I85PREP
⌘ K
Power Eng 4th Class 4B study materials
Power Eng 4th Class 4B study resources

Power Eng 4th Class 4B practice questions

One question at a time. Build your Power Eng 4th Class 4B understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Boilers and Boiler Fittings

A boiler operator notices the water level glass showing a steady level, but the try-cocks below the glass indicate water is present. What is the most appropriate first action before trusting the glass reading?

  1. Open the bottom blowdown valve on the boiler for two minutes
  2. Close the steam drum vent to pressurize the glass
  3. Blow down the water column and glass, then confirm the level returns promptly Correct answer.
  4. Increase firing rate to raise the water level in the glass

Explanation

The gauge glass and water column must be proven clear before the indicated level is trusted, because sludge or blockage can give a false steady reading. Blowing down the column and glass and watching for a prompt, correct return confirms the connections are open. Firing changes or long bottom blows would not verify the glass and could create hazardous conditions.

Question 2Boilers and Boiler Fittings

On a fire-tube boiler, the main purpose of the fusible plug is to:

  1. Warn of low water by releasing steam to the firebox
  2. Drain condensate from the steam header
  3. Provide a soft-metal relief point that melts when the crown sheet is uncovered Correct answer.
  4. Indicate the boiler pressure on a remote gauge

Explanation

The fusible plug contains a low-melting alloy that fails when the metal around it overheats, typically because the crown sheet has been uncovered by low water. Its melting releases steam and water toward the fire side, giving an audible warning and reducing further overheating. It is a last-resort protective device, not a gauge or drain fitting.

Question 3Boilers and Boiler Fittings

The safety valve on a boiler is set to lift at the maximum allowable working pressure. What is the primary function of this valve?

  1. To maintain steady steam pressure for the plant
  2. To admit air during cooldown
  3. To drain water from the steam space
  4. To discharge steam and prevent pressure from exceeding the allowable limit Correct answer.

Explanation

A boiler safety valve is the final overpressure protection device and must open at or below the maximum allowable working pressure to prevent an unsafe pressure rise. It is not a regulating valve and does not control normal operating pressure. It discharges steam, not water or air.

Question 4Boilers and Boiler Fittings

A low-water fuel cutoff on a steam boiler should be tested by:

  1. Closing the steam outlet valve and watching pressure rise
  2. Opening the safety valve by hand
  3. Lowering the water level in the gauge glass to the cutoff point and observing the burner shut down Correct answer.
  4. Raising the water level above normal and checking for carryover

Explanation

The low-water cutoff must be proven to trip the fuel supply when the water level falls to the cutoff point. The accepted field test lowers the level in the float chamber or gauge glass and confirms the burner stops. Pressure, safety valve, or high-level checks do not test the low-water trip function.

Question 5Boilers and Boiler Fittings

Which fitting is installed on a boiler to allow visual confirmation of the water level when the gauge glass is not readable?

  1. Blowdown valve
  2. Air cock
  3. Try-cocks Correct answer.
  4. Feedwater regulator

Explanation

Try-cocks are small valves mounted at the normal water level and at points above and below it, allowing the operator to confirm level by observing water or steam discharge. They are a backup to the gauge glass. Blowdown valves, air cocks, and feedwater regulators serve different purposes.

Question 6Boilers and Boiler Fittings

The main steam stop valve on a boiler is opened slowly during warm-up primarily to:

  1. Prevent thermal shock and water hammer in the steam line Correct answer.
  2. Test the low-water cutoff
  3. Increase the boiler's maximum allowable working pressure
  4. Reduce the safety valve set pressure

Explanation

Opening the main steam stop slowly allows the steam line to warm gradually, limiting thermal stress and avoiding water hammer from rapid steam admission against condensate. It does not change the safety valve setting or the boiler's allowable pressure. Low-water cutoff testing is unrelated.

Question 7Boilers and Boiler Fittings

A steam boiler is fitted with a water column. What is the function of this component?

  1. To separate moisture from steam before the superheater
  2. To store feedwater for sudden demand
  3. To collect scale from the boiler water
  4. To steady the water level in the gauge glass and reduce turbulence so a true level is shown Correct answer.

