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MN Boiler Operator practice questions

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Question 1Boiler construction, design and fittings

A low-water fuel cutoff on a steam heating boiler is required by the Minnesota rules to perform which primary safety function?

  1. Sound an alarm only, while allowing the burner to keep firing
  2. Start the feed pump automatically whenever the burner fires
  3. Open the safety valve to relieve pressure when the water level falls
  4. Stop the fuel supply before the water level drops out of sight in the gauge glass Correct answer.

Explanation

The low-water fuel cutoff is a safety control whose purpose is to cut off the burner fuel supply before the water level falls below the lowest safe visible level in the gauge glass. It is not a feed-water regulator and it does not relieve pressure; those duties belong to the feed controls and the safety valve.

Question 2Boiler construction, design and fittings

On a power boiler, the main purpose of the safety valve is to:

  1. Indicate the water level in the gauge glass
  2. Drain condensate from the steam header
  3. Vent air when the boiler is filling
  4. Limit internal pressure by discharging steam at the set pressure Correct answer.

Explanation

A safety valve is the final overpressure protection on a boiler; it opens automatically at its set pressure and recloses after the pressure falls. It must be sized to discharge more steam than the boiler can generate so pressure cannot continue to rise.

Question 3Boiler construction, design and fittings

Which fitting is installed on a boiler to show the operator the water level inside the drum?

  1. Gauge glass Correct answer.
  2. Blowdown valve
  3. Steam pressure gauge
  4. Feedwater regulator

Explanation

The gauge glass, also called the water column glass, gives a direct visual indication of the water level in the boiler. Its upper and lower connections must be kept clear so the indicated level matches the true drum level.

Question 4Boiler construction, design and fittings

In a fire-tube boiler, the hot gases of combustion:

  1. Never contact any heat transfer surface
  2. Pass through tubes surrounded by water Correct answer.
  3. Pass over tubes that carry water internally
  4. Flow only through the external economizer

Explanation

In a fire-tube design the products of combustion travel through tubes or flues that are submerged in the surrounding water, so the water absorbs heat through the tube walls. This is the reverse of a water-tube boiler, where water flows inside the tubes and hot gas passes outside them.

Question 5Boiler construction, design and fittings

A fusible plug installed in the crown sheet of a fire-tube boiler is intended to:

  1. Indicate the boiler water temperature to the operator
  2. Melt and release steam into the furnace if the metal overheats from low water Correct answer.
  3. Relieve excess steam pressure to atmosphere
  4. Automatically restart the burner after a lockout

Explanation

The fusible plug contains a low-melting alloy that melts when the crown sheet is uncovered and overheats, allowing steam and water to quench the furnace and warn the operator. It is a last-resort protection against overheating from low water, not a pressure relief device.

Question 6Boiler construction, design and fittings

The maximum allowable working pressure stamped on a boiler nameplate represents:

  1. The highest pressure at which the boiler may be safely operated Correct answer.
  2. The hydrostatic test pressure used at the factory
  3. The normal operating pressure of the plant steam header
  4. The pressure at which the safety valve must be gagged

Explanation

The maximum allowable working pressure is the highest gauge pressure at which the boiler is designed to operate safely, based on its material and thickness calculations. Safety valves are set at or below this value and it is not the same as the hydrostatic test pressure.

Question 7Boiler construction, design and fittings

The main steam stop valve on a boiler is normally installed:

  1. In the steam line between the boiler and the plant steam header Correct answer.
  2. Between the safety valve and the boiler drum
  3. On the blowdown line below the water column
  4. On the feedwater line ahead of the feed pump

Explanation

The main steam stop valve isolates the boiler from the plant steam system and is located in the main steam outlet line. It allows the boiler to be cut in or out of service without disturbing the rest of the plant.

Question 8Boiler construction, design and fittings

Boiler blowdown piping is provided primarily to:

  1. Supply steam to the deaerator
  2. Remove accumulated sludge and dissolved solids from the boiler water Correct answer.
  3. Add chemicals to the boiler water
  4. Drain the steam header of condensate

Explanation

Blowdown removes water carrying concentrated dissolved solids and settled sludge, which keeps the boiler water chemistry within limits. Bottom blowdown removes sludge from the low points and surface blowdown skims dissolved solids from the water surface.

Question 9Boiler construction, design and fittings

A boiler operator notices the water level in the gauge glass is difficult to read and the glass appears empty while the plant is running steadily. Which construction feature should the operator check first?

  1. The safety valve set pressure
  2. The gauge glass connections and water column for blockage Correct answer.
  3. The burner flame scanner sensitivity
  4. The feedwater pump discharge temperature

Explanation

An empty or erratic gauge glass on a running boiler often means the upper or lower gauge glass connection, or the water column, is plugged with scale or sludge. The operator should verify the connections and blow down the glass to confirm the true level before concluding the boiler is low on water.

Question 10Boiler construction, design and fittings

Why must a steam pressure gauge be fitted with a siphon or pigtail between the gauge and the boiler?

  1. To allow the gauge to be removed without shutting the boiler down
  2. To increase the gauge reading for accuracy
  3. To dampen pressure pulsations from the burner
  4. To keep live steam from damaging the Bourdon tube and to trap condensate Correct answer.

Explanation

The siphon loop fills with condensate, which protects the gauge internals from high-temperature live steam and steadies the reading. A gauge cock is the separate fitting that allows removal or isolation of the gauge.

