POWER PLANT

POWER PLANT


A power plant is an industrial facility for the generation of electric power. Power plant is also used to refer to the engine in ships, aircraft and other large vehicles. Some prefer to use the term energy center because it more accurately describes what the plants do, which is the conversion of other forms of energy, like chemical energy, gravitational potential energy or heat energy into electrical energy.
At the center of nearly all power stations is a generator, a rotating machine that converts mechanical energy into electrical energy by creating relative motion between a magnetic field and a conductor. The energy source harnessed to turn the generator varies widely. It depends chiefly on what fuels are easily available and the types of technology that the power company has access to.
Classification:
By fuel
• Nuclear power plants use a nuclear reactor's heat to operate a steam turbine generator.
• Fossil fuelled power plants may also use a steam turbine generator or in the case of natural gas fired plants may use a combustion turbine.
• Geothermal power plants use steam extracted from hot underground rocks.
• Renewable energy plants may be fuelled by waste from sugar cane, municipal solid waste, landfill methane, or other forms of biomass.
• Waste heat from industrial processes is occasionally concentrated enough to use for power generation, usually in a steam boiler and turbine.
By prime mover
• Steam turbine plants use the dynamic pressure generated by expanding steam to turn the blades of a turbine. Almost all large non-hydro plants use this system.
• Gas turbine plants use the dynamic pressure from flowing gases to directly operate the turbine. Natural-gas fuelled turbine plants can start rapidly and so are used to supply "peak" energy during periods of high demand, though at higher cost than base-loaded plants. These may be comparatively small units, and sometimes completely unmanned, being remotely operated.
• Combined cycle plants have both a gas turbine fired by natural gas, and a steam boiler and steam turbine which use the exhaust gas from the gas turbine to produce electricity. This greatly increases the overall efficiency of the plant, and many new base load power plants are combined cycle plants fired by natural gas.
• Internal combustion Reciprocating engines are used to provide power for isolated communities and are frequently used for small cogeneration plants. Hospitals, office buildings, industrial plants, and other critical facilities also use them to provide backup power in case of a power outage. These are usually fuelled by diesel oil, heavy oil, natural gas and landfill gas.
• Microturbines, Stirling engine and internal combustion reciprocating engines are low cost solutions for using opportunity fuels, such as landfill gas, digester gas from water treatment plants and waste gas from oil production.
Out of the above mentioned power plants the P.P.C.L plant is a combined cycle power plant of 330 MW capacity and it is consist of two gas turbine units each of 104 MW capacity and one steam turbine unit of 122 MW generation capacity










GAS TURBINE



A gas turbine is a rotary machine, which consists of three main components - a compressor, a combustion chamber and a turbine. The air after being compressed into the compressor is heated either by directly burning fuel in it or by burning fuel externally in a heat exchanger. The heated air with or without products of combustion is expanded in a turbine resulting in work output, a substantial part, about two-thirds, of which is used to drive the compressor. The rest, about one-third, is available as useful work output. A gas turbine extracts energy from a flow of hot gas produced by combustion of gas or fuel oil in a stream of compressed air. It has an upstream air compressor (radial or axial flow) mechanically coupled to a downstream turbine and a combustion chamber in between. "Gas turbine" may also refer to just the turbine element.
Energy is released when compressed air is mixed with fuel and ignited in the combustor. The resulting gases are directed over the turbine's blades, spinning the turbine, and mechanically powering the compressor. Finally, the gases are passed through a nozzle, generating additional thrust by accelerating the hot exhaust gases by expansion back to atmospheric pressure. Energy is extracted in the form of shaft power, compressed air and thrust, in any combination, and used to power aircraft, trains, ships, electrical generators, and even tanks.
The gas turbine at P.P.C.L is of B.H.E.L model MS 9000, is a simple cycle, single shaft gas turbine with a 14 combustion chambers, reverse flow combustion system. The turbine assembly is consist of following six major sections or groups :
*Air inlet section.
*Compressor section.
*Combustion system.
*Turbine section.
*Exhaust section.
*Support systems.


Fig: Main components of a gas turbine
Theory of operation:
Gas turbines are described thermodynamically by the Brayton cycle, in which air is compressed isentropically, combustion occurs at constant pressure, and expansion over the turbine occurs isentropically back to the starting pressure.
In practice, friction, and turbulence cause:
1. Non-isentropic compression: for a given overall pressure ratio, the compressor delivery temperature is higher than ideal.
2. Non-isentropic expansion: although the turbine temperature drop necessary to drive the compressor is unaffected, the associated pressure ratio is greater, which decreases the expansion available to provide useful work.
3. Pressure losses in the air intake, combustor and exhaust: reduces the expansion available to provide useful work.


