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Transcript of Full Repor
1
Introduction
About Kapco:-
KAPCO(Kot Addu Power Company limited) is Pakistan’s largest independent power
producer (IPP) with a name plate capacity of 1600 MW. In April 1996, KAPCO was
incorporated as a public limited company under the companies’ ordinance, 1984 with the
objective acquiring the power plant from WAPDA. The power plant is situated in District
Muzaffargarh, Punjab, and 90KM north west of Multan of the left bank of the Indus River by a
distance of 16KM from Taunsa Barrage. The area is surrounded by agricultural land on the north
and West Side of Kot Addu. The principal activities of KAPCO include ownership, operation,
and maintenance of the power plant.
Share Holders:-
On June 27, 1996, following international competitive biding by the privatisation commission
Government of Pakistan. (The ―Privatisation Commission‖ ), the management of KAPCO was
transferred to National Power (now International Power ) of the United Kingdom, which
acting through its subsidiary National Power (KOT ADDU) Limited (―NPKAL‖), bought shares
representing a 26% stake in KAPCO. Later, NPKAL bought a further 10% share holding in
KAPCO, increasing its total share holding to 36%.
The other majority shareholder in KAPCO is WAPDA with present share holding of 46%.
Following the successful completion of the offer for sale by the Privatisation Commission (on
behalf of WAPDA) in February 2005, 18% of KAPCO’s share holding is now held by General
Public.
On April 18, 2005, KAPCO was formally listed on all three stock exchanges of Pakistan.
VISION AND MISSION:-
Vision Statement
To be a leading power generation company, driven to exceed our shareholder’s
expectations and meet our customer’s requirements.
Mission Statement
To be a responsible corporate citizen.
To maximize shareholder’s return.
To provide reliable and economical power for our customer.
2
Plant Management:- To run the power plant in an efficient way the whole organization is divided into
following main departments.
I&C Block 2 Department:-
Hierarchy
Senior Manager
Foreman GT
Instrument engineer Instrument engineer
Principal engineer
Foreman STG
Test Inspector Test Inspector
Technician Technician
3
Plant Overview
Kot Addu power plant comprises of ten multifuel-fired gas turbine and 5 steam turbines installed
in 5 phases between 1985 and 1996. These turbines are divided into 3 energy blocks with each
block having a combination of gas and steam turbines. The power plants combined cycle
technology enables KAPCO to use the waste heat from the gas turbine exhaust to produce steam
in the heat recovery steam generator which in turn is used to run the steam turbines thereby
resulting in fuel cost, efficiency and minimum wastage. The power plant is a multifuel gas
turbine power plant with the capacity of using three different fuels to generate electricity,
namely: natural gas, low sulphur furnace oil and high speed diesel to generate electricity. The
power plant is also the only major plant in Pakistan with the ability to self-start in case of a
countrywide blackout.
Plant Distribution:-
Block 1:-
This block comprises of four Gas Turbines (GT1, GT2, GT3, GT4) and two Steam
Turbines (STG 9, STG10).
GT 1 &2:-
Manufacturer......................................... Siemens (Germany)
Model........................................................………..V 94.2
Block 1
GT 1,3
STG 9
GT 2,4
STG 10
Block 2
GT 5,6
STG 11
GT 7,8
STG 12
Block 3
GT 13,14
STG 15
4
Starting Device.................................................... Generator runs as motor initially
Starting time up to 3000RPM..............................4 minutes
Turbine stages..........................................4
Flue gas mass flow...................................426 Kg/sec
Control....................................................Iskamatic
Capacity (MW) IDC...............................95MW
GT 3&4:-
Manufacturer............................. FIAT M/s GIE(Italy)
Model......................................................TG-50
Starting Device.......................................11KV_1915kW
Starting time up to 3000RPM..............25 minutes
Turbine stages.........................................4
Flue gas mass flow..................................322 Kg/sec
Control....................................................conventional relay type; FIAT HI Tech
Capacity (MW) IDC...............................82MW
STG 9&10:-
Manufacturer.........................................ABB, Germany
Model......................................................DK2056
Rated Power...........................................112.2 MW
Vacuum (Bar)........................................0.091
HP Steam Inlet Press/Temp..................47.9bar/495°C
LP Steam Inlet Press/Temp..................3.99bar/190.6°C
Capacity (MW) IDC...............................87MW (STG 9)
Capacity (MW) IDC...............................97MW (STG 10)
Block 2:-
This block comprises of four Gas Turbines (GT5, GT6, GT7, GT8) and two Steam
Turbines (STG 11, STG12).
GT 5 -8:-
Manufacturer...............................ALSTOM France
5
Model...................................................... MS 9001 E
Starting Device......................................... 6.6kV-1000kW
Starting time upto 3000RPM..................10 minutes
Turbine stages...........................................3
Flue gas mass flow...................................406 Kg/sec
Control....................................................Mark four Speedtronic
Capacity (MW) IDC...............................79MW (GT5)
Capacity (MW) IDC...............................82MW (GT6)
Capacity (MW) IDC...............................77MW (GT 7)
Capacity (MW) IDC...............................79MW (GT8)
STG 11&12:-
Manufacturer......................................... RATEAU, France
Model...................................................... VEGA209 110B
Rated Power...........................................103.4 MW
Vacuum (Bar)........................................0.091
HP Steam Inlet Press/Temp..................40bar/510°C
Capacity (MW) IDC...............................76MW (STG11)
Capacity (MW) IDC...............................82MW (STG12)
Block 3:-
This block comprises of two Gas Turbines (GT13-14) and one Steam Turbines (STG15).
GT 13-14:-
Manufacturer...............................Siemens (Germany)
Model...................................................... V-94.2
Starting Device.........................................Generator runs as motor
Starting time up to 3000RP....................4 minutes
Turbine stages...........................................4
Flue gas mass flow...................................471 Kg/sec
Control....................................................TELEPERM
Capacity (MW) IDC...............................106 MW
6
STG 15:-
Manufacturer......................................... Siemens, Germany
Model........................................................030-16, N30-2X5-B-9
Rated Power...........................................148.6 MW
Vacuum (Bar)........................................0.091
HP Steam Inlet Press/Temp....................57bar/528°C
LP Steam Inlet Press/Temp....................5.78bar/221°C
Capacity (MW) IDC...............................120MW
Combined Cycle Power plant:-
The combined cycle power plant consists of two gas turbine-generator units, a
steam turbine – generator complete with a condenser and condensate/ feed water
system and all required auxiliaries.
Two gas turbines that drive their own generators exhaust into a special boiler
called a Heat Recovery Steam Generator (HRSG) that generates steam for use is
Steam Turbine.
One of the principal reasons for the popularity of combined cycle power plant is
their high thermal efficiency. Combined cycle plant with thermal efficiencies as
high as 52% have been built. Combined cycle plants can achieve these efficiencies
because much of the heat from the gas turbines is captured and used in the Rankine
cycle portion of plant. The heat from the exhaust gases would normally be lost to
the atmosphere in an open cycle gas turbine.
Another reason for the popularity of combined cycle plant is that it requires less
time for their construction as compared to a conventional steam power plant of
same output. Although it takes longer time to built the combined cycle plant than a
simple gas turbine plant.
Thermodynamic Cycles:-
Gas turbine Cycle..................................Joule-Brayton cycle
Steam-water Cycle................................Rankine cycle
7
Joule-Brayton Cycle:-
It consists of four processes
Compression of Air(Work is done on the air by the compressor, the air
stores this energy in the form of temperature and pressure)
Addition of heat at constant Pressure(by burning of fuel)
Expansion (energy of hot pressurized gas is used to perform work)
Cooling of hot gases that exhaust to the atmosphere.
