Boiler Operator's Exam Prep Guide (PB)

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Contents

Introduction: Taking the Test

Chapter One Heat

Chapter Two Boilers

Chapter Three Boiler Trim

Chapter Four Combustion

Chapter Five Fuel-Burning Equipment

Chapter Six Fuel-Saving Equipment

Chapter Seven Steam Traps and Pipes

Chapter Eight Water Treatment

Chapter Nine Turbines

Chapter Ten Pumps and Motors

Chapter Eleven Bearings

Chapter Twelve Sample Multiple Choice Questions

Chapter Thirteen Essay Questions

Chapter Fourteen Answers to Multiple Choice Questions

Chapter Fifteen Essay Answers

Appendix Various Steam Tables

Glossary

Index

INTRODUCTION

Taking the Test

Here are a few tips when taking your licensing examination:

Select what appears to be the most correct answer.

While examiners swear that there is only one obvious correct answer to a question, you may find that either none or more than one of the choices is obviously correct. Instead of trying to figure out which is the correct answer, eliminate those that are obviously wrong and then narrow it down to the most correct answer.

Don’t get hung up on one question.

Go through the test and answer the questions you definitely know. Then go back and work on the ones you skipped. The last thing you want to do is expend all your nervous energy on a hard question at the beginning of the test. Don’t be surprised if, after going through the test several times, there are still a few questions that make no sense. If that’s the case, just give them your best guess.

Learn definitions.

A lot of multiple choice questions are based on definitions. Take the following example:

Convection means

a) the movement of liquids or gases created by a temperature difference.

b) transfer of heat by direct molecular contact.

c) the weight of a substance as compared to unit.

d) a form of heat transfer by waves.

The correct answer is a, but change the question to radiation means or conduction means, and the correct answer is then d and b, respectively.

MATH PROBLEMS AND UNITS

When working a math problem, always assign units to the numbers in the problem. When the equation is set up, the units should work out to the units you expect. If not, then the equation is not set up properly. Consider the following problem:

The water surface in a fire tank is 135 feet above the suction of a fire pump. What is the psi at the suction of the fire pump?

You remember the conversion is 2.31, but the question seems to be, do you multiply or divide 2.31 by 135 feet. But the question is really 2.31 what?! Every number that is written down should have units attached to it. When these numbers, especially conversions, have units, then the math becomes much easier.

The correct measurement is actually 2.31 feet/psi. Let’s say your first guess is to multiply 135 feet by 2.31 feet/psi (at this point don’t worry about the numbers, just concentrate on the units):

Obviously feet² per psi is not the answer you were looking for. Now divide 135 feet by 2.31 feet/psi:

This is more like it. The unit you were expecting was psi. Now that the problem is set up properly, just add numbers to the units and let the calculator do the number crunching:

Some examiners insist that you show all steps to the solution. This method automatically makes you show all your work. Even if you don’t come up with what the examiner thinks is the correct result, if your work is laid out in an easily followed fashion partial credit might be given.

Now after all this talk about units, there are some numbers that are dimension-less. It’s not that these numbers started out lacking units; it’s that all the units canceled themselves out. Percentages, efficiencies, and ratios like n (pi) are examples.

From elementary school you learned that the equation for the area of a circle is πr². This same equation can also be written as 0.7854d². Use whichever one you feel most comfortable with.

With that in mind, let’s find the area of a 6 foot diameter circle. The answer must be in square inches.

The same problem can also be worked out as follows:

Notice the answers are close but not exact. The problem here is how many decimals to use with π. Usually two decimal places is plenty. The important thing is to show all your work.

Another advantage of using units is that you are forced to do all the conversions. In the previous example, diameter was given in feet, but the answer had to be in inches. You can bet your bottom dollar that if this were a multiple choice question, it would include the answer that did not have feet converted to inches. You would work the problem without converting, then be relieved that the incorrect answer you came up with was included in the choices.

Make sure your calculator can do square roots. There are a few problems where this feature comes in handy.

CONVERSIONS

Now for conversions. Since most tests are closed book, these will have to be learned or memorized:

One British thermal unit (Btu) is required to raise the temperature of one pound of water 1°F and is equal to 778 ft-lb of work

Atmospheric pressure of 14.7 psia will sustain a column of water 34 feet high and a column of mercury (Hg) 29.92 inches high.

EQUATIONS

The following are a few of the equations you will have to memorize:

Boilers

The equation for maximum allowable working pressure (MAWP) is:

where:

For bursting pressure, set the factor of safety to one.

