in length and have an internal diameter of 2 - 4mm.
  67. 67. Gas Turbine for Power Generation  The use of gas turbines for generating electricity dates back to 1939 Today, gas turbines are one of the most widely-used power generating technologies. Gas turbines are a type of internal combustion (IC) engine in which burning of an air-fuel mixture produces hot gases that spin a turbine to produce power. It is the production of hot gas during fuel combustion, not the fuel itself that the gives gas turbines the name. Gas turbines can utilize a variety of fuels, including natural gas, fuel oils, and synthetic fuels. Combustion occurs continuously in gas turbines, as opposed to reciprocating IC engines, in which combustion occurs intermittently.  How Do Gas Turbines Work? Gas turbines are comprised of three primary sections mounted on the same shaft: the compressor, the combustion chamber (or combustor) and the turbine. The compressor can be either axial flow or centrifugal flow. Axial flow compressors are more common in power generation because they have higher flow rates and efficiencies. Axial flow compressors are comprised of multiple stages of rotating and stationary blades (or stators) through which air is drawn in parallel to the axis of rotation and incrementally compressed as it passes through each stage. The acceleration of the air through the rotating blades and diffusion by the stators increases the pressure and reduces the volume of the air. Although no heat is added, the compression of the air also causes the temperature to increase.
  68. 68. Turbines Work
  69. 69. Principle of turbine  The compressed air is mixed with fuel injected through nozzles. The fuel and compressed air can be pre-mixed or the compressed air can be introduced directly into the combustor. The fuel-air mixture ignites under constant pressure conditions and the hot combustion products (gases) are directed through the turbine where it expands rapidly and imparts rotation to the shaft. The turbine is also comprised of stages, each with a row of stationary blades (or nozzles) to direct the expanding gases followed by a row of moving blades. The rotation of the shaft drives the compressor to draw in and compress more air to sustain continuous combustion. The remaining shaft power is used to drive a generator which produces electricity. Approximately 55 to 65 percent of the power produced by the turbine is used to drive the compressor. To optimize the transfer of kinetic energy from the combustion gases to shaft rotation, gas turbines can have
  70. 70. Turbine field installation
  71. 71. Final instrument of process control valve  Topics:-  Introduction  Control Valve Characteristics and Types  Control Valve Parts  Control Valve Accessories  Control Valve Operation  Valve Hand Jack and Minimum Stop
  72. 72. Introduction of control valve  Introduction:-  The control valve is composed of a valve with an externally powered actuator. The control valve is designed specifically for reliable continuous throttling with minimum backlash and packing friction. The control valve is involved with the disposition of energy in a process. It dispenses energy from the source, dissipated energy that exists within the system, or distributes energy in the system in one way or another.  The chemical and petroleum industries have many applications requiring control of gases, liquids, or vapors processes. Many process operation require regulation of such quantities as density and composition, but by far the most important control parameter is flow rate. A regulation of flow rate emerges as the regulatory parameters for reaction rate, temperature, composition, or a host of other fluid properties. For this purposes the control valve is using as the process control element.
  73. 73. Control Valve Characteristics and Types  The different types of control valves are classified by a relationship between the valve stem position and the flow rate through the valve. This control valve characteristic is assigned with the assumptions that the stem position indicates the extent of the valve opening and that the pressure difference is determined by the valve alone. There are three basic types of control valves whose relationship between stem position (as percentage of full range) and the flow rate (as a percentage of maximum) • Quick Opening:- This types of valve is used predominantly for full ON/full OFF control applications. The valve characteristic shows that a relatively small motion of valve stem results in maximum possible flow rate
  74. 74.  Through the valve. Such a valve, for Example , may allow 90% of the flow rate with only a 30% travel of the stem.  2. Linear:- This type of valve, as shown in picture, has a flow rate that varies linearly with the stem position. It depends the ideal situation where the valve alone determines the pressure drop.  3. Equal Percentage:- Equal percentage is the characteristic most commonly used in process control. The change in flow per unit of valve stroke is directly proportional to the flow occurring just before the change is made. While the flow characteristic of the valve itself may be equal percentage, most control loops will produce an installed characteristic approaching linear when the overall system pressure drop is large relative to that across the valve.
  75. 75. The types of the valves as follows,  Globe valve  Butterfly valves  Gate valves  Diaphragm valves  Ball valves  Knife edge valves
  76. 76. Control Valve Parts  The Valves has Two Main Parts  Body Assembly  Actuator Assembly
  77. 77. Body Assembly Parts  Body: The pressure retaining housing through which the service fluid flows. It has inlet and outlet connections, and houses the trim components  Seat Ring: Trim component the plug makes contact with to close the valve.  Seat Retainer: Trim component which clamps the seat ring in place. The seat retainer does not guide the plug, and should not be confused with a cage  All Gaskets: are used in control valves to prevent leakage. around the seat ring, bonnet or pressure- balanced sleeve.
  78. 78.  Plug: Part that moves in and out of the seat ring to open and close the valve. It can also be used to characterize the flow.  Bonnet: The valve component which houses the guides and packing. It also seals one opening to the body.  Bonnet Flange: Flange that attaches the bonnet to the body.  Guides — Bushings contained in the packing box which align the plug with the seat ring.  Guides — Bushings contained in the packing box which align the plug with the seat ring.  Packing — Material used to seal the valve from leaking around the plug stem.  Packing Box — Internal bore of bonnet which contains guiding and packing.
