Fundamentals Of Boiler Design 5s6n6w

FUNDAMENTALS OF BOILER DESIGN NAREN SUKAIH

Algerian LNG plant explosion 19 Jan 2004

Algerian LNG plant explosion • ALGIERS (AFP) - At least 27 people were killed and 72 injured when a huge explosion, apparently caused by a defective boiler, ripped through a liquefied natural gas plant near the eastern Algerian port of Skikda, a government minister said

Fireman extinguish remaining fires at the vast petrochemical complex in the port city of Skikda, January 20, 2004, following a huge explosion on Monday evening. Rescue workers searched through rubble for missing workers at Algeria's largest refinery and export port on Tuesday after a blast at a nearby liquefied natural gas (LNG) plant killed at least 23

Algerian LNG plant explosion

Algeria's largest refinery and export port on Tuesday after a blast at a nearby liquefied natural gas (LNG) plant killed at least 23 people

15.1 WATER TUBE BOILERS • These range in capacity – from small low pressure heating units generating a few thousand kilograms of steam per hour – to large central station boilers operating in the supercritical pressure region and serving turbine-generators in the million kilowatt range.

• Capacity may be magnified more than a thousand times from the smallest to the largest, • Pressure may extend from – just above atmospheric to – values of 35MPa or higher,

• Steam temperatures may vary from – the boiling point to a – highly superheated condition at 650 ºC or above.

15.1 WATER TUBE BOILERS • What are the common elements in boilers having such a diversity of design parameters? • Necessary to define the primary function of a boiler. – simply to generate steam at pressures above the atmospheric. – Steam is generated by the absorption of heat produced in the combustion of fuel. – In some instances, such as waste heat boilers, heated fluids serve as the heat source.

15.1 WATER TUBE BOILERS • Generation of steam by heat absorption from products of combustion suggests that a boiler must have – – – – – –

a pressure parts system to convert incoming feed water into steam; a structure within which the combustion reaction may take place, at the same time facilitating heat transfer and ing boiler components; means of introducing fuel and removing waste products; and controls and instruments to regulate and monitor operation.

• The elements a boiler designer has to deal with are — using other terminology — such things as – – – – – – –

drums, headers and tubing which make up the pressure parts system and enclose the furnace in which combustion takes place; burners and related fuel and ash handling equipment; and fans to supply combustion air and exhaust waste gases. Various types of instruments and controls - link these elements together in a physical and an operational sense.

15.1 WATER TUBE BOILERS • These elements vary with the size and purpose of the power plant in which the boiler is installed. – A large central station in which high thermal efficiency as a primary requisite has many more refinements and auxiliaries than – a small heating plant in which minimum capital investment may be an important criterion.

• No matter how many of these elements may be present, – – – –

the boiler designer must integrate them so that the boiler as a whole can function as a carefully adjusted, complex system which is capable of efficient operation over a wide load range

Criteria Affecting Boiler Design • In addition to the basic elements which are of concern to the boiler designer there are many other factors which must be taken into consideration. – Some of these are linked to the type of power plant and its thermodynamic cycle; – others are related to fundamental considerations of engineering science; – the remainders have to do with such things as fuels, manufacturing techniques and controls.

• All of them have a bearing on the practicality and economy of a particular boiler design.

Criteria Affecting Boiler Design • • • • • • • •

Size and type of plant Thermodynamic cycle Fluid flow Materials and metallurgy Manufacturing processes Fuels Auxiliaries Controls and instruments

Method of Operation Heat Transfer Feed water quality Structural design Testing results Fuel burning equipment Calculations Specific boiler types

Criteria Affecting Boiler Design • For specific boiler applications, these design criteria have a more pronounced effect on some types of boilers than on others. – For example, in combined circulation boilers operating at supercritical pressures, – metallurgy and water treatment have far more stringent requirements than – for shop assembled boilers.

• The latter, however, have been responsible for many improvements in shop fabrication techniques, – and the designer takes advantage of this, – for example, by specifying wall construction for central station boilers.

