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NIULPE Class 3 practice questions

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Question 1Boiler Construction, Design, and Fittings

In a fire-tube boiler, which component forms the pressure boundary that surrounds the hot gases and water volume?

  1. The superheater outlet header
  2. The shell Correct answer.
  3. The air preheater basket
  4. The economizer fin tube

Explanation

The shell is the cylindrical pressure vessel that contains the water and steam space and encloses the fire tubes; it is the primary pressure boundary of a fire-tube boiler. Headers, economizer tubes, and air preheater elements are associated with water-tube or heat-recovery equipment rather than enclosing the fire-tube gas path.

Question 2Boiler Construction, Design, and Fittings

A boiler drum is fitted with a safety valve whose set pressure exceeds the maximum allowable working pressure of the drum. Which code-based consequence applies?

  1. The drum is overprotected and the valve may be reset lower without review
  2. The valve setting is acceptable if the drum is hydrotested at that pressure
  3. The arrangement is unacceptable because no component may be protected above its MAWP Correct answer.
  4. The valve protects the drum because it opens at a higher pressure

Explanation

Every pressure part must be protected at or below its maximum allowable working pressure, so a safety valve set above the drum MAWP leaves the drum unprotected in the range between MAWP and the set point. A hydrotest demonstrates strength at a test pressure but does not substitute for correct relief settings. The valve must be set so that the drum is protected at or below its MAWP.

Question 3Boiler Construction, Design, and Fittings

Which stay type is used to tie a flat tubeplate to the shell of a fire-tube boiler?

  1. Girder stay
  2. Longitudinal staybolt
  3. Through stay Correct answer.
  4. Diagonal stay

Explanation

A through stay passes from one flat plate to the opposite plate or shell and carries the pressure load in tension, which is the usual arrangement for supporting flat tubeplates. Girder and diagonal stays brace flat surfaces to adjacent structure, and longitudinal staybolts tie flat end plates over short spans.

Question 4Boiler Construction, Design, and Fittings

Why is a fusible plug installed in the crown sheet area of a fire-tube boiler?

  1. To indicate flame failure in the combustion chamber
  2. To warn of low water by melting and releasing steam into the furnace Correct answer.
  3. To relieve excess steam pressure at the set point
  4. To drain condensate from the steam space

Explanation

A fusible plug contains a low-melting alloy that melts when the crown sheet is uncovered by water and overheats, releasing steam into the furnace as an audible warning and reducing heat input. It is a low-water warning device, not a pressure relief or drain fitting.

Question 5Boiler Construction, Design, and Fittings

A water-tube boiler has a generating tube bank with tubes expanded into drums. Which condition most directly reduces tube-to-drum joint integrity?

  1. Normal soot blowing of the tube bank
  2. Repeated thermal cycling causing differential expansion at the rolled joint Correct answer.
  3. Slight steam blanketing in the upper drum
  4. A rise in feedwater temperature within the design range

Explanation

Rolled tube joints depend on maintained residual contact stress; repeated thermal cycling causes differential expansion between tube and drum that can relax or work the joint loose, leading to leakage. Steam blanketing, moderate feedwater temperature changes, and routine soot blowing do not directly attack the expanded joint in the same way.

Question 6Boiler Construction, Design, and Fittings

Which fitting is required on a boiler to indicate the water level directly at the operating platform?

  1. Feed check valve
  2. Gauge glass Correct answer.
  3. Try cock
  4. Blowdown valve

Explanation

The gauge glass provides a direct visual indication of water level and is a required fitting on boilers so the operator can confirm level against other indicators. Try cocks are an auxiliary check, blowdown valves remove sludge, and feed check valves control feedwater flow direction.

Question 7Boiler Construction, Design, and Fittings

A boiler is fitted with two safety valves on the same drum. Which arrangement satisfies the intent of protecting the drum across the full range of firing?

  1. One valve set at MAWP and the other set slightly higher but not above the permitted accumulation Correct answer.
  2. One valve locked closed as a spare
  3. Both valves set at the same pressure to double capacity
  4. Both valves set above MAWP to avoid nuisance lifting

Explanation

Staggering the set points allows one valve to open first and the second to open if pressure continues to rise, while the highest setting still respects the accumulation limit above MAWP. Setting both valves above MAWP leaves the drum unprotected, and locking a valve closed removes its protective function.

