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Page | 2

BOILERS

ACKNOWLEDGEMENT

It is a great pleasure to present this Seminar project report carried out in the QUANTUM

SCHOOL OF TECHNOLOGY ROORKEE. I express my heartily gratitude to Dr.

V.K.GOEL (HOD, Mechanical Department) who gave me an opportunity to undergo seminar. I am very thankful to Mr. ASHWANI KUMAR for his excellent guidance and co-

operation for the successful completion of my seminar.

(Aditya Aggarwal)

10430104031

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BOILERS

DDEEPPAARRTTMMEENNTT OOFF

MMEECCHHAANNIICCAALL EENNGGIINNEEEERRIINNGG

CERTIFICATE

This is to certify that ADITYA AGGARWAL Roll No. 10430104031 of

7th

Semester BTech in Mechanical Engineering has completed the seminar work

Satisfactory towards fulfilment of academic year 2013-14 as prescribed in UTU

curriculum.

SEMINAR GUIDE HOD

Page | 4

BOILERS

ABSTRACT

A boiler is a closed vessel in which water or other fluid is heated. The fluid does not

necessarily boil. (The term "furnace" is normally used if the purpose is not actually to boil

the fluid.) The heated or vaporized fluid exits the boiler for use in various processes or

heating applications, including central heating, boiler-based power generation, cooking,

and sanitation.

Boiler is an apparatus use to produce steam. It is a device in which thermal energy released

by combustion of fuel is used to make steam at the desired temperature and pressure.

A boiler or steam generator is used wherever a source of steam is required. The form and size

depends on the application: mobile steam engines such as steam locomotives, portable

engines and steam-powered road vehicles typically use a smaller boiler that forms an integral

part of the vehicle; stationary steam engines, industrial installations and power stations will

usually have a larger separate steam generating facility connected to the point-of-use by

piping. A notable exception is the steam-powered fireless locomotive, where separately-

generated steam is transferred to a receiver (tank) on the locomotive

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BOILERS

CONTENTS

Sr. No. Name of Chapter Page No.

i. Acknowledgement 2

ii. Certificate 3

iii. Abstract 5

1. Introduction 6

2. Working Principle 7

3. Classification of Boilers 9

4. Important Boilers

a. Cochran Boilers 13

b. Locomotive Boilers 14

c. Lancashire Boilers 15

d. Babcock Wilcox Boilers 16

e. Hydronic Boilers 16

5. Boiler fitting and Accessories 18

6. Materials of Boiler 20

7. Superheated Steam Boiler 21

8. Supercritical Steam generator 22

9. Safety 23

10. Controlling Drought 25

11. Best Practices for efficient operation 26

12. Best Practices for maintenance 32

13. Reference 34

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BOILERS

INTRODUCTION

The steam generator or boiler is an integral component of a steam engine when considered as

a prime mover. However it needs be treated separately, as to some extent a variety of

generator types can be combined with a variety of engine units. A boiler incorporates

a firebox or furnace in order to burn the fuel and generate heat. The generated heat is

transferred to water to make steam, the process of boiling. This produces saturated steam at a

rate which can vary according to the pressure above the boiling water. The higher the furnace

temperature, the faster the steam production. The saturated steam thus produced can then

either be used immediately to produce power via a turbine and alternator, or else may be

further superheated to a higher temperature; this notably reduces suspended water content

making a given volume of steam produce more work and creates a greater temperature

gradient, which helps reduce the potential to form condensation. Any remaining heat in

the combustion gases can then either be evacuated or made to pass through an economiser,

the role of which is to warm the feed water before it reaches the boiler.

Boilers have several strengths that have made them a common feature of buildings. They

have a long life, can achieve efficiencies up to 95% or greater, provide an effective method of

heating a building, and in the case of steam systems, require little or no pumping energy.

However, fuel costs can be considerable, regular maintenance is required, and if maintenance

is delayed, repair can be costly.

Guidance for the construction, operation, and maintenance of boilers is provided primarily by

the ASME (American Society of Mechanical Engineers), which produces the following

resources:

Rules for construction of heating boilers, Boiler and Pressure Vessel Code, Section

IV-2007

Recommended rules for the care and operation of heating boilers, Boiler and Pressure

Vessel Code, Section VII-2007

Boilers are often one of the largest energy users in a building. For every year a boiler system

goes unattended, boiler costs can increase approximately 10%.

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BOILERS

WORKING PRINCIPLE

Both gas and oil fired boilers use controlled combustion of the fuel to heat water. The key

boiler components involved in this process are the burner, combustion chamber, heat

exchanger, and controls.

The burner mixes the fuel and oxygen together and, with the assistance of an ignition device,

provides a platform for combustion. This combustion takes place in the combustion chamber,

and the heat that it generates is transferred to the water through the heat exchanger. Controls

regulate the ignition, burner firing rate, fuel supply, air supply, exhaust draft, water

temperature, steam pressure, and boiler pressure.

Hot water produced by a boiler is pumped through pipes and delivered to equipment

throughout the building, which can include hot water coils in air handling units, service hot

water heating equipment, and terminal units. Steam boilers produce steam that flows through

Firetube Boiler

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BOILERS

pipes from areas of high pressure to areas of low pressure, unaided by an external energy

source such as a pump. Steam utilized for heating can be directly utilized by steam using

equipment or can provide heat through a heat exchanger that supplies hot water to the

equipment.