Explanation

The water column provides a stilling chamber that dampens turbulence and surging in the boiler so the gauge glass shows a representative level. It also provides a mounting point for level controls and try-cocks. It is not a feedwater store, a steam separator, or a scale collector.

Question 8Boilers and Boiler Fittings

The gauge glass on a boiler is connected to the steam space and the water space. Why must both connections be kept clear?

  1. To allow the glass to be drained completely during firing
  2. To ensure the glass reflects the true boiler water level under pressure Correct answer.
  3. To permit steam to escape from the glass during operation
  4. To allow the glass to act as a pressure relief point

Explanation

The glass indicates level only because both the steam and water connections are open, so pressure is equalized and water rises in the glass to the boiler level. A blocked connection causes a false reading, which is dangerous. The glass is not a drain, vent, or relief device.

Question 9Boilers and Boiler Fittings

During a routine bottom blowdown, the operator should watch the gauge glass closely. What is the main reason?

  1. To test the safety valve setting
  2. To confirm the blowdown line is discharging and to stop before the level falls too far Correct answer.
  3. To increase the boiler pressure quickly
  4. To warm the feedwater line

Explanation

Watching the glass during bottom blowdown confirms that water is actually leaving the boiler and lets the operator stop before the level drops below a safe point. Losing too much water can expose heating surfaces. Blowdown does not raise pressure, test the safety valve, or warm the feedwater line.

Question 10Boilers and Boiler Fittings

A boiler has two safety valves. If the valves are of unequal size, how should they normally be arranged?

  1. The larger valve set at the maximum allowable pressure and the smaller set slightly lower Correct answer.
  2. The smaller valve set at the maximum allowable pressure and the larger set lower
  3. Only one valve left connected and the other blanked off
  4. Both set to the same pressure and lifted together

Explanation

Where two safety valves of unequal size are fitted, the larger valve is set at the maximum allowable working pressure and the smaller valve is set at a slightly lower pressure. This gives staged relief and ensures the smaller valve opens first. Both valves must remain connected and functional.

Question 11Boilers and Boiler Fittings

An operator suspects the gauge glass is showing a false high level because the water connection is partially plugged. Which action best confirms the actual level?

  1. Close the steam connection to the glass and read the level
  2. Reduce firing rate and wait for the glass to drop
  3. Blow down the glass and observe whether the level returns and fluctuates with firing Correct answer.
  4. Increase feedwater flow to see if the level rises

Explanation

Blowing down the glass clears the connections and lets the operator see whether water returns promptly and responds to boiler conditions. A partially plugged water connection often shows a sluggish or stuck level. Closing a connection defeats the purpose of the glass, and changing feed or firing does not prove the glass is clear.

Question 12Boilers and Boiler Fittings

A steam boiler is equipped with a high-water-level alarm. What is the principal hazard this alarm helps the operator avoid?

  1. Loss of flame in the burner
  2. Overheating of the crown sheet
  3. Water carryover into the steam line Correct answer.
  4. Excessive blowdown rate

Explanation

Excessive water level allows water to be carried over with steam, which can damage turbines, piping, and process equipment and cause water hammer. A high-level alarm warns the operator to reduce feed and correct the level. Crown sheet overheating is associated with low water, and flame loss or blowdown rate are separate concerns.

Question 13Boilers and Boiler Fittings

Why is a siphon or pigtail fitted between the boiler and a pressure gauge?

  1. To increase the gauge reading accuracy at high pressure
  2. To allow the gauge to be used as a relief valve
  3. To trap condensate and protect the gauge from live steam temperature Correct answer.
  4. To reduce the boiler pressure during firing

Explanation

The siphon loop collects condensate so the Bourdon tube sees water rather than high-temperature steam, protecting the gauge mechanism. It does not change calibration, act as a relief device, or affect boiler pressure. A gauge cock is used for isolation and testing.

Question 14Boilers and Boiler Fittings

A boiler operator observes the water level in the gauge glass rising and falling in step with the burner cycling. What is the most likely explanation?