As with all cyclic heat engines, higher combustion temperature means greater efficiency. The limiting factor is the ability of the steel, nickel, ceramic, or other materials that make up the engine to withstand heat and pressure. Considerable engineering goes into keeping the turbine parts cool. Most turbines also try to recover exhaust heat, which otherwise is wasted energy. Recuperators are heat exchangers that pass exhaust heat to the compressed air, prior to combustion. Combined cycle designs pass waste heat to steam turbine systems. And combined heat and power (co-generation) uses waste heat for hot water production. Mechanically, gas turbines can be considerably less complex than internal combustion piston engines. Simple turbines might have one moving part: the shaft/compressor/turbine/alternative-rotor assembly (see image above), not counting the fuel system.
More sophisticated turbines (such as those found in modern jet engines) may have multiple shafts (spools), hundreds of turbine blades, movable stator blades, and a vast system of complex piping, combustors and heat exchangers. As a general rule, the smaller the engine the higher the rotation rate of the shaft(s) needs to be to maintain tip speed. Turbine blade tip speed determines the maximum pressure that can be gained, independent of the size of the engine. Jet engines operate around 10,000 rpm and micro turbines around 100,000 rpm. Thrust bearings and journal bearings are a critical part of design. Traditionally, they have been hydrodynamic oil bearings, or oil-cooled ball bearings.



CYCLES
Brayton cycle:
The 'Brayton cycle' is a constant-pressure cycle that describes the workings of the gas turbines, basis of the jet engine and others. It is also sometimes known as the Joule cycle. The Ericsson cycle is also similar but uses external heat and incorporates the use of a regenerator.
A Brayton-type engine consists of three components:
• A gas compressor
• A mixing chamber
• An expander
In the original 19th-century Brayton engine, ambient air is drawn into a piston compressor, where it is compressed; ideally an isentropic process. The compressed air then runs through a mixing chamber where fuel is added, a constant-pressure isobaric process. The heated (by compression), pressurized air and fuel mixture is then ignited in an expansion cylinder and energy is released, causing the heated air and combustion products to expand through a piston/cylinder; another theoretically isentropic process. Some of the work extracted by the piston/cylinder is used to drive the compressor through a crankshaft arrangement.
The term Brayton cycle has more recently been applied to the gas turbines. This also has three components:
• A gas compressor
• A burner (or combustion chamber)
• An expansion turbine
\Ideal Brayton cycle:
• Isentropic process (1-2) - Ambient air is drawn into the compressor, where it is pressurized.
• Isobaric process (2-3) - The compressed air then runs through a combustion chamber, where fuel is burned, heating that air—a constant-pressure process, since the chamber is open to flow in and out.
• Isentropic process (3-4) - The heated, pressurized air then gives up its energy, expanding through a turbine (or series of turbines). Some of the work extracted by the turbine is used to drive the compressor.
• Isobaric process (4-1) - Heat Rejection (in the atmosphere).

Actual Brayton cycle:
Since there are always some heat and other losses, so as a result the actual cycle differs from ideal cycle. In the actual brayton cycle the various processes are:
• Adiabatic process (1-2) - Compression.
• Isobaric process (2-3) - Heat Addition.
• Adiabatic process (3-4) - Expansion.
• Isobaric process (4-1) - Heat Rejection.
Since neither the compression nor the expansion can be truly isentropic, losses through the compressor and the expander represent sources of inescapable working inefficiencies. In general, increasing the compression ratio is the most direct way to increase the overall power output of a Brayton system.






STEAM TURBINE


Introduction:
A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into useful mechanical work. The available heat energy of the steam first is converted into kinetic energy by the expansion of the steam in suitably shaped passage, or nozzle, form which it issues as a jet, at a proper angle, against curved blades mounted on a revolving disk or cylinder and by the reaction of the jet itself as it leaves the curved passage. The pressure on the blades, causing rotary motion, is solely due to the change of momentum of the steam jet during its passage through these blades.
The steam energy is converted mechanical work by expansion through the turbine. The expansion takes place through a series of fixed blades (nozzles) and moving blades each row of fixed blades and moving blades is called a stage. The moving blades rotate on the central turbine rotor and the fixed blades are concentrically arranged within the circular turbine casing which is substantially designed to withstand the steam pressure. On large output turbines the duty too large for one turbine and a number of turbine casing/rotor units are combined to achieve the duty. These are generally arranged on a common centre line (tandem mounted) but parallel systems can be used called cross compound systems.
Energy in the steam after it leaves the boiler is converted into rotational energy as it passes through the turbine. The turbine normally consists of several stages with each stage consisting of a stationary blade (or nozzle) and a rotating blade. Stationary blades convert the potential energy of the steam (temperature and pressure) into kinetic energy (velocity) and direct the flow onto the rotating blades. The rotating blades convert the kinetic energy into forces, caused by pressure drop, which results in the rotation of the turbine shaft. The turbine shaft is connected to a generator, which produces the electrical energy. The rotational speed is near to 3000 rpm for generating electricity at 50 Hz.