The discussion of different types of boilers, below, provides more detail on the designs of

specific boiler systems.

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BOILERS

CLASSIFICATION OF BOILERS

Boilers are classified according certain condition. Following figure shows classification o f

boiler.

Two primary types of boilers include Firetube and Watertube boilers. In a Firetube boiler, hot

gases of combustion flow through a series of tubes surrounded by water. Alternatively, in

Water tube boiler, Water flows in the inside of the tubes and the hot gases from combustion

flow around the outside of the tubes.

Boilers

Acc to Flow

Water Tube

Fire Tube

Acc to position of

furnace

Externally Fired

Internally Fired

Acc to principle axis

Vertical

Horizontal

Inclined

Acc to Application

Stationary

Mobile

Acc to circulation

Natural

Forced

Acc to Pressure

Low Pressure

Medium Pressure

High Pressure

Watertube Boiler

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BOILERS

Firetube boilers are more commonly available for low pressure steam or hot water

applications, and are available in sizes ranging from 500,000 to 75,000,000 BTU input.

Watertube boilers are primarily used in higher pressure steam applications and are used

extensively for comfort heating applications. They typically range in size from 500,000 to

more than 20,000,000 BTU input.

Cast iron sectional boilers are another type of boiler commonly used in commercial space

heating applications. These types of boilers don’t use tubes. Instead, they’re built up from

cast iron sections that have water and combustion gas passages. The iron castings are bolted

together, similar to an old steam radiator. The sections are sealed together by gaskets.

They’re available for producing steam or hot water, and are available in sizes ranging from

35,000 to 14,000,000 BTU input.

Cast iron sectional boilers are advantageous because they can be assembled on site, allowing

them to be transported through doors and smaller openings. Their main disadvantage is that

because the sections are sealed together with gaskets, they are prone to leakage as the gaskets

age and are attacked by boiler treatment chemicals.

Cast Iron Sectional Boiler

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BOILERS

Working Pressure and Temperature

Boilers are classified as either low pressure or high pressure and are constructed to meet

ASME Boiler and Pressure Vessel Code requirements. Low-pressure boilers are limited to a

maximum working pressure of 15 psig (pound-force per square inch gauge) for steam and

160 psig for hot water (2). Most boilers used in HVAC applications are low-pressure boilers.

High-pressure boilers are constructed to operate above the limits set for low-pressure boilers,

and are typically used for power generation. Operating water temperatures for hot water

boilers are limited to 250o F (2).

Fuel Type

In commercial buildings, natural gas is the most common boiler fuel, because it is usually

readily available, burns cleanly, and is typically less expensive than oil or electricity. Some

boilers are designed to burn more than one fuel (typically natural gas and fuel oil). Dual fuel

boilers provide the operator with fuel redundancy in the event of a fuel supply interruption.

They also allow the customer to utilize the fuel oil during “peak time” rates for natural gas. In

times when the rates for natural gas are greater than the alternate fuel, this can reduce fuel

costs by using the cheaper alternate fuel and limiting natural gas use to occur only during “off

peak” times.

Electric boilers are used in facilities with requirements for a small amount of steam or where

natural gas is not available. Electric boilers are known for being clean, quiet, and easy to

install, and compact. The lack of combustion results in reduced complexity in design and

operation and less maintenance. Heating elements are easily replaced if they fail. These types

of boilers can be used to produce low or high pressure steam or water, and may be good

alternatives for customers who are restricted by emissions regulations. Sizes range from

30,000 to 11,000,000 BTU input with overall efficiency generally in the range of 92% to

96%.

Draft Methods

The pressure difference between the boiler combustion chamber and the flue (also called the

exhaust stack) produces a draft which carries the combustion products through the boiler and

up the flue. Natural draft boilers rely on the natural buoyancy of hot gasses to exhaust

combustion products up the boiler flue and draw fresh air into the combustion chamber.

Mechanical draft boilers include: Forced Draft, where air is forced into the combustion

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BOILERS

chamber by a fan or blower to maintain a positive pressure; and Induced Draft, where air is

drawn through the combustion chamber by a fan or blower to maintain a negative pressure.

Size and Capacity

Modular Boilers are small in size and capacity and are often intended to replace a large single

boiler with several small boilers. These modular boilers can easily fit through a standard

doorway, and be transported in elevators and stairways. The units can be arranged in a variety

of configurations to utilize limited space or to accommodate new equipment. Modular boilers

can be staged to efficiently meet the demand of the heating load.

Condensing Method

Traditional hot water boilers operate without condensing out water vapor from the flue gas.

This is critical to prevent corrosion of the boiler components. Condensing Boilers operate at a

lower return water temperature than traditional boilers, which causes water vapor to condense

out of the exhaust gasses. This allows the condensing boiler to extract additional heat from

the phase change from water vapor to liquid and increases boiler efficiency. Some carbon

dioxide dissolves in the condensate and forms carbonic acid. While some condensing boilers

are made to handle the corrosive condensation, others require some means of neutralizing the

condensate. Traditional non-condensing boilers typically operate in the range of 75% – 86%

combustion efficiency, while condensing boilers generally operate in the range of 88% to

95% combustion efficiency

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BOILERS

IMPORTANT BOILERS

1. COCHRAN BOILER

a. It is very compact and requires minimum floor area

b. Any type of fuel can be used with this boiler

c. Well suited for small capacity requirements

d. It gives about 70% thermal efficiency with coal firing and about 75% with oil firing.