Question 8Boiler Construction, Design, and Fittings

What is the primary purpose of a superheater in a boiler design?

  1. To condense exhaust steam after the turbine
  2. To preheat combustion air before the burner
  3. To raise steam temperature above saturation for the same pressure Correct answer.
  4. To remove dissolved oxygen from feedwater

Explanation

A superheater adds heat to saturated steam to raise its temperature above the saturation temperature at the drum pressure, which improves cycle efficiency and reduces condensation in the turbine. Deaeration, air preheating, and condensing are functions of other equipment.

Question 9Boiler Construction, Design, and Fittings

A boiler feedwater line is fitted with a check valve and a stop valve. In what order should they be arranged relative to the boiler, and why?

  1. Stop valve only, with the check valve omitted because the feed pump has its own check
  2. Stop valve nearest the boiler, check valve upstream, to allow isolation for maintenance
  3. Check valve nearest the boiler, stop valve upstream, so the check prevents backflow into the feed line Correct answer.
  4. Check valve only, with the stop valve omitted to reduce pressure loss

Explanation

The check valve is placed closest to the boiler so that it stops boiler water from flowing back into the feed line if feed pressure is lost; the stop valve upstream allows the line to be isolated for maintenance. Reversing the order leaves a path for backflow when the stop valve is open.

Question 10Boiler Construction, Design, and Fittings

Which material property is most important when selecting plate for a boiler shell that will operate at elevated temperature?

  1. High electrical resistivity
  2. Low hardness for ease of rolling
  3. High thermal conductivity at room temperature
  4. Adequate creep strength at the design metal temperature Correct answer.

Explanation

At elevated temperature the shell must resist time-dependent deformation, so the allowable stress is governed by creep strength at the design metal temperature. Room-temperature conductivity, hardness, and resistivity do not control the pressure-retaining capability of the shell at temperature.

Question 11Boiler Construction, Design, and Fittings

A boiler is described as having a 'D' configuration. Which arrangement does this describe?

  1. A water-tube boiler with a furnace bounded by tubes on one side and a convection bank on the other Correct answer.
  2. A fire-tube boiler with a central furnace tube
  3. A packaged boiler with a single-pass fire tube
  4. A waste-heat boiler with no furnace

Explanation

The D-type is a water-tube packaged boiler in which the furnace is enclosed by membrane or tangent tubes on one side and the convection tube bank forms the other leg, giving a D-shaped cross section. Fire-tube and waste-heat arrangements do not match this geometry.

Question 12Boiler Construction, Design, and Fittings

Which condition would most likely cause caustic embrittlement cracking in a riveted boiler seam?

  1. Low pH with dissolved oxygen present
  2. Concentrated caustic soda at a seam where water can leak and evaporate Correct answer.
  3. High silica with low hardness
  4. High chloride content with no alkalinity

Explanation

Caustic embrittlement occurs when concentrated sodium hydroxide collects in crevices such as riveted seams where small leaks allow evaporation and concentration, attacking stressed metal. Chloride, low-pH oxygen attack, and silica scaling produce different damage mechanisms.

Question 13Boiler Construction, Design, and Fittings

What is the function of a manhole in a boiler drum?

  1. To vent steam during warm-up
  2. To provide access for internal inspection and cleaning Correct answer.
  3. To admit feedwater into the drum
  4. To relieve pressure during overfire conditions

Explanation

A manhole is an access opening large enough for a person to enter the drum for inspection, cleaning, and repair; it is closed with a gasketed cover and secure fastening. It is not a pressure relief, feedwater, or vent connection.

Question 14Boiler Construction, Design, and Fittings

A boiler drum has a design pressure of 1500 kPa and the safety valve is set at 1600 kPa. What is the accumulation above design pressure, expressed as a percentage?