Working cycle:
The steam turbine works on rankine cycle, the Rankine cycle is a thermodynamic cycle which converts heat into work. The heat is supplied externally to a closed loop, which usually uses steam as the working fluid. A Rankine cycle describes a model of the operation of steam heat engines most commonly found in power generation plants. Common heat sources for power plants using the Rankine cycle are coal, natural gas, oil, and nuclear.
The Rankine cycle is sometimes referred to as a practical Carnot cycle as, when an efficient turbine is used, the TS diagram will begin to resemble the Carnot cycle. The main difference is that a pump is used to pressurize liquid instead of gas. This requires about 100 times less energy than that compressing a gas in a compressor (as in the Carnot cycle).
The efficiency of a Rankine cycle is usually limited by the working fluid. Without the pressure going super critical the temperature range the cycle can operate over is quite small, turbine entry temperatures are typically 565°C (the creep limit of stainless steel) and condenser temperatures are around 30°C. This gives a theoretical Carnot efficiency of around 63% compared with an actual efficiency of 42% for a modern coal-fired power station. This low turbine entry temperature (compared with a gas turbine) is why the Rankine cycle is often used as a bottoming cycle in combined cycle gas turbine power stations.

Fig: working of steam turbine
The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. The water vapor often seen billowing from power stations is generated by the cooling systems (not from the closed loop Rankine power cycle) and represents the waste heat that could not be converted to useful work. Note that steam is invisible until it comes in contact with cool, saturated air, at which point it condenses and forms the white billowy clouds, seen leaving cooling towers. While many substances could be used in the Rankine cycle, water is usually the fluid of choice due to its favorable properties, such as nontoxic and unreactive chemistry, abundance, and low cost, as well as its thermodynamic properties.
One of the principal advantages it holds over other cycles is that during the compression stage relatively little work is required to drive the pump, due to the working fluid being in its liquid phase at this point. By condensing the fluid to liquid, the work required by the pump will only consume approximately 1% to 3% of the turbine power and so give a much higher efficiency for a real cycle. The benefit of this is lost somewhat due to the lower heat addition temperature. Gas turbines, for instance, have turbine entry temperatures approaching 1500°C. Nonetheless, the efficiencies of steam cycles and gas turbines are fairly well matched.
Processes in rankine cycle:


Fig: Ts diagram of a typical Rankine cycle operating between pressures of 0.06bar and 50bar
There are four processes in the Rankine cycle, each changing the state of the working fluid. These states are identified by number in the diagram to the right.
• Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a liquid at this stage the pump requires little input energy.
• Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapor.
• Process 3-4: The dry saturated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapor, and some condensation may occur.
• Process 4-1: The wet vapor then enters a condenser where it is cooled at a constant pressure and temperature to become a saturated liquid. The pressure and temperature of the condenser is fixed by the temperature of the cooling coils as the fluid is undergoing a phase-change.
In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the pump and turbine would generate no entropy and hence maximize the net work output. Processes 1-2 and 3-4 would be represented by vertical lines on the Ts diagram and more closely resemble that of the Carnot cycle. The Rankine cycle shown here prevents the vapor ending up in the superheat region after the expansion in the turbine, which reduces the energy removed by the condensers.
Constructional features:
The steam turbine at P.P.C.L has a tandem compound shaft arrangement with H.P and L.P sections. The H.P section is a single flow turbine where as the L.P section is double flow. The individual turbine rotors and generator rotors are connected by rigid couplings. The H.P turbine has been constructed for throttle control governing. The initial steam is admitted before the blading by two combined main steam stop and control valves.
The steam from H.P turbine exhaust is led to the L.P turbine through cross around pipes. Additional steam from L.P stage of waste heat recovery steam generator is passed to the L.P turbine via two combined L.P stop and control valve. Following are some important components and systems of the steam turbine











HEAT RECOVERY STEAM GENERATOR (HRSG)


