instrumentation control process
TRANSCRIPT
Instrumentation & Control
IMRAN KHAN
Registration No: - 10D2-113031
Program: - B-Tech (Pass) Electronics
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
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.
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 .
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 .
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>
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).
Define the types of loop control ?
Open loop control system .
Close loop control system .
Cascade loop control system .
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 .
Open loop control system Diagram
Open loop
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.
Close loop system diagram .
Close loop Block diagram
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.
Cascade loop control system Diagram.
Cascade loop
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
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.
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
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
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.
Gauge Construction types .
“C” Type Bourdon
Helical Bourdon
Coiled Bourdon
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
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.
Pressure Transmitter
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.
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.
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.
Orifice Plate
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
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.
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.
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
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
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.
Turbine Flow meter
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
Level Measurement Types
Level Gauges
Guided Wave Radar
Radar
Differential Pressure
Float / Displacer
Ultrasonic
Capacitance
Nuclear
Level Gauges
Tubular
Glass Tube with Option of Graduations
Not Popular for Process Applications
Typically Used for Calibrating Metering Pumps (Calibration Tubes)
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
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
Magnetic Level Gauge
Consists of a Non-Magnetic Chamber, Internal Float with Magnet and Bi-
Colored Indicator Wafers
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
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
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
Types of Temperature Instrument
Thermocouple T/C
Resistance Temperature Detector (RTD)
Thermo well
Bi-metallic Thermometers
Filled Thermal Systems
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.
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
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).
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
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.
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.
RTD Elements
Wire Wound ElementPrecise 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 ElementWires 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 ElementMetallic 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.
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.
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.
R1 R
2
R3
AMMETER
RT
D
Temperature Element Assembly
Head Nipple-Union-Nipple Thermowell
Thermo wells
Straight Shank
FlangedVan Stone
Step Shank
Tapered Shank
Threaded
Weld-in
Plug
Plug
with
Chain
Accessories
Thermo wells
Thermo well Insertion Modification
TYPICAL THERMOWELL
CONSTRUCTION
SHORTENED
THERMOWELL
CONSTRUCTION
STEPPED THERMOWELL
CONSTRUCTION
Temperature Transmitters
Signal Conditioner
Low level inputs
mV from thermocouples
from RTD’s
High level outputs
4-20mA current
Digital (i.e. Fieldbus)
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.
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
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.
schematic diagram of a gas chromatograph
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;
The split /injector
Detectors
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.
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.
Turbines Work
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
Turbine field installation
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
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.
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
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.
The types of the valves as follows,
Globe valve
Butterfly valves
Gate valves
Diaphragm valves
Ball valves
Knife edge valves
Control Valve Parts
The Valves has Two Main Parts
Body Assembly
Actuator Assembly
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.
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.
Actuator Assembly
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.
Butterfly Valve Body Assembly
Spring Cylinder Rotary Actuator
Valve Accessories
Positioner
I/P Transducer
Volume Booster
Quick Exhaust
Lockup Realy
Solenoid
Limit Switch
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.
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.
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
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 allownormal positioner air flow (and normal actuation) with small changes in the positioner input signal. Depending on actuator size, packing set and the numberused, boosters can decrease valve stroking times up to 90 percent.
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
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.
• 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
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