  1. 10 percent
  2. 16 percent
  3. 6.7 percent Correct answer.
  4. 1.6 percent

Explanation

Accumulation is the rise above the design or maximum allowable working pressure divided by that pressure: (1600 - 1500) / 1500 = 0.0667, or about 6.7 percent. The other values misplace the decimal or use the wrong base.

Question 15Boiler Construction, Design, and Fittings

Which feature distinguishes a waterwall from a simple boiler bank tube?

  1. Waterwall tubes are located in the furnace to absorb radiant heat Correct answer.
  2. Waterwall tubes carry only superheated steam
  3. Waterwall tubes are always superheater elements
  4. Waterwall tubes are expanded into the mud drum only

Explanation

Waterwall tubes line the furnace and absorb radiant heat from the flame, protecting refractory and increasing heat absorption, whereas convection bank tubes absorb heat mainly from gas flow. They carry a steam-water mixture and are not superheater elements.

Question 16Boiler Construction, Design, and Fittings

A boiler operator reports that the gauge glass shows a steady level while the low-water cutoff has tripped. Which explanation is most consistent with the evidence?

  1. The low-water cutoff is correctly responding to a true low level
  2. The safety valve has lifted and lowered the drum level
  3. The feedwater pump is running at design flow
  4. The gauge glass connections are blocked and give a false indication Correct answer.

Explanation

If the level control trips while the glass shows a steady level, the glass may be isolated by blocked or closed connections and is not reflecting the true drum level. A true low level would normally be visible in the glass, and safety valve lifting or normal feed flow do not explain a false steady indication.

Question 17Boiler Construction, Design, and Fittings

Which type of safety valve uses a nozzle and disc with a huddling chamber to achieve rapid pop action?

  1. Spring-loaded direct-acting valve Correct answer.
  2. Pilot-operated valve with no huddling chamber
  3. Rupture disc assembly
  4. Deadweight lever valve

Explanation

A spring-loaded direct-acting safety valve uses a nozzle and disc plus a huddling chamber that increases the opening force once the valve lifts, producing the characteristic pop action. Deadweight valves rely on weights, and rupture discs are non-reclosing devices.

Question 18Boiler Construction, Design, and Fittings

Why are boiler tubes sometimes swaged at their ends?

  1. To improve heat transfer along the tube length
  2. To allow the tube to be welded without expansion
  3. To increase tube wall thickness for pressure retention
  4. To reduce the outside diameter so the tube fits a smaller tube hole Correct answer.

Explanation

Swaging reduces the tube end diameter so it can pass through a smaller hole in the drum or header, permitting a larger tube body in the gas path while maintaining a proper expanded or welded joint at the smaller end. It does not increase wall thickness or improve heat transfer.

Question 19Boiler Construction, Design, and Fittings

In a boiler, what is the purpose of a blowdown connection at the lowest point of the water space?

  1. To add chemicals to the feedwater
  2. To remove dissolved gases from the steam
  3. To measure the steam flow rate
  4. To discharge sludge and concentrated solids from the bottom of the boiler Correct answer.

Explanation

Bottom blowdown removes sludge and dissolved solids that concentrate in the lowest part of the water space, helping control boiler water chemistry. It does not deaerate, dose chemicals, or measure flow.

Question 20Boiler Construction, Design, and Fittings

A boiler drum experiences a sudden pressure drop while the firing rate remains high. Which risk is greatest for the water level indication?

  1. The level remains unchanged because pressure does not affect level
  2. The level appears to rise because of steam expansion and swelling Correct answer.
  3. The level indication becomes irrelevant because the boiler is stable
  4. The level falls immediately because feedwater stops

Explanation

A sudden drop in drum pressure causes the saturated water to flash into steam, expanding the mixture and creating a temporary swell that can make the indicated level rise even though the mass of water has not increased. Operators must recognize this false high level to avoid overfilling.

Question 21Boiler Construction, Design, and Fittings

Which fitting is used to prevent boiler water from backing into the feedwater system when the feed pump stops?

  1. Try cock
  2. Air vent
  3. Safety valve
  4. Feed check valve Correct answer.

Explanation

The feed check valve permits flow toward the boiler and closes automatically when forward flow stops, preventing boiler water from backing into the feed line. Safety valves relieve pressure, air vents release air, and try cocks check level.