A heat recovery steam generator or HRSG is a heat exchanger that recovers heat from a hot gas stream. It produces steam that can be used in a process or used to drive a steam turbine. A common application for an HRSG is in a combined-cycle power station, where hot exhaust from a gas turbine is fed to an HRSG to generate steam which in turn drives a steam turbine. This combination produces electricity more efficiently than either the gas turbine or steam turbine alone. Another application for an HRSG is in diesel engine combined cycle power plants, where hot exhaust from a diesel engine is fed to an HRSG to generate steam which in turn drives a steam turbine. The HRSG is also an important component in cogeneration plants. Cogeneration plants typically have a higher overall efficiency in comparison to a combined cycle plant. This is due to the loss of energy associated with the steam turbine.
HRSG is consist of three major components. They are the Evaporator, Superheater, and Economizer. The different components are put together to meet the operating requirements of the unit. Based on the flow of exhaust gases, HRSGs are categorized into vertical and horizontal types. In vertical type HRSGs, exhaust gas flows horizontally over vertical tubes whereas in horizontal type HRSGs, exhaust gas flow vertically over horizontal tubes. Based on pressure levels, HRSGs can be categorized into single pressure and multi pressure. Single pressure HRSGs have only one steam drum and steam is generated at single pressure level whereas multi pressure HRSGs employ two (double pressure) or three (triple pressure) steam drums. As such triple pressure HRSGs consist of three sections: an LP (low pressure) section, a reheat/IP (intermediate pressure) section, and an HP (high pressure) section. Each section has a steam drum and an evaporator section where water is converted to steam. This steam then passes through superheaters to raise the temperature and pressure past the saturation point.
The heat recovery steam generator at P.P.C.L is a horizontal, natural circulation, single drum, dual pressure, unfired water tube boiler. It is designed to generate steam quantities as furnished in operating parameters at main steam stop valve under specified modes of operation. Feed water temperature is 151.7˚c for H.P section and 150.1˚c for L.P section at the design point. It is consist of the following main components:
1) H.P superheater and components:
A superheater is a device in a HRSG that heats the steam generated again, increasing its thermal energy and decreasing the likelihood that it will condense inside the steam turbine. Superheaters increase the efficiency of the whole system, and are widely adopted. Steam which has been superheated is logically known as superheated steam; non-superheated steam is called saturated steam or wet steam. H.P superheater is the first heat transfer surface arranged in the direction of gas flow. H.P superheater is constructed of modules consisting of finned tubes welded to the top and bottom headers two rows maximum per module. The H.P superheater is designed for single pass flow on the gas side and on the tube side. Interconnection of modules will be by multiple 180 degrees bent tubes of similar material as header. The H.P superheater modules are equipped with low point 1 inches drain for a fully drainable design. The H.P superheater outlet is equipped with a vent for start up.



2) H.P evaporator:
An evaporator changes liquids into gaseous state. For instance, water is heated and changed into steam. Therefore, it is the opposite of a condenser. H.P evaporator is the second heat transfer surface arranged in the direction of the gas flow. In the circulation system, the heated feed water from H.P economizer goes to the steam drum. The boiler water from the steam drum flows to the common feed headers through large down corners and is distributed to the bottom headers of the evaporator through the individual feeders. Steam water mixture generated in the evaporator tubes due to the heat transfer from the gas turbine exhaust, flows back to the drum through the riser tubes. The saturated steam is separated by centrifugal separator and final scrubbers placed inside the drum.
3) H.P economizer:
Economizers are mechanical devices intended to reduce energy consumption, or to perform another useful function like preheating a fluid. The term economizer is used for other purposes as well. Boiler, power plant, and heating, ventilating, and air-conditioning (HVAC) uses are discussed in this article. In simple terms, an economizer is a heat exchanger.
H.P economizer 2 is the third and H.P economizer 1 is the sixth heat transfer surface. The H.P economizer is constructed of modules, consisting of spiral finned tubes welded to the top and bottom headers, two rows per module. The H.P economizer is designed for single pass flow on the gas side and multipass flow on the tube side.
4) L.P superheater:
L.P superheater is the fourth heat transfer surface arranged in the direction of the gas flow. L.P superheater is constructed of a module consisting of finned tubes, welded to top and bottom headers single row module. The L.P superheater is designed for single pass flow on the gas side and on the tube side.

5) L.P evaporator:
It is fifth heat transfer surface, in the circulation system, the heated feed goes to the steam drum. The boiler water from the steam drum flow to the common feed headers through the individual feeders steam water mixture generated in the evaporator due to heat transfer from the gas turbine exhaust flows back to the drum through the riser tubes.
6) Condensate preheater:
It is the seventh and the final heat transfer surface arranged in the gas flow. Condensate preheater is constructed of modules consisting of spiral finned tubes welded to the top and bottom headers two rows maximum per module. Condensate preheater is designed for single pass flow of the gas side and multipass flow on the tube side.



7) Attemperator:
Attemperator reduces steam temperature by bringing superheated steam into direct contact with water. The steam is cooled through the evaporation of the water. Attemperators can be mounted either horizontally or vertically and are normally used for relatively steady load conditions where pressure losses must be minimized. These desuperheaters are a modification of the venturi-type unit, without the venturi tail, and offer increased turndown when mounted vertically up.
Water enters the attemperator and is preheated in the circulatory chamber around the water diffuser tube. It is then introduced in many small jets to assist final atomization by the steam flow through the center of the throat. After leaving the throat, the mixture of steam and water enters the main steam flow in a fog-like condition where final heat transfer is achieved without contacting the sidewalls-providing maximum desuperheating effectiveness with minimum of pipe wear. Water pressure into the attemperator should equal steam line pressure. Spray attemperator is utilised to control the H.P steam temperature to 520˚C. For this application interstage attemperator is used after the H.P superheater.
8) Expansion joints:

