Its complete report about KAPCO Internship

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Internship Report
Dated 06/08/2012-to-25/09/2012
Submitted By:
Muhammad Arshad
Reg.no. 10ES-155
Department Of Electronics Engineering.
University College Of Engineering & Technology (IUB)
Bahawalpur.

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ACKNOWLEDGEME
In the Name of Allah, the Most Gracious, the Most Merciful.
I am very thankful to Almighty ALLAH who gave me power and ambitious mind
to make this report possible. With Allah’s will and mercy I have been able to
achieve all of this. As is the case in most human productions, this thesis was the
result of the collective efforts of a number of important and valued people who
directly or indirectly supported me during my doctoral studies. To these people, I
owe my gratitude and thanks.
I wish to express my sincere appreciation and gratitude to my Senior Engineers
Mr.Amjad, Mr.Imran & Mr.Irfan. Thank you for your guidance, encouragement,
and support. I have appreciated your patience, input, and positive criticism and
comments throughout the development of this study. I should also like to thank the
Management of KAPCO for giving me the chance to do work with them.

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ABSTRACT
The report is about my internship tenure which made me learn the basics of
Instrumentation and control system of a combined cycle power plant. The Power
Plant comprises of 10 multi fuel (HSD, BFO &Gas) fired gas turbines and 5 steam
turbines. These turbines are divided into 3 energy Blocks with each Block having a
combination of gas and steam turbines. The Power Plant's 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.

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Table of Contents:
1) ACKNOWLEDGEME…………………………………………………………….2
2) ABSTRACT………………………………………………………………………………………………….3
3) Table of contents………………………………………………………………………………………...4
4) Over View Of The Power Plant……………………………………………………………………..5
5) Kot Addu Power Company Limited (KAPCO)……………………………..6
6) Inside KAPCO……………………………………………………………………………………………….7
7) GAS TURBINE POWER STATION (1-4)…………………………………………………………..9
8) TREATMENT OF FURNACE OIL……………………………………………………………………10
9) FORWARDING SKID…………………………………………………………………………………….11
10) Combine Cycle…………………………………………………………………………………………….12
11) Gas Turbine…………………………………………………………………………………………………13
12) Steam Turbine……………………………………………………………………………………………..14
13) Boiler (HRSG)……………………………………………………………………………………………..16
14) INSTRUMENTATION AND CONTROL…………………………………………………………….17
15) Process Control Loop…………………………………………………………………………………...18
16) The Element Used In The Process Control Loop…………………………………………….19
17) Controller…………………………………………………………………………………………………….20
18) Sensors ……………………………………………………………………………………………………………….21
19) Temperature Sensor…………………………………………………………………………………….21
20) Pressure Sensor…………………………………………………………………………………………...24
21) Flow Sensor………………………………………………………………………………………………...28
22) Level Sensor…………………………………………………………………………………………………31

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Over View of The Power Plant:

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Kot Addu Power Company Limited (KAPCO)
Introduction: Kot Addu Power Plant (the "Power Plant") was built by the
Pakistan Water and Power Development Authority ("WAPDA") in five phases between
1985 and 1996 at its present location in Kot Addu, District Muzaffargarh, Punjab ,Pakistan.
KAPCO is Pakistan's largest Independent Power Producer (IPP) with a name plate capacity
of 1600 MW. . In April 1996, Kot Addu Power Company Limited ("KAPCO") was
incorporated as a public limited company under the Companies Ordinance, 1984 with the
objective of acquiring the Power Plant from WAPDA. The principal activities of KAPCO
include the ownership, operation and maintenance of the Power Plant.
In KAPCO three type of fuel are used in the gas turbine to produce electricity these
three are Natural gas, Low Sculpture Furnace Oil and High Speed Diesel. The Power Plant is
also the only major plant in Pakistan with the ability to self-start in case of a country wide
blackout.
On June 27, 1996, following international competitive bidding by the Privatization
Commission Government of Pakistan (the "Privatization 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%
shareholding in KAPCO increasing its total shareholding to 36%.The other majority
shareholder in KAPCO is WAPDA with a present shareholding of 46%.
Following the successful completion of the offer for sale by the Privatization
Commission (on behalf of WAPDA) in February 2005, 20% of KAPCO’s shareholding is now
held by the General Public. On April 18, 2005 KAPCO was formally listed on all three Stock
Exchanges of Pakistan.
The Power Plant is situated in District Muzaffargarh, Punjab, 90 K.M. north west of
Multan on the left bank of the River Indus at a distance of 16 K.M. from Taunsa Barrage.
The area is surrounded by agricultural land on the north and west side of Kot Addu.