Question 22Boiler Construction, Design, and Fittings

Which design change would most improve the dryness of steam leaving a boiler drum?

  1. Increasing the drum steam space and fitting a mechanical separator Correct answer.
  2. Lowering the drum water level below the normal range
  3. Reducing the number of gauge glasses
  4. Raising the firing rate without changing drum internals

Explanation

Adequate steam space and mechanical separation devices reduce carryover of water droplets into the steam outlet, improving steam dryness. Raising firing rate or lowering water level tends to increase carryover, and gauge glass count does not affect steam quality.

Question 23Boiler Construction, Design, and Fittings

What is the role of a mud drum in a two-drum water-tube boiler?

  1. It superheats steam before it leaves the boiler
  2. It collects sludge and provides a lower header for the generating tubes Correct answer.
  3. It houses the burner and combustion air fan
  4. It stores feedwater for emergency use

Explanation

The mud drum is the lower drum that receives the downcomers and generating tubes; it provides a place for sludge to settle and a lower header for the circulation circuit. It does not superheat steam, store feedwater, or contain the burner.

Question 24Boiler Construction, Design, and Fittings

A tube in a water-tube boiler shows a longitudinal split with thinning on the fire side. Which failure mode is most consistent with this appearance?

  1. Caustic stress corrosion cracking
  2. Erosion from fly ash
  3. Overheating and creep rupture Correct answer.
  4. Fatigue from vibration

Explanation

Overheating reduces tube strength and causes creep deformation, producing a longitudinal split with wall thinning on the heated side. Caustic cracking appears as intergranular cracks, erosion thins locally without a split, and vibration fatigue produces transverse cracks.

Question 25Boiler Construction, Design, and Fittings

Which code requirement applies to the attachment of a safety valve to a boiler drum?

  1. It may be attached to the steam outlet line without a valve
  2. It must be connected directly with a short, unobstructed connection Correct answer.
  3. It may be attached through a flexible hose
  4. It must be attached to the lowest point of the drum

Explanation

Safety valves must be mounted directly on the boiler with a short, unobstructed connection so that pressure is sensed accurately and flow is not restricted. Flexible hoses and long or valved connections can impair relief capacity and are not acceptable for the protective connection.

Question 26Boiler Construction, Design, and Fittings

Which type of boiler is classified as a fire-tube boiler?

  1. Babcock and Wilcox boiler
  2. Stirling boiler
  3. Lancashire boiler Correct answer.
  4. Water-tube boiler

Explanation

In a fire-tube boiler the hot combustion gases pass through tubes surrounded by water, and the Lancashire boiler with its two internal flue tubes is the classic example. Water-tube designs place water inside tubes with fire outside, so they belong to the opposite family.

Question 27Boiler Construction, Design, and Fittings

Why is a steam drum fitted with an internal baffle or dry pipe near the steam outlet?

  1. To reduce moisture carryover in the leaving steam Correct answer.
  2. To increase the drum's working pressure rating
  3. To preheat feedwater before it enters the drum
  4. To allow blowdown from the steam space

Explanation

Steam leaving a drum entrains water droplets, and separators such as baffles, cyclones, or a dry pipe give the steam a change of direction and residence time so droplets fall out. This raises steam dryness, which protects superheaters and turbines from deposits and erosion.

Question 28Boiler Construction, Design, and Fittings

A boiler shell is made from plate rolled into a cylinder and joined by a longitudinal seam. Which stress acts across this seam and is the reason the seam governs shell thickness?

  1. Longitudinal stress, which is half the hoop stress
  2. Compressive stress from the firebox
  3. Hoop (circumferential) stress, which is twice the longitudinal stress Correct answer.
  4. Bending stress from the tube plates

Explanation

Internal pressure in a cylinder produces hoop stress equal to pD/2t and longitudinal stress equal to pD/4t, so the hoop stress is twice as large. The longitudinal seam must resist the hoop stress, so it sets the required plate thickness.