  79. 79. Actuator Assembly
  80. 80. Actuator Assembly Parts  Actuator — Device which develops sufficient thrust to open or close the valve. Common designs include piston, diaphragm, hydraulic, manual hand wheel and electro-hydraulic actuators.  Lifting Ring: Used for Lifting The Valve  Adjusting Screw: Part used to compress the actuator spring.  Cylinder: Actuator part used for containing air pressure and enclosing the piston.  Spring Button: The part that prevents movement of the actuator spring and permits the adjusting screw to compress the spring.  Spring: In piston actuators, the part which provides force for fail-safe operation; in diaphragm actuators, the part that provides force to counteract air pressure from the opposing chamber.  Piston: Part used to separate two air chambers of piston actuator.  Actuator Stem: Part used to connect the valve plug with the piston actuator.  Yoke: A component which secures the actuator to the valve body.
  81. 81. Butterfly Valve Body Assembly
  82. 82. Spring Cylinder Rotary Actuator
  83. 83. Valve Accessories  Positioner  I/P Transducer  Volume Booster  Quick Exhaust  Lockup Realy  Solenoid  Limit Switch
  84. 84. Positioner  A valve Positioner is like a proportional controller. The set point is the control signal from the primary controller and the controlled variable is the valve position. The Positioner compensates for disturbances and nonlinearities.  The use of positioner are as follows,  When the signal pressure is not enough to operate the control valve.  To make split range between the valves.  It can be used to reverse the action of the actuator from air to open to air to close and vice versa.  To minimize the effect of hysterisis effect.  To minimize the response time for the valve.  If the actuator is spring less positioner will be used.  If the valve has high friction.
  85. 85. Operation:-  The operation of the most common positioner as follows. In construction, pneumatic valve positioners have diaphragms or bellows to sense the incoming signal from the controller and feed back devices from the valve stem. The unit may be position balanced or force balanced. Any error in the two signals causes a proportional change in the output of a pilot valve.  In our plant we are using Valtek beta positioner and the main parts are shown in the picture.
  86. 86.  Instrument Signal Capsule: It will receive The Signal from I/P Transducer & move The Pilot Stem.  Spool Valve:  Feedback Spring  Cam  Range Arm  Range Adjustment Locking Screw  Range Adjustment Gear  Zero Adjustment Locking Knob  Zero Adjustment
  87. 87. I/P Transducer  Transducers convert a current signal to a pneumatic signal. The most common transducer converts a 4-20 mA electric signal to a 3-15 psig pneumatic signal. An increase in the dc signal to the coil increases the magnetic field around the coils. This field increases the magnetic strength in the armature and the magnetic attraction across the air gap between the armature and the pole pieces. The magnetic attraction will therefore downward at the nozzle end and upward at the feed back bellows end, resulting in a torque that rotates the armature about the torsion rod to cover the armature nozzle. The resulting restriction produces an increased pressure in the nozzle, in the upper chamber of the relay, and in the feed back bellows. The relay responds to the increase in nozzle pressure to increase the output pressure to the actuator and control valve.  Volume Booster: Volume Boosters are used on throttling control valves to provide fast stroking action with large input signal changes. At the same time, the flow boosters allow normal positioner air flow (and normal actuation) with small changes in the positioner input signal. Depending on actuator size, packing set and the number used, boosters can decrease valve stroking times up to 90 percent.
  88. 88. Valve Operation:-  Air to Open  Air to Close  Air fail to Lock in the same position Fail Safe System for Valves:-  Where service conditions exceed the capabilities of the standard fail-safe spring to drive the valve to its failure position, and where specially designed, extra-strong  failure springs may be both mechanically and economically unfeasible, air spring fail-safe systems on Valtek control valves provide the thrust necessary to drive the
  89. 89.  plug to its failure position. An air spring provides a pressurized volume of air to drive the actuator piston in the failure direction. The volume of air is sometimes provided within the actuator itself, or where the cylinder volume is insufficient, a separate external volume tank is provided.  Air spring systems are used primarily to close valves upon air failure. And sometimes they must open valves upon air failure. A fail-closed valve is customarily operated with the flow direction over the plug. Thus, with the plug on the seat, the upstream pressure acts to hold the valve closed.  Fail-open valves customarily operate with the flow direction under the plug. Thus, when a general system failure occurs, the upstream pressure will keep the plug off the seat and the valve open.  Air springs on valves are practical because the locked-up air is used only at the instant of air failure to drive the valve to the fail position. Line pressure will insure that the valve stays either closed or open.
  90. 90. • Occasionally, service conditions require that the valve remain in the last operating position upon loss of air supply. For such applications, valves can be equipped with a fail-in-place lock-up system. If air failure occurs, the system activates two pilot-operated lock-up valves that trip and lock the existing cylinder pressures on both sides of the piston, thus maintaining the last throttling position. Signal-to-open, Fail-closed Signal-to-close, Fail-open
  91. 91. -------------The End ------------