Advantages

a. The feed pipe projecting into the boiler is perforated to ensure uniform water

distribution.

b. Its heating surface area per unit volume at the boiler is considerably large.

c. Its maintenance is easy.

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BOILERS

d. It is suitable where a large reserve of hot water is needed. This boiler due to the large

reserve capacity can easily meet load fluctuations.

e. Super-heater and economizer can be easily incorporated into the system, therefore;

overall efficiency of the boiler can be considerably increased (80-85%).

2. LOCOMOTIVE BOILERS

Locomotive boiler is a horizontal fire tube type mobile boiler. The main requirement of

this boiler is that it should produce steam at a very high rate. Therefore, this boiler

requires a large amount of heating surface and large grate area to burn coa l at a rapid

rate. In order to provide the large heating surface area, a large number of fire tubes are

setup and heat transfer rate is increased by creating strong draught by means of steam jet.

Advantages

a. Large rate of steam generation per square metre of heating surface. To some extent

this is due to the vibration caused by the motion.

b. It is free from brickwork, special foundation and chimney. This reduces the cost of

installation.

c. It is very compact.

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BOILERS

3. LANCSHIRE BOILER

It is stationary fire tube, internally fired, horizontal, natural circulation boiler. This is a

widely used boiler because of its good steaming quality and its ability to burn coal of

inferior quality. These boilers have a cylindrical shell 2 m in diameters and its length

varies from 8 m to 10 m. It has two large internal flue tubes having diameter between 80

cm to 100 cm in which the grate is situated. This boiler is set in brickwork forming

external flue so that the external part of the shell forms part of the heating surface.

Advantages

a. The feed pipe projecting into the boiler is perforated to ensure uniform water

distribution.

b. Its heating surface area per unit volume at the boiler is considerably large.

c. Its maintenance is easy.

d. It is suitable where a large reserve of hot water is needed. This boiler due to the large

reserve capacity can easily meet load fluctuations.

e. Super-heater and economizer can be easily incorporated into the system, therefore;

overall efficiency of the boiler can be considerably increased (80-85%).

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BOILERS

4. BABCOCK WILCOX BOILER

a. The evaporative capacity of this boiler is high compared with other boilers (20,000 to

40,000 kg/hr). The operating pressure lies between 11.5 to 17.5 bar.

b. The draught loss is minimum compared with other boilers.

c. The defective tubes can be replaced easily.

d. The entire boiler rests over an iron structure, independent of brick work, so that the

boiler may expand or contract freely. The brick walls which form the surroundings of

the boiler are only to enclose the furnace and the hot gases.

5. HYDRONIC BOILERS

Hydronic boilers are used in generating heat for residential and industrial purposes. They

are the typical power plant for central heating systems fitted to houses in

northern Europe (where they are commonly combined with domestic water heating), as

opposed to the forced-air furnaces or wood burning stoves more common in North

America. The hydronic boiler operates by way of heating water/fluid to a preset

temperature (or sometimes in the case of single pipe systems, until it boils and turns to

steam) and circulating that fluid throughout the home typically by way ofradiators,

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BOILERS

baseboard heaters or through the floors. The fluid can be heated by any means...gas,

wood, fuel oil, etc., but in built-up areas where piped gas is available, natural gas is

currently the most economical and therefore the usual choice. The fluid is in an enclosed

system and circulated throughout by means of a pump. The name "boiler" can be a

misnomer in that, except for systems using steam radiators, the water in a properly

functioning hydronic boiler never actually boils. Some new systems are fitted

with condensing boilers for greater efficiency. These boilers are referred to

as condensing boilers because they are designed to extract the heat of vaporization of the

flue gas water vapor. As a result of the lower flue gas temperatures, flue gas water vapor

condenses to liquid and with dissolved carbon dioxide forms carbonic acid. The carbonic

acid would damage a typical boiler by corroding the flue and fireside boiler heating

surfaces. Condensing boilers solve this problem by routing the carbonic acid down a

drain and by making the flue exposed to the corrosive flue gas of stainless steel or PVC.

Although condensing boilers are becoming more popular, they are still less common than

other types of hydronic boilers as they are more expensive.

Hydronic systems are being used more and more in new construction in North America for

several reasons. Among those are:

They are more efficient and more economical than forced-air systems (although initial

installation can be more expensive, because of the cost of the copper and aluminum).

The baseboard copper pipes and aluminum fins take up less room and use less metal than

the bulky steel ductwork required for forced-air systems.

They provide more even, less fluctuating temperatures than forced-air systems. The

copper baseboard pipes hold and release heat over a longer period of time than air does,

so the furnace does not have to switch off and on as much.

They tend to not dry out the interior air as much as forced air systems, but this is not

always true. When forced air duct systems are air-sealed properly, and have return-air

paths back to the furnace (thus reducing pressure differentials and therefore air movement

between inside and outside the house), this is not an issue.

They do not introduce any dust, allergens, mold , or (in the case of a faulty heat

exchanger) combustion by products into the living space.

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BOILERS

BOILER FITTINGS AND ACCESSORIES

Safety valve: It is used to relieve pressure and prevent possible explosion of a boiler.

Water level indicators: They show the operator the level of fluid in the boiler, also

known as a sight glass, water gauge or water column is provided.