A part of expansion joint Positions of expansion joints
One expansion joint at HRSG inlet and one more expansion joint at chimney or stack inlet are provided to take care of expansions.
9) Chimney or Stacks:
The gas turbine exhaust gases after passing through all the heat transfer surfaces are discharged into the atmosphere at safe height through a 70 m high steel chimney or stack. On each HRSG there are two stacks one main stack and other bypass stack. When enough steam is produced in the HRSG than the gas turbine exhaust gases are diverted to bypass stack by closing




GENERATOR


A steam turbine generator
In electricity generation, an electrical generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction. The reverse conversion of electrical energy into mechanical energy is done by a motor, and motors and generators have many similarities. A generator forces electric charges to move through an external electrical circuit, but it does not create electricity or charge, which is already present in the wire of its windings. It is somewhat analogous to a water pump, which creates a flow of water but does not create the water inside. The source of mechanical energy may be a reciprocating engine, a steam or gas turbine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, the sun or solar energy, compressed air or any other source of mechanical energy. At P.P.C.L there are 3 electrical generators, each coupled with two gas turbines and one steam turbine. The two main parts of a generator or motor can be described in either mechanical or electrical terms:
Mechanical:
• Rotor:
The rotor is the non-stationary part of a rotary electric motor or a A.C generator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis. In some designs, the rotor can act to serve as the motor's armature, across which the input voltage is supplied. The stationary part of an electric motor is the stator.



Stator:
The stationary part of an alternator, generator, dynamo or motor is called stator. The non-stationary part on an electric motor is the rotor.
Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the armature, receiving its influence from moving field coils on the rotor.
Electrical:
• Armature: The power-producing component of an alternator, generator, dynamo or motor. In a generator, alternator, or dynamo the armature windings generate the electrical current. The armature can be on either the rotor or the stator.
• Field: The magnetic field component of an alternator, generator, dynamo or motor. The magnetic field of the dynamo or alternator can be provided by either electromagnets or permanent magnets mounted on either the rotor or the stator.
Because power transferred into the field circuit is much less than in the armature circuit, AC generators nearly always have the field winding on the rotor and the stator as the armature winding. Only a small amount of field current must be transferred to the moving rotor, using slip rings. Direct current machines necessarily have the commutator on the rotating shaft, so the armature winding is on the rotor of the machine.
EXITATION:
An electric generator or electric motor that uses field coils rather than permanent magnets will require a current flow to be present in the field coils for the device to be able to work. If the field coils are not powered, the rotor in a generator can spin without producing any usable electrical energy, while the rotor of a motor may not spin at all. Very large power station generators often utilize a separate smaller generator to excite the field coils of the larger. The generators at P.P.C.L also use an exiter. In the event of a severe widespread power outage where islanding of power stations has occurred, the stations may need to perform a black start to excite the fields of their largest generators, in order to restore customer power service.
WORKING PRINCIPLE:
The electrical generator works on faraday’s law of electromagnetic induction. Faraday's law of induction describes an important basic law of electromagnetism, which is involved in the working of transformers, inductors, and many forms of electrical generators. The law states:
“The induced electromotive force or EMF in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.”
The law was discovered by Michael Faraday in 1831 and independently at the same time by Joseph Henry.
Quantitatively, the law takes the following form:
.
Where
is the electromotive force (EMF) in volts
ΦB is the magnetic flux through the circuit (in webers).
The direction of the electromotive force (the negative sign in the above formula) is given by Lenz's law. The meaning of "flux through the circuit" is elaborated upon in the examples below.
Traditionally, two different ways of changing the flux through a circuit are recognized. In the case of transformer EMF the idea is to alter the field itself, for example by changing the current originating the field (as in a transformer), or by sweeping a magnet past a loop of wire. In the case of motional EMF, the idea is to move all or part of the circuit through the magnetic field, for example, as in a homopolar generator.
When the magnetic field around a conductor changes, a current is induced in the conductor. Typically, a rotating magnet called the rotor turns within a stationary set of conductors wound in coils on an iron core, called the stator. The field cuts across the conductors, generating an electrical current, as the mechanical input causes the rotor to turn. The rotating magnetic field induces a AC voltage in the stator windings. Often there are three sets of stator windings, physically offset so that the rotating magnetic field produces three phase currents, displaced by one-third of a period with respect to each other.
The rotor magnetic field may be produced by induction (in a "brushless" alternator), by permanent magnets (in very small machines), or by a rotor winding energized with direct current through slip rings and brushes. The rotor magnetic field may even be provided by stationary field winding, with moving poles in the rotor. Automotive alternators invariably use a rotor winding, which allows control of the alternator generated voltage by varying the current in the rotor field winding. Permanent magnet machines avoid the loss due to magnetizing current in the rotor, but are restricted in size, owing to the cost of the magnet material. Since the permanent magnet field is constant, the terminal voltage varies directly with the speed of the generator. Brushless AC generators are usually larger machines like those in power plants than those used in automotive applications