Brayton Cycle
Rankine cycle:-
A simple rankine cycle of conventional steam power plant consists of five
processes.
Increase in pressure of condensate from condenser(Boiler-feed pumps)
Addition of heat to convert water into steam at constant Pressure(Boiler)
Additional energy is added to steam(Super heater)
Steam is expanded and cooled as it passes through turbine(energy is used
to perform work)
Condensation of steam that exhausts from the turbine
Compression
Heat addition
Expansion
Cooling
8
Process control system & Instrumentation
Why Automatic control?
The need for automatic control system has become steadily more and more important in the
modern power plant. It is quite impossible to build and operate the large power plants safely
without automatic control system. It is fairly clear that the control of the bigger power plants is
too much complicated and cannot be left solely in the hands of operator. It is therefore obvious
that one of the main objectives of the process instrumentation is the automatic control of
measured process variable (temperature, flow, level, pressure etc).
Process control system:-
It is defined as the function and operation necessary to change a material either physically or
chemically to maintain a process. Control can be discrete or analog, manual or automatic, and
periodic or continuous
The automatic control systems have four basic elements. They are primary element, the
measuring element, the controlling element and the final control element. The brief description
of all these elements is given here under
Primary elements
This is device that detects the change in the measured variable. For example the thermocouple,
Borden tube, flow orifice etc. are considered as primary elements.
Boiler feed pumps
Boiler
SuperheaterSteam turbine
condensor
9
Measuring Element
This is the device that receives the signal from primary element. It measures the amount of
measured variable that has drifted from the set point. It receives the signal from sensing element
and sends it to controller. For example the pressure transmitter, pneumatic transmitter,
thermocouple transducer and level transmitter can be considered as measuring element.
Controlling Elements
The controlling element (controller) uses changes in the value of measured variable to alter the
mechanical, pneumatic or electrical source of power. The controlling element can actuate the
source of power, increase or decrease its output, or turn it off, depending upon its setting.
Final Control Element
The final control element is a device that varies the energy supplied to the process. This energy is
manipulated variable so that the value of the measured or controlled variable is maintained
within the desired range e.g. control valve, actuators etc.
Control system can be classified as open loop or closed loop control system.
Open loop control system:-
The process is controlled by inputting to the controller the desired set point believed necessary to
achieve the ideal operating point for the process and accepting whatever the output results. Since
the only input to the controller is the set point, it is apparent that an open loop system controls
the process blindly. That is, the controller receives no information concerning the present status
of the process or the need for any corrective action. The effect of the disturbance is never seen in
the process output. Open loop control systems are considerably cheaper and less complex than
their closed loop counterparts.
Set point disturbance
Feed-back control system:-
A closed loop control system is one in which the output of a process affects the input. The
system measures the actual output of the process and compares it to the desired output.
Adjustments are made continuously by the control system until the difference between the
desired and actual output is as small as practical. Disturbance signals are all those signals, which
Controller Output signal
conditioning
Output
actuator
Process
10
influence the control value in an unwanted way. Feed back control system looks to these
disturbances and control the process according to given set points. Set point is the input that
determines the desired operating point for the process. It is normally provided by a human
operator, altough it may also be supplied by another electronic circuit. Process variable is the
signal that contains the information about the current process status. Error amplifier
determines whether the process operation matches the set point. The magnitude and polarity
of the error signal will determine how the process will be brought back under control.
Error amplifier disturbance
Set point +
-
Process variable signal
Basic Process control system Block diagram:-
In short, the major components of process control system are
Sensors
Signal conditioning
Operator-machine interface
Controller
Actuators
Input signal
conditioning Controller
Output signal
conditioning
Process
sensor
External
sensor
Operator-machine
interface External
actuators
Process
actuators
Controlled
process
Controller Output actuator
Input sensors
Process
Input signal conditioning
11
Sensors:-
A sensor is a device that measures a physical quantity and converts it into a signal which can be
read by an observer or by an instrument.
Sensors convert physical phenomena to measurable signals, typically voltages or currents
Sensors are also called transducers. This is because they convert input physical
phenomena to an electrical output.
Provides input from the process and from the external environment.
There are many types of sensors used in power industry. In KAPCO, following parameter
sensors are extensively used.
Temperature sensors
Pressure sensors
Level sensors
Flow sensors
For gas turbine and generator protection, following sensors are used.
Speed sensors
Vibration sensors
Proximity sensors
Flame detectors
Temperature sensors:-
There are four basic types of temperature sensors.
Thermocouple
Resistance temperature detector (RTD)
Thermistor
pyrometers
Thermocouples:-
Construction:-
A thermocouple is constructed of two dissimilar metal
wires joined at one end. When one end of each wire is
connected to a measuring instrument, the thermocouple
becomes a sensitive and highly accurate measuring
device. Thermocouples may be constructed of several
different combinations of materials.
12
Basic Principle:-
―If a temperature gradient is present in electric conductor, the heat flow will create the movement
of electrons and an electromotive force will be generated in that region. Direction of emf will be
dependent on the magnitude and direction of temperature gradient and material forming the
conductor.‖ This is called thermoelectric effect.
Operation:-
Thermocouples will cause an electric current to flow in the attached circuit when subjected to
changes in temperature. The amount of current that will be produced is dependent on the
temperature difference between the measurement and reference junction; the characteristics of
the two metals used; and the characteristics of the attached circuit. Heating the measuring
junction of the thermocouple produces a voltage which is greater than the voltage across the
reference junction. The difference
between the two voltages is proportional
to the difference in two temperatures and
can be measured on the voltmeter (In
milli volts).
Types of thermocouples:-
In KAPCO,K types are extensively used.
Type Temperature Range
K 0 to 1100
T -185 to 300
J 20 to 700
E 0 to 800
Advantages:-
They have wide temperature range.
They are self powered.
Disadvantages:-
Voltage-temperature graph nonlinear
Low voltage
Reference required
13
Resistance Temperature detector:-
The RTD incorporates pure metals or certain
alloys that
increase in resistance as temperature
increases
Decrease in resistance as temperature decreases.
RTDs act somewhat like an electrical transducer, converting changes in temperature to voltage
signals by the measurement of resistance. RTD elements are normally constructed of platinum,
copper, or nickel. These metals are best
suited for RTD applications because of
their linear resistance temperature
characteristics, their high coefficient of
resistance, and their ability to withstand
repeated temperature cycles. In
KAPCO, RTD are installed on thermal
oil heaters in the forwarding skid.
Construction:-
RTD elements are usually long, spring-
like platinum wires surrounded by an
insulator and enclosed in a sheath of
metal. The insulator prevents a short
circuit between the wire and the metal
sheath. The change in temperature will
cause the platinum wire to heat or cool,
resulting in a proportional change in
resistance. RTD use protective well.
The well protects the RTD from
damage by the gas or liquid being
measured. Protecting wells are
normally made of stainless steel, carbon
steel or cast iron, and they are used for
temperatures up to 1100°C.
Advantages:-
More linear than thermocouples
More stable
Disadvantages
Expensive
Power supply required
Small change in resistance
Self heating
14
Thermistor:-
It is a thermally sensitive resistor that usually has a negative temperature
co efficient. As
Temperature increases, its resistance decreases
Temperature decreases, its resistance increases
Thermistors differ from resistance temperature detectors (RTD) in that
the material used in a thermistor is generally a ceramic or polymer, while
RTDs use pure metals. The temperature response is also different; RTDs are useful over larger
temperature ranges, while thermistor typically achieve a higher precision within a limited
temperature range [usually -90C to 130C]. They are not extensively used in KAPCO.