Bottom blowdown valves: They provide a means for removing solid particulates

that condense and lie on the bottom of a boiler. As the name implies, this valve is usually

located directly on the bottom of the boiler, and is occasionally opened to use the

pressure in the boiler to push these particulates out.

Continuous blowdown valve: This allows a small quantity of water to escape

continuously. Its purpose is to prevent the water in the boiler becoming saturated with

dissolved salts. Saturation would lead to foaming and cause water droplets to be carried

over with the steam - a condition known as priming. Blowdown is also often used to

monitor the chemistry of the boiler water.

Flash Tank: High pressure blowdown enters this vessel where the steam can 'flash'

safely and be used in a low-pressure system or be vented to atmosphere while the

ambient pressure blowdown flows to drain.

Automatic Blowdown/Continuous Heat Recovery System: This system allows the

boiler to blowdown only when makeup water is flowing to the boiler, thereby transferring

the maximum amount of heat possible from the blowdown to the makeup water. No flash

tank is generally needed as the blowdown discharged is close to the temperature of the

makeup water.

Hand holes: They are steel plates installed in openings in "header" to allow for

inspections & installation of tubes and inspection of internal surfaces.

Steam drum internals, A series of screen, scrubber & cans (cyclone separators).

Low- water cutoff: It is a mechanical means (usually a float switch) that is used to turn

off the burner or shut off fuel to the boiler to prevent it from running once the water goes

below a certain point. If a boiler is "dry-fired" (burned without water in it) it can cause

rupture or catastrophic failure.

Surface blowdown line: It provides a means for removing foam or other lightweight

non-condensible substances that tend to float on top of the water inside the boiler.

Circulating pump: It is designed to circulate water back to the boiler after it has

expelled some of its heat.

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BOILERS

Feedwater check valve or clack valve: A non-return stop valve in the feedwater line.

This may be fitted to the side of the boiler, just below the water level, or to the top of the

boiler.

Top feed: In this design for feedwater injection, the water is fed to the top of the boiler.

This can reduce boiler fatigue caused by thermal stress. By spraying the feedwater over a

series of trays the water is quickly heated and this can reduce limescale.

Desuperheater tubes or bundles: A series of tubes or bundles of tubes in the water

drum or the steam drum designed to cool superheated steam. Thus is to supply auxiliary

equipment that does not need, or may be damaged by, dry steam.

Chemical injection line : A connection to add chemicals for controlling feedwater pH.

STEAM ACCESSORIES

Main steam stop valve : It is use to regulate supply of steam.

Steam traps

Main steam stop/Check valve: It is used on multiple boiler installations.

COMBUSTION ACCESSORIES

Fuel oil system or fuel oil heaters

Gas system

Coal system

Soot blower

OTHER ESSENTIAL ITEMS

Pressure gauges

Feed pumps

Fusible plug

Inspectors test pressure gauge attachment

Name plate

Page | 20

BOILERS

MATERIALS

The pressure vessel of a boiler is usually made of steel (or alloy steel), or historically

of wrought iron. Stainless steel, especially of the austenitic types, is virtually prohibited by

the ASME Boiler Code for use in wetted parts of modern boilers, but ferritic stainless steel is

used often in superheater sections that will not be exposed to liquid boiler water. However

electrically-heated stainless steel shell boilers are allowed under the European "Pressure

Equipment Directive" for production of steam for sterilizers and disinfectors.

In live steam models, copper or brass is often used because it is more easily fabricated in

smaller size boilers. Historically, copper was often used for fireboxes (particularly for steam

locomotives), because of its better formability and higher thermal conductivity; however, in

more recent times, the high price of copper often makes this an uneconomic choice and

cheaper substitutes (such as steel) are used instead.

For much of the Victorian "age of steam", the only material used for boiler making was the

highest grade of wrought iron, with assembly by rivetting. This iron was often obtained from

specialistironworks, such as at Cleator Moor (UK), noted for the high quality of their rolled

plate and its suitability for high-reliability use in critical applications, such as high-pressure

boilers. In the 20th century, design practice instead moved towards the use of steel, which is

stronger and cheaper, with welded construction, which is quicker and requires less labour.

Cast iron may be used for the heating vessel of domestic water heaters. Although such heaters

are usually termed "boilers" in some countries, their purpose is usually to produce hot water,

not steam, and so they run at low pressure and try to avoid actual boiling. The brittleness of

cast iron makes it impractical for high pressure steam boilers.

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BOILERS

SUPERHEATED STEAM BOILER

Most boilers produce steam to be used

at saturation temperature; that is, saturated

steam. Superheated steam boilers vaporize the

water and then further heat the steam in

a superheater. This provides steam at much

higher temperature, but can decrease the overall

thermal efficiency of the steam generating plant

because the higher steam temperature requires a

higher flue gas exhaust temperature. There are

several ways to circumvent this problem, typically by providing an economizer that heats the

feed water, a combustion air heater in the hot flue gas exhaust path, or both. There are

advantages to superheated steam that may, and often will, increase overall efficiency of both

steam generation and its utilisation: gains in input temperature to a turbine should outweigh

any cost in additional boiler complication and expense. There may also be practical

limitations in using wet steam, as entrained condensation droplets will damage turbine blades.

