B O I L E R & S T E A M S Y S T E M M A N A G E M E N T

Future Solutions Training Center (1)

BOILER FUNDAMENTALS

What is the boiler?

A boiler is equipment that generates steam by heating water in a closed vessel with heat produced by combustion of fuel.

To do this, heat must be moved and there are three basic ways of moving or transferring heat:

Radiation is the transfer of heat by direct transmission similar to the

transfer of heat by the sun's rays. (Radiant heat transfer can

occur when there is no medium to carry it.)

Conduction is the movement of heat through direct contact. Most heat

exchangers have a metal wall or walls that separate two

fluids of different temperatures. Heat from the hotter fluid

passes through the wall(s) to the cooler fluid.

Chapter

1

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Convection is the heat transfer taking place in fluids such as water or air from

natural or forced mixing of the fluid's hotter and cooler parts

Steam generated by a boiler is used for the following two purposes:

1. For generating electric power

2. For utilization of steam itself

The process of heating a liquid until it reaches it's gaseous state is called evaporation.

Heat is transferred from one body to another by means of (1) radiation, which is the

transfer of heat from a hot body to a cold body through a conveying medium without

physical contact, (2) convection, the transfer of heat by a conveying medium, such as air

or water and (3) conduction, transfer of heat by actual physical contact, molecule to

molecule. The heating surface is any part of the boiler metal that has hot gases of

combustion on one side and water on the other. Any part of the boiler metal that

actually contributes to making steam is heating surface. The amount of heating surface a

boiler has is expressed in square feet. The larger the amount of heating surface a boiler

has the more efficient it becomes. The measurement of the steam produced is generally

in pounds of water evaporated to steam per hour.

Gallons of water evaporated x 8.3 pounds/gallon water = Pounds of steam

In firetube boilers the term boiler horsepower is often used. A boiler horsepower is 34.5

pounds of steam. This term was coined by James Watt a Scottish inventor. The measurement

of heat is in British Thermal Units (Btu’s). A Btu is the amount of heat required to raise the

The heat required to change the temperature of a substance is called its sensible heat. In the

teapot illustration to the left the 70 oF water contains 38 Btu’s and by adding 142 Btu’s the

water is at 32 F it is assumed that its heat value is zero.

Boilers and pressure vessels are built under requirements of the American Society of

Mechanical Engineers or ASME referred to as the "ASME Code." High pressure boilers

are fired vessels for an operation greater than 15 psig and 160oF and are built in accordance

with Section I of the ASME Code with the ASME S stamp. Vessels with design pressures

below 15

psig steam and 180oF hot water are low pressure and are built to Code Section IV. All unfired

vessels are built in accordance with Code Section VIII, Division I and with the ASME U

stamp attached. Repairs to all boilers and pressure vessels are governed by the state boiler

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jurisdictions which for the US and Canada have universally adopted the National Board of

Boiler & Pressure Vessel Inspectors (National Board Code) and affixed with the national

board R stamp.

1. THE CHANGES IN ENERGY APPLIED TO A FOSSIL-FUEL POWER

PLANT.

These changes are:

The chemical energy contained in the combustible is going to change into thermal energy thanks to the combustion which took place in the boiler. This energy is going to be transmitted to the water/steam through the steam generator, the energy stocked by the fluid is called: thermoplastic energy. This thermoplastic energy will then be changed into mechanical energy thanks to a pressure reduction phenomenon in the turbine. Finally, the kinetic energy is going to be changed into electrical energy in the alternator.

CHANGES OF

ENERGIES

applied to a fossil-fuel

power plant

CHEMICAL

ENERGY

CALORIFIC

MACHINE

THERMAL

ENERGY

THERMO-DYNAMIC

MACHINE

MECHANICAL

ENERGY

ELECTRICAL

MACHINE

ELECTRIC

ENERGY

COMBUSTIBLE

STEAM

SHAFT

ROTATION

WATERBOILER

STEAM

TURBINE

ALTERNATOR

ELECTRICAL

POWER

CONDENSER

WATER/STEAM

LOOP

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Types of Boilers:

Boilers can be fire tube or water tube.

Fire tube boiler, the hot gases pass through the inside of the tubes located within the

shell of the boiler. The water around the tubes absorbs the heat of hot gases. The fire tube

boiler is limited in size and pressure.