Advantages:-
Fast response
High output
Disadvantages:-
Nonlinear
Limited temperature range
Power supply required
Pyrometers:-
A pyrometer is a non-contacting device that
intercepts and measures thermal radiation. This
device can be used to determine the temperature of an object's surface. There is no need for
direct contact between the pyrometer and the object, as there is with thermocouple and
Resistance temperature detector (RTDs). Pyrometers are suited especially to the measurement of
moving objects or any surfaces that cannot be reached or cannot be touched.
Temperature detection circuit:-
The bridge circuit is used whenever extremely accurate resistance measurements are required
(such as RTD measurements).
The basic bridge circuit consists of:
Two known resistors (R1 and R2) that are used for rationing the adjustable and known
resistances
One known variable resistor (R3) that is used to match the unknown variable resistor. In
balanced bridge configuration, we have to change the resistance of the R3 in such a way
that ammeter shows zero current. This change in resistance is used to measure
temperature. In unbalanced bridge the value of R3 is fixed and the amount of current
flowing through a balanced circuit is calibrated according to temperature change.
15
One unknown resistor (Rx) that is used to measure temperature
A sensing ammeter that indicates the current flow through the bridge circuit
Pressure sensors:-
Many processes are controlled by measuring pressure. Pressure is defined as a force per unit
area, and can be measured in units such as Psi (pounds per square inch), millimeters of mercury,
Pascal (Newton per square meter) or bar. In industry Bar is normally common.
Gauge pressure:-
It is equal to absolute pressure minus atmospheric pressure. Gauge pressure is measured relative to
prevailing atmospheric pressure (approximately 14.7 psi).Hence, if we check a pressure gauge
and it says 30 psi, the pressure being measured is 30 psi above atmospheric pressure.
Absolute pressure:-
It is equal to gauge pressure plus atmospheric pressure. If one completely evacuates from a container,
then it has an absolute pressure of 0 (zero) psi (or any other unit of pressure). So, in the previous
example of 30 psi gauge pressure, the absolute pressure would be about 47.7 psi.
Differential pressure:-
Differential pressure is the difference in pressure between two points.
16
There are numerous types of pressure sensors. Some of the pressure sensors are
Bellow type sensors
Bourdon tube type sensors
Bellow type sensors:-
The need for a pressure sensing element that was extremely sensitive to low pressures and
provided power for activating recording and indicating mechanisms resulted in the development
of the metallic bellows pressure sensing element.
Pressure range:-
The metallic bellows is most accurate when measuring pressures from 0.5 to 75 psig. However,
when used in conjunction with a heavy range
spring, some bellows can be used to measure
pressures of over 1000 psig.
Construction:-
The bellows is a one-piece, collapsible
metallic unit that has deep folds formed from
very thin-walled tubing. The elastic elements
in bellows gauges are made of brass, phosphor
bronze, stainless steel, beryllium-copper, or
other metal that is suitable for the intended
purpose of the gauge. The diameter of the
bellows ranges from 0.5 to 12 in. and may
have as many as 24 folds. System pressure is
applied to the internal volume of the bellows. As the inlet pressure to the instrument varies, the
bellows will expand or contract. The moving end of the bellows is connected to a mechanical
linkage assembly. As the bellows and linkage assembly moves, either an electrical signal is
generated or a direct pressure indication is provided.
Bourdon tube type sensors:-
The bourdon tube pressure instrument is one of the oldest pressure sensing instruments in use
today. In KAPCO, temperature controllers have bourdon type tube in it. Mostly pressure sensors
in KAPCO are of Bourdon type.
17
Construction and operation:-
The bourdon tube consists of a thin-walled tube that is flattened diametrically on opposite sides
to produce a cross-sectional area elliptical in shape, having two long flat sides and two short
round sides. The tube is bent lengthwise into an arc of a circle of 270 to 300 degrees. Pressure
applied to the inside of the tube causes distention of the flat sections and tends to restore its
original round cross-section. This change in cross-section causes the tube to straighten slightly.
Since the tube is permanently fastened at one end, the tip of the tube traces a curve that is the
result of the change in angular position with respect to the center. Within limits, the movement of
the tip of the tube can then be used to position a pointer or to develop an equivalent electrical
signal to indicate the value of the applied internal pressure.
In short, in a bourdon tube type sensor
System pressure is applied to the inside of a slightly flattened arc shaped tube. As
pressure increases, the tube tends to restore to its original round cross-section. This
change in cross-section causes the tube to straighten.
Since the tube is permanently fastened at one end, the tip of the tube traces a curve that is
the result of the change in angular position with respect to the center. The tip movement
can then be used to position a pointer or to develop an electrical signal.
18
Pressure detection circuitry:- Any of the pressure detectors previously discussed can be joined to an electrical device to form a
pressure transducer. Following detection circuitry is used.
Resistance type transducer
Inductance type transducer
Capacitive type transducer
Resistance type transducer:-
1. First method:-
In this method strain gauge is used. A strain gauge measures the external force (pressure) applied
to a fine wire. The fine wire is usually arranged in the form of a grid. The pressure change causes
a resistance change due to the distortion of the wire.
The value of the pressure can be found by measuring
the change in resistance of the wire grid.
𝑅 = 𝑘𝐿
𝐴
R = resistance of the wire grid in ohms
K = resistivity constant for the particular type of wire
grid
L = length of wire grid
A = cross sectional area of wire grid
As the wire grid is distorted by elastic deformation, its
length is increased, and its cross-sectional area decreases. These changes cause an increase in the
resistance of the wire of the strain gauge. This change in resistance is used as the variable
resistance in a bridge circuit that provides an electrical signal for indication of pressure.
An increase in pressure at the inlet of the bellows causes the bellows to expand. The expansion
of the bellows moves a flexible beam to which a strain gauge has been attached. The movement
of the beam causes the resistance of the strain gauge to change. The temperature compensating
gauge compensates for the heat produced by current flowing through the fine wire of the strain
gauge.
19
2. Second method:-
Other resistance-type transducers combine a
bellows or a bourdon tube with a variable resistor.
As pressure changes, the bellows will either
expand or contract. This expansion and
contraction causes the attached slider to move
along the slide wire, increasing or decreasing the
resistance, and thereby indicating an increase or
decrease in pressure.
Inductive type transducer:-
The inductance-type transducer consists of three parts: a coil, a movable magnetic core, and a
pressure sensing element. The element is attached to the core, and, as pressure varies, the
element causes the core to move inside the coil. An AC voltage is applied to the coil, and, as the
core moves, the inductance of the coil changes. The current through the coil will increase as the
inductance decreases according this formula
𝐿 = 𝑁(𝑓𝑙𝑢𝑥)
𝑖
Another type of inductance transducer, utilizes two coils wound on a single tube and is
commonly referred to as a Linear Variable Differential Transformer.
Operation of Differential transformer:-
The primary coil is wound around the center of the tube. The secondary coil is divided with one
half wound around each end of the tube. Each end is wound in the opposite direction, which
causes the voltages induced to oppose one another. A core, positioned by a pressure element, is
movable within the tube. When the
core is in the lower position, the
lower half of the secondary coil
provides the output. When the core
is in the upper position, the upper
half of the secondary coil provides
the output. The magnitude and
direction of the output depends on
the amount the core is displaced
from its center position. When the
core is in the mid-position, there is
no secondary output. The
excitation for an LVDT varies.
Typical values range from 50HZ
20
to 30 kHz. If the transducer must accurately track rapidly changing displacement, the higher
frequencies are advantageous. The voltage applied to the primary is usually around 10V.
Displacement of as little as 50 inches is detected by LVDT’s.