. DEAERATOR


A Deaerator is a device that is widely used for the removal of air and other dissolved gases from the feedwater to steam generating boilers. In particular, dissolved oxygen in boiler feedwaters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming oxides (rust). It also combines with any dissolved carbon dioxide to form carbonic acid that causes further corrosion. Most deaerators are designed to remove oxygen down to levels of 7 ppb by weight (0.0005 cm³/L) or less.
The removal of dissolved gases from boiler feedwater is an essential process in a steam system. The presence of dissolved oxygen in feedwater causes rapid localized corrosion in boiler tubes. Carbon dioxide will dissolve in water, resulting in low pH levels and the production of corrosive carbonic acid. Low pH levels in feedwater causes severe acid attack throughout the boiler system. While dissolved gases and low pH levels in the feedwater can be controlled or removed by the addition of chemicals, it is more economical and thermally efficient to remove these gases mechanically. This mechanical process is known as deaeration and will increase the life of a steam system dramatically.
The purposes of deaeration are:
1. To remove oxygen, carbon dioxide and other non-condensable gases from feed water.
2. To heat the incoming makeup water and return condensate to an optimum temperature for:
a. Minimizing solubility of the undesirable gases
b. Providing the highest temperature water for injection to the boiler
Deaerators are typically elevated in boiler rooms to help create head pressure on pumps located lower. This allows hotter water to be pumped without vapor locking should some steam get into the pump.
The most common source of corrosion in boiler systems is dissolved gas: oxygen, carbon dioxide and ammonia. Of these, oxygen is the most aggressive. The importance of eliminating oxygen as a source of pitting and iron deposition cannot be over-emphasized. Even small concentrations of this gas can cause serious corrosion problems.
Makeup water introduces appreciable amounts of oxygen into the system. Oxygen can also enter the feed water system from the condensate return system. Possible return line sources are direct air-leakage on the suction side of pumps, systems under vacuum, the breathing action of closed condensate receiving tanks, open condensate receiving tanks and leakage of non-deaerated water used for condensate pump seal and/or quench water. With all of these sources, good housekeeping is an essential part of the preventive program.
One of the most serious aspects of oxygen corrosion is that it occurs as pitting. This type of corrosion can produce failures even though only a relatively small amount of metal has been lost and the overall corrosion rate is relatively low. The degree of oxygen attack depends on the concentration of dissolved oxygen, the pH and the temperature of the water.
The influence of temperature on the corrosivity of dissolved oxygen is particularly important in closed heaters and economizers where the water temperature increases rapidly. Elevated temperature in itself does not cause corrosion. Small concentrations of oxygen at elevated temperatures do cause severe problems. This temperature rise provides the driving force that accelerates the reaction so that even small quantities of dissolved oxygen can cause serious corrosion.
Operating Principle:
Deaeration is based on two scientific principles. The first principle can be described by Henry's Law. Henry's Law asserts that gas solubility in a solution decreases as the gas partial pressure above the solution decreases. The second scientific principle that governs deaeration is the relationship between gas solubility and temperature. Easily explained, gas solubility in a solution decreases as the temperature of the solution rises and approaches saturation temperature. A deaerator utilizes both of these natural processes to remove dissolved oxygen, carbon dioxide, and other non-condensable gases from boiler feedwater. The feedwater is sprayed in thin films into a steam atmosphere allowing it to become quickly heated to saturation. Spraying feedwater in thin films increases the surface area of the liquid in contact with the steam, which, in turn, provides more rapid oxygen removal and lower gas concentrations. This process reduces the solubility of all dissolved gases and removes it from the feedwater. The liberated gases are then vented from the deaerator. Deaerators use steam to heat the water to the full saturation temperature corresponding to the steam pressure in the deaerator and to carry away dissolved gases. The deaeration system consists of deaeration tank, a storage tank and a vent. In the deaeration tank, water is heated and agitated by steam bubbling through the water. Steam is cooled by the incoming water and condensed at the vent condenser. Non-condensable gases and some steam are released through the vent. Steam provided to the deaerator provides physical stripping action and heats the mixture of returned condensate and boiler feedwater make-up to saturation temperature. While most of the steam condenses, a small percentage of steam (5-14%) must be vented to accommodate the stripping requirements.
There are two basic types of deaerators, the tray-type and the spray-type:
• The tray-type deaerator (also called the cascade-type) includes a vertical domed deaeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feedwater storage tank.
• The spray-type deaerator consists only of a horizontal (or vertical) cylindrical vessel which serves as both the deaeration section and the boiler feedwater storage tank.
Types of deaerators:
There are many different horizontal and vertical designs available from a number of manufacturers, and the actual construction details will vary from one manufacturer to another. The deaerator at P.P.C.L is a Tray type deaerator. The two types of deaerator are described below:
Tray-type deaerator:


Figure: A schematic diagram of a typical tray-type deaerator.
The typical horizontal tray-type deaerator has a vertical domed deaeration section mounted above a horizontal boiler feedwater storage vessel. Boiler feedwater enters the vertical dearation section above the perforated trays and flows downward through the perforations. Low-pressure dearation steam enters the below the perforated trays and flows upward through the perforations. Some designs use various types of packing material, rather than perforated trays, to provide good contact and mixing between the steam and the boiler feed water.
The steam strips the dissolved gas from the boiler feedwater and exits via the vent at the top of the domed section. Some designs may include a vent condenser to trap and recover any water entrained in the vented gas. The vent line usually includes a valve and just enough steam is allowed to escape with the vented gases to provide a small and visible telltale plume of steam.
The deaerated waster flows down into the horizontal storage vessel from where it is pumped to the steam generating boiler system. Low-pressure heating steam, which enters the horizontal vessel through a sparger pipe in the bottom of the vessel, is provided to keep the stored boiler feedwater warm. External insulation of the vessel is typically provided to minimize heat loss.

Deaeration steam:
The deaerators in the steam generating systems of most power plants use low pressure steam obtained from an extraction point in their steam turbine system. However, the steam generators in many large industrial facilities such as petroleum refineries may use whatever low-pressure steam that is available.
Oxygen scavengers:
Oxygen scavenging chemicals are very often added to the deaerated boiler feedwater to remove any last traces of oxygen that were not removed by the deaerator. The most commonly used oxygen scavenger is sodium sulfite (Na2SO3). It is very effective and rapidly reacts with traces of oxygen to form sodium sulfate (Na2SO4) which is non-scaling.
Limitations:
Inlet water should be virtually free of suspended solids that could clog spray valves and ports of the inlet distributor and the deaerator trays. In addition, spray valves, ports and deaerator trays may become plugged with scale that forms when the water being deaerated has high hardness and alkalinity levels. In this case, routine cleaning and inspection of the deaerator is very important.


COOLING TOWER



An induced draft cooling tower
Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or rely solely on air to cool the working fluid to near the dry-bulb air temperature. Common applications include cooling the circulating water used in oil refineries, chemical plants, power plants and building cooling. The towers vary in size from small roof-top units to very large hyperboloid structures that can be up to 200 metres tall and 100 metres in diameter, or rectangular structures that can be over 40 metres tall and 80 metres long. Smaller towers are normally factory-built, while larger ones are constructed on site.
In a combined cycle power plant the steam after doing useful work in steam turbine is condensed by means of a cooling tower, the steam after passing through steam turbine goes into the condenser, the condenser is consist of a number of coils in which cold water from cooling tower runs, the steam passes over from these coils and thus the heat is absorbed from the steam and it condenses in to water, this water is again sent to the HRSG after mixing it with pre-treated makeup water. The water after absorbing heat becomes hot, so it is again sent to the cooling tower where it again cools and is re-circulated. If cooling tower is not present in a plant than water requirement of the plant will become very large. The primary task of a cooling tower is to reject heat into the atmosphere. This heat rejection is accomplished through the natural process of evaporation that takes place when air and water are brought into direct contact in the cooling tower. The evaporation is most efficient when the maximum water surface area is exposed to the maximum flow of air, for the longest possible period of time.
Cooling towers are designed in two different configurations, counter flow and cross flow. The specific configuration indicates the direction of air flow through the tower relative to the direction of the water flow. Cooling tower water and air distribution systems are designed in concert, with each playing an equally important role in determining the efficiency and proper application of the cooling tower.
The overall efficiency of a cooling tower is directly related to the design of the tower's hot water distribution system. The primary consideration in selecting the type of hot water distribution system for a specific application is pump head. The pump head imposed by a cooling tower consists of the static lift (related to the height of the inlet) plus the pressure necessary to move the water through the distribution system and over the fill. The pump head varies according to the cooling tower configuration. The cooling tower at P.P.C.L is a induced draught cross flow cooling tower, the cooling towers can be classified as:
Air flow generation methods:
With respect to drawing air through the tower, there are three types of cooling towers:
• Natural draft, which utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due to the density differential to the dry, cooler outside air. Warm moist air is less dense than drier air at the same temperature and pressure. This moist air buoyancy produces a current of air through the tower.
• Mechanical draft, which uses power driven fan, motors to force or draw air through the tower.
o Induced draft: A mechanical draft tower with a fan at the discharge which pulls air through tower. The hot water falls down from top of the cooling tower through nozzles; the fan is situated at top of these nozzles. The interaction of air and water results in absorption of heat from the hot water.