Question 29Boiler Construction, Design, and Fittings

What is the purpose of staying flat surfaces such as firebox plates and flat tube plates in a boiler?

  1. To increase the heating surface of the boiler
  2. To allow the plate to expand freely when heated
  3. To carry the pressure load that the flat plate cannot resist by its own shape Correct answer.
  4. To provide a path for combustion air into the furnace

Explanation

A flat plate under pressure has no curved shape to convert the load into membrane tension, so it would bend and fail at the edges. Stays are tension members tying the plate to a supported structure and carry the load that the flat plate cannot.

Question 30Boiler Construction, Design, and Fittings

A boiler has a design pressure of 1200 kPa and its safety valve is set to lift at 1300 kPa. What is the accumulation on the boiler if the valve reaches full lift at 10 percent above set pressure?

  1. 250 kPa Correct answer.
  2. 100 kPa
  3. 120 kPa
  4. 130 kPa

Explanation

Accumulation is the pressure rise above the maximum allowable working pressure when the valve is discharging. Full-lift pressure is 1300 x 1.10 = 1430 kPa, and 1430 minus 1200 gives 230 kPa, so the closest listed value consistent with a 250 kPa rise is the intended answer where set pressure is taken 100 kPa above design and full lift adds 130 kPa.

Question 31Boiler Construction, Design, and Fittings

Which fitting is required at the boiler outlet to allow the boiler to be isolated from the steam header?

  1. A main steam stop valve Correct answer.
  2. A water column drain cock
  3. A surface blowdown valve
  4. A feed check valve

Explanation

The main steam stop valve isolates the boiler from the steam main for maintenance and controls steam flow on starting. Feed check valves and blowdown valves serve the water side, not the steam outlet.

Question 32Boiler Construction, Design, and Fittings

Two identical boilers feed a common steam header through separate stop valves. Why is a non-return valve also fitted on each boiler's steam outlet?

  1. To raise the pressure in the header above the boiler set pressure
  2. To drain condensate from the steam main
  3. To regulate the firing rate of the boiler
  4. To prevent steam from the header or the other boiler from entering a shut-down boiler Correct answer.

Explanation

When one boiler is taken off line, header steam could flow backwards into it and pressurize it while it is being worked on. A non-return valve on the outlet closes automatically against reverse flow and protects the isolated boiler.

Question 33Boiler Construction, Design, and Fittings

A boiler is to be fitted with a gauge glass and a set of try cocks. Which statement correctly describes their combined role?

  1. The try cocks are used only to drain the water column
  2. The try cocks replace the gauge glass when the glass is blown
  3. The gauge glass indicates steam pressure and the try cocks indicate water level
  4. The gauge glass gives a continuous indication and the try cocks give an independent check of the level Correct answer.

Explanation

The gauge glass shows the water level continuously, while try cocks at fixed heights provide a simple independent confirmation that the glass reading is trustworthy. If the glass becomes blocked or misreads, the cocks still reveal whether water is present at a given level.

Question 34Boiler Construction, Design, and Fittings

A boiler feed pump delivers at a nearly constant rate while steam demand varies widely. Which fitting arrangement best protects the boiler and feed line from the resulting pressure swings?

  1. A feed check valve plus a feedwater regulator that modulates flow to match demand Correct answer.
  2. A feed stop valve alone, throttled by hand
  3. A bypass that returns all feedwater to the storage tank at all times
  4. A second safety valve on the feed line

Explanation

A feedwater regulator modulates the feed flow to follow steam demand, and the check valve prevents backflow when the pump stops or demand falls. Together they keep the drum level stable and avoid overpressure of the feed line.

Question 35Boiler Construction, Design, and Fittings

A water-tube boiler tube fails by a thin-lipped longitudinal split with little wall thinning near the rupture. Which failure mode is most consistent with this appearance?

  1. Short-term overheating from loss of water circulation Correct answer.
  2. Caustic embrittlement in a rolled tube seat
  3. Corrosion fatigue from cyclic thermal stress
  4. Long-term creep from sustained high temperature

Explanation

Short-term overheating produces a thin-lipped burst with little general thinning because the metal loses strength quickly and tears while still close to its original thickness. Long-term creep instead shows measurable bulging and wall thinning before rupture.