Capacitive type transducer:-
In KAPCO, few pressure sensors have capacitive type transducer. Capacitive-type transducers
consist of two flexible conductive plates and a dielectric. The Dielectric used in KAPCO is fluid.
As pressure increases, the flexible conductive plates will move farther apart, changing the
capacitance of the transducer. This change in capacitance is measurable and is proportional to the
change in pressure.
Piezoelectric
When a crystal undergoes strain it displaces a small amount of charge. In other words, when the
distance between atoms in the crystal lattice changes some electrons are forced out or drawn in.
This also changes the capacitance of the crystal. This is known as piezoelectric effect. The
charge generated is a function of the force applied and the strain in the material. When using
piezoelectric sensors charge amplifiers are needed to convert the small amount of charge to a
larger voltage.
Its current force relationship is
𝐼 = 𝜀𝑔𝑑𝐹
𝑑𝑇
Where g is material constant and e is dielectric constant. This is not commonly used in KAPCO.
Level sensors:-
Level sensors detect the level of liquid in the tank. The level measurement can be either
Continuous values
Point values.
Continuous level sensors:-
It measure level within a specified range and determine the exact amount of substance in a
certain place
Point level sensors:-
Point-level sensors only indicate whether the substance is above or below the sensing point.
Generally it detects levels that are excessively high or low.
Following level sensors are used in KAPCO.
Gauge glass
Ball float
Chain float
Magnetic bond
21
Bubbler gauge
Conductive probes
Differential pressure
Gauge glass:-
In the gauge glass method, a transparent tube is attached to the bottom and top (top connection
not needed in a tank open to atmosphere) of the tank that is monitored. The height of the liquid in
the tube will be equal to the height of water in the tank. The gauge glasses in the form of ―U"
tube manometer where the liquid seeks its own level due to the pressure of the liquid in the
vessel is commonly used in KAPCO. Gauge glasses made from tubular glass or plastic are used
for service up to 450 psig and 400°F. If it is desired to measure the level of a vessel at higher
temperatures and pressures, a different type of gauge glass is used.
Refraction gauge glass is used in the dark areas. Lights are usually used in refraction gauge.
Operation is based on the principle that the bending of light, or refraction, will be different as
light passes through various medium. Light is bent, or refracted, to a greater extent in water than
in steam. The portion of the gauge containing water appears green; the portion of the gauge from
that level upward appears red. This type is used in the region of steam turbine auxiliaries to
measure the level of water.
Ball float:-
The ball float method is a direct reading liquid level
mechanism. The most practical design for the float is a hollow
metal ball or sphere. The design consists of a ball float attached
to a rod, which in turn is connected to a rotating shaft which
indicates level on a calibrated scale. The ball floats on top of
the liquid in the tank. If the liquid level changes, the float will
follow and change the position of the pointer attached to the
rotating shaft.
The travel of the ball float is limited by its design to be within
±30 degrees from the horizontal plane which results in optimum response and performance. The
actual level range is determined by the length of the connecting arm. These are mostly used in
KAPCO steam turbine unit.
Chain float:-
The operation of the chain float is similar to the ball
float except in the method of positioning the pointer
and in its connection to the position indication. The
float is connected to a rotating element by a chain with
a weight attached to the other end to provide a means
of keeping the chain taut during changes in level.
These are not used much in KAPCO.
Magnetic Bond method:-
The magnetic bond mechanism consists of a magnetic
float which rises and falls with changes in level. The
float travels outside of a non-magnetic tube which
22
houses an inner magnet connected to a level indicator. When the float rises and falls, the outer
magnet will attract the inner magnet, causing the inner magnet to follow the level within the
vessel.
Conductivity probe method:-
It is normally used for two types of alarm
Low level
High level
It consists of one or more level detectors, an operating relay, and a controller. When the liquid
makes contact with any of the electrodes, an electric current will flow between the electrode and
ground. The current energizes a relay which causes the relay contacts to open or close depending
on the state of the process involved. The relay in turn will actuate an alarm, a pump, a control
valve, or all three. This is used in the tank where the fluid inside is conductive.
Bubbler gauge:-
In this case, liquid level is determined by bubbling air through the liquid. The amount of pressure
required to force the air out of the bottom of the dip tube depends on the level of the liquid.
Differential pressure level detectors:-
Differential pressure level detectors are extensively used in KAPCO. They are used in steam
drum, feed water tank, lube oil tank level detection etc.
Operation:-
The differential pressure detector method of liquid level measurement uses a differential Pressure
detector connected to the bottom of the tank being monitored. The higher pressure, caused by the
fluid in the tank, is compared to a lower reference pressure (usually atmospheric). This
comparison takes place in the DP detector. A differential pressure (D/P) transmitter which
consists of a diaphragm with the high pressure (H/P) and low pressure (L/P) inputs on opposite
sides. As the differential pressure changes, the diaphragm will move. The transducer changes this
mechanical motion into an electrical signal.
If tank is not exposed to atmosphere:-
When the tank is not exposed to
atmosphere, then both the sides of
differential pressure transmitter are
connected to the tank. The high pressure
connection is connected to the tank at or
below the lower range value to be
measured. The low pressure side is
connected to a "reference leg" that is
connected at or above the upper range value
to be measured. The reference leg is
pressurized by the gas or vapor pressure,
but no liquid is permitted to remain in the
reference leg. The reference leg must be maintained dry so that there is no liquid head pressure
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on the low pressure side of the transmitter. The high pressure side is exposed to the hydrostatic
head of the liquid plus the gas or vapor pressure exerted on the liquid’s surface. The gas or vapor
pressure is equally applied to the low and high pressure sides. Therefore, the output of the DP
transmitter is directly proportional to the hydrostatic head pressure, that is, the level in the tank.
Drawback of differential pressure level measurement:-
When the liquid density changes with temperature then it will give us a wrong level
measurement because of the pressure difference increase or decrease due to density changes. We
know that
∆𝑃 = 𝜌𝑔
Where ρ is the density of the fluid
The density of steam (or vapor) above the liquid level will have an effect on the weight of the
steam or vapor bubble and the hydrostatic head pressure. As the density of the steam or vapor
increases, the weight increases and causes an increase in hydrostatic head even though the actual
level of the tank has not changed. As the temperature of the fluid in the tank is increased, the
density of the fluid decreases. As the fluid’s density decreases, the fluid expands, occupying
more volume. If the fluid in the tank changes temperature, and therefore density, some means of
density compensation must be incorporated in order to have an accurate indication of tank level.
Density compensation is accomplished by using either:
Electronic circuitry
Instrument calibration
Ultrasonic level measurement:-
An ultrasonic sensor operates by sending sound waves toward the target and measuring the time
it takes for the pulses to bounce back. The time taken for this echo to return to the sensor is
directly proportional to the distance or height of the object because sound has a constant velocity.
The echo signal is electronically converted to a 4-20 mA output. Density factor doesn’t affect the
reading of the sensor as in DP level transmitter. These are installed in main fuel oil tanks in
KAPCO. They are very accurate as compared to DP level transmitter.
Speed sensors:-
For the over speed protection of the turbine we use speed sensors KAPCO. Normally shaft speed
is monitored for operational, control, and protection of the machine. Speed is normally
monitored at the turning gear section or at the exciter end of the turbine.
Operation:-
A tooth wheel of magnetic material is mounted on the turbine shaft. Speed pickups are installed
in perpendicular to the tooth surface. When a teeth passes through the pick up a pulse signal is
generated.
There are two types of magnet speed sensors.
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Induction type sensor
Eddy current type sensor
Induction type sensor:-
A magnet is attached to a shaft. As a tooth of wheel passes the sensor (coil). It induces voltage in
it and gave pulse each time. The voltage output of the pickup is usually very small and requires
little amplification to be measured.