Water tube boiler is used in big installations mainly in steam power plants. It is

composed of drums and tubes. The water passes through the tubes surrounded by hot gases.

The tubes contain the entire heating surfaces.

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Heat recovery steam generator

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 a 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

HRSGs consist of three major components. They are the Evaporator, Superheated, and Economizer. The

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1.1 Steam - The Energy Fluid

It is useful to introduce the topic of steam by considering its many uses and benefits, before entering an overview of the steam plant or any technical explanations.

Steam has come a long way from its traditional associations with locomotives and the Industrial Revolution. Steam today is an integral and essential part of modern technology. Without it, our food, textile, chemical, medical, power, heating and transport industries could not exist or perform as they do.

Steam provides a means of transporting controllable amounts of energy from a central, automated boiler house, where it can be efficiently and economically generated, to the point of use. Therefore as steam moves around a plant it can equally be considered to be the transport and provision of energy.

For many reasons, steam is one of the most widely used commodities for conveying heat energy. Its use is popular throughout industry for a broad range of tasks from mechanical power production to space heating and process applications.

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1.1.1 Steam is efficient and economic to generate.

Water is plentiful and inexpensive. It is non-hazardous to health and environmentally sound. In its gaseous form, it is a safe and efficient energy carrier. Steam can hold five or six times as much potential energy as an equivalent mass of water.

When water is heated in a boiler, it begins to absorb energy. Depending on the pressure in the boiler, the water will evaporate at a certain temperature to form steam. The steam contains a large quantity of stored energy which will eventually be transferred to the process or the space to be heated.

It can be generated at high pressures to give high steam temperatures. The higher the pressure, the higher is the temperature. More heat energy is contained within high temperature steam so its potential to do work is greater.

Modern shell boilers are compact and efficient in their design, using multiple passes and efficient burner technology to transfer a very high proportion of the energy contained in the fuel to the water, with minimum emissions.

The boiler fuel may be chosen from a variety of options, including combustible waste, which makes the steam boiler an environmentally sound option amongst the choices available for providing heat. Centralized boiler plant can take advantage of low interruptible gas tariffs, because any suitable standby fuel can be stored for use when the gas supply is interrupted.

Highly effective heat recovery systems can virtually eliminate blowdown costs, return valuable condensate to the boiler house and add to the overall efficiency of the steam and condensate loop.

The increasing popularity of Combined Heat and Power (CHP) systems demonstrates the high regard for steam systems in today's environment and energy-conscious industries.

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1.1.2 Steam can easily and cost effectively be distributed to the

point of use

Steam is one of the most widely used media to convey heat over distances. Because steam flows in response to the pressure drop along the line, expensive circulating pumps are not needed.

Due to the high heat content of steam, only relatively small bore pipework is required to distribute the steam at high pressure. The pressure is then reduced at the point of use, if necessary. This arrangement makes installation easier and less expensive than for some other heat transfer fluids.

Overall, the lower capital and running costs of steam generation, distribution and condensate return systems mean that many users choose to install new steam systems in preference to other energy media, such as gas fired, hot water, electric and thermal oil systems.

1.1.3 Steam is easy to control

Because of the direct relationship between the pressure and temperature of saturated steam, the amount of energy input to the process is easy to control, simply by controlling the saturated steam pressure. Modern steam controls are designed to respond very rapidly to process changes.

The item shown in Figure 1.1.4 is a typical two port control valve and pneumatic actuator assembly, designed for use on steam. Its accuracy is enhanced by the use of a pneumatic valve positioner.

The use of two port valves, rather than the three port valves often necessary in liquid systems, simplifies control and installation, and may reduce equipment costs.

1.1.4 Energy is easily transferred to the process

Steam provides excellent heat transfer. When the steam reaches the plant, the condensation process efficiently transfers the heat to the product being heated.

Steam can surround or be injected into the product being heated. It can fill any space at a uniform temperature and will supply heat by condensing at a constant temperature; this eliminates temperature gradients which may be found along any heat transfer surface - a problem which is so often a feature of high temperature oils or hot water heating, and may result in quality problems, such as distortion of materials being dried.