A cooling tower fan
The fan produces low entering and high exiting air velocities, reducing the possibility of recirculation in which discharged air flows back into the air intake. This fan/fill arrangement is also known as draw-through.

o Forced draft: A mechanical draft tower with a blower type fan at the intake. The fan forces air into the tower, creating high entering and low exiting air velocities. The low exiting velocity is much more susceptible to recirculation. With the fan on the air intake, the fan is more susceptible to complications due to freezing conditions. Another disadvantage is that a forced draft design typically requires more motor horsepower than an equivalent induced draft design. The forced draft benefit is its ability to work with high static pressure. They can be installed in more confined spaces and even in some indoor situations. This fan/fill geometry is also known as blow-through.
• Fan assisted natural draft. A hybrid type that appears like a natural draft though airflow is assisted by a fan.

Categorization by air-to-water flow:
Crossflow:

Crossflow is a design in which the air flow is directed perpendicular to the water flow). Air flow enters one or more vertical faces of the cooling tower to meet the fill material. Water flows (perpendicular to the air) through the fill by gravity. The air continues through the fill and thus past the water flow into an open plenum area. A distribution or hot water basin consisting of a deep pan with holes or nozzles in the bottom is utilized in a crossflow tower. Gravity distributes the water through the nozzles uniformly across the fill material. Cross flow towers utilize a distinctly different type of water distribution system. Hot water is distributed to the fill by gravity through metering orifices in the floor of the inlet basin. There is no pressure spray distribution system. The air movement is horizontally through the fill, across the downward fall of the water. In cross flow towers, the internal pressure component of pump head is insignificant because maximum flow is achieved by gravity.
ADVANTAGES:
Advantages of cross flow cooling towers due to their gravity flow hot water distribution
system:
1) Low pumping head.
2) Lower first cost pumping systems.
3) Lower annual energy consumption and operating costs.
4) Accepts larger variation in water flow without adverse effect on the water distribution pattern (flat plate heat exchanger operation in winter).
5) Easy maintenance access to distribution nozzles.



Advantages of cross flow cooling towers due to their horizontal air distribution system:
1) Low static pressure drop.
2) Reduced drift.
3) Reduced recirculation.
4) More air per fan horsepower.
5) Larger diameter fans can be used so that fewer cells are required for a given capacity.
6) Lower energy and operating costs.
Disadvantages of cross flow cooling towers due to their gravity flow hot water distribution system:
1) Low pressure head on the distribution pan may encourage orifice clogging and less water break-up at spray nozzle.
2) Exposure to air in the hot water basin may accelerate algae growth. Larger footprint.
Disadvantage of cross flow cooling towers due to their horizontal air distribution system:
1) Larger louver surface area makes icing more difficult to control.

Counterflow:

In a counterflow design the air flow is directly opposite of the water flow. Air flow first enters an open area beneath the fill media and is then drawn up vertically. The water is sprayed through pressurized nozzles and flows downward through the fill, opposite to the air flow. Counter flow towers use a high pressure spray nozzle hot water distribution system to achieve water coverage of the fill. The nozzle spray pattern is sensitive to changes in water flow, and consequent change in nozzle pressure. The air movement is vertically upward through the fill, counter to the downward fall of the water. Counter flow towers typically have a smaller footprint than cross flow towers, but require additional height, static lift, and dynamic head to achieve the same cooling effect.
Common to both designs:
• The interaction of the air and water flow allows a partial equalization and evaporation of water.
• The air, now saturated with water vapor, is discharged from the cooling tower.
• A collection or cold water basin is used to contain the water after its interaction with the air flow.
Both crossflow and counterflow designs can be used in natural draft and mechanical draft cooling towers.
Advantages of counter flow cooling towers due to their pressurized spray water distribution system:
• Increased tower height accommodates longer ranges and closer approaches.
• More efficient use of air due to finer droplet size from pressure sprays.
Advantages of counter flow cooling towers due to their vertical air distribution system:
The vertical air movement across the fill allows the coldest water to be in contact with the driest air maximizing tower performance.
Disadvantages of counter flow cooling towers due to their pressurized spray water distribution system:
• Increased system pumping head requirements.
• Increased energy consumption and operating costs.
• Distribution nozzles difficult to inspect and clean.
• Requires individual risers for each cell, increasing external piping costs.
Disadvantages of counter flow cooling towers due to their vertical air distribution system:
• The resistance to upward air travel against the falling water results in higher static pressure loss and a greater fan horsepower than in cross flow towers.
• The restricted louver area at the base with high velocity of inlet air increases the fan horsepower.
• Tendency for uneven distribution of air through the fill with very little movement near the walls and center of the tower.
• High inlet velocities are liable to suck airborne trash and dirt into the tower.