Eddy current type sensor:-
Voltage output of the probe is changed due to the eddy current losses in the teeth, when it passes
the sensor.
Pulses of output signal from speed sensor is counted in a set time interval by the electronic
circuit and is converted into speed signal. As the speed increases, the frequency of the pulses
increases. Therefore the number of pulses counted in a fixed time interval increases.
𝑃𝑢𝑙𝑠𝑒 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 =𝑛𝑜 𝑜𝑓 𝑡𝑒𝑒𝑡 ∗ 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑡𝑒 𝑚𝑎𝑐𝑖𝑛𝑒
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Tachometer generators:-
Tachometers coupled to motors are commonly used in motor speed control application to
provide a feedback voltage to the controller that is proportional to motor speed.
A Tachometer normally refers to a small permanent magnet dc generators. When the generator is
rotated, it produces a dc voltage directly proportional to speed.
Vibration sensors:- ―Vibration is a response to some form of excitation‖
The free movement of shaft in a journal bearing will cause it to vibrate when a forcing function
is applied. In kapco vibration sensors are used to protect the turbine if vibrations are too much.
When vibration is out of the range it will shut down the machine.
Vibration measurement:- o Displacement measurement
o Velocity measurement
o Acceleration measurement
Displacement measurement:- Measurement of total movement in relation to a reference point.
Sensors used in this type of measurement are Proximity probes.
Velocity measurement:-
Measurement of rate of movement. Units are mm/sec or inch/sec
Measurement of distance covered by a sine wave in unity time is velocity
measurement.
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Velometers are used.
Acceleration measurement:-
Rate of change in velocity measurement.
Accelerometers are used to measure rate of change of velocity of vibrating body.
Velocity sensors:-
It consists of an electric coil mass suspended by a spring in a field of permanent magnet. The
magnet vibrates in direct accordance with the machine case. The inertial mass coil senses the
magnet movement and induces electric voltage that is proportional to the machine case velocity.
Velocity has amplitude in proportion to the vibration velocity and frequency equal to frequency
of vibration.
Sensor output= 150mV/inch per sec
Proximity sensors:- These types of sensors are used to in KAPCO to measure the position
of the damper which are BYD (bypass damper) and BID (Boiler
inlet damper). They are pilot devices that detect the presence of an
object without any physical contact. They are solid state electronic
devices that are completely encapsulated to protect against excessive
vibration, corrosive agent found in industrial environment. They have
high switching rates. There are generally two types of proximity
sensors.
Inductive proximity sensor
Capacitive proximity sensor
Inductive Proximity sensors:- When energy is supplied, the oscillator operates to generate high frequency field. There must not
be any conductive material in high frequency field. When a metal object enters high frequency
field, eddy currents are induced in the surface of the target. This result in the loss of energy in the
oscillator circuit, this causes smaller amplitude of an oscillator. Detector recognizes a specific
change in amplitude and generates a signal that will turn the solid state output turn ON or OFF.
When metal object leaves sensing area, the oscillator regenerates allowing a sensor to return to
the normal state.
Capacitive proximity sensors:- It is a sensing device that is actuated either by conductive or non conductive materials. Instead of
a coil, active faces of a capacitor sensor are formed by two metallic electrodes-rather like an
opened capacitor. They are in a feedback loop of high frequency oscillator that is inactive with
―no target‖ present. As target approaches the face of the sensor, it enters in the electrostatic field
formed by the electrodes. This cause an increase in the coupling capacitance and circuit begin to
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oscillate. The amplitude of these oscillations is measured by evaluating circuit that generates a
signal to turn the solid state output ON or OFF.
Flame sensors:-
Flame detectors are used to sense the presence or absence
of flame. In KAPCO, during the starting sequence, it is
essential that an indication of the presence or absence of
flame be transmitted to the control system. There are four
flame sensors. The ultraviolet flame sensor consists of a
flame sensor, containing a gas filled detector. The gas
within this flame sensor detector is sensitive to the
presence of ultraviolet radiation which is emitted by a
hydrocarbon flame. A DC voltage supplied by the
amplifier is impressed across the detector terminals. If
flame is present, the ionization of gas in the detector
allows conduction of current in the circuit which
activates the electronics to give an output defining flame.
Conversely the absence of flame will generate an
opposite output defining ―no flame‖. In KAPCO if a loss
of flame in the combustion chamber is indicated by two flame detector sensor, the control
circuitry will cause an annunciation only of this condition.
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Signal conditioning:-
Input signal conditioning involves converting input and output signal to a usable form. In other
words signal conditioning means manipulating an analog signal in such a way that it meets the
requirements of the next stage for further processing. It includes following techniques.
Amplification
Attenuation
Filtering
A/D converters
D/A converters
Current to voltage converter
Voltage to current converter
Signal inputs accepted by signal conditioners include DC voltage and current, AC voltage and current, frequency and electric charge. Sensor inputs can be accelerometer, thermocouple, thermistor, resistance thermometer, strain gauge or bridge, and LVDT or RVDT. Outputs for signal conditioning equipment can be voltage, current, frequency, timer or counter, relay, resistance or potentiometer.
Filtering:- Filtering is the most common signal conditioning function, as usually not all the signal frequency
spectrum contains valid data. Electronic filters are used in filtering. Electronic filters are
electronic circuits which perform signal processing functions, specifically to remove unwanted
frequency components from the signal, to enhance wanted ones. Filters can be analogue or
digital, active or passive, low pass or high pass.
Amplification:-
Signal amplification performs two important functions: increases the resolution of the input
signal, and increases its signal-to-noise ratio. For example, the output of an electronic
temperature sensor, which is probably in the millivolts range is probably too low for an
controller to process directly. In this case it is necessary to bring the voltage level up to that
required by the controller. Normally used amplifiers are log amplifiers, antilog amplifiers, simple
op amp etc.
Multiplexers:- It is a device that performs multiplexing; it selects one of many analog or digital input signals
and forwards the selected input into a single line
A/D converter:-
Analogue to digital converter converts continuous
quantity on to discrete quantity. Sampling is done in
order to convert analogue in to digital signal.
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Attenuation:- It is the opposite of the amplification. An attenuator is an electronic device that reduces the
amplitude or power of a signal without appreciably distorting its waveform. It provides gain less
than 1.Simple attenuators are voltage divider circuit. In measuring signals, attenuator pads are
used to lower the amplitude of the signal a known amount to enable measurements
Digital-to-analog converter It converts digital input in to analogue output. This can be done through R-2R ladder.
Operator Machine Interface:- In modern automatic control system, few parameters can be changed by the operator. All modern
control systems have operator machine interface. This facility is in all the LCR,CCR of KAPCO.
It
Allows input from a human to setup the starting condition or alter the control of the
process
Allows human input through various type of switches, controls and keypads
Operates using supplied input information that may include emergency shutdown or
changing the load, speed, the type of the fuel it use, to gave set points to the controller.
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Controller Controller is a device which makes system’s decisions based on the input signals. They generates
output signal which operate actuator to carry out the decision. Controllers may be open loop or
closed loop as described above. There are two types of control.
Discrete control
Analogue control
Digital control:- Discrete control deals with systems in which each element can only exist in certain defined
states. This type of control is implemented with logic diagrams and circuits.
Analogue control:- Analog control deals with systems in which variables can have a continuous range of values, rather than
simply discrete states. Basic analog control consists of the process of measuring the actual output of a
system, comparing it to the desired value of that output, and taking control action based on the difference
to cause the output to return to the desired value.