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Inside KAPCO:
General information of the power plant
Gas Turbines 10
HRSGs 10
Steam Turbines 5
Installed Capacity 1600MW
Max. Load Generation 1541MW
Load According to IDC Test (1996) 1345MW
Load According to ADC Test (2010) 1355MW
No. of Feeders 6 x132KV; 6 x220KV
Max. Generation in one day 35,667Mwh
*IDC (Initial Dependable Capacity) test
*ADC (Annual Dependable Capacity) test
+Feeder Line: a feeder line is peripheral Route
or Branch in a Network, Which connects smaller
or more remote nodes with route or Branch carrying
heavier traffic.

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GAS TURBINE POWER STATION (1-4)
Three different type of fuels used in KAPCO .These are given below.
1) Natural Gas.
2) High Speed Diesel (HSD)
3) Blended Furnace Oil (BFO)
KAPCO use natural gas which is given by the pipe lines and for the HSD and the BFO
storage it has Storage Tanks.
Tanks:
There are total 27 tanks for the storage of BFO and HSD. 5 tanks are used for the
storage of HSD. The numbering of these are 4, 5, 6, 11&20. HEH keep ambient
temperature.
Other 22 tanks are use for the storage of BFO. The viscosity of the BFO is too much high
to decrease the viscosity of the BFO special type of electric heater is used in the BFO
tanks. If we will not use this electric heater the viscosity of the BFO is increased and this
oil can freeze in the pipe lines. BFO is not in pure form so before using the BFO some
type of treatment taken place on BFO.
The distribution of the Tanks is as follow,
1) For untreated Oil , 11 Tanks
2) For treated Oil , 11 Tanks
3) For HSD ,5 Tanks

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TREATMENT OF FURNACE OIL:
Treatment of LSFO is quite necessary before it can be used in the combustion
chambers because it may contain a no. of impurities in the form of water soluble salts, oil
soluble salts and compounds, and suspended particles. These impurities can cause a
considerable damage to combustion chambers, Turbine blades and other necessary
equipment that are very expensive and these impurities thus lower the overall efficiency
and increase the maintenance costs. Generally these impurities are classified in three types:
1) Water Soluble ( Na, K and other 2nd
group elements)
2) Oil Soluble (Pb, Ni, Co, V etc.)
3) Suspended particles and dust particles
Now there are different methods to remove these impurities. We first discuss the method
to remove water soluble impurities as follows:
There are two methods to remove water soluble impurities from raw FO:
 Electrostatic Method
 Centrifugal Method
There are five FUEL OIL TREATMENT PLANTS (FOTPs) at KAPCO for the treatment of
Furnace Oil. FOTPs nos. 1, 2, 3 and 4 are for Electrostatic Method and FOTP no. 5 is for
Centrifugal Method. KAPCO is the only company that has the facility of removing water
soluble impurities through Centrifugal method. Centrifugal Method is so far the most
efficient method for the purpose.
 Electrostatic Method :
In an electrostatic method basically the salts are ionized and made to collect at their
respective electrodes. A large drum is used for this purpose in which there are Cathode and
Anode. Untreated FO is mixed with water in the drum at a specific temperature. The water
soluble salts get mixed with water and +ve and –ve ions are formed. For example in case of
NaCl two ions are formed i.e. Na+ & Cl−. When electrodes are connected to power supply of
high voltage, these ions move to their respective electrode. Thus these salts are thus
removed from the electrodes later on.
(LSFO.Low Sulphur Fuel Oil)

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Now a chemical called DEMULSIFIER is added into the oil water mixture. The function of
Demulsified is to make the oil droplets separate from water droplets. It makes water
droplets larger in terms of its diameter due to which its surface area increases and due to
gravity water droplets settle down and drained to effluent water tanks. This water is called
EFFULANT WATER. Thus this treated Oil is obtained and pumped to treated oil tanks.
 Centrifugal Method:
In centrifugal Method, water is first added in Oil in the same way as before. Water
soluble salts get dissolved in water and are separated from Untreated Oil. This water containing
salts is removed from oil by centrifugal action. The water Oil mixture is rotated at a very large
speed in a large drum. The centrifugal force pushes the heavier fluid i.e. water containing salts in
it towards outer radial direction and oil gets deposited in the center and drained to Treated oil
tanks.
FORWARDING SKID
Booster pumps carry FO from treated tanks or from FOTP to Forwarding skid at
pressure of 3 bars. Pumps to maintain pressure of 6.5 to 7 bar. Nitrogen
Accumulator to bear the pressure jerks while changing the pump. Pumps to carry FO
from Forwarding Skid to Filtration Skid.
FILTERATION SKID
Dosing of KI200 (with 25% Mg.) to FO to eliminate vanadium effect. Plate type Heat
Exchanger to recover heat of fuel coming back from GT (Excess fuel).