The equation for boiler horsepower is as follows:

Where the factor of evaporation is:

The equation for boiler efficiency is as follows:

Pumps

To calculate horsepower, use the following equation:

Or the following equation may be used to calculate horsepower:

To calculate the gpm of a reciprocating pump, use the following equation:

where:

Note: A stroke is a piston moving once over its path. For a duplex pump, a stroke is both pistons moving once over their path.

Turbines

For condenser cooling water, use the following equation:

where:

The equation for torque is:

where:

5,252 = constant that converts radians to rpm

For pipe expansion, use the following equation:

(T1 – T2) × L × Coefficient of expansion × 12 in./ft

where:

coefficient of expansion for steel = 0.0000065 in./in.-°F

To calculate pipe size, use the following equation:

where:

Become familiar with different steam pressures and their corresponding temperatures and heat content.

CHAPTER ONE

Heat

While the concept of thermodynamics may seem a little daunting, it is necessary to know a little about it, as it lays the foundation for everything that follows. What is more important is that it also helps with some test questions. Understanding terms such as sensible heat and latent heat and knowing what happens when water changes to steam will help you understand what happens in boilers and steam traps.

HEAT TRANSFER

Let’s begin by examining the process of changing water into steam. The heat energy from fuel is delivered to the water by three methods of transfer: radiation, conduction, and convection:

• Radiation does not require a transmission medium; it travels like light waves through a vacuum and through air. The most common example of radiation is the heat we feel from the sun.

• Conduction is the transfer of heat from a warm molecule to a cooler one. Some materials conduct heat better than others; for example, gases and vapors are poor conductors, liquids are better, and metals are best. Materials that are poor heat conductors, like asbestos and calcium silicate, are called insulators. Heat travels through insulators but at a slower rate.

• Convection heat transfer takes place by movement of the heated material itself. In a heated room, warm air rises and the cold air falls. In a boiler, the hot water rises and the cold water falls to the bottom.

Now it is possible to see how the three forms of heat transfer work in a boiler. The tubes in the furnace section of the boiler receive their heat by radiation from the visible flame. In fact, about half of the steam in an industrial boiler and all the steam in a utility boiler is generated by the furnace tubes. The part of the boiler that contains most of the tubes is called the convection section. This section receives its heat by convection from the hot flue gas. Heat is then transferred through the tube metal and into the water by conduction.

Water to Steam

To demonstrate how water is transformed into steam, pour one pound of 32°F water into a pot sitting on a stove burner. Because this demonstration takes place in an open pot, the pressure of the water and any steam produced remains at atmospheric pressure. (Standard atmospheric pressure is 14.7 pounds per square inch absolute (psia), which is explained later in this chapter. Experiments performed at different pressures yield different results.) Place a thermometer in the water to monitor its temperature. You must imagine placing a device (let’s call it a heat-o-meter) in the water that measures the amount of heat absorbed by the water. The heat-o-meter would be calibrated in Btu (British thermal units). One British thermal unit is the heat required to raise one pound of water one degree Fahrenheit, Figure 1-1.

Figure 1-1. One British thermal unit raises one pound of water one degree Fahrenheit

With the dial on the heat-o-meter set to zero and the thermometer reading 32°F, turn on the burner (Point 3, Figure 1-2). The readings on both the thermometer and heat-o-meter will increase. The heat absorbed by the water that causes the temperature increase is called sensible heat. If you put your finger in the water, you can detect or sense the sensible heat. Sensible heat changes the temperature of a substance but not its state. This means that water absorbing sensible heat stays water and will not turn to steam.

Figure 1-2. Water changing to steam

As the water temperature reaches 212°F (Point 4, Figure 1-2), there will still be a pound of water in the pot because boiling hasn’t started yet.¹ The heat-o-meter reads 180 Btu. This matches the definition of a British thermal unit, because we increased the temperature of one pound of water 180°F (212° -32°F = 180°F).

Just beyond Point 4 in Figure 1-2, the water is still absorbing heat because the burner is still on, but the temperature of the water remains constant at 212°F. This is called latent heat (latent means hidden). Latent heat changes the state of a substance but not its temperature. It takes extra energy to change the state of a substance, and at this point, latent heat is necessary to convert water into steam.