  1. The glass connections are fully blocked
  2. The feedwater pump has stopped
  3. Normal level surging caused by firing changes and steam demand Correct answer.
  4. The safety valve is leaking

Explanation

Level surging that follows firing cycles is commonly caused by changing steam generation and demand, which temporarily swells or collapses the level. This is normal to a degree, but persistent severe surging may indicate carryover or control issues. Blocked connections would give a stuck reading, and a stopped pump would cause a falling level.

Question 15Boilers and Boiler Fittings

What is the purpose of the slow-opening characteristic built into many large safety valves?

  1. To allow the valve to be used as a pressure regulator
  2. To lower the set pressure automatically as load increases
  3. To reduce the pressure surge and chatter when the valve lifts and reseats Correct answer.
  4. To permit manual lifting during normal operation

Explanation

Slow-opening or easing gear designs reduce the sudden pressure surge and valve chatter that can occur when a large valve lifts and reseats. They improve stability and reduce wear on the seat. They do not convert the valve into a regulator, change set pressure with load, or encourage routine manual lifting.

Question 16Boilers and Boiler Fittings

A boiler is being placed in service after a shutdown. Before lighting off, the operator confirms the drum vent is open. Why is this important?

  1. To allow air to escape and prevent a vacuum or pressure buildup during warm-up Correct answer.
  2. To increase the steam pressure faster
  3. To allow the safety valve to be tested
  4. To permit blowdown through the vent line

Explanation

With the vent open, air can escape as the boiler warms and steam forms, and the vent prevents a vacuum from forming during later cooling. Leaving it open avoids pressure buildup from trapped air expanding. It does not speed pressurization, test the safety valve, or serve as a blowdown path.

Question 17Boilers and Boiler Fittings

An operator finds the water level in the gauge glass difficult to read and the glass appears dirty internally. What is the correct procedure?

  1. Increase the boiler pressure to clear the glass
  2. Blow down the glass, then isolate, drain, and clean or replace the glass with the boiler secured as required Correct answer.
  3. Open the drain on the glass and leave it open during operation
  4. Close both glass connections and continue firing

Explanation

The glass should first be blown down to clear loose deposits; if it remains obscured, the glass must be isolated, drained, and cleaned or replaced under the proper safe procedure. Operating with an unreadable glass is unacceptable. Leaving the drain open or raising pressure does not correct the problem.

Question 18Boilers and Boiler Fittings

A boiler is fitted with a low-water cutoff and a separate low-water alarm. Why are both devices provided?

  1. They perform identical functions and provide redundancy only
  2. The cutoff warns the operator, while the alarm stops the fuel
  3. The alarm controls feedwater while the cutoff controls firing
  4. The alarm warns the operator, while the cutoff independently stops fuel to prevent overheating Correct answer.

Explanation

The low-water alarm alerts the operator to an abnormal level, while the low-water cutoff independently shuts off fuel to protect heating surfaces if the level continues to fall. The two functions are distinct: warning versus automatic trip. They are not interchangeable, and neither controls feedwater.

Question 19Boilers and Boiler Fittings

Which condition would cause a safety valve to simmer or leak below its set pressure?

  1. Correctly installed easing gear
  2. A gauge with a siphon loop
  3. A perfectly adjusted set pressure with a clean seat
  4. Foreign material on the seat or a damaged seating surface Correct answer.

Explanation

Simmering or leakage below set pressure usually indicates a seat that is not tight, often from scale, dirt, or damage on the seating faces. A clean, properly adjusted valve holds tight until set pressure. Easing gear and gauge siphons do not cause leakage.

Question 20Boilers and Boiler Fittings

The water level in a boiler is found to be below the bottom of the visible gauge glass. What is the correct operator response?

  1. Open the bottom blowdown to check for water
  2. Secure the fuel immediately and follow the low-water emergency procedure Correct answer.
  3. Add feedwater slowly and continue firing
  4. Increase firing to raise steam pressure

Explanation

A level below the visible glass is a low-water emergency because heating surfaces may be uncovered. Fuel must be secured immediately and the approved emergency procedure followed; feedwater may be added only as the procedure directs. Blowing down or increasing firing would worsen the hazard.