The four most popular types of control response used in the process industry are
ON/OFF
Proportional
Integral
Derivative
ON/OFF:-
With ON/OFF control, the final control element is either ON or OFF-one for the occasion when
the value of the measured variable is above the set point and the other when the value of the
measured variable is below the set point. The controller will never keep the final control element
in the intermediate position. E.g. we are using ON/OFF controller when a liquid is heated by the
steam. If the liquid temperature goes below the set point the steam valve opens and the steam is
turned ON or vice versa.
This type of controller is usually inexpensive but not very accurate to be used in accurate control
applications.
Drawbacks:-
It can position a valve in only two different settings
It treats a large offset and small offset the same way
It allows oscillation to the process and cannot give control for steady states for the
process.
Proportional:-
These controllers are designed to eliminate the hunting or cycling associated with the ON/OFF
controller. Proportional (P) controllers produce an output that is directly proportional to the error
signal.
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It allows the final control element to take the intermediate positions between ON or OFF
depending on the error signal. This permits the analogue control of the final control element to
vary the amount of energy to the process, depending upon how much the value of the measured
variable has shifted from the desired variable. Proportional controllers are tune able. Their
response to process changes can be adjusted to suit the time constants of specific process system.
In theory, a proportional controller should be all that is needed for process control any change in
the system output is corrected by an appropriate change in controller output. Unfortunately the
operation of a proportional controller leads to a process deviation known as offset. This steady
state error is the difference between the attained value of the controller and the required value.
To compensate the steady state error it is used in conjunction with the integral or derivative
controller. Process with the long time lags and a large maximum rate of rise (e.g. heat exchanger)
require wide proportional bands to eliminate oscillation.
Integral action:-
It is sometimes called reset action respond to the size and the time duration of the error signal
therefore the output signal from an integral controller is the mathematic integral of the error. An
error signal exists when there is difference between the process variable and the set point, so the
integral action will cause to change and continue to change until the error no longer exists.
Integral action eliminates steady state error. The amount of integral action is measured as
minutes per repeat.
Derivative action:-
The derivative mode controller respond to the speed at which error signal is changing-that is
greater the error change , the greater the correcting output. The derivative action is measured in
term of time.
Proportional-Integral controller:-
PI controller combines the characteristics of both types of control. A step change in the
measurement causes the controller to respond in a proportional manner followed by the integral
response, which is added to the proportion response. Because the integral mode determines the
output changes as a function of time, the more the integral action in the control, the faster the
output changes. It is used
To eliminate the offset error, the controller needs to change its output until the process
variable reaches to zero
Reset (integral) control action changes the controller output by the amount needed to
derive the process variable back to the set point value.
Proportional- derivative controller:-
It is used in process control systems that have errors that change very rapidly. By adding
derivative control to proportional control, we get a controller output that responds to the
measurement’s rate of change as well as to its size.
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PID controller:- Proportional integral derivative controllers produce outputs that depend on the magnitude,
duration and rate of change of the system error signal. Sudden system disturbances are met with
an aggressive attempt to correct the condition. A PID controller can reduce the system error to
zero faster than any other controller because it has an integrator and a differentiator. During start
up the set point, proportional band, reset, and rate as well as the output limits are specified. All
these can be changed during operation to tune the process.PID controllers are used in the PLC’s
of KAPCO.
In KAPCO, Pneumatic PI controllers are installed in the fuel and gas lines of heat exchanger in
the filtering skid and forwarding skid. They sense the temperature with the help of fluid which
increases the pressure of the bourdon tube which tells the pneumatic PI controller to decide the
flow of air to open the valve which will try to follow set points given by operator.
Actuators:- An actuator is any device that converts an electric signal in to mechanical movement.
There are many types of actuators.
A pneumatic, hydraulic, or electrically powered device that supplies force and motion to
open or close a valve.
Actuators can create a linear motion, rotary motion, or oscillatory motion. That is, they can
create motion in one direction, in a circular motion, or in opposite directions at regular intervals.
Pneumatically operated control valve actuators are the most popular type in use, but electric,
hydraulic, and manual actuators are also widely used. The spring-and-diaphragm pneumatic
actuator is most commonly specified due to its dependability and simplicity of design.
Pneumatically operated piston actuators provide high stem force output for demanding service
conditions. Adaptations of both spring-and-diaphragm and pneumatic piston actuators are
available for direct installation on rotary-shaft control valves.
Electric and electro-hydraulic actuators are more complex and more expensive than pneumatic
actuators.
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They offer advantages where no air supply
source is available.
Pneumatic operated diaphragm actuator:-
Pneumatically operated diaphragm actuators
use air supply from controller, positioner, or
other source. Various styles include
Direct acting (increasing air pressure
pushes down diaphragm and extends
actuator stem);
reverse-acting (increasing air
pressure pushes up diaphragm and
retracts actuator stem,
Direct-acting unit for rotary valves (increasing air pressure pushes down on diaphragm, which
may either open or close the valve, depending on orientation of the actuator lever on the valve
shaft. In KAPCO, they are used in the filtering skid.
Piston Actuators Piston actuators are pneumatically operated using high-pressure plant
air to 150 psig, often eliminating the need for supply pressure
regulator. Piston actuators furnish maximum thrust output and fast
stroking speeds. They are used for pneumatic trip valves and lock-up
systems. In KAPCO, these types of actuator are used in speed ratio
valve and gas control valve.
Valves:-
A valve is a device that regulates the flow of a fluid (gases, liquids or slurries) by
opening, closing, or partially obstructing various passageways
The control valve regulates the rate of fluid flow as the position of the valve plug or disk
is changed by force from the actuator.
The most common final control element in the process control industries is the control valve. The
control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to
compensate for the load disturbance and keep the regulated process variable as close as possible
to the desired set point. There are many types of control valves like sliding stem and rotary shaft
control valve.
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Sliding stem control valve:-
Single ported valve body:
This valve contains one plug means it allows only one
path for fluid to flow i.e. one inlet and one outlet. It is simple in
construction and valve plug can be moved in or out by
applying pressure pneumatically and electrically through the
actuator.
Three way valve body:
They are designed specifically to blend (mix) or split two flowing
streams. There are two inlet ports and one output port for blending
while one inlet port and two output port for diverting (splitting).
Rotary shaft control valve:-
Butterfly Valve Bodies
It consists of a shaft supported disc which rotates within cylindrical body.
They are now being designed for high and low pressure drops high static
pressure and tight shutoff. Tight shutoff is accomplished using soft seating
as rubber lining. They are being used for low pressure drop requirements,
high capacity and minimum space for installation. Butterfly valve bodies
might require high-output or large actuators if the valve is big or the
pressure drop is high, because operating torques might be quite large.
Ball type valve:-
It consists of a ball in place of plug to open and close the path of fluid. Its design provides self-cleaning; it provides tight shutoff and wide range ability for accurate controlling of flow.
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Control Valve Accessories Positioner:-
Pneumatically operated valves depend on a positioner to take an input signal from a process
controller and convert it to valve travel.
Pneumatic Positioner—A pneumatic signal (usually 3-15 psig) is supplied to the
positioner. The positioner translates this to a required valve position and supplies the
valve actuator with the required air pressure to move the valve to the correct position.
Analog I/P Positioner—This positioner performs the same function as the one above,
but uses electrical current (usually 4-20 mA) instead of air as the input signal.
Digital Controller—Although this instrument functions very much as the Analog I/P
described above, it differs in that the electronic signal conversion is digital rather than
analog.
Supply Pressure Regulator Supply pressure regulators commonly called air sets; reduce plant air supply to valve positioner
and other control equipment. Common reduced-air-supply pressures are 20, 35 and 60 psig. The
regulator mounts integrally to the positioner or nipple-mounts or bolts to the actuator. 67CF
series filter regulators are used to regulate air pressure in various Gas turbine units in KAPCO.