Because the heat transfer properties of steam are so high, the required heat transfer area is relatively small. This enables the use of more compact plant, which is easier to install and takes up less space in the plant. A modern packaged unit for steam heated hot water, rated to 1 200 kW and incorporating a steam plate heat exchanger and all the controls, requires only 0.7 m² floor space. In comparison, a packaged unit incorporating a shell and tube heat exchanger would typically cover an area of two to three times that size.

1.1.5 The modern steam plant is easy to manage

Increasingly, industrial energy users are looking to maximize energy efficiency and minimize production costs and overheads. The Kyoto

Agreement for climate protection is a major external influence driving the energy efficiency trend, and has led to various measures around the globe, such as the Climate Change Levy in the UK. Also, in todays competitive markets, the organization with the

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lowest costs can often achieve an important advantage over rivals. Production costs can mean the difference between survival and failure in the marketplace.

Ways of increasing energy efficiency include monitoring and charging energy consumption to relevant departments. This builds an awareness of costs and focuses management on meeting targets. Variable overhead costs can also be minimized by ensuring planned, systematic maintenance; this will maximize process efficiency, improve quality and cut downtime.

Most steam controls are able to interface with modern networked instrumentation and control systems to allow centralized control, such as in the case of a SCADA system or a Building /Energy Management System. If the user wishes, the components of the steam system can also operate independently (standalone).

With proper maintenance a steam plant will last for many years, and the condition of many aspects of the system is easy to monitor on an automatic basis. When compared with other systems, the planned management and monitoring of steam traps is easy to achieve with a trap monitoring system, where any leaks or blockages are automatically pinpointed and immediately brought to the attention of the engineer.

This can be contrasted with the costly equipment required for gas leak monitoring, or the time-consuming manual monitoring associated with oil or water systems.

In addition to this, when a steam system requires maintenance, the relevant part of the system is easy to isolate and can drain rapidly, meaning that repairs may be carried out quickly. In numerous instances, it has been shown that it is far less expensive to bring a long established steam plant up to date with sophisticated control and monitoring systems, than to replace it with an alternative method of energy provision, such as a decentralized gas system. The case studies referred to in Module 1.2 provide real life examples. Today's state-of-the-art technology is a far cry from the traditional perception of steam as the stuff of steam engines and the Industrial Revolution. Indeed, steam is the preferred choice for industry today. Name any Fig. 1.1.6 Just some of the products well known consumer brand, and in nine cases out of manufactured using steam as an essential ten, steam will have played an important part in part of the process production.

1.1.6 Steam is flexible

Not only is steam an excellent carrier of heat, it is also sterile, and thus popular for process use in the food, pharmaceutical and health industries. It is also widely used in hospitals for sterilization purposes. The industries within which steam is used range from huge oil and petrochemical plants to small local laundries. Further uses include the production of paper, textiles, brewing, food production, curing rubber, and heating and humidification of buildings. Many users find it convenient to use steam as the same working fluid for both space heating and for process applications. For example, in the brewing industry, steam is used in a variety of ways during different stages of the process, from direct injection to coil heating. Steam is also intrinsically safe - it cannot cause sparks and presents no fire risk. Many petrochemical plants utilize steam fire-extinguishing systems. It is therefore ideal for use in hazardous areas or explosive atmospheres.

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1.1.7 Other methods of distributing energy

The alternatives to steam include water and thermal fluids such as high temperature oil. Each method has its advantages and disadvantages, and will be best suited to certain applications or temperature bands.

Compared to steam, water has a lower potential to carry heat, consequently large amounts of water must be pumped around the system to satisfy process or space heating requirements. However, water is popular for general space heating applications and for low temperature processes (up to 120°C) where some temperature variation can be tolerated.

Thermal fluids, such as mineral oils, may be used where high temperatures (up to 400°C) are required, but where steam cannot be used. An example would include the heating of certain chemicals in batch processes. However thermal fluids are expensive, and need replacing every few years - they are not suited to large systems. They are also very searching and high quality connections and joints are essential to avoid leakage.

Different media are compared in Table 1.1.1, which follows. The final choice of heating medium depends on achieving a balance between technical, practical and financial factors, which will be different for each user.