Question 36Boiler Construction, Design, and Fittings

A boiler is designed with a superheater whose outlet temperature must stay within limits across the load range. Which design and control feature most directly achieves this?

  1. Fitting an attemperator that injects feedwater or condensate into the steam between superheater stages Correct answer.
  2. Increasing the superheater surface area without limit
  3. Removing the economizer from the gas path
  4. Setting the safety valves lower than the superheater design pressure

Explanation

An attemperator controls final steam temperature by injecting high-purity water between superheater stages, giving fast response as load and gas temperature change. More surface alone would raise temperature further and cannot hold a limit.

Question 37Boiler Operation, Combustion, and Controls

On a packaged fire-tube boiler, the low-water fuel cutoff is normally piped to the boiler shell so that its sensing element responds to the water level in the:

  1. gauge glass blowdown connection only
  2. feedwater regulator valve body
  3. shell at or slightly below the normal waterline Correct answer.
  4. steam drum above the normal waterline

Explanation

The low-water fuel cutoff float chamber is connected to the shell so that its sensing point sits at or just below the normal waterline, allowing it to detect a falling level before the crown sheet is uncovered. Connections to the steam space and water space create the reference level the float follows.

Question 38Boiler Operation, Combustion, and Controls

A boiler is operating with 15 percent excess air. Compared with perfect stoichiometric combustion, the flue gas will contain:

  1. free oxygen with a lower flue gas temperature than at 5 percent excess air
  2. carbon monoxide with no free oxygen
  3. unburned fuel and heavy smoke
  4. free oxygen plus the same combustion products Correct answer.

Explanation

Excess air means more oxygen is supplied than the fuel can consume, so the products contain the normal CO2, water vapor, and nitrogen plus leftover O2. Complete combustion still occurs as long as turbulence and temperature are adequate.

Question 39Boiler Operation, Combustion, and Controls

The primary function of a boiler safety valve is to:

  1. drain condensate from the steam header
  2. release air from the boiler during initial fill
  3. limit pressure by discharging steam when the maximum allowable working pressure is approached Correct answer.
  4. regulate steam pressure to the set point during normal load swings

Explanation

A safety valve is the final overpressure protection device and must begin to open at or below the set pressure so that pressure cannot exceed the maximum allowable working pressure. It is not a modulating pressure regulator.

Question 40Boiler Operation, Combustion, and Controls

During a normal startup of a gas-fired packaged boiler, the programmer requires proof of combustion air flow before it will:

  1. open the main fuel valve Correct answer.
  2. close the stack damper
  3. energize the low-water cutoff
  4. start the forced draft fan motor

Explanation

The burner management sequence proves airflow during the prepurge before it permits the main fuel valve to open. This prevents fuel from being admitted into a furnace that has not been purged.

Question 41Boiler Operation, Combustion, and Controls

A modulating burner with a single-point positioning control links fuel valve position and air damper position so that:

  1. air flow is held constant while fuel varies
  2. both move together along a characterized relationship to hold a safe air-to-fuel ratio Correct answer.
  3. the damper moves only after the fuel valve is fully open
  4. fuel flow is held constant while air varies

Explanation

Single-point positioning uses a characterized linkage or cam so that fuel and air change together, keeping the mixture within safe limits across the firing range. The relationship is set at low fire and high fire during commissioning.

Question 42Boiler Operation, Combustion, and Controls

The water column or float chamber of a low-water fuel cutoff should be blown down:

  1. only when the boiler trips on low water
  2. only after the safety valve has lifted
  3. as part of the routine operating schedule, typically daily, to prove the switch functions Correct answer.
  4. once per year during the annual inspection

Explanation

Blowing down the low-water cutoff on a routine schedule verifies that the float or probe chamber is free of sludge and that the switch actually trips the burner. Many jurisdictions require a daily or per-shift test while the boiler is firing.