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Combine Cycle:
In a combined cycle power plant (CCPP), or combined cycle gas turbine (CCGT) plant, a gas
turbine generator generates electricity and the waste heat is used to make steam to
generate additional electricity via a steam turbine; this last step enhances the efficiency of
electricity generation. Most new gas power plants in North America and Europe are of this
type. In a thermal power plant, high-temperature heat as input to the power plant, usually
from burning of fuel, is converted to electricity as one of the outputs and low-temperature
heat as another output. As a rule, in order to achieve high efficiency, the temperature
difference between the input and output heat levels should be as high as possible. This is
achieved by combining the Rankin (steam) and Braxton (gas) thermodynamic cycles. Such
an arrangement used for marine propulsion is called combined gas (turbine) and steam
(turbine) (COGAS).

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Gas Turbine:
Now first we discuss gas turbine of the combine cycle. In an open cycle gas turbine, air and
fuel follow several types of thermodynamic cycles. The simplest gas turbine cycle consists
of Compression of air, burning the fuel, Expansion of flue gases etc.
The gas turbine unit consists of the following main components: The name of main
components and the working principle are given below.
 Starting motor
 Intake air filters
 Compressor
 Combustion chamber
 Turbine
 Generator
Starting Motor:
A starting motor is an Asynchronous motor; it used to start the machine. This motor is
designed at 6.6 KV and it takes 1MW power. In starting it take power from the unit (1-4)
common bus bar or black start diesel engine bus bar (COBDA). After that when this
machine is able to produce power. Then automatically change its own power.
Approximately at 1900 rpm it becomes cut off from the system. Because at this time burnt
gases become able to rotate itself to the machine.

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Steam Turbine:
Main components of a steam Turbine are:
1. Boiler(HRSG)
2. Turbine
3. Condenser
4. Pump
A steam turbine is a prime mover which continuously converts energy of high
pressure, high temperature steam supplied by the steam generator in to the shaft work
with the low temperature steam exhausted to a condenser. The energy conversion
essentially occurs into steps:
1. The high pressure high temperature steam first expands in nozzles and comes out at
high velocity
2. The high velocity jets of steam coming out of the nozzles, impinge on the blades
mounted on a wheel, get deflected an angle and suffer a loss of momentum which is
absorbed by the rotating wheel in producing torques.
Steam turbine power plants operate on a Rankin cycle. The steam is created by a
boiler, where pure water passes through a series of tubes to capture heat from the
firebox and then boils under high pressure to become superheated steam. The heat in
the firebox is normally provided by burning fossil fuel (e.g. coal, fuel oil or natural gas).
However, the heat can also be provided by biomass, solar energy or nuclear fuel. The
suprerheated steam leaving the boiler then enters the steam turbine throttle, where it
powers the turbine and connected generator to make electricity. After the steam
expands through the turbine, it exits the back end of the turbine, where it is cooled and
condensed back to water in the surface condenser. This condensate is then returned to
the boiler through high-pressure feed pumps for reuse. Heat from the condensing steam
is normally rejected from the condenser to a body of water, such as a river or cooling
tower.
Steam turbine plants generally have a history of achieving up to 95% availability and
can operate for more than a year between shutdowns for maintenance and inspections.
Their unplanned or forced outage rates are typically less than 2% or less than one week
per year.
Modern large steam turbine plants (over 500 MW) have efficiencies approaching 40-
45%. These plants have installed costs between $800 and$2000/kW, depending on
environmental permitting requirement
(Rankin Cycle idealized thermodynamic cycle of heat engine that converts the heat into mechanical work)

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Boiler (HRSG)
The exhaust gases of a gas turbine are at a high temperature and such hot gases are
utilized in Heat Recovery Steam Generator (HRSG). The efficiency of HRSG is relatively
higher than conventional boilers i.e. 40% to 45 % whereas the efficiency of
conventional boilers is about 35 % and that of Burners Boiler is 32% to 40 %.
The HRSG has the capacity of about 200 tons of steam production in it. There are
different stages of a HRSG. There are generally four stages:
1. LP Evaporator
2. HP Economizer
3. HP Evaporator
4. Super Heater