Broadly speaking, for commercial heating and ventilation, and industrial systems, steam remains the most practical and economic choice.

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1.2 The Steam Cycle (Rankine Steam Cycles)

Rankine Steam Cycles

Simple Cycle

One Reheat Stage

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One Open Feed water Heater

Boiler Bases:

One Reheat Stage & One Open Feed water Heater

The Boiler House Block of the Steam and Condensate Loop will concentrate on the design and contents of the boiler house, and the applications within it.

A well designed, operated and maintained boiler house is the heart of an efficient steam plant. However, a number of obstacles can prevent this ideal. The boiler house and its contents are sometimes viewed as little more than a necessary inconvenience and even in today's energy-conscious environment, accurate steam flow measurement and the correct allocation of costs to the various users, is not universal. This can mean that efficiency improvements and cost-saving projects related to the boiler house may be difficult to justify to the end user.

In many cases, the boiler house and the availability of steam are the responsibility of the Engineering Manager, consequently any efficiency problems are seen to be his.

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It is important to remember that the steam boiler is a pressurised vessel containing scalding hot water and steam at more than 100°C, and its design and operation are covered by a number of complex standards and regulations.

These standards vary as follows:

Location - For example, the UK, Australia, and New Zealand all have individual standards. The variations between standards may seem small but can sometimes be quite significant.

Over time - For example, technology is changing at a tremendous rate, and improvements in the capabilities of equipment, together with the frequent adjustment of operating standards demanded by the relevant legislative bodies, are resulting in increases in the safety of boiler equipment.

Environmental terms - Many governments are insisting on increasingly tight controls, including emission standards and the overall efficiency of the plant. Users who chose to ignore these (and pending controls) do so with an increasing risk of higher penalties being imposed on them.

Cost terms - Fuel costs are continually increasing, and organisations should constantly review alternative steam raising fuels, and energy waste management.

For the reasons listed above, the user must confirm national and local and current legislation.

The objective of this Module is to provide the designer, operator, and maintainer of the boiler house with an insight into the considerations required in the development of the boiler and its associated equipment.

Modern steam boilers come in all sizes to suit both large and small applications. Generally, where more than one boiler is required to meet the demand, it becomes economically viable to house the boiler plant in a centralized location, as installation and operating costs can be significantly lower than with decentralized plant.

For example, centralization offers the following benefits over the use of dispersed, smaller boilers:

More choices of fuel and tariff.

Identical boilers are frequently used in centralized boiler rooms reducing

Spares, inventory and costs.

Heat recovery is easy to implement for best returns.

A reduction in manual supervision releases labour for other duties on site.

Economic sizing of boiler plant to suit diversified demand.

Exhaust emissions are more easily monitored and controlled.

Safety and efficiency protocols are more easily monitored and controlled.

1.2.1 Fuel for boilers

The three most common types of fuel used in steam boilers, are coal, oil, and gas. However, industrial or commercial waste is also used in certain boilers, along with electricity for electrode boilers.

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1.2.1.1 Coal

Coal is the generic term given to a family of solid fuels with high carbon content. There are several types of coal within this family, each relating to the stages of coal formation and the amount of carbon content. These stages are:

Peat.

Lignite or brown coals.

Bituminous.

Semi bituminous.

Anthracite.

The bituminous and anthracite types tend to be used as boiler fuel.

In the UK, the use of lump coal to fire shell boilers is in decline.

There are a number of reasons for this including:

Availability and cost - With many coal seams becoming exhausted, smaller quantities of coal are produced in the UK than formerly, and its decline must be expected to continue.

Speed of response to changing loads - With lump coal, there is a substantial time lag between:

- Demand for heat occurring.

- Stoking of coal into the boiler.

- Ignition of the coal.

- Steam being generated to satisfy the demand.

To overcome this delay, boilers designed for coal firing need to contain more water at saturation temperature to provide the reserve of energy to cover this time lag. This, in turn, means that the boilers are bigger, and hence more expensive in purchase cost, and occupy more valuable product manufacturing space.

Ash - Ash is produced when coal is burned.

The ash may be awkward to remove, usually involving manual intervention and a reduction in the amount of steam available whilst de-ashing takes place.

The ash must then be disposed of, which in itself may be costly.