Superheated steam presents unique safety concerns because, if any system component fails

and allows steam to escape, the high pressure and temperature can cause serious,

instantaneous harm to anyone in its path. Since the escaping steam will initially be

completely superheated vapor, detection can be difficult, although the intense heat and sound

from such a leak clearly indicates its presence.

Superheater operation is similar to that of the coils on an air conditioning unit, although for a

different purpose. The steam piping is directed through the flue gas path in the boiler furnace.

The temperature in this area is typically between 1300–1600 degrees Celsius (2372–2912 °F).

Some superheaters are radiant type; that is, they absorb heat by radiation. Others are

convection type, absorbing heat from a fluid. Some are a combination of the two types.

Through either method, the extreme heat in the flue gas path will also heat the superheater

steam piping and the steam within. While the temperature of the steam in the superheater

rises, the pressure of the steam does not and the pressure remains the same as that of the

boiler.[5] Almost all steam superheater system designs remove droplets entrained in the steam

to prevent damage to the turbine blading and associated piping.

A superheated boiler on a steam locomotive

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BOILERS

SUPERCRITICAL STEAM GENERATOR

Supercritical steam generators are frequently

used for the production of electric power. They

operate at supercritical pressure. In contrast to a

"subcritical boiler", a supercritical steam

generator operates at such a high pressure (over

3,200 psi or 22 MPa) that the physical

turbulence that characterizes boiling ceases to

occur; the fluid is neither liquid nor gas but a

super-critical fluid. There is no generation of

steam bubbles within the water, because the

pressure is above the critical pressure point at

which steam bubbles can form. As the fluid expands

through the turbine stages, its thermodynamic state drops below the critical point as it does

work turning the turbine which turns electrical generator from which power is ultimately

extracted. The fluid at that point may be a mix of steam and liquid droplets as it passes into

the condenser. This results in slightly less fuel use and therefore less greenhouse

gas production. The term "boiler" should not be used for a supercritical pressure steam

generator, as no "boiling" actually occurs in this device.

Steam generation power plant

Page | 23

BOILERS

SAFETY

To define and secure boilers safety, some professional specialized organizations such as

the American Society of Mechanical Engineers (ASME) develop standards and regulation

codes. For instance, the ASME Boiler and Pressure Vessel Code is a standard providing a

wide range of rules and directives to ensure compliance of the boilers and other pressure

vessels with safety, security and design standards.

Historically, boilers were a source of many serious injuries and property destruction due to

poorly understood engineering principles. Thin and brittle metal shells can rupture, while

poorly welded or riveted seams could open up, leading to a violent eruption of the pressurized

steam. When water is converted to steam it expands to over 1,000 times its original volume

and travels down steam pipes at over 100 kilometres per hour. Because of this steam is a

great way of moving energy and heat around a site from a central boiler house to where it is

needed, but without the right boiler feed water treatment, a steam-raising plant will suffer

from scale formation and corrosion. At best, this increases energy costs and can lead to poor

quality steam, reduced efficiency, shorter plant life and unreliable operation. At worst, it can

lead to catastrophic failure and loss of life. Collapsed or dislodged boiler tubes can also spray

scalding-hot steam and smoke out of the air intake and firing chute, injuring the firemen who

load the coal into the fire chamber. Extremely large boilers providing hundreds of

horsepower to operate factories can potentially demolish entire buildings. The Locomotive, by

Hartford Steam Boiler Inspection and Insurance Company, Published by Hartford Steam

Boiler Inspection and Insurance Co., 1911, Item notes: n.s.:v.28 (1910-11), Original from

Harvard University, Digitized December 11, 2007 by Google Books, Link to digitized

document: an article on a massive Pabst Brewing Company boiler explosion in 1909 that

destroyed a building, and blew parts onto the roof of nearby buildings. This documents also

contains a list of day-by-day boiler accidents and accident summaries by year, and

discussions of boiler damage claims.

A boiler that has a loss of feed water and is permitted to boil dry can be extremely dangerous.

If feed water is then sent into the empty boiler, the small cascade of incoming water instantly

boils on contact with the superheated metal shell and leads to a violent explosion that cannot

be controlled even by safety steam valves. Draining of the boiler can also happen if a leak

occurs in the steam supply lines that is larger than the make-up water supply could replace.

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BOILERS

All combustion equipment must be operated properly to prevent dangerous conditions or

disasters from occurring, causing personal injury and property loss. The basic cause of boiler

explosions is ignition of a combustible gas that has accumulated within the boiler. This

situation could arise in a number of ways, for example fuel, air, or ignition is interrupted for

some reason, the flame extinguishes, and combustible gas accumulates and is reignited.

Another example is when a number of unsuccessful attempts at ignition occur without the

appropriate purging of accumulated combustible gas.

There is a tremendous amount of stored energy within a boiler. The state change of

superheated water from a hot liquid to a vapor (steam) releases an enormous amount of

energy. For example, 1 ft3 of water will expand to 1600 ft3 when it turns to steam. Therefore,

“if you could capture all the energy released when a 30 gallon home hot water tank flashes

into explosive failure at 332oF, you would have enough force to send the average car

(weighing 2,500 lbs) to a height of nearly 125 feet. This is equivalent to more than the height

of a 14 story apartment building, starting with a lift off velocity of 85 miles per hour!” (5).

Boiler safety is a key objective of the National Board of Boiler and Pressure Vessel

Inspectors. This organization reports and tracks boiler safety and the number of incidents

related to boilers and pressure vessels each year. Their work has found that the number one

incident category resulting in injury was poor maintenance and operator error (5). This

stresses the importance of proper maintenance and operator training.