Question 11Boiler construction, design and fittings

A water-tube boiler differs from a fire-tube boiler in that the water-tube design:

  1. Contains a large steam drum surrounded by fire
  2. Circulates water inside tubes heated externally by combustion gases Correct answer.
  3. Cannot be fitted with a superheater
  4. Operates only at pressures below 15 psi

Explanation

In a water-tube boiler, water circulates through tubes that are heated on the outside by combustion gases, which allows higher pressures and faster response to load changes. Fire-tube boilers instead pass hot gas through tubes surrounded by water.

Question 12Boiler construction, design and fittings

A boiler is equipped with two safety valves. To help ensure both valves relieve at the design pressure, the practice is to:

  1. Set both valves well below the maximum allowable working pressure
  2. Set one valve at the maximum allowable working pressure and the other slightly above it within the permitted tolerance Correct answer.
  3. Set both valves at the identical pressure and gag one during operation
  4. Remove the nameplate rating so the valves can be set freely

Explanation

When multiple safety valves are used, one is set at the maximum allowable working pressure and the additional valve is set slightly higher, within the tolerance allowed by the code, so the valves open in sequence and total relieving capacity is adequate. Gagging a valve or setting valves below the rating is not acceptable practice.

Question 13Boiler construction, design and fittings

The purpose of a superheater in a boiler is to:

  1. Raise steam temperature above the saturation temperature at the drum pressure Correct answer.
  2. Remove moisture from the feedwater before it enters the drum
  3. Reduce the boiler water level during blowdown
  4. Cool the combustion gases before they reach the stack

Explanation

A superheater adds heat to saturated steam so its temperature rises above the saturation temperature for that pressure, which improves cycle efficiency and reduces condensation in the turbine. Moisture removal is the job of separators or the drum internals, not the superheater.

Question 14Boiler construction, design and fittings

An economizer installed in the boiler flue gas path is used to:

  1. Provide a final safety relief for overpressure
  2. Filter suspended solids from the boiler water
  3. Superheat the steam before it leaves the drum
  4. Heat the incoming feedwater using heat from the exiting flue gases Correct answer.

Explanation

The economizer recovers sensible heat from the flue gas leaving the boiler and transfers it to the feedwater, lowering stack temperature and improving overall efficiency. It does not superheat steam or act as a relief device.

Question 15Boiler construction, design and fittings

The water column attached to a boiler serves to:

  1. Provide a stilling chamber so the gauge glass shows a steady, accurate water level Correct answer.
  2. Reduce the pressure at which the safety valve opens
  3. Increase the boiler storage capacity for steam
  4. Separate steam from water before it leaves the drum

Explanation

The water column is a vertical chamber connected near the normal water level; it dampens turbulence so the gauge glass shows a steady level and it provides a mounting point for the low-water cutoff and gauge glass. It does not change steam storage or safety valve settings.

Question 16Boiler construction, design and fittings

A boiler is rated at 15 psi steam. Which statement about its construction is most accurate?

  1. It needs no safety valve because the pressure is low
  2. It is a high-pressure power boiler requiring a superheater
  3. It is a low-pressure heating boiler and must have a safety valve set at or below the rating Correct answer.
  4. It must be built as a water-tube boiler only

Explanation

A boiler limited to 15 psi steam is classified as a low-pressure heating boiler, and its safety valve must be set at or below the maximum allowable working pressure so it cannot be overpressured. Low pressure does not eliminate the need for safety devices.

Question 17Boiler construction, design and fittings

In a fire-tube boiler, stays are used mainly to:

  1. Support the tubes inside the shell
  2. Tie flat or stayed surfaces together so they resist pressure loads Correct answer.
  3. Hold the safety valve to the boiler shell
  4. Guide the combustion air into the burner

Explanation

Stays are structural members that tie flat plates and stayed surfaces to opposing plates or to the shell so those surfaces can resist the pressure load without excessive deflection. They are a construction feature of the pressure boundary, not a guide for air or tubes.

Question 18Boiler construction, design and fittings

A boiler has a gauge glass with a lower try cock that is opened and only steam escapes. What does this most likely indicate?

  1. The water level is above the try cock
  2. The steam pressure is too high
  3. The water level is below the try cock and the boiler may be low on water Correct answer.
  4. The try cock is plugged with scale

Explanation

Each try cock is located at a known level; if only steam comes out of a lower cock, the water level is below that cock, which is a low-water warning. The operator should verify with the gauge glass and act according to procedure before adding water or securing the burner.

Question 19Boiler construction, design and fittings

Why is a non-return valve often installed in the feedwater line between the feed pump and the boiler?

  1. To measure the feedwater flow rate
  2. To reduce the feedwater temperature before it enters the drum
  3. To relieve excess feedwater pressure to the drain
  4. To prevent boiler water from flowing back into the feed line when the pump stops Correct answer.

Explanation

The feedwater check valve permits flow toward the boiler but blocks reverse flow, protecting the pump and feed line from hot boiler water when the pump stops or when pressure in the drum exceeds the pump discharge. Flow measurement and pressure relief are separate functions.

Question 20Boiler construction, design and fittings

A boiler operator finds the burner firing while the water level is below the lowest visible level in the gauge glass and the low-water cutoff has not tripped. What is the correct immediate action?

  1. Increase firing rate to raise steam temperature
  2. Add feedwater slowly and continue firing until the level returns
  3. Open the safety valve to reduce pressure
  4. Secure the burner and follow the low-water emergency procedure Correct answer.

Explanation

A low-water condition with the burner still firing is an emergency; the operator must stop the fuel supply immediately and follow the plant low-water procedure rather than adding water to a hot, uncovered boiler. Adding water to an overheated boