- Steam generating unit is defined as "A combination of apparatus for producing, furnishing or recovering heat together with the apparatus for transferring the heat. So made available to the fluid being heated and vapourised.
Classification of Boilers
- Horizontal, Vertical or Inclined: The parts of the horizontal boiler can be inspected and repaired easily but it occupies more space. The vertical boiler occupies less floor area.
- Fire Tube and water Tube: In the fire tube boilers; the hot gases are inside the tubes and a water surrounds the tubes. Example Cochran, Lancashire, Locomotive etc. In the water tube boilers, the water is inside the tubes and hot gas surrounding them Example Babcock and Wilcox, Stirling, yarrow boiler etc.
- Externally and Internally fired: If the fire is outside the shell Example Babcock and Wilcox boiler, Stirling boiler etc. In case of internally fired boilers, the furnace is located inside the boiler shell Example Cochran, Lancashire boiler etc.
- Forced Circulation and Natural Circulation: In forced circulation, circulation of water is done by a forced pump Example Velox, Lamont, Benson etc. In natural circulation type of boiler water in boilers takes place due to natural convection currents Example Lancashire, Babcock & Wilcox etc.
- High Pressure and Low pressure Boilers: The boilers which produce steam at pressures of 80 bar and above are called high-pressure boilers Example Babcock & Wilcox, Velox, Lamont, Benson boilers. The pressure below 80 bar in a boiler is called low-pressure boilers Example; Cochran, Cornish, Lancashire, Locomotive etc.
- Stationary and Portable: Stationary Boilers are used for power plants or processes steam in plants.Portable Boilers are small units of mobile and are used for temporary uses at the sites.
- Single Tube and Multitube Boilers: Only cornish is single tube vertical boiler.
- Shell: The shell of a boiler consists of one or more steel plates bent into a cylindrical form and riveted or welded together.
- Setting: The primary function of setting is to confine heat to boiler and form a passage for gases.
- Grate: It is the platform in the furnace upon which fuel is burnt and it is made of cast iron bars.
- Mountings: The items such as stop valve, safety valves, water level, gauges, fusible plug, blow of cock, pressure gauges, water level indicator.
- Accessories: The items such as superheaters, econom feed pumps etc. are termed as accessories, they form integral part of boiler.
- Foaming: Formation of steam bubbles on the surface of boiler water due to high surface tension of water.
- Scale: A deposit of medium to extreme hardness occurring on water heating surface of a boiler.
- Blowing off: The removal of the mud and other impurities of water from the lowest part of the boiler is formed as 'blowing off'.
- Logging: Blocks of asbestos or magnesia insulation wrapped on outside of a boiler shell or steam piping.
- High Pressure Boiler: Where steam is needed at pressure, 30 bar and individual boilers are required to raise less than about 30000 kg of steam per hour, shell boilers are considerably cheaper than the water tube Boilers. Therefore water tube boilers are generally preferred for high pressure and high output whereas shell boilers for low pressure and low output. The modern high-pressure boilers employed for power generation are for steam capacities 30 to 650 tons/hr and above with a press drop up to 160 bar and maximum steam temperature of about 540ºC.
HIGH PRESSURE BOILERS
Steam Drum: The functions of the drum in a water-tube boiler can then be summerized as follows:
- To store water and steam sufficiently to meet varying load requirement.
- To aid in circulation
- To separate vapour or steam from water-steam mixture discharged by the risers.
- To provide enough surface area for liquid-vapour disengagement.
- To maintain a certain desired ppm in the drum water by phosphate injection and blowdown.
- Mechanical separators like baffles, screens and cyclones which are housed inside the drum for separation of steam-water mixture are known as drum internals.They changes or reverse the steam flow direction.
- Cyclone separators utilize the centrifugal forces for separation of two-phase mixture.
- The flow of water and steam within the boiler circuit is called circulation. If circulation is caused by density difference, the boiler is said to have natural circulation. If it is caused by a pump, it has forced or controlled circulation. The down, which is insulated, is outside the furnace and the riser is inside it.
- Nearly saturated water falls by gravity from the drum through the downcomer into the bottom header.
- The pressure head available for natural circulation is given by
- Higher is the density difference, more will be the pressure head available for natural circulation. However, the density differential decreases as pressure increases.
- At the critical pressure, vg = v1 and rg = r1 and there can be no natural circulation.
- Circulation Ratio
m = mass of saturated water flowing through the down riser circuit during a certain time.
mg = mass of steam released from the drum during same time.
ml = m – mg = mass of liquid (saturated) at the riser exit.
TDF = top Dryness fraction i.e. the quality of liquid vap mixture discharged from riser into the drum.
- Economisers: Economizer utilizes the outgoing heat of flue gases to heat the feed water before it enters into the boiler. It brings the fluid from feed water pump to saturation temperature. The rate of heat transfer from the flue gases to feedwater is given by
l = length of one coil
n = number of coils
Uo = overall heat transfer coefficient.