Question 21Boilers and Boiler Fittings

Why must the blowdown valve on a gauge glass be opened with care and closed firmly after use?

  1. To drain the steam drum completely
  2. To increase the boiler pressure during the test
  3. To reset the safety valve
  4. To prevent erosion of the glass fittings and avoid a steam or water leak at the glass Correct answer.

Explanation

The small blowdown valve and its seat are easily eroded by high-velocity steam and water, so it must be opened carefully and closed firmly to prevent leakage at the glass. A leaking glass valve gives a false reading and creates a hazard. The test does not affect boiler pressure, the safety valve, or drain the drum.

Question 22Boilers and Boiler Fittings

A boiler's safety valve is found to have a set pressure higher than the maximum allowable working pressure stamped on the boiler. What is the correct action?

  1. Reduce the boiler operating pressure to match the valve
  2. Have the valve adjusted and resealed by qualified personnel to the correct set pressure Correct answer.
  3. Blank off the valve and rely on the low-water cutoff
  4. Leave it as is because higher set pressure gives more margin

Explanation

The safety valve set pressure must not exceed the boiler's maximum allowable working pressure, so the valve must be adjusted and resealed by qualified personnel. Leaving it high removes overpressure protection. Reducing operating pressure does not correct the valve setting, and blanking off the valve is prohibited.

Question 23Boilers and Boiler Fittings

During warm-up of a cold boiler, the gauge glass shows a level that gradually rises even though no feedwater is being added. Which explanation is most consistent with this observation?

  1. Water in the boiler is expanding and steam bubbles are forming as the contents heat Correct answer.
  2. The feedwater pump is running at full capacity
  3. The safety valve is leaking steam
  4. The blowdown valve is stuck open

Explanation

As a cold boiler warms, the water expands and steam bubbles form within the water, both of which raise the apparent level in the glass without any feedwater addition. A leaking safety valve or open blowdown would tend to lower the level, and a running feed pump would be adding water.

Question 24Boilers and Boiler Fittings

A boiler is fitted with a pressure gauge, a safety valve, and a low-water cutoff. Which statement best describes how these fittings work together to protect the boiler?

  1. The pressure gauge trips the fuel supply when pressure is high
  2. The low-water cutoff adjusts the safety valve set pressure
  3. The safety valve warns of low water while the cutoff relieves pressure
  4. Each device operates independently, so failure of one does not affect the protection given by the others Correct answer.

Explanation

These protective fittings are independent: the gauge indicates pressure, the safety valve relieves overpressure, and the low-water cutoff trips fuel on low level. Independence is important so a single failure does not remove all protection. The devices do not perform each other's functions.

Question 25Boilers and Boiler Fittings

Why is the water column blown down regularly on a steam boiler?

  1. To reduce the boiler's maximum allowable working pressure
  2. To increase the feedwater temperature
  3. To keep the steam and water connections clear so the gauge glass and level controls read correctly Correct answer.
  4. To test the safety valve lifting pressure

Explanation

Regular blowdown of the water column clears sludge and sediment from the connections so the gauge glass and level controls respond to the true boiler level. Blocked connections cause false readings and can prevent the low-water cutoff from operating. Blowdown does not change allowable pressure, test the safety valve, or heat feedwater.

Question 26Boilers and Boiler Fittings

On a firetube boiler, what is the primary purpose of the fusible plug installed in the crown sheet or upper tubesheet?

  1. To drain condensate from the furnace during a cold start-up
  2. To act as a secondary pressure relief device if the safety valve fails to lift
  3. To melt and release a warning of steam when the metal overheats from low water Correct answer.
  4. To allow air to escape while the boiler is being filled with feedwater

Explanation

A fusible plug contains a low-melting alloy that softens and blows out when the surrounding metal becomes overheated, typically because water has dropped below the crown sheet. The escaping steam and water give the operator a loud warning and can quench the hot metal. It is a low-water warning device, not a pressure relief or vent fitting.