Their specifications are
Maximum inlet pressure…..250 psi
Outlet pressure ranges………0 to 35 psig/0 to 60 psig/0 to 125 psig
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Solenoid valve:- A solenoid valve is a combination of two basic
functional units:
A solenoid (electromagnet) with its core or
plunger.
A valve body containing an orifice in which a
disc or plug is positioned to restrict or allow
flow.
A solenoid is a device used to convert an electrical
signal or an electric current into linear mechanical
motion. It is basically an actuator. The solenoid is made up of a coil with a moveable iron core.
When the coil is energized, the core is pulled inside the coil. The amount of pulling or pushing
force produced by the solenoid is determined by the number of turns of copper wire and the
amount of the current flowing through it.
Now coming back to solenoid valve, flow through an orifice is OFF or allowed by the movement
of the core and depends on whether the solenoid is energized or de energized. Solenoid valves
are available to control hydraulics, pneumatics
Symbols:-
Normally Closed
Inlet
Normally open
Inlet
Electro hydraulic servo valve:- Servo-controlled hydraulic systems provide exceptional control over very large forces. The most
critical element of the system is the electro hydraulic controller or servo. Common servo-valves
consist of a two stage spool whose position is controlled by electromagnetic coils. Energizing the
coils allows fluid flow in one direction or the other depending on the input signal. The basic
servo-valve produces a control flow proportional to input current for a constant load. Two stage
servo-valves may be further divided into nozzle-flapper and jet pipe types. Nozzle-flapper type
servo-valves are currently by far the most common in high performance servo applications. So
we will discuss nozzle type servo valve.
In KAPCO, we are using Moog G77XK series industrial servo valve are used.
Construction and Operation:- It consists of a polarized electric torque motor and two stages of hydraulic power amplification.
The motor armature extends in to air gap of the magnetic flux circuit and is supported in this
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position by a flexture tube member. The flexure tube act as a seal between the electromagnetic
and hydraulic sections of the valve. The two motor coils surround the armature, one on each side
of the flexure tube.
The flapper of the first stage hydraulic amplifier is rigidly attached to the midpoint of the
armature. The flapper extends through the flexure tube and passes between two nozzles crating
two variable orifices between the nozzle tip and the flapper. The pressure controlled by the
flapper and the nozzle variable orifice is fed to the end areas of the second stage spool. The
second stage is a conventional four way spool design in which output flow from the valve, at a
fixed pressure drop, is proportional to the spool displacement from the null position.
Input signal induces a magnetic charge in the armature and causes a deflection of the armature
and flapper. This assembly pivots about the flexure tube and increases the size of one nozzle
orifice and decreases the size of the other.
This action creates a differential pressure from one end of a spool to the other and result in spool
displacement. Spool movement continues until the feedback wire force equals the input signal
force.
A is the inlet of the hydraulic oil, B is the controlled output hydraulic oil, T is drain and P is to
control the movement of spool.
Servo controller:-
In servo controller the error amplifier continuously monitors the input reference signal (Ur) and
compares it against the actuator position (Up) measured by a displacement transducer (LVDT) to
yield an error signal. The error is manipulated by the servo controller according to a pre-defined
control law to generate a command signal (Uv) to drive the hydraulic flow control valve.
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Relays:-
Relays are used primarily as switching devices in a circuit. In KAPCO, relays are used in
SPEEDTRONIC control system of GTs for switching. In world there are various type of relays
are used but some relays which are used in KAPCO with respect to process control or switching
are following
Electromechanical relays
Solid state relay
Timing relay
Latching relay
Electromechanical relay:-
It is magnetic switch. It turns a load circuit ON or OFF by energizes an electromagnet. A relay
usually has one coil, but it has any number of different contacts. They contain a stationary and
the moving part. The moving contact is attached to the plunger. Contacts are referred as normally
open (NO) and normally close (NC). Action of this field, in turn,
causes the plunger to move through the coil closing the NO contact and
opening NC contacts.
NO contacts are open when coil is de energized
NO contacts are close when coil is energized
Control relays are used as auxiliary device to switch control circuits
and load such as small motors, solenoid and pilot lights. They can be
used to control a high voltage load circuit with a low voltage control circuit. Coils and contacts
are well insulated.
Some important terminologies of relays:-
Pick up voltage:-
Level of voltage at which relay coil is energized, resulting in contact switching.
Drop out voltage:-
Level of voltage at which relay coil is de energized.
Inrush current:-
When the coil is energized the plunger is in an out position. Because of an open gap in the
magnetic path, the initial current in the coil is high. This high current is called in rush current.
Sealed current:-
As plunger moves in to the coil, closing the gap, the current level drop to a lower level. The
lower value is called sealed current.
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Solid state relays:- They are not used in KAPCO. EMR and SSR both perform the same function but with different
mechanism. They use bipolar transistor, MOSFET, SCR (silicon controlled devices) or triacs.
Solid state relay has no moving part. They are resistant to shock and vibration. Like EMR, SSE
found its application in isolating a low voltage control circuit from a high power load circuit.
Optically coupled SSR:-
LED glows when condition are correct to actuate the relay. LED shines on phototransistor, which
then conducts, causing the trigger current to be applied to the triac. Thus output is isolated from
the input by simple LED and phototransistor arrangement.
Timing relays:-
It is used in industry when time delay is required e.g. machine in which start of an event must be
delayed until another event has occurred ( a machine must be on gas until the fuel has been
heated up to 100°C temperature). Timing relays are conventional relay that are equipped with an
additional hardware mechanism or circuitry to delay the opening or closing of load contact.
There are two types of timing relays.
ON delay
OFF delay
They are further divided in to NO or NC contact. Let us take the example of OFF delay NO
timing relay.
S1 (switch) TD
NO lamp L1
When S1 opens, TD de energized, TD opens, and L1 is OFF
S1 close, TD close instantaneously and L1 is switched ON
S1 opens, TD de energized, timing period start, TD is still closed and lamp is still on
After 10s or any timer set value, TD open and lamp is switch OFF
Latching Relay:-
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Electromechanical latching relays are designed to hold the relay closed after power has been
removed from the coil. The latching coil is momentarily energized to set the latch and hold the
relay in latched position
Advantages:-
In control circuit to have to remember when particular event take place and not permit
certain functions once this event occurs. E.g. Shutdown.
Power failure of the control system
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System architecture:- Control system architecture can range from simple local control to highly redundant distributed control.
SCADA systems.
Local control It describes a system architecture in which sensors, controller, and controlled equipment are
within close proximity and the scope of each controller is limited to a specific system or
subsystem. Local controllers are typically capable of accepting inputs from a supervisory
controller to initiate or terminate locally-controlled automatic sequences, or to adjust control set
points, but the control action itself is determined in the local controller.
Centralized control Centralized control describes a system in which all sensors, actuators, and other equipment
within the facility are connected to a single controller or group of controllers located in a
common control room. Locating all controls, operator interfaces and indicators in a single
control room improves operator knowledge of system conditions and speeds response to
contingencies. This type of system architecture was common for power plants. In this if the main
controller fails then the whole machine trips. No exchange of controller status or data is sent to
other controllers.
Distributed control Distributed control system architecture offers the best features of both local control and
centralized control. In a distributed control system, controllers are provided locally to systems or
groups of equipment, but networked to one or more operator stations in a central location
through a digital communication circuit. Control action for each system or subsystem takes place
in the local controller, but the central operator station has complete visibility of the status of all
systems and the input and output data in each controller, as well as the ability to intervene in the
control logic of the local controllers if necessary.
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There are a number of characteristics of distributed control architecture which enhance
reliability:
Input and output wiring runs are short and less vulnerable to physical disruption or
electromagnetic interference.