- Superheaters: Supertransfer heat to the saturated vapour from boiler and raises it temperature. It increases the overall efficiency and improves the dryness fraction of exhaust from turbine.
- Superheaters are classified as either convective superheaters, radiant superheaters or combined superheaters; depending on how heat is transferred from the gases to steam.
Steam Generator Control
- The object of steam generator control is to provide the steam flow required by the turbine at design pressure and temperature. The key measurements that describe the plant performance are steam flow rate, steam pressure, steam temperature, primary and secondary air flow rates, fuel firing rate, feed water flow rate and steam drum level, and electrical power output. The control system must act on the measurement of these plant parameters so as to maintain plant operation at the desired condition.
Feed Water and Drum Level Control
- Feed water and, therefore, steam flow is controlled to meet load demand by the turbine and at the same time maintain the level of water in the steam drum within relatively narrow limits. A high steam consumption by the turbine combined with low feed water supply would lower the water level in the drum. A three element automatic control system, of which the drain level is one element, is shown in the figure.
Steam Pressure Control
- The steam pressure control system, sometimes called the "boiler master", maintains steam pressure by adjusting fuel and combustion air flows to meet the desired pressure (Figure). When pressure drops, the flows are increased. A steam pressure sensor acts directly on the fuel flow and air flow controls, such as the pulverized coal power drives and forced drought fan, to affect the desired changes. A trimming single from fuel flow and air flow sensors maintains the proper fuel-air ratio, since it is often difficult to obtain accurate fuel flows, a steamy flow sensor is sometimes substituted for the fuel flow sensor.
Steam Temperature Control : The principal variables affecting superheat temperature are :
- Furnace temperature.
- Cleanliness of radiant and pendant superheaters.
- Temperature of gases entering the convective superheater.
- Cleanliness of convective superheater.
- Mass flow rate of gases through the convective superheater.
- Feedwater temperature.
- Variation of load on the unit.
- A reduction in steam temperature result is loss in plant-efficiency.
- Electro Static Precipitator :
- The principal components of an Electrostatic precipitator are two sets of electrodes insulated from each other. The first set is composed of rows of electrically grounded vertical parallel plates, called the collection electrodes, between which the dust laden gas flows. The second set of electrodes consists of wire, called the discharge or emitting electrodes that are centrally located between each pair of parallel plates.
- Ash Handling System : The large ash particles are collected under the furnace in a water-filled ash hopper. Fly ash is collected in dust-collectors with either an electrostatic precipitator or a baghouse. A pulverized coal (PC) boiler generates approximately 80% fly ash and 20% bottom ash. Three major factor should be considered for ash disposal system :
- Plant site
- Fuel source
- Environmental regulations
- Needs for water and land are important considerations for many ash hand high systems.
- The sluice conveyor system is the most widely used for bottom ash handling, while the hydraulic vacuum conveyor is the most frequently used for fly ash system.
- Feedwater Treatment : The make-up water needs to be treated prior to feeding it to the boiler for :
- Prevention of hard scale formation on the heating surfaces.
- Elimination of corrosion.
- Prevention of silica deposition and corrosion damage to turbine blades.
- Raw water contains a variety of impurities such as :
- Suspended solids and turbidity
- hardness (salts of calcium and magnesium)
- alkanility (bicarbonates, carbonates, hydrates)
- other dissolved ions (sodium, sulphate, chloride etc.)
- dissolved gas (O2, CO2).
- Deaeration :Deareation (designification) is one of the most important steps in boiler water treatment. It depends on the decrease in solubility of dissolved gases notably O2 and CO2 as the water temperature is increased.
- Internal treatment : The dissociation and recombination of water go on till equilibrium is reached.
HOH = H+ + (OH)–
- The presence of excess hydrogen ions makes the water acidic and hence corrosive.
- Trisodium phosphate; Na3 PO4, is injected to water in suitable doses to increase alkalinity. Monosodium phosphate NaH2PO4, is used to decrease alkalinity.
- A pH 10.5 is usually maintained for boiler water in order to minimize corrosion.
- Two principal steps in scale prevention are :
- Periodic or continuous blowdown
- External or internal treatment to eliminate the scale producer by chemical means.
- Internal treatment to prevent scale deposition is usually carried out with some form of sodium phosphate, once in the boiler; all these phosphates react with the calcium salts to form tricalcium phosphate which is rather a flocculent precipitate.
TYPES OF HIGH PRESSURE BOILER
- This boiler worked on forced circu and circulation is maintained by a centrifugal pump, driven by a steam turbine using steam from the boiler. The feed water passes through economizer to the drum from which it is drawn to the circulation pump. The pump delivers the feed water to the tube vapourising section which in turn sends a mixture of steam and water to drum. The steam in the drum is then drawn through the superheater. The saturated superheated so obtained is then supplied to the prime mover.