Question 27Boilers and Boiler Fittings

A boiler operator notes that the water in the gauge glass appears still and clear, while the actual drum level is known to be higher. Which fitting is most likely to have a restricted steam-space connection?

  1. The main steam non-return valve
  2. The water column drain valve
  3. The feedwater check valve
  4. The gauge glass steam connection Correct answer.

Explanation

The gauge glass indicates level only when steam pressure is free to act on the top connection and water pressure on the bottom connection. A plugged or restricted steam connection traps water or condensate in the upper fitting, so the glass reads lower than the true drum level. The glass must be blown down and the connections proven clear before the reading is trusted.

Question 28Boilers and Boiler Fittings

What is the effect of fitting a baffle or deflector plate above the boiler steam outlet nozzle?

  1. It increases the steam velocity through the nozzle to improve carryover
  2. It prevents the boiler from being blown down while under load
  3. It separates moisture droplets from the steam before the steam leaves the drum Correct answer.
  4. It raises the safety valve set pressure by reducing drum pressure

Explanation

A baffle or deflector plate changes the direction of steam flow so entrained water droplets impinge on the plate and drain back to the drum. This reduces carryover, which protects superheaters, turbines, and steam lines from water hammer and deposits. It does not change pressure settings or blowdown capability.

Question 29Boilers and Boiler Fittings

A boiler is fitted with a high-pressure alarm on the steam drum in addition to the safety valve. What is the correct interpretation of this arrangement?

  1. The alarm is set below the safety valve set pressure to warn the operator before the valve lifts Correct answer.
  2. The alarm replaces the safety valve on small boilers where a relief valve cannot be fitted
  3. The alarm is set above the safety valve set pressure to confirm the valve opened
  4. The alarm is set at exactly the same pressure as the safety valve to give a duplicate signal

Explanation

A high-pressure alarm is a warning device, so it is set below the safety valve lifting pressure to give the operator time to reduce firing before relief occurs. The safety valve remains the final overpressure protection and is set at or below the maximum allowable working pressure. An alarm set above the safety valve setpoint would serve no protective purpose.

Question 30Boilers and Boiler Fittings

Why must a boiler superheater be drained and vented before being filled with water during start-up?

  1. To increase the steam temperature produced during the first hour of operation
  2. To allow the superheater to be chemically cleaned while the boiler is firing
  3. To reduce the pressure at which the safety valve on the superheater outlet lifts
  4. To ensure the superheater tubes fill completely and do not trap air that would cause overheating Correct answer.

Explanation

Air trapped in superheater tubes during filling can prevent cooling steam flow and cause local overheating of the tube metal. Draining and venting the superheater before filling ensures it fills with water and that air is displaced. Once firing begins, steam flow then provides continuous cooling of the superheater tubes.

Question 31Boiler Combustion and Draft Systems

In a boiler furnace, what is the primary purpose of supplying excess air beyond the theoretical air requirement?

  1. To ensure complete combustion of the fuel by supplying enough oxygen to every fuel molecule Correct answer.
  2. To increase the dew point of the flue gas so that stack corrosion is reduced
  3. To reduce the volume of flue gas passing through the economizer
  4. To lower the furnace temperature so that tube overheating is prevented

Explanation

Perfect mixing of fuel and air never occurs in a real furnace, so some air must pass through without contacting fuel. Excess air provides the reserve oxygen needed to burn the fuel that would otherwise leave as CO or unburned carbon. The amount is kept just high enough for complete combustion because extra air carries heat up the stack.

Question 32Boiler Combustion and Draft Systems

A boiler is firing natural gas and the flue gas analysis shows 3% oxygen with no measurable carbon monoxide. What does this indicate about the combustion air setting?

  1. The furnace is operating with insufficient draft to support combustion
  2. Excess air is higher than necessary and stack heat loss is being wasted
  3. Air supply is close to the minimum for complete combustion with little surplus Correct answer.
  4. The burner is starved of air and producing CO that the analyser cannot detect

Explanation

Low oxygen with zero CO means combustion is essentially complete but there is very little oxygen left over, so the air setting is near the minimum. This is efficient but leaves little margin for load swings or burner adjustment. Operators normally keep a small safety margin of oxygen to avoid CO formation.