As the water continues to boil, the heat-o-meter reading would continue to increase, but the thermometer would stay at 212°F. When the last drop of water evaporates (Point 5, Figure 1-2), the heat-o-meter would read 1,150.3 Btu. This is the total amount of heat required to evaporate one pound of water starting at 32°F. Subtract the sensible heat from this total (1,150.3 Btu -180 Btu = 970.3 Btu) and you have the amount of latent heat required to evaporate one pound of water from 212°F at 14.7 psia.

During the evaporation process, the volume of one pound of water changes drastically. It starts as a liquid at 0.01672 ft³ (about one pint) and changes to steam (gas) at 26.79 ft³ (about one cubic yard), which is an increase of more than 1,600 times!

Steam may be called a gas, because that is what it is. What you see around the top of a pot of boiling water is water vapor condensing out of the steam. Condensation is the process by which steam gives up its latent heat and turns back into water.

Now that all the water has evaporated from our pot, turn off the burner. While evaporating, the water was absorbing heat fast enough to keep the metal pot at a safe level. Without the heat-absorbing water, meltdown occurs. This is an extremely important point, because the same thing can happen in a boiler; if the water gets too low, property damage or even personal injury can take place.

Steel holds its strength up to 700°F, and it weakens rapidly above that point. Since the flame temperature is over 2,500°F, how does the boiler survive under these conditions? It all has to do with how fast heat travels through the metal. If there is something on the other side of the metal that can absorb large amounts of heat quickly (water, for example), then the metal temperature will stay at a safe level. If there is nothing on the other side that can absorb a lot of heat quickly (air or steam, for example), then the metal overheats.

Events in nature may be explained by the theory just described. For example, in the geyser Old Faithful in Yellowstone National Park, water under the Earth’s surface absorbs heat from rocks heated by Earth’s molten core. After a time, the water absorbs enough heat so that some of it flashes off into steam. Since steam occupies so much more volume than water, pressure increases sharply. This pressure is relieved by forcing the mixture of hot water and steam out of openings and cracks in the Earth’s surface. It is because of this principle that it is dangerous to sparge (directly inject) steam into a closed pressure vessel such as a tank or pipe. Although this might seem a good way to heat water, the water under pressure absorbs heat until some of it flashes into steam. The sudden increase in volume results in damaged equipment if the pressure has no place to go.

STEAM TABLES

Steam tables show various pressures, temperatures, heat content, and specific volumes. Notice in Table 1-1 that as water pressure increases, the corresponding boiling temperature also increases. Also notice that water at a specific pressure always boils at a specific temperature. For example, water at 15 psig always boils at 250°F, and water at 250 psig always boils at 406°F.

Table 1-1. Properties of saturated steam

Saturation Line

The curve shown in Figure 1-3 is known as the saturation line. Every point below the curve is water, and every point above the curve is superheated steam. Every point on the line is water and/or steam at its saturated temperature and pressure (where the gas is at the same pressure/temperature as the liquid it contacts). A point on this saturation curve is actually on the line between Points 4 and 5 in Figure 1-2. Point 4 is 100% water at its saturation temperature. Halfway to Point 5 is 50% water and 50% steam. Point 5 is 100% steam. Point 5 is dry steam (all gas and no liquid), while wet steam is anywhere in between, but not including, Points 4 and 5. The closer to Point 4, the wetter the steam. The steam and water are both at the same temperature and pressure, but instead of having 100% gas, there is a mixture of liquid and gas at the same temperature.

Figure 1-3. Saturation line

Refrigerants also have pressure-temperature tables and saturation curves. Refrigeration gauges have dials calibrated to measure pressure and temperature. In the steam table shown in Table 1-1, both gauge pressure and absolute pressure are given. (Absolute pressure and gauge pressure are explained later.) In unabridged versions, only absolute pressures are listed. Absolute pressure must be used in most engineering calculations; gauge pressure is listed on the steam table for convenience.

Absolute and Gauge Pressures

A vacuum is any pressure less than atmospheric pressure. Zero pounds per square inch absolute (psia) is a perfect vacuum. Zero pounds per square inch gauge (psig or psi) is atmospheric pressure. When stating absolute pressure, psia must be used. When stating gauge pressure, psi is all that is required, but sometimes psig is used. At sea level, absolute pressure is 14.7. When converting absolute pressure to gauge pressure, subtract 14.7. When converting gauge pressure to absolute pressure, add 14.7. Remember, gauge pressure plus atmospheric pressure equals absolute pressure. Figure 1-4 shows a comparison between absolute and gauge pressures.