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INSTRUMENTATION AND CONTROL:
Process control is the automatic control of an output variable by sensing the amplitude of
the output parameter from the process and comparing it to the desired or set level and
feeding an error signal back to control an input variable.
Instrumentation is the base of the process control in industry .However it comes in many
forms from domestic water heaters and HVAC, where the variable temperature is
measured and used to control gas, oil, or electricity flow to the water heater, or heating
system, or electricity to the compressor for refrigeration, to complex industrial process
control applications such as used in the petroleum or chemical industry.
Importance The instrumentation and control (I&C) systems of power plant (PP) have
three major roles.
Firstly, they are the ‘eyes and ears’ of the operator. If properly planned, designed,
constructed and maintained, they provide accurate and appropriate information and
permit judicious action during both normal and abnormal operation. They are therefore,
with the human operator, vital for the safe and efficient operation of the plant.
Secondly, under normal operating conditions they provide automatic control, both of the
main plant and of many ancillary systems. This allows the operator time to observe plant
behavior and monitor what is happening so that the right corrective action can be taken
quickly, if required.
Thirdly, the I&C safety systems protect the plant from the consequences of any mistakes
which the operator or the automatic control system may make. Under abnormal conditions
they provide rapid automatic action to protect both the plant and the environment. The I&C
requirements of an PP are, in most cases, more complex and diverse than those of a
conventional power plant
Process Control Loop:

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The Element Used In The Process Control Loop
The following are used in the process control loop.
Feedback loop is the signal path from the output back to the input to correct for any
variation between the output level from the set level. In other words, the output of a
process is being continually monitored, the error between the set point and the output
parameter is determined, and a correction signal is then sent back to one of the process
inputs to correct for changes in the measured output parameter.
Controlled or measured variable is the monitored output variable from a process.
The value of the monitored output parameter is normally held within tight given limits.
Manipulated variable is the input variable or parameter to a process that is varied by
a control signal from the processor to an actuator. By changing the input variable the value
of the measured variable can be controlled.
Set point is the desired value of the output parameter or variable being monitored
by a sensor. Any deviation from this value will generate an error signal.
Instrument is the name of any of the various device types for indicating or
Measuring physical quantities or conditions, performance, position, direction and the like.
Sensors are devices that can detect physical variables, such as temperature, light
intensity, or motion, and have the ability to give a measurable output that varies in relation
to the amplitude of the physical variable.
Transducers are devices that can change one form of energy to another, e.g., a resistance
thermometer converts temperature into electrical resistance, or a thermocouple converts
temperature into voltage.
Converters are devices that are used to change the format of a signal without changing
the energy form, i.e., a change from a voltage to a current signal.
Actuators are devices that are used to control an input variable in response to a signal
from a controller. Atypical actuator will be a flow-control valve that can control the rate of
flow of a fluid in proportion to the amplitude of an electrical signal from the controller.
Other types of actuators are magnetic relays that turn electrical power on and off.
Examples are actuators that control power to the fans and compressor in an air-
conditioning system in response to signals from the room temperature sensors

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Controller
Controller are devices that monitor signals from transducers and take the necessary action
to keep the process within specified limits according to a predefined program by activating
and controlling the necessary actuators.
Programmable logic controllers (PLC) are used in process-control applications,
and microprocessor-based systems. Small systems have the ability to monitor several
variables and control several actuators, with the capability of being expanded to monitor
60 or 70 variables and control a corresponding number of actuators, as may be required in
a petrochemical refinery.
PLCs, which have the ability to use analog or digital input information and output analog or
digital control signals, can communicate globally with other controllers, are easily
programmed on line or off line, and supply an unprecedented amount of data and
information to the operator. Ladder networks are normally used to program the
controllers.
An error signal is the difference between the set point and the amplitude of the measured
variable.
A correction signal is the signal used to control power to the actuator to set the level of the
input variable.
Transmitters are devices used to amplify and format signals so that they are suitable for
transmission over long distances with zero or minimal loss of information. The transmitted
signal can be in one of the several formats, i.e., pneumatic, digital, analog voltage, analog
current, or as a radio frequency (RF) modulated signal. Digital transmission is preferred in
newer systems because the controller is a digital system, and as analog signals can be
accurately digitized, digital signals can be transmitted without loss of information. The
controller compares the amplitude of the signal from the sensor to a predetermined set
point.