- In Benson Boiler major difficulty is deposition of salt and sediment on the inner surface of tubes. In this, it is removed by preventing the flow of water into boiler tubes. (forced circulation). The principle is evaporating off the feed water by means of superheated steam from the reheater, the hot gases from the furnace being primarily used for superheating purpose. This boiler can carry higher salt concentration than any other type and is more compact than indirectly heated boilers having natural circulations. These qualities fit is for land or sea transport power generation (140 bar).
- In Lamont Boiler main difficulties is formation and attachment of bubbles on the inner surface of heating tubes. The attached bubbles to the tube surfaces reduce the heat flow and steam generation it offers high thermal resistance than water film. This problem is reduced in Benson boiler by increasing the boiler pressure to critical pressure (225 atm), on transit section steam-water mixture converted into steam. (500 atm).
- It is well-known fact that when the gas velocity exceeds the sound velocity the heat is transferred from the gas at a much higher rate than rates achieved with the subsonic flow. The advantage of this theory is taken to effect the large heat transfer from a smaller surface area in this boiler. This boiler makes use of pressurized combustion.
Super Critical Boiler
- A large number of the steam generating plants are designed between working ranges of 125 atm and 510°C to 300 am and 660°C. These are basically characterized as subcritical and supercritical. Subcritical consists of preheater, evaporator and superheater while supercritical boiler requires only preheater and superheater.
Fluidized bed Boiler
- Fluidized bed boilers produce steam from fossil and waste fuel by using fluidized bed combustion technique.
- In this technique, pulverized coal is put in form of bed on a grate and air is passed from dew side.
- Due to air drag, the small particles are suspended in air and complete coal bed is appeared to be flowing in air like fluid. Due to this the intermixing of particle is very good and complete combustion is achieved so higher efficiency.
- The air velocity at which the coal bed becomes fluidized is called terminal velocity
- These are of two types
1. Bubbling fluidized bed boiler (BFB)
2. Circulating fluidized bed boiler (CFB)
- Advantage of CFB boilers :
(a) Fuel flexibility
(b) High combustion efficiency
(c) Efficient sulphur removal
(d) Low NOx emission
(e) Simple fuel handling and feed system
(f) High availability
(a) Crosion of reactor walls
(b) Attrition of particles
(c) Complexities of the hydrodynamics
- The function of a water level indicator is to indicate the level of water in the boiler constantly. It is also called water gauge. Normally two water level indicators are fitted at the front end of every boiler
- The function of a pressure gauge is to measure the pressure exerted inside the vessel. The gauge is usually mounted on the front top of the shell. These are two types Burdon tube pressure gauge and Diaphragm type pressure gauge.
- The function of safety valve is to release the excess steam when the pressure of steam inside the boiler exceeds the rated pressure. Safety valve generally mounted on the top of shell.
- The function of a fusible plug is to protect the boiler against damage due to overheating for low water level Gun metal is used for fusible plug
Blow off cock
- A blow off cock perform two functions.
- it may discharge a portion of water when the boiler is in operation to blow out mud, scale or sediments periodically.
- it may empty the boiler when necessary for cleaning; inspection and repair.
- It is fitted on the boiler shell at the lowest part of water space.
Feed check valve
- The function of a feed check valve is to control the supply of water to the boiler and to prevent the escaping of water from the boiler when the pump pressure is less or pump is stopped. It is fitted in the water space of the boiler slightly below the normal level of the water.
Junction or Stop Valve
- A junction valve is a valve which is placed directly over a boiler and connected to a steam pipe which carries steam to the engine. Junction valve and stop valve are essentially the same; the larger sizes are called junction valves and the smaller size stop valves. The function of the stop valve is to regulate the flow of steam from one steam pipe to the other or from the boiler to the steam pipe.
- The feed pump is pump which is used to deliver feed water to the boiler.
- The function of an injector is to feed water into the boiler. It is also used where space is not available for the installation of a feed pump. It is not used in large capacity high-pressure boilers.
- An economiser is a device in which the waste heat of the flue gases is utilised for heating the feed water.
- The temperature range between various parts of the boiler is reduced which results in reduction of stresses due to irregular expansion.
- If the boiler is feed with cold water it may result in chilling the boiler metal. Hot feed water checks it.
- Overall efficiency of plant is increased.
- The function of the air preheater is to increase the temperature of air before it enters the furnace. It is generally placed after economiser so that flue gases passes through the economizer and then to Air Preheater.
- The function of a superheater is to increase the temperature of the steam above its saturation point. Superheaters are located in the path of furnace gases so that heat is recovered by the superheater from the hot gases. Advantages are:
- Steam consumption of engine or Turbine is reduced.
- Loses due to condensation in the cylinders and the steam pipes are reduced.
- Erosion of turbine blade is eliminated.
- Efficiency of steam plant is increased.
- The function of the steam separator is to remove the entertained water particles from the steam conveyed to the steam engine or turbine. It is installed as close to the steam engine as possible on the main steam pipe from the boiler.
- The function of steam trap is to drain away automatically the condensed steam from the steam pipes steam jackets and steam separators without permitting any steam to escape.