Boilers must be inspected regularly based on manufacturer’s recommendations. Pressure

vessel integrity, checking of safety relief valves, water cutoff devices and proper float

operation, gauges and water level indicators should all be inspected. The boiler’s fuel and

burner system requires proper inspection and maintenance to ensure efficient operation, heat

transfer and correct flame detection. The Federal Energy Management Project (FEMP) O&M

Best Practices Guide to Achieving Operation Efficiency is a good resource describing a

preventive maintenance plan and also explaining the importance of such a plan.

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BOILERS

CONTROLLING DROUGHT

Most boilers now depend on mechanical draught equipment rather than natural draught. This

is because natural draught is subject to outside air conditions and temperature of flue gases

leaving the furnace, as well as the chimney height. All these factors make proper draught

hard to attain and therefore make mechanical draught equipment much more economical.

There are three types of mechanical draught:

Induced draught: This is obtained one of three ways, the first being the "stack effect" of

a heated chimney, in which the flue gas is less dense than the ambient air surrounding the

boiler. The denser column of ambient air forces combustion air into and through the

boiler. The second method is through use of a steam jet. The steam jet oriented in the

direction of flue gas flow induces flue gasses into the stack and allows for a greater flue

gas velocity increasing the overall draught in the furnace. This method was common on

steam driven locomotives which could not have tall chimneys. The third method is by

simply using an induced draught fan (ID fan) which removes flue gases from the furnace

and forces the exhaust gas up the stack. Almost all induced draught furnaces operate with

a slightly negative pressure.

Forced draught: Draught is obtained by forcing air into the furnace by means of a fan

(FD fan) and ductwork. Air is often passed through an air heater; which, as the name

suggests, heats the air going into the furnace in order to increase the overall efficiency of

the boiler. Dampers are used to control the quantity of air admitted to the furnace. Forced

draught furnaces usually have a positive pressure.

Balanced draught: Balanced draught is obtained through use of both induced and forced

draught. This is more common with larger boilers where the flue gases have to travel a

long distance through many boiler passes. The induced draught fan works in conjunction

with the forced draught fan allowing the furnace pressure to be maintained slightly below

atmospheric.

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BOILERS

BEST PRACTICES FOR EFFICIENT OPERATION

1. Efficiency

The percentage of the heat energy contained in the fuel that is captured by the working fluid

(e.g. water) in the boiler is defined as the combustion efficiency of the boiler. Combustion

efficiencies of 80% or higher are usually possible for hot water boilers and low pressure

steam boilers for commercial buildings.

Complete combustion results when a hydrocarbon fuel such as natural gas or oil burns and

produces only carbon dioxide, water and heat. If there is insufficient oxygen and/or poor

mixing of fuel and oxygen, then incomplete combustion will occur resulting in other products

of combustion including carbon monoxide and unburned fuel.

When incomplete combustion occurs, the chemical energy of the fuel is not completely

released as heat and the combustion efficiency is reduced. This is also a safety concern as

unburned fuel could ignite in the stack and cause an explosion. Boilers must be tuned to

achieve complete combustion. One strategy to ensure complete combustion is to provide

some amount of excess air. However, as shown in the figure below, a small amount of excess

air will improve combustion efficiency, but a large amount will reduce efficiency.

Stoichiometric air:fuel ratio Most efficient air:fuel ratio air fuel Percent Excess Air Boiler

efficiency

Combustion Efficiency vs. Excess Air

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For high overall boiler efficiency, the heat released by combustion must be efficiently

transferred into the working fluid. Any heat not transferred into the fluid will be lost through

the boiler shell or the flue gas. The temperature of the flue gasses in the boiler stack is a good

indicator of this heat transfer and thus the efficiency. There are practical limits to how low

the stack temperature can be. The temperature will be higher than the working fluid in the

boiler. In non-condensing boilers, it must be high enough so that the water vapor in the

exhaust gas does not condense and bathe the heat transfer surface in the corrosive condensate.

Condensing natural gas boilers are designed and built with materials designed to resist

corrosion. As such, they may have exhaust temperatures less than 150°F. Capturing the heat

from the condensate can result in combustion efficiencies of greater than 90%.

2. Use Boiler Controls for Optimized Air-to-Fuel Ratio

To ensure that complete combustion occurs, extra air is introduced at the burner. But too

much will result in air being wastefully heated and exhausted out of the boiler flue,

penalizing combustion efficiency, and creating a safety issue. When a boiler is tuned, the goal

is to maximize combustion efficiency by providing just enough excess air to assure complete

combustion but not too much to reduce efficiency. How much excess air is enough to assure

complete combustion? That varies with the design and condition of the burner and boiler, as

well as with the different firing rates of the burner, but is typically considered to be between

2% - 3%. Excess air must also be adjusted to allow for variations in temperature, density, and

humidity of the boiler combustion air throughout any daily and seasonal variations. It’s

desirable to maintain a constant amount of excess air across the entire firing range.

The important idea to remember is that complete combustion is critical to ensuring efficient

boiler operation. Incomplete combustion of the fuel can significantly reduce boiler efficiency

by 10% or more, while increasing excess air by 10% may only impact boiler efficiency by

about 1%. Signs of incomplete combustion are a smoky exhaust, a yellow flame, flame

failures, and sooty boiler tubes. It is a good idea to tune up a boiler annually to ensure the

combustion process is optimized.