Boiler Output • The output or capacity of a boiler is often expressed in – tons of steam per hour or – in the power output of a turbine a generator in those cases where a single boiler provides the entire steam supply for an electric generating unit. – Neither term is a true measure of the thermal energy supplied by the boiler. • Actual boiler output in of heat energy depends on several factors other than quantity of steam. – Those include temperature of feedwater entering the economizer, – steam pressure and steam temperature at the superheater outlet, and – the quantity, temperature and pressure of steam entering and leaving the reheater. • Similarly, because boiler output is affected – by turbine and generator efficiencies, – generator output in kilowatts or megawatts is not entirely a true measure of the energy output of the boiler alone.

Boiler Functions • In addition to its primary function of generating steam under pressure, the modern boiler may be required to perform some or all of the following functions; – 1. Provide steam of exceptionally high purity by removing impurities from the saturated steam. – 2. Superheat the steam generated in the boiler to the desired temperature and – maintains this temperature constant over a specified range of load. – 3. In power plants operating on the re-heat cycle, resuperheat the steam which is returned to boiler – after expanding through the high pressure stages of the turbine and – maintain the desired reheat temperature constant over a specified range of load.

Boiler Efficiency • One of the best ways to comprehend the significance of boiler efficiency is by means of the Sankey diagram • which is generally associated with the evaluation of thermo dynamic cycles.

Boiler Efficiency shows the distribution of heat energy in a coal fired re-heat boiler for a central station. • It can be seen that the primary source of heat is the fuel (A) and that preheated air also contributes to the total heat in the furnace. • The amount of heat in the preheated air corresponds to that extracted from the exhaust gases by the air preheater.

Boiler Efficiency • For central station boilers of the reheat type, – losses for about ten per cent of the total heat supplied in the fuel, – thereby realizing boiler efficiencies of 90 per cent or higher.

• becomes quite evident that boiler efficiency may be represented by – the ratio of heat absorbed by water and steam (B) – to the heat in fuel (A).

Boiler Efficiency • Sankey diagrams for various types of boilers

Heat absorbing surfaces • The objective of the boiler designer is – to arrange heat transfer surface and fuel burning equipment in such a manner – as to optimize thermal efficiency and economic investment. • He has a choice of a number of types of surface for absorbing and recovering heat. • Waterwalls, superheaters and reheaters – all absorb heat from the furnace gas as – they perform their respective functions of heating water to the saturation point and – of superheating and resuperheating steam. • Air heaters and economizers – recover heat from the furnace exit gases – in order to preheat combustion air and increase the temperature of incoming feedwater.

Heat absorbing surfaces • The boiler designer must proportion – heat absorbing and heat recovery surfaces in such a manner as – to make best use of the heat released by the fuel.

• Waterwalls, superheaters and reheaters are exposed to convection and radiant heat, • Convection heat transfer predominates in air heaters and economizers. • The relative amounts of these types of surface vary with the size and operating conditions of the boiler. – A small low pressure heating plant with no heat recovery equipment has – quite a different boiler arrangement from a large high pressure central station – operating on a reheat regenerative cycle and incorporating heat recovery equipment.

Heat absorbing surfaces

Heat absorbing surfaces Low pressure heating boiler

box-like low pressure heating boiler has a considerable amount of evaporative surface, in the form of waterwalls, closely spaced tubing ing the two drums

Central station reheat boiler

tower-like central station boiler, incorporates superheater and reheater surface plus an air heater and an economizer in addition to a single drum and waterwalls.

WATERWALLS WATERWALLS • Practically all, modern power boilers are equipped with waterwalls. – In large central station boilers, – waterwalls completely cover the interior surfaces of the furnace, – thus providing practically complete elimination of exposed refractory surface.

• In addition, the waterwalls serve – as the sole means of heating and evaporating the feedwater – supplied to the boiler from the economizer

WATERWALLS • Waterwalls usually consist of – substantially vertical tubes arranged tangent or – approximately so and are connected at top and bottom to headers.