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Sensors
Sensors are devices that can detect physical variables, such as temperature, light intensity,
or motion, and have the ability to give a measurable output that varies in relation to the
amplitude of the physical variable.
In industry major four type of sensors are used,
1) Temperature Sensor
2) Pressure Sensor
3) Flow Sensor
4) Level Sensor
Now we discuss these four types of sensors.
 Temperature Sensor: Name of the different temperature sensors are
 Resistance temperature detectors (RTD)
 Thermistor NTC and PTC
 Thermocouples
 Pyrometers
RTD (Resistance Temperature Detector)

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Thermocouple:
Thermocouples (T/C) are formed when two dissimilar metals are joined together to form a
junction. Joining together the other ends of the dissimilar metals to form a second junction
completes an electrical circuit. A current will flow in the circuit if the two junctions are at
different temperatures. The voltage difference between the two junctions is measured, and
this difference is proportional to the temperature difference between the two junctions.
Resistance Temperature Detector: RTD’s are built from selected metals (typically
Platinum), which change resistance with temperature change. The resistance temperature
detector (RTD) measures the electrical conductivity as it varies with temperature. The
electrical resistance generally increases with temperature, and the device is defined as
having a positive temperature coefficient. The magnitude of the temperature coefficient
determines the sensitivity of the RTD. Apart from Platinum, other metals are used for RTD’s
such as Copper and Nickel. Platinum is the most common and has the best linear
characteristics of the three, although Nickel has a higher temperature coefficient giving it
greater sensitivity.

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Thermistors:
A thermistor is a semiconductor device formed from metal oxides. The principle of
temperature measurement with a thermistor is that its resistance changes with
temperature. Most thermistors differ from normal resistors in that they have a negative
coefficient of resistance, this means that the resistance decreases with an increase in
temperature. Negative (NTC) thermistors are the more common although positive(PTC)
are also available.
Pyrometer: Pyrometric methods of temperature measurement use the electromagnetic
radiation that is emitted from a material. The emitted radiation is proportional to the
temperature. Any object with a temperature above absolute zero will radiate
electromagnetic energy. Infrared pyrometers measure the amount of energy radiated from
an object in order to determine its temperature. There are a number of different types of
infrared pyrometers:
1)-Total radiation
2)-Single wavelength
3)-Dual wavelength

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Pressure Sensor
Pressure is defined as a force per unit area, and can be measured in units such as psi
(pounds per square inch), inches of water, and milli meters of mercury, Pascal (Pa, or
N/m²) or bar. Until the introduction of SI units, the 'bar' was quite common.
Pressure= Force/Area
Pressure Transducers and Elements
 Bourdon tube
 Helix and spiral tubes
 Spring and bellows
 Diaphragm
 Manometer
 Single and Double inverted bell
 Piezoelectric
Bourdon Tube The Bourdon tube works on a simple principle that a bent tube will change
its shape when exposed to variations of internal and external pressure. As pressure is
applied internally, the tube straightens and returns to its original form when the pressure
is released.

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Helix and spiral tubes are fabricated from tubing into shapes as per their naming. With
one end sealed, the pressure exerted on the tube causes the tube to straighten out. The
amount of straightening or uncoiling is determined by the pressure applied.
Manometer Manometers are good examples of pressure measuring instruments. U-tube
manometers consist of “U” shaped glass tubes partially filled with a liquid. When there are
equal pressures on both sides, the liquid levels will correspond to the zero point on a scale.

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Piezoelectric When pressure is applied to crystals, they are elastically deformed.
Piezoelectric pressure sensing involves the measurement of such deformation. When a
crystal is deformed, an electric charge is generated for only a few seconds. The electrical
signal is proportional to the applied force. Quartz is commonly used as the sensing crystal
as it is inexpensive, stable and insensitive to temperature variations. Tourmaline is an
alternative which gives faster response speeds, typically in the order of microseconds

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Flow Sensors
Basic concept
Velocity: This is the speed at which the fluid passes a point along the pipe. The velocity is
used to calculate volume and mass flow rates.
Volumetric flow rate: The volumetric flow rate represents that volume of fluid which
passes through a pipe per unit of time. This form of measurement is most frequently
achieved by measuring the velocity of a fluid with a DP sensor as it travels through a pipe of
known cross sectional area.
Mass flow rates: Mass flow is a measure of the actual amount of mass of the fluid that
passes some point per unit time.
Different sensors are used to measure the flow of the fluid.
 Orifice Plate
 Venturi Tube
 Pitot Tube
 Rota meter
 Annubar Tube
 Turbine meter
 Magnetic Flow meter
Orifice Plate A standard orifice plate is simply a smooth disc with a round, sharp-edged
inflow aperture and mounting rings. In the case of viscous liquids, the upstream edge of the
bore can be rounded. The shape of the opening and its location do vary widely, and this is
dependent on the material being measured. Standard orifice meters are primarily used to
measure gas and vapor flow. Measurement is relatively accurate.