A catastrophic environmental failure in one area of the facility will not affect controllers or
wiring located in another area.
Types of DCS:-
Plant distributed control system (DCS):-
While the term DCS applies in general to any system in which controllers are distributed rather
than centralized, in the power generation it has come to refer to a specific type of control system
able to execute complex analog process control algorithms at high speed, as well as provide
routine monitoring, reporting and data logging functions. In most applications, the input and
output modules of the system are distributed throughout the facility, but the control processors
themselves are centrally located in proximity to the control room.
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Direct digital control:- They consist of local controllers connected to a network with a personal computer (PC) based
central station which provides monitoring, reporting, data storage and programming capabilities.
They are normally used in HVAC plants.
Remote terminal unit (RTU) based SCADA:
RTU-based systems are common in the electric distribution industries where monitoring and
control must take place across large geographical distances. The RTUs were developed primarily
to provide monitoring and control capability at unattended sites such as substations, metering
stations. They communicate with a central station over telephone lines, fiber-optics, radio or
microwave transmission. Monitored sites tend to be relatively small, with the RTU typically used
mainly for monitoring and only limited control.
Programmable logic controller (PLC) based systems:
PLCs can be networked together to share data as well as provide centralized monitoring and
control capability. Control systems consisting of networked PLCs are supplanting both the plant
DCS and the RTU-based systems in many industries. They were developed for factory
automation and have traditionally excelled at high speed discrete control, but have now been
provided with analog control capability as well. The recommended controller for SCADA
systems is the programmable logic controller (PLC).
PLCs are general-purpose microprocessor based controllers that provide logic, timing, counting,
and analog control with network communications capability. They provide high speed
processing, which is important in generator applications.
A PLC consists of the required quantities of the following types of modules or cards, mounted
on a common physical support and electrical interconnection structure known as a rack.
Power supply: The power supply converts facility electrical distribution voltage, such as
120 VAC or 125 VDC to signal level voltage used by the processor and other modules.
Processor: The processor module contains the microprocessor that performs control
functions and computations, as well as the memory required to store the program.
Input/output (I/O): These modules provide the means of connecting the processor to
the field devices.
Communications: Communications modules are available for a wide range of industry-
standard communication network connections. These allow digital data transfer between
PLCs and to other systems within the facility
Redundancy: Many PLCs are capable of being configured for redundant operation in
which one processor backs up another.
S3 PLC is used for fuel dosing pump in the KAPCO.
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Mark four SPEEDTRONIC:-
This type of control systems are used in the local control room of
the GT5-8. It is a plant distributed control system. It controls the
operation and monitoring of the GT 5-8. It employs three
computers, identified as controller R, S and T which performs all
calculations necessary to keep the gas turbine running after it has
reached a complete sequence and also for the shutdown of the unit.
Each computer is designed such that it drives its output in a defined
direction on loss of power or failure of any computer. The two out
of three logic is provided in this system configuration.
A fourth computer called as ―communicator‖ or ―human machine
interface‖ supervises the three controllers and initiates an audible
alarm where there is any disagreement between any control
parameter or logic signal in the three controller. The turbine
however continues to run because the control is responding to the median value.
The field trip contacts are wired to the two contact input modules. From here the signals are
paralleled to the three optical couplers which feed them to the separate digital input cards in R, S
and T. the C computer monitors the input seen by R, S and T and performs the majority vote and
feed the voted values to the CRT for display.
For analogue control system the temperature input signal are connected to the I/O module where
they are filtered prior to being fed to the controller R, S, T or C. These incoming signals are then
multiplexed on the computer card dedicated to specific functions and then wired to another input
analogue card for second stage of multiplexing and the final analogue to digital conversion.
1 & 2
2 & 3
3 & 1
Channel 1
Channel 3
Channel 2
Machine emergency trip
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The thermocouple inputs are processed similar to other analogue inputs. Here the trip and
essential control thermocouples are connected to separate input modules.
Two types of relays are used soft relays and hardware relays. Soft relays are used for soft
contacts in the ladder diagram while hardware relays are used for switching.
256 alarm messages are available and additional alarms are dedicated to internal diagnostics. The
status of all the contact inputs can be displayed simultaneously to assist in trouble shooting.
STG 11-12 control system:- STG 11-12 has Distributed control system. It is a very huge and complex control system. The
very brief introduction of the STG control system is as following.
T20:-
T20 cards controllers are used for steam turbine auxiliary control e.g. boiler, feed water tank,
steam drum etc. They are open loop control systems. The detail of the open loop system is
mentioned above.
Micro z controllers:-
Micro z controllers are also used in steam turbine auxiliary. They are closed loop control
systems.
Turbine control:-
These control cards are used to control the steam turbine.
STG controller DIGIREC 920
STG safety REC 920
Temperature rack
Vibratory monitoring and protection
Stress calculator
ALSPA C100 PLC
STG controller DIGIREC 920:-
DIGIREC920 control following parameters
Speed control
Acceleration control
HP pressure control
Valves position control
Gland steam control (gland steam is used to stop the lost of vacuum generated in
condenser)
Safety REC 920:-
Over speed protection
Lube oil pressure low
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Lube oil tank level low
Condenser pressure high
Generator failure
Internal tripping orders (vibration rack, bearing temperature high, generator excitation
temperature high)
Functions of ALSPA C100 PLC:-
Logics for fundamental safeties test
Start up checks
Stress calculator:-
Monitoring of inlet steam pressure, temperature and turbine casing temperatures.
Calculate turbine rotor and casing stress
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Power plant safety rules:-
Electric and electronic circuit can be dangerous. Safe practices are necessary to prevent electric
shocks, fires, explosions, mechanical damage and injuries resulting from the improper use of
tools.
Perhaps the greater hazard is electrical shock. A current through the human body in excess of
100 milli amperes can paralyze the victim and make it impossible to let go of a ―live‖ conductor
or component.
High voltage can force enough current through the skin in order to produce a shock. The danger
of shock increases with the increase in voltage. Any voltage above 30V is considered dangerous.
The pathway through the body is another factor influencing the effect of electric shock. E.g. a
current from hand to foot, which passes through the heart and part of the central control system,
is far more dangerous than a shock between two points on the same arm.
Safety in the workplace:-
Many statistics shows that 98 percent of all accidents are avoidable. So following things must be
kept in mind while working in KAPCO or any other power plant.
Red is used to designate fire protection equipment
Yellow is used to designate caution and physical hazards
Orange is used to designate dangerous part of machines
Purple is used to designate radiation Hazards
Green is used to tell the location of first aid equipments
Personal safety:-
The clothing worn at work is important for personal safety. The following things should be
observed.
Hard hats
Safety shoes
Goggles
Metal jewelry should not be worn while working on energized circuit; gold and silver are
excellent conductor.
Grounding:-
It refers to the deliberate connections of parts of a wiring installation to a common earth
detection. Grounding guards us against the shock hazard. It arises when there is a little or no
leaking current but the potential for the abnormal current flow present exists. E.g. if the exposed
live wire touched the metal frame of an ungrounded piece of electrical equipment , the voltage of
the live wire would charge the metal plate. If we touch the charged metal frame, our body can
provide it a current path and we suffer a serious shock. For this reason in KAPCO every machine
is grounded. When there is abnormal current flowing through the metal body it will blow a fuse
or trips a circuit breaker to immediately open the circuit. Grounding has nothing to do with the
operation of electrical equipment. Its sole purpose is the protection of life and property.
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Electrical lock out:-
They are normally known as permit. They are necessary so that someone will not inadvertently
turn the equipment to the ON position while it is being worked.