Typically, excess air of around 10% for a natural gas boiler is optimal to ensure complete

combustion and peak efficiency. This corresponds to excess O2 of around 2% to 3%.

Operating with excess air beyond 10% is undesirable, as it can result in reduced efficiency

and higher emissions. Therefore maintaining the optimum level of excess air across the entire

firing range is preferred. This can be accomplished with the use of burner controls including

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parallel positioning controls, cross-limiting controls, and oxygen trim controls. These types of

controls are superior alternatives to traditional mechanical jackshaft controls. A brief

description of each burner control type is provided below (3):

a. Mechanical jackshaft control is the simplest type of modulating burner control,

typically used on smaller burners. Also called single point control because one

mechanical linkage assembly controls both air and fuel. These controls cannot

measure airflow or fuel flow. The range of control is limited, resulting in excessive

levels of excess air to ensure safe operation under all conditions and firing rates. Slop

in the linkages makes accurate and repeated control difficult, and requires regular

maintenance and adjustment.

b. Parallel positioning controls use separate motors to adjust fuel flow and airflow

allowing each to be adjusted over the entire firing range of the boiler. During setup,

many points are “mapped,” typically 10 to 25 points, to create a curve of airflow and

corresponding fuel flow. The air-fuel ratio can therefore vary across the entire firing

range to provide the optimal ratio under all firing conditions. Also, with the use of

electronic servo-motors, this method of control is highly repeatable.

c. Cross-limiting controls, usually applied to larger boilers, use controls to sense and

compensate for some of the factors that affect optimum air to fuel ratio. Air flow and

fuel flow are measured and adjusted to maintain the optimum value determined during

initial calibration.

d. Oxygen trim control is used in conjunction with standard parallel positioning or

cross- limiting controls. It analyzes the oxygen in the flue gas and adjusts the air-fuel

ratio accordingly to maintain a set amount of excess oxygen. These controls are

usually installed on larger boilers with high annual fuel usage, and can increase

energy efficiency by one or two percent beyond what is achieved with the standard

control alone.

3. Monitor Boiler Gauges

It is possible that a leak will develop in the hot water distribution loop. Such leaks will

increase the system’s energy and water consumption, and may also result in water damage.

Hot water and steam distribution systems should be provided with make-up water to replace

any steam or water that is lost through a leak in the system. This will provide an easy way to

ensure the system is fully charged with water at all times. It is best practice to install a meter

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on the make-up line to the system. The meter should be read weekly to check for unexpected

losses of water from the system.

In steam systems, it is a best practice to monitor make-up water volume daily. As steam leaks

from the system, additional make-up water is required to replace the loss. Monitoring the

make-up water will ensure that you are maximizing the return of condensate, thereby

reducing the need for make-up water.

4. Seasonal Operation

If a steam or hot water system is not used for a portion of the year, shutting the system down

can result in significant savings. Maintaining a boiler at its operating temperature consumes

energy equivalent to its standby losses. In the case of a hot water system, energy use may also

include pump operation.

5. Operating Multiple Boiler Plants

Boiler loads in commercial buildings vary greatly from summer to winter, from day to night,

and from weekday to weekend. With a single boiler it is difficult to efficiently supply these

varying loads. When the building heating needs drop below the heat supplied by the boiler at

its lowest firing rate, the boiler cycles off. Cycling a boiler on and off is very inefficient

because there is a pre- ignition purge and a post- ignition purge that draw heat out of the boiler

with each cycle. Also, in the case of a non-modulating boiler, cycling doesn’t allow the boiler

to operate at part load and steady firing rate when combustion efficiencies are at their best.

If a facility has multiple boilers, it may be possible to sequence the boilers to avoid frequent

cycling. If using non-modulating boilers, it may be better to stage subsequent boilers on once

the primary boiler has reached full capacity, rather than cycling multiple boilers on and off to

meet the load. On the other hand, with modulating boilers, boiler efficiency increases at part

load conditions. Therefore it may be advantageous to operate multiple boilers simultaneously

at part load conditions rather than one boiler at 100% output. Figure 7 below shows the

relationship between firing rate and efficiency for a boiler with the ability to modulate both

airflow and fuel input.

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BOILERS

Finally, automatic sequencing of boilers is essential to efficient operation. When building loads are

reduced at night and on weekends, increased boiler cycling will likely occur if no-one is available to

turn boilers off as necessary.

If your facility has multiple boilers you should assess if it is truly necessary to keep any boilers on

standby (at pressure or temperature), since this has an energy penalty. The standby boiler will not only

cycle on and off, it will lose heat to the surroundings through radiation losses, which increase

significantly as a percentage of boiler input at reduced firing rates. At low firing rates, such as when a

boiler is maintained in a standby condition, efficiency loss can be as much as 15% (7). Having a

standby boiler will allow quick recovery if the lead boiler fails, but this must be weighed against this

large energy penalty. If a standby boiler is not critical to your operation, or if the need for a standby

boiler is seasonal, you should consider shutting off any unnecessary boilers to prevent these energy

losses.

6. Implement Boiler Lockout Control Sequence

Including boiler lockout in the HVAC system’s sequence of operation is important in

achieving energy efficiency. With the common application of VAV sys tems in commercial

buildings today, simultaneous heating and cooling, and excessive reheating of primary air can

often go unnoticed. Implementing a boiler lockout based on outside air temperature, for

example when outside air is greater than 65oF, is an effective way to prevent these

conditions.