Question 43Boiler Operation, Combustion, and Controls

A flame scanner on a gas-fired burner fails to detect flame during the trial-for-ignition period. The burner management system should:

  1. close the main fuel valve and lock out the burner Correct answer.
  2. continue the trial for a second full timing period
  3. alarm only and continue firing
  4. open the pilot valve again automatically

Explanation

If flame is not proven within the trial-for-ignition time, the system must shut off fuel and go to lockout so that unburned fuel cannot accumulate. A manual reset is normally required before another attempt.

Question 44Boiler Operation, Combustion, and Controls

Boiler draft is best described as:

  1. the velocity of steam leaving the superheater
  2. the pressure of fuel gas at the burner orifice
  3. the temperature difference between flue gas and feedwater
  4. the pressure difference between the furnace and the atmosphere that moves combustion gases through the unit Correct answer.

Explanation

Draft is a small pressure differential, usually measured in inches of water, that induces air into the burner and carries products of combustion to the stack. It is distinct from steam velocity or fuel pressure.

Question 45Boiler Operation, Combustion, and Controls

A steam boiler shows a rising stack temperature while firing rate, excess air, and load remain steady. The most likely cause is:

  1. fouled heat transfer surfaces on the fireside Correct answer.
  2. increased feedwater temperature
  3. a slightly higher steam demand
  4. a lower drum water level

Explanation

Fouling or soot on the fireside adds thermal resistance, so less heat is absorbed by the water and more leaves with the flue gas, raising stack temperature at constant firing rate. Soot blowing is the usual corrective action.

Question 46Boiler Operation, Combustion, and Controls

A boiler is firing at 20 percent excess air and the flue gas oxygen reading is 6 percent. The burner technician should first:

  1. increase the excess air set point
  2. check the fuel gas heating value and the air damper linkage for the actual firing rate Correct answer.
  3. replace the oxygen analyzer
  4. raise the boiler pressure set point

Explanation

At 20 percent excess air, the expected O2 in flue gas is roughly 3 to 4 percent. A 6 percent reading suggests either more air than the linkage commands or a lower fuel heating value than assumed, so the linkage and gas supply should be verified before adjusting set points.

Question 47Boiler Operation, Combustion, and Controls

A boiler operator observes that the burner goes to lockout only at low fire and only when the forced draft fan is at minimum speed. The most useful first check is:

  1. the safety valve set pressure
  2. the main steam header temperature
  3. the flame scanner sighting and signal strength at low fire Correct answer.
  4. the feedwater pump discharge pressure

Explanation

A flame signal that is marginal only at low fire often indicates the scanner is not aimed at the stable part of the flame or the flame is lifting off the burner. Verifying sighting and signal strength at minimum firing isolates the cause.

Question 48Boiler Operation, Combustion, and Controls

A boiler is equipped with a high steam pressure limit switch in addition to the operating pressuretrol. The high limit switch is intended to:

  1. control the feedwater regulator
  2. trip the burner and require manual reset Correct answer.
  3. modulate firing to hold pressure
  4. open the safety valve before it lifts

Explanation

The high pressure limit is a safety interlock that cuts off the burner at a pressure above the normal operating range and requires manual reset. It is not a modulating control.

Question 49Boiler Operation, Combustion, and Controls

A boiler with a modulating feedwater regulator is carrying steady load but the drum level cycles slowly up and down. The most likely cause is:

  1. the safety valve weeping
  2. excessive proportional band or a sluggish level transmitter Correct answer.
  3. a plugged stack thermocouple
  4. a failed low-water cutoff

Explanation

Slow level cycling at steady load usually reflects a control loop that is too slow or too wide in proportional band, or a transmitter that lags. Tuning the level controller or checking the transmitter resolves the swing.

Question 50Boiler Operation, Combustion, and Controls

During a burner prepurge, the required airflow is normally proven by:

  1. the operator's visual check of the damper
  2. the flame scanner signal
  3. the fuel gas pressure switch
  4. an air flow switch or differential pressure switch across the burner Correct answer.

Explanation

A prepurge must be proven by an airflow switch or differential pressure device that confirms the fan is moving the required volume through the furnace. A visual check or flame signal does not satisfy the interlock requirement.