Question 33Boiler Combustion and Draft Systems

Which condition causes the greatest loss of boiler efficiency through the stack?

  1. Low feedwater temperature entering the economizer
  2. High excess air combined with high flue gas temperature Correct answer.
  3. A small amount of CO in the flue gas
  4. Slightly low furnace draft

Explanation

Stack loss is the product of the mass of flue gas leaving and its temperature above ambient. Excess air adds mass, and a high exit temperature adds energy per unit mass, so together they produce the largest avoidable loss. Reducing either one cuts the loss.

Question 34Boiler Combustion and Draft Systems

What is the function of the forced draft fan in a balanced draft boiler?

  1. To supply combustion air to the windbox at a pressure above atmospheric Correct answer.
  2. To deliver secondary air directly to the overfire air ports only
  3. To circulate flue gas back into the furnace to control steam temperature
  4. To pull flue gas from the furnace and maintain a negative pressure in the setting

Explanation

The forced draft fan takes ambient air and raises its pressure so it can be pushed through the air preheater and into the windbox. In a balanced draft unit the induced draft fan handles the downstream side and holds the furnace slightly negative. The two fans work together to establish the pressure balance across the furnace.

Question 35Boiler Combustion and Draft Systems

A boiler furnace is being held at a steady negative pressure of about 0.5 kPa (2 in. water). What is the main reason for maintaining this slightly negative pressure?

  1. To reduce the speed required from the forced draft fan
  2. To prevent hot combustion gases from escaping through openings in the setting Correct answer.
  3. To increase the oxygen content of the flue gas
  4. To raise the furnace temperature above the ash fusion point

Explanation

A slightly negative furnace pressure means the pressure inside is below the surrounding atmosphere, so any leakage flows inward rather than outward. This protects personnel and equipment from hot gas and keeps the boiler room free of combustion products. Too much negative pressure, however, wastes heat by drawing in cold air.

Question 36Boiler Combustion and Draft Systems

During a sudden increase in steam demand, the furnace pressure rises toward positive. Which control action restores the balanced draft condition?

  1. Reduce the fuel flow to the burners immediately
  2. Increase the induced draft fan speed to match the higher gas flow Correct answer.
  3. Reduce the induced draft fan speed and increase the forced draft fan speed
  4. Close the air preheater bypass damper

Explanation

A rising furnace pressure means the gas leaving the furnace is being restricted relative to the gas being produced. The induced draft control must increase fan speed or damper opening to remove the extra gas and pull the pressure back to its setpoint. Forced draft and fuel flow follow the demand signal separately.

Question 37Boiler Combustion and Draft Systems

What is the theoretical (stoichiometric) air requirement for combustion?

  1. The minimum air needed to burn all the fuel completely with no oxygen left over Correct answer.
  2. The air quantity that produces 3% oxygen in the flue gas
  3. The air flow measured at the forced draft fan outlet
  4. The air actually supplied to the burners at full load

Explanation

Theoretical air is calculated from the fuel's chemical composition and represents exactly enough oxygen to convert all carbon, hydrogen and sulphur to their fully oxidised forms. It contains no surplus oxygen, so in practice more than theoretical air must be supplied. The ratio of actual to theoretical air is the excess air factor.

Question 38Boiler Combustion and Draft Systems

A burner produces a long, lazy, yellow-orange flame that impinges on the rear wall. Which adjustment is most likely to correct this condition?

  1. Lower the furnace draft to a smaller negative value
  2. Increase the secondary air and improve fuel atomisation Correct answer.
  3. Reduce the atomising steam or air supply to the burner
  4. Increase the fuel pressure to the burner

Explanation

A long, lazy, luminous flame indicates poor fuel-air mixing and incomplete combustion, often from coarse atomisation or too little secondary air. Increasing secondary air and improving atomisation shorten the flame and complete combustion before it reaches the wall. Flame impingement can cause tube overheating and slagging if left uncorrected.