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BOILERS

7. Condensing Boilers

Both the system design and operating conditions are critical to the successful operation and

performance of a condensing boiler. Return water temperatures below 130oF are typically

required to get the rated efficiency out of a condensing boiler. Return water temperatures

above 130oF prevent condensation of the flue gas and result in the boiler operating no more

efficiently than a traditional boiler.

8. Flue Gas Economizers

Flue gas economizers offer the best opportunity for heat recovery .These are essentially heat

exchangers in the boiler exhaust which transfer heat from the flue gas to either the boiler

feedwater or combustion air. Even with efficient boilers that operate with a relatively low

flue gas temperature, there is ample room to recover some of the flue gas heat that would

otherwise go up the stack. Economizers typically increase overall boiler efficiency by three to

four percent.

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BOILERS

BEST PRACTICES FOR MAINTENANCE

1. Keep the Boiler Clean

As mentioned previously, any residue, such as soot or scale, that coats the heat transfer

surfaces of the boiler will reduce its efficiency and also increase the likelihood of equipment

failure. Cleaning this surface according to manufacturer’s recommendations is important to

maintaining optimum boiler performance and equipment life. Residue that coats the tubes of

a boiler will interfere with heat transfer and elevate the flue gas temperature. If incomplete

combustion occurs, the resulting soot accumulates on the combustion side of the tubes.

Similarly, poor water treatment practices can result in scale accumulation on the water side of

the tubes. A layer of soot or scale only 0.03 inches thick can reduce heat transfer by 9.5%. A

layer 0.18 inches thick can reduce heat transfer by 69%.

2. Water Chemical Treatment Plan

Good boiler water chemical treatment is essential to maintain efficient ope ration. Each

chemical treatment plan must be customized based on the dissolved minerals in the make-up

water, the percentage of condensate returned, and the presence or absence of a de-aerator.

Dissolved solids in the boiler water and the level of treatment chemicals should be tested

daily in small low pressure plants and hourly in larger high pressure plants. Instruments

should be calibrated monthly. Annual inspections of boilers should include a thorough

examination of the water side surfaces for evidence of scaling and corrosion. Even a thin

layer of scale interferes with heat transfer and thereby decreases combustion efficiency.

An upward trend in flue gas temperatures over weeks or months usually indicates that a

deposit has built up on either the fireside or waterside of the boiler heat exchange surfaces. If

this condition is observed the boiler should be inspected promptly.

3. Minimize Boiler Blowdown

Having too many total dissolved solids (TDS’s) in the boiler water can cause scale and

reduce boiler efficiency. Therefore, it is necessary to maintain the solids below certain limits.

As TDS concentration increases, it becomes more likely that the dissolved solids will

precipate out of the water and form scale. Draining of the water, called boiler blowdown, is

required to remove some of those dissolved solids and keep the TDS concentration below the

level where they will precipitate. Consistent and frequent small volume blowdowns is a better

practice than infrequent high volume blowdowns, because it conserves energy, water, and

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chemicals. Large steam boilers with steady loads should have continuous blowdown, where a

small amount of water is drained continuously from the boiler while fresh make-up water is

introduced.

4. Inspect and Repair Insulation

Insulation is critical for steam and condensate piping. Un-insulated pipes, valves, or fittings

carry a heavy energy penalty. It’s generally cost effective to insulate any surface with a

temperature greater than 130oF (4). Steam, condensate, and hot water pipes in air conditioned

spaces produce a double penalty if un-insulated because the heat loss from the pipes must be

removed by additional air conditioning.

5. Sample Maintenance Logs & Boiler Checklists

Boiler O&M best practices begin with maintaining regularly scheduled inspection logs and

checklists to ensure proper equipment operation. Pressure, water temperature, and flue gas

temperatures should be recorded daily, as they can serve as a baseline reference for system

operation and troubleshooting problems. More detailed inspections and checks should be

performed to document system performance, which can be very important since a gradual

change in system operating conditions over time may not be readily apparent without the use

of such documentation.

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REFERENCES 1. Capehart, B., Turner, W. and Kennedy, W., 2006. Guide to Energy Management.

2. ASHRAE Handbook, HVAC Systems and Equipment, 2008.

3. Boiler and Heaters, Improving Energy Efficiency, Canadian Industry Program for Energy

Conservation, August 2001.

http://oee.nrcan.gc.ca/publications/infosource/pub/cipec/boilersheaters.PDF

4. Federal Energy Management Program Fact Sheet, PNNL, January 2005.

http://www1.eere.energy.gov/femp/pdfs/om_combustion.pdf

5. FEMP O&M Best Practices, a Guide to Achieving Operational Efficiency, U.S.

Department of Energy, August 2010.

http://www1.eere.energy.gov/femp/pdfs/omguide_complete.pdf

6. Efficient Boiler Operations Sourcebook, Fourth Edition, F. William Payne and Richard E.

Thompson, 1996.

7. The Control of Boilers, 2nd Edition, Sam G. Dukelow, 1991.

Other Resources

1. The National Board of Boilers and Pressure Vessel Inspectors,

http://www.nationalboard.org/default.aspx.

2. 2010 ASME Boiler and Pressure Vessel Code