Figure 1-4. Comparison between absolute and gauge pressures

Superheated Steam

Unabridged steam tables are divided into two parts. The first part lists the saturated pressure-temperature relationships between liquid and vapor phases. In the first part, the first column lists temperature, while the second column lists pressure. The second part of the steam table describes the properties of superheated steam.

Although it was stated that for a given pressure there is only one temperature for steam, the temperature of superheated steam is higher than saturated steam at a given pressure. This is because after steam is produced, additional heat can be added to increase its temperature and heat content. Any point beyond Point 5 in Figure 1-2 is superheated steam.

Superheated steam is used for two main reasons: 1) to provide extra energy that is used for driving a steam turbine; and 2) its higher temperature means that less of it condenses when transported over long distances, such as airports, military bases, and large campuses. Superheated steam is rarely used for space and process heating. To understand why, let’s look at the steam cycle and how steam is used.

Consider that 15 psi steam is transported to its point of use (for example, a heating coil in an hvac system, a kettle in a kitchen, or a fuel oil heater). As heat is transferred from the steam and into the process, the steam condenses back into water. For every pound of steam that condenses, 946 Btu are delivered (see Table 1-1). This is the great advantage of steam; a lot of heat can be transferred with a little amount of effort.

If we add 50°F to a pound of superheated steam at 250°F and 15 psi, around 25 Btu will be added. This is a very small percentage compared to the 946 Btu already contained in the saturated steam. The superheat is used very quickly, and most heating is still done at the saturated temperature of 250°F.

Flash Steam

Nature will neither permit water to remain in the liquid state at temperatures higher than 212°F, nor to contain more than 180 Btu/lb at atmospheric pressure. Saturated water at 0 psi at 212°F contains 180 Btu/lb. Saturated water at 150 psi at 366°F contains 339 Btu/lb. In the latter case, the Btu/lb exceeding 180 Btu/lb must be jettisoned. Nature takes care of this surplus by converting a fraction of the water to flash steam. Live steam is generated in a boiler, while flash steam is produced when hot water at its saturated temperature is released to a lower pressure.

The percentage of flash steam can be calculated by finding the difference in heat content between the high and low pressure waters, then dividing by the latent heat of the steam at the lower pressure. To convert this decimal answer to a percentage, multiply by 100. For example, a steam trap in a 100 psi system discharges condensate to atmospheric pressure (0 psi). The steam table (Table 1-1) shows that 100 psi water contains 309 Btu/lb, 0 psi water contains 180 Btu/lb, and that the latent heat of 0 psi steam is 970.3 Btu/lb. What is the percentage of water flashed into steam?

Wasting 13% of the water is bad enough, but now calculate how much heat is lost by letting the flash steam get away. What percentage of heat does the flash steam contain compared to the saturated water at 100 psi?

Figure 1-5 is a graphic representation of the flash steam equation.

Figure 1-5. Percentage of flash steam formed when condensate is discharged to a lower pressure (Courtesy, Armstrong International, Inc.)

Unabridged Steam Tables

For those of you who must take more advanced examinations, steam table questions are fair game. It is sometimes asked how the tables are organized. The tables are divided into three sections: saturated steam listed by temperature, Table 1-2; saturated steam listed by pressure, Table 1-3; and superheated steam, Table 1-4. The complete steam tables may be found in the appendix.

Table 1-2. Properties of saturated steam -temperature table (Courtesy, Cameron Hydraulic Data book. Reproduced with permission of Ingersoll-Dresser Pump Company.)

Table 1-3. Properties of saturated steam -pressure table (Courtesy, Cameron Hydraulic Data book. Reproduced with permission of Ingersoll-Dresser Pump Company.)

Table 1-4. Properties of superheated steam (Courtesy, Cameron Hydraulic Data book. Reproduced with permission of Ingersoll-Dresser Pump Company.)

The tables start at the liquid phase at the triple point of water, which is where water can exist as a solid, liquid, and vapor simultaneously. At this point, the specific internal energy and the specific entropy are each exactly zero.

The tables go up to the critical temperature and pressure, which are 705.47°F and 3,208.2 psia, respectively. At these points, the figures for specific volume, enthalpy, and entropy are the same for liquid as they are for vapor.

The explanations for the subscript abbreviations are as follows:

In the superheated steam table (Table 1-4), the top row of temperatures represent the total steam temperature, while the temperatures to the left of the abbreviation sh show the amount the steam is superheated.

TEMPERATURE

In addition to absolute pressure, another