• These tubes receive their water supply – from the boiler drum by means of downcomer tubes – connected between bottom of drum and lower headers.

• In a typical central station boiler, – roughly 50 per cent of the heat released – by the combustion of fuel in the furnace – is absorbed by the waterwalls,

• Heat so absorbed is utilized in – evaporation of all or a relatively large percentage of the water supplied to the boiler.

• The steam generated and a substantial quantity of accompanying water – is discharged from the top of the water- wall tubes into the upper waterwall headers – and then es through riser tubes to the boiler drum. – Here the steam is separated and the accompanying water together with the incoming feedwater is returned to the waterwalls through the downcomers.

Superheaters and Reheaters • The function of the superheater is – to increase the temperature of the steam generated in the boiler.

• Steam enters the superheater at – saturated temperature at practically dry saturated condition, and – consequently the absorption of heat appears as sensible heat in increasing the steam temperature.

• The reheater receives superheated steam which has partly expanded through the turbine. – The function of the reheater is to re-superheat this steam to a desired temperature.

• Designs of superheaters and reheaters vary – depending largely on the duty to be performed. – For relatively low final temperatures, superheaters of the convection type are generally used.

Design Considerations • Fuels • Coal is to a large extent the base fuel in power generating stations – although natural gas and oil are used where costs are favorable. – In contrast to gas, coal contains ash which consists of a number of objectionable chemical elements and compounds. – Oil, generally contains only small amounts of ash. The objection able constituents of the ash, however, may have a far reaching effect on the design.

Design Considerations • Fuels Ash is of concern since – at the high temperatures resulting from the burning of fuel in the furnace, fractions of the ash become partially fused and sticky. – Depending on the quantity and fusion temperature, the partially fused ash may adhere to surfaces ed by the ash-containing combustion gases, – causing objectionable buildup of slag on or bridging between tubes. – Chemicals in the ash may attack materials such as alloys used in superheaters and reheaters. • In addition to the deposits in the high temperature sections of the unit, – the air heater which is, the coolest part may he subject to corrosion and plugging of gas ages due primarily to sulfur compounds in the fuel acting in combination with moisture present in the flue gas.

Design Considerations • Furnaces • Furnace design must take into consideration – water heating and steam generation in the wall tubes – as well as the processes of combustion. • Practically all large modern boilers are designed – with walls comprised of water cooled tubes to form complete metal coverage of furnace enclosure – in addition,areas outside of the furnace which form enclosures for sections of superheaters, reheaters and often economizers are also designed in a manner similar to the furnace using either water or steam cooled tube surfaces. • Present practice is to use – tube arrangements and configurations – which permit practically complete elimination of – refractories in all areas that are exposed to high temperature gases.

Design Considerations • Furnaces • Tube diameter and thickness are of concern from the standpoints of circulation and metal temperatures. – Natural circulation boilers generally use larger diameter tubes than controlled circulation or once through boilers. – This practice is dictated largely by the need for more liberal flow area to provide the lower velocities necessary with the limited head available. – The use of small diameter tubes becomes advantageous in high pressure boilers since the lesser tube thicknesses required result in lower outside tube metal temperatures. – Small diameter tubes are advantageously used in controlled or forced circulation boilers where pumps provide adequate head for circulation and maintenance of desired velocities. – Factors such as velocity of water and water steam mixtures, quality of water and steam mixtures are of particular concern in high heat absorption areas of high pressure boilers.

Design Considerations • Circulation • In practically all types of boilers, – the difference in density between steam and water – is utilized to provide or to assist in providing water circulation.

• Generally, the waterwall downtakes – serve as the high density leg of the U-tube, – the waterwalls or the boiler bank containing mixture of steam and water constitutes the low density leg.

• The available head for providing circulation is also affected by frictional and entrance and exit losses in the several circuits. – adequate circulation can only be provided if losses are low enough to make sufficient circulating head available. – Generally, boilers are designed in such a manner that steam discharged from the waterwall headers and to the drum is accompanied by limited quantity of water.