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Venturi Tube A number of obstruction devices are available that are specially designed to
minimize the pressure loss in the measured fluid. These have various names such as
Venturi, flow nozzle and Dallflow tube. The Venturi Tube is often selected because pressure
drop is not as significant as with the orifice plate and accuracy is better maintained.
Pitot tube: The Pitot tube measures flow based on differential pressure and is primarily
used with gas flows. The Pitot tube is a small tube that is directed into the flow stream.
Pitot tubes have the advantage that they cause negligible pressure loss in the flow. They are
also cheap, and the installation procedure consists of the very simple process of pushing
them down a small hole drilled in the flow-carrying pipe.

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Turbine Meter A turbine flow meter consists of a multi-bladed wheel mounted in a pipe
along an axis parallel to the direction of fluid flow in the pipe. The flow of fluid past the
wheel causes it to rotate at a rate that is proportional to the volume flow rate of the fluid.
As an important application of the turbine meter is in the petrochemical industries, where
gas/oil mixtures are common.
Magnetic Flow meter Electromagnetic flow meters, also known as magmeters, use
Faradays’ law of electromagnetic induction to sense the velocity of fluid flow. Faradays
law states that moving a conductive material at right angles through a magnetic field
induces a voltage proportional to the velocity of the conductive material. The conductive
material in the case of a magmeter is the conductive fluid. The advantages of magnetic flow
meters are that they have no obstructions or restrictions to flow, and therefore no pressure
drop and no moving parts to wear out.

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Level Sensors
Level sensing devices can be divided into four categories
Direct sensing, in which the actual level is monitored
Indirect sensing, in which a property of the liquid, such as pressure, is sensed to
determine the liquid level
Single point measurement, in which it is only necessary to detect the presence or absence
of a liquid at a specific level.
Different types of sensors are used in Level measurement.
 Float Sensor
 Displacer Switches
 Conductive Probes
 Capacitive Probes
 Ultrasonic Level Sensor
 Radar measurements
Float Sensors There are two types of floats shown: the angular arm and the pulley .The
advantages of the float sensor are that they are almost independent of the density of the
liquid or solid being monitored, are accurate and robust, and have a linear output with level
height. One of the significant types of float is a magnetrolfloat level switch which consists a
plain float and operates via a magnetic coupling action.

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Conductive Probes Conductive probes are used in single and multiple point
measurements systems. Low voltages are applied to the electrodes as they are immersed in
the liquids. The conductive liquid completes the electrical circuit of the control, which
activates a semiconductor switch. The advantage of conductive probes is their low cost and
simple design. The disadvantage is that they are limited to point measurement and can be
only used with conductive liquids.
Capacitive Probes They are used for continues measurements. The value of the
capacitance can be change by varying the dielectric. The probe and the metal wall of the
tank form the two plates of a capacitor, and the contents in the tank are the dielectric.
When the tank is empty the dielectric is the air. As the level changes the dielectric constant
changes and causes the capacitance change.

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Ultrasonic Level Sensors Ultrasonic or sonic devices can be used for single point or
continuous level measurement of a liquid or a solid. A pulse of sonic waves (approximately
10 kHz) or ultrasonic waves (more than 20 kHz) from the transmitter is reflected from the
surface of the liquid to the receiver, and the time for the echo to reach the receiver is
measured. The time delay gives the distance from the transmitter and receiver to the
surface of the liquid, from which the liquid level can be calculated, knowing the velocity of
ultrasonic waves (approximately 340 m/s).
Radar Measurement Radar gauges differ from ultrasonic in that they use microwaves
instead of sound waves. Like ultrasonic devices they measure from the top of the vessel to
determine the product level. Two examples of radar gauges are the 5.8GHz and 24GHz
systems. The higher transmission frequency can be used to detect dry, non-conductive
materials with very low bulk density.

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