Stoking equipment - A number of different arrangements exist including stepper stokers, sprinklers and chain-grate stokers. The common theme is that they all need substantial maintenance.

Emissions - Coal contains an average of 1.5% sulphur (S) by weight, but this level may be as high as 3% depending upon where the coal was mined.

During the combustion process:

1. Sulphur will combine with oxygen (O2) from the air to form SO2 or SO3.

2. Hydrogen (H) from the fuel will combine with oxygen (O2) from the air to form water (H2O).

After the combustion process is completed, the SO3 will combine with the water (H2O) to produce sulphuric acid (H2SO4), which can condense in the flue causing corrosion if

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the correct flue temperatures are not maintained. Alternatively, it is carried over into the atmosphere with the flue gases. This sulphuric acid is brought back to earth with rain, causing:

1. Damage to the fabric of buildings.

2. Distress and damage to plants and vegetation.

The ash produced by coal is light, and a proportion will inevitably be carried over with the exhaust gases, into the stack and expelled as particulate matter to the environment.

Coal, however, is still used to fire many of the very large water-tube boilers found in power stations.

Because of the large scale of these operations, it becomes economic to develop solutions to the problems mentioned above, and there may also be governmental pressure to use domestically produced fuels, for national security of electrical supply.

The coal used in power stations is milled to a very fine powder, generally referred to as pulverized fuel, and usually abbreviated to pf.

The small particle size of pf means that its surface area-to-volume ratio is greatly increased, making combustion very rapid, and overcoming the rate of response problem encountered when using lump coal.

The small particle size also means that pf flows very easily, almost like a liquid, and is introduced into the boiler furnace through burners, eliminating the stokers used with lump coal.

To further enhance the flexibility and turndown of the boiler, there may be 30+ pf burners around the walls and roof of the boiler, each of which may be controlled independently to increase or decrease the heat in a particular area of the furnace. For example, to control the temperature of the steam leaving the superheater.

With regard to the quality of the gases released into the atmosphere:

The boiler gases will be directed through an electrostatic precipitator where electrically charged plates attract ash and other particles, removing them from the gas stream.

The sulphurous material will be removed in a gas scrubber.

The final emission to the environment is of a high quality.

Approximately 8 kg of steam can be produced from burning 1 kg of coal.

1.2.1.2 Oil

Oil for boiler fuel is created from the residue produced from crude petroleum after it has been distilled to produce lighter oils like gasoline, paraffin, kerosene, diesel or gas oil. Various grades are available, each being suitable for different boiler ratings; the grades are as follows:

Class D - Diesel or gas oil.

Class E - Light fuel oil.

Class F - Medium fuel oil.

Class G - Heavy fuel oil.

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Oil began to challenge coal as the preferred boiler fuel in the UK during the 1950s. This came about in part from the then Ministry of Fuel and Powers sponsorship of research into improving boiler plant. The advantages of oil over coal include:

A shorter response time between demand and the required amount of steam being generated.

This meant that less energy had to be stored in the boiler water. The boiler could therefore be smaller, radiating less heat to the environment, with a consequent improvement in efficiency.

The smaller size also meant that the boiler occupied less production space.

Mechanical stokers were eliminated, reducing maintenance workload.

Oil contains only traces of ash, virtually eliminating the problem of ash handling and disposal.

The difficulties encountered with receiving, storing and handling coal were eliminated.

Approximately 15 kg of steam can be produced from 1 kg of oil, or 14 kg of steam from 1 litre of oil.

1.2.1.3 Gas

Gas is a form of boiler fuel that is easy to burn, with very little excess air. Fuel gases are available in two different forms:

Natural gas - This is gas that has been produced (naturally) underground. It is used in its natural state, (except for the removal of impurities), and contains a high proportion of methane.

Liquefied petroleum gases (LPG) - These are gases that are produced from petroleum refining and are then stored under pressure in a liquid state until used. The most common forms of LPG are propane and butane.

In the late 1960s the availability of natural gas (such as from the North Sea) led to further developments in boilers.

The advantages of gas firing over oil firing include:

Storage of fuel is not an issue; gas is piped right into the boiler house.

Only a trace of sulphur is present in natural gas, meaning that the amount of sulphuric acid in the flue gas is virtually zero.