Design Considerations • Metallurgy • The selection of materials for superheaters and reheaters in boilers – designed for high pressures and temperatures in excess of 538 ºC – requires the use of high strength alloy tubing. • In addition to matters – of strength and oxidation resistance in superheater and reheater materials, – the use of high steam pressure requires increased tube thicknesses in all tubes subject to steam pressure. • Furthermore, the thicker tubes • are subject to higher outside metal temperatures. • Since chemical action is accelerated at higher temperatures, the tube metal is more subject to external corrosion. • This is of particular concern when burning fuels containing objectionable impurities. • The designer takes of such conditions in selecting material and tube sizes.

Design Considerations • Controlled Steam Temperature • The desire to maintain turbine efficiency – over a wide range of load and – to avoid fluctuations in turbine metal temperatures – has resulted in a need to maintain constant steam and reheat temperatures over the anticipated operating load range. • In order to satisfy this requirement – it is necessary that the boiler be equipped with means for controlling and maintaining constant steam and reheat temperatures – over the desired range. • If uncontrolled, steam temperatures rise with in creasing steam output. • This is characteristic of convection type superheaters which necessarily for the major share of the heat absorbed by the super heater.

Design Considerations • Boiler Drum • The boiler drum serves two functions, – the first and primary one being that of separating steam from the, mixture of water and steam discharged into it. – Secondly, the drum is used to house the equipment used for purification of steam after being separated from the water.

• The quantity of water contained in the boiler below the water level – is relatively small compared to the total steam output. – Consequently, regardless of the drum size, the matter of water storage is not significant.

• Primarily the drum size is – determined by the space required to accommodate steam separating and purifying equipment. – Drum diameter and length should be sufficient to provide accessibility for installation and inspection. – Length generally depends on furnace width, or in the case of high capacity units, it may be controlled by the space required for the steam separating devices

Design Considerations • Heat generated in the combustion of fuel – appears as sensible and – latent heat in the products of combustion. – Roughly 50 percent of the heat so generated is absorbed by water circulated through tubes forming the furnace wall lining. – Heat absorbed generates steam by the evaporation of part of the circulated water.

• To a limited degree the furnace heat absorbing surface is dictated by – the size and dimensions required for proper combustion of the fuel.

• Other limitations are often imposed – such as ash content and – fusion temperature, in the case of coal firing.

• For coal’s containing low fusion temperature ash, – the gas temperature leaving the furnace may be limited by considerations of slag accumulations and – bridging between tubes of boiler or superheater.

Design Considerations • Determination of the Surface • The amount of heating surface to be provided in each of the several elements of the boiler depends on a number of factors. – It is of course necessary that surface be adequate to satisfy design and operating requirements. – Also of importance are proper applications and arrangements of surfaces to insure effective utilization. The latter is to a large degree dictated by economic considerations.

• The present practice of using steam and reheat temperatures of 538 ºC and higher – not only requires the use of expensive alloys in sections of these elements attaining metal temperatures of 593ºC and higher – but also requires the use of large amounts of tubing. – It is thus important to give careful consideration to designs which provide the desired performance and reflect economic considerations

Design Considerations • Furnace • Heat generated in the combustion of fuel – appears as sensible and – latent heat in the products of combustion. – Roughly 50 percent of the heat so generated is absorbed by water circulated through tubes forming the furnace wall lining. – Heat absorbed generates steam by the evaporation of part of the circulated water. • To a limited degree the furnace heat absorbing surface is – dictated by the size and dimensions required for proper combustion of the fuel.

• Other limitations are often imposed – such as ash content and – fusion temperature, in the case of coal firing.

• For coal’s containing low fusion temperature ash, – the gas temperature leaving the furnace – may be limited by considerations of slag accumulations and – bridging between tubes of boiler or superheater.

Design Considerations • • • • • •

Welded Walls Superheaters and Reheaters Economizers Tubular Air Heaters Regenerative Air Preheater Calculations for Materials Selection