Question 39Boiler Combustion and Draft Systems

Why must the furnace be purged with air before a burner is ignited?

  1. To remove any accumulated combustible gases that could ignite explosively Correct answer.
  2. To warm the furnace refractory gradually to prevent thermal shock
  3. To raise the oxygen level in the flue gas to the normal operating value
  4. To cool the burner tip before fuel is admitted

Explanation

Unburned fuel can accumulate in the furnace and gas passages during a shutdown or a failed light-off. If a burner is then lit, the accumulated mixture can ignite as a furnace explosion. A purge of several volume changes of air removes these combustibles before ignition is attempted.

Question 40Boiler Combustion and Draft Systems

A flame scanner on a gas burner loses its signal for several seconds while the burner is firing. What is the correct response of the burner management system?

  1. Trip the fuel supply to the burner and require a purge before relighting Correct answer.
  2. Switch automatically to the standby burner without any fuel interruption
  3. Continue firing and alarm the operator only
  4. Increase the fuel flow to re-establish a stable flame

Explanation

Loss of flame signal means the system can no longer prove that combustion is occurring, so fuel must be shut off promptly to prevent an explosive accumulation. The interlock requires a purge and a fresh light-off sequence before fuel is readmitted. Flame failure response is one of the most important safety functions on a boiler.

Question 41Boiler Combustion and Draft Systems

Which statement best describes the relationship between excess air and carbon monoxide in flue gas?

  1. CO rises steadily as excess air is increased from low to high values
  2. CO decreases as the furnace temperature falls, regardless of air supply
  3. CO is independent of excess air and depends only on fuel sulphur content
  4. As excess air falls below the level needed for good mixing, CO rises sharply Correct answer.

Explanation

When air supply or mixing becomes insufficient, some carbon cannot find oxygen and leaves as CO instead of CO2. This produces a sharp rise in CO as the excess air drops toward and below the practical minimum. Operating with a small margin above that point keeps CO low while limiting stack loss.

Question 42Boiler Combustion and Draft Systems

What is the purpose of the air preheater in the flue gas path of a boiler?

  1. To remove particulates from the flue gas before the induced draft fan
  2. To cool the flue gas below its acid dew point before it reaches the stack
  3. To add moisture to the combustion air for better mixing
  4. To transfer heat from the leaving flue gas to the incoming combustion air Correct answer.

Explanation

The air preheater recovers sensible heat from the flue gas and gives it to the combustion air entering the furnace. This lowers the stack temperature and improves boiler efficiency while also helping combustion. It must be operated so the metal temperature stays above the acid dew point to limit corrosion.

Question 43Boiler Combustion and Draft Systems

A boiler burning heavy fuel oil shows a rising flue gas dew point and corrosion on the cold end of the air preheater. Which change in operation would best reduce the corrosion?

  1. Increase the induced draft fan speed to sweep the gas through faster
  2. Reduce the fuel oil temperature at the burner
  3. Lower the excess air substantially to reduce gas volume
  4. Raise the combustion air inlet temperature or bypass part of the air Correct answer.

Explanation

Cold-end corrosion occurs when the metal surface falls below the acid dew point of the flue gas. Raising the incoming air temperature keeps the metal above that point, and an air bypass achieves the same effect at low load. This is why preheaters are fitted with steam air heaters or bypass arrangements.

Question 44Boiler Combustion and Draft Systems

Which parameter is normally used to express the draft produced by a boiler fan?

  1. Cubic metres per second of volumetric flow
  2. Kilopascals of static pressure measured in millimetres or inches of water Correct answer.
  3. Degrees Celsius of temperature rise across the fan
  4. Kilowatts of shaft power at the fan coupling

Explanation

Draft is a very small pressure difference, so it is measured in units of water column such as millimetres or inches of water. A typical furnace draft is only a fraction of a kilopascal. Flow and power are separate fan characteristics used in different calculations.