Approximately 42 kg of steam can be produced from 1 Therm of gas (equivalent to 105.5 MJ) for a 10 bar g boiler, with an overall operating efficiency of 80%.

1.3 WASTE AS THE PRIMARY FUEL

There are two aspects to this:

Waste material - Here, waste is burned to produce heat, which is used to generate steam. The motives may include the safe and proper disposal of hazardous material. A hospital would be a good example:

o In these circumstances, it may be that proper and complete combustion of the waste material is difficult, requiring sophisticated burners, control of air ratios and monitoring of emissions, especially particulate matter. The cost

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of this disposal may be high, and only some of the cost is recovered by using the heat generated to produce steam. However, the overall economics of the scheme, taking into consideration the cost of disposing of the waste by other means, may be attractive.

o Using waste as a fuel may involve the economic utilisation of the combustible waste from a process. Examples include the bark stripped from wood in paper plants, stalks (bagasse) in sugar cane plants and sometimes even litter from a chicken farm.

The combustion process will again be fairly sophisticated, but the overall economics of the cost of waste disposal and generation of steam for other applications on site, can make such schemes attractive.

Waste heat - here, hot gases from a process, such as a smelting furnace, may be directed through a boiler with the objective of improving plant efficiency. Systems of this type vary in their level of sophistication depending upon the demand for steam within the plant. If there is no process demand for steam, the steam may be superheated and then used for electrical generation.

This type of technology is becoming popular in Combined Heat and Power (CHP) plants:

o A gas turbine drives an alternator to produce electricity.

o The hot (typically 500°C) turbine exhaust gases are directed to a boiler, which produces saturated steam for use on the plant.

Very high efficiencies are available with this type of plant. Other benefits may include either security of electrical supply on site, or the ability to sell the electricity at a premium to the national electricity supplier.

Which fuel to use?

The choice of fuel(s) is obviously very important, as it will have a significant impact on the costs and flexibility of the boiler plant.

Factors that need consideration include:

Cost of fuel - For comparison purposes the cost of fuel is probably most conveniently expressed in £ / kg of steam generated.

Cost of firing equipment - The cost of the burner(s) and associated equipment to suit the fuel(s) selected, and the emission standards which must be observed.

1.3.1 Security of Supply

What are the consequences of having no steam available for the plant? Gas, for example, may be available at advantageous rates, provided an interruptible supply can be accepted. This means that the gas company will supply fuel while they have a surplus. However, should demand for fuel approach the limits of supply, perhaps due to seasonal variation, then supply may be cut, maybe at very short notice.

As an alternative, boiler users may elect to specify dual fuel burners which may be fired on gas when it is available at the lower tariff, but have the facility to switch to oil firing when gas is not available. The dual fuel facility is obviously a more expensive capital option, and the likelihood of gas not being available may be small. However, the cost of

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plant downtime due to the non-availability of steam is usually significantly greater than the additional cost.

1.3.2 Fuel storage

This is not an issue when using a mains gas supply, except where a dual fuel system is used.

However it becomes progressively more of an issue if bottled gas, light oils, heavy oils and solid fuels are used.

The issues include:

How much is to be stored, and where.

How to safely store highly combustible materials.

How much it costs to maintain the temperature of heavy oils so that they are at a suitable viscosity for the equipment.

How to measure the fuel usage rate accurately.

Allowance for storage losses.

1.3.3Boiler design

The boiler manufacturer must be aware of the fuel to be used when designing a boiler. This is because different fuels produce different flame temperatures and combustion characteristics.

For example:

Oil produces a luminous flame, and a large proportion of the heat is transferred by radiation within the furnace.

Gas produces a transparent blue flame, and a lower proportion of heat is transferred by radiation within the furnace.

On a boiler designed only for use with oil, a change of fuel to gas may result in higher temperature gases entering the first pass of fire-tubes, causing additional thermal stresses, and leading to early boiler failure.

Boiler types

The objectives of a boiler are:

To release the energy in the fuel as efficiently as possible.

To transfer the released energy to the water, and to generate steam as efficiently as possible.

To separate the steam from the water ready for export to the plant, where the energy can be transferred to the process as efficiently as possible.

A number of different boiler types have been developed to suit the various steam applications.