Question 45Boiler Combustion and Draft Systems

A boiler is operating with 12% excess air but the flue gas oxygen analyser reads 6%. Which explanation is most consistent with these readings?

  1. The fuel flow meter is over-reading the actual fuel burned
  2. Air is leaking into the flue gas path downstream of the furnace Correct answer.
  3. The analyser is correct and the excess air calculation is wrong
  4. The burners are producing large amounts of carbon monoxide

Explanation

Tramp air entering through leaks in the convection pass, economizer or ductwork raises the measured oxygen without taking part in combustion. The true excess air at the furnace is therefore lower than the analyser suggests. Leak surveys and sealing the setting restore the reading's usefulness for combustion control.

Question 46Boiler Combustion and Draft Systems

What is the main function of overfire air in a stoker-fired or spreader-stoker furnace?

  1. To cool the grate surface and prevent clinker formation
  2. To supply all the oxygen required by the fuel bed
  3. To burn the volatile gases and fine particles carried above the fuel bed Correct answer.
  4. To provide the primary air that passes up through the grate

Explanation

Under-grate air burns the fuel on the bed, but volatile matter and fine particles are released above it and need additional oxygen. Overfire air is injected above the bed to complete combustion of these gases and reduce smoke and CO. Proper overfire air also helps shape the flame in the furnace.

Question 47Boiler Combustion and Draft Systems

A pulverised coal boiler shows high unburned carbon in the fly ash along with a high furnace exit gas temperature. Which combination of changes should be investigated first?

  1. Reduce the coal fineness and lower the primary air temperature
  2. Lower the furnace draft and increase the overfire air
  3. Increase the coal feed rate and reduce secondary air
  4. Improve coal fineness and adjust the air-fuel distribution to the burners Correct answer.

Explanation

Coarse pulverised coal burns slowly and can leave the furnace before it is consumed, raising unburned carbon. Poor air-fuel distribution between burners also creates fuel-rich zones that produce unburned carbon. Improving fineness and balancing the burner air and fuel are the first steps before changing firing rate.

Question 48Boiler Combustion and Draft Systems

Which device is used to measure the small negative pressure in a boiler furnace and is typically connected to a draft gauge?

  1. A thermocouple in the superheater outlet header
  2. An orifice plate in the main steam line
  3. A sensing tap in the furnace roof or side wall connected by tubing Correct answer.
  4. A pitot tube in the forced draft fan discharge

Explanation

Furnace draft is sensed through a tap in the furnace enclosure and transmitted by tubing to a draft gauge or transmitter. The gauge compares furnace pressure with atmosphere to show the small negative value. This signal is used by the draft control system to position the induced draft fan.

Question 49Boiler Combustion and Draft Systems

In a balanced draft boiler, the forced draft fan and induced draft fan are interlocked so that the induced draft fan must be running before the forced draft fan can start. Why is this interlock provided?

  1. To prevent the furnace from being pressurised and forcing hot gas out through openings Correct answer.
  2. To ensure the air preheater reaches operating temperature before firing
  3. To guarantee that the flue gas oxygen analyser is calibrated
  4. To reduce the starting current drawn by the forced draft fan motor

Explanation

If the forced draft fan ran alone, it would push air into a furnace with no path for the gas to leave, raising the pressure and forcing hot gas and flame out of openings. Starting the induced draft fan first establishes a flow path and a negative pressure. The interlock is therefore a personnel and equipment protection measure.

Question 50Boiler Combustion and Draft Systems

A boiler flue gas analysis gives 4% O2 and 1200 ppm CO at high load. Which action is most appropriate before making any other change?

  1. Reduce the induced draft fan speed to hold gas in the furnace longer
  2. Increase the fuel firing rate to raise furnace temperature
  3. Check burner air distribution and mixing, then adjust air to bring CO down Correct answer.
  4. Reduce excess air further to lower the stack loss

Explanation

Elevated CO with moderate oxygen points to poor mixing or maldistribution rather than an overall air shortage. Correcting burner air registers and distribution allows CO to be burned out without simply adding more excess air. Only after mixing is corrected should the oxygen setpoint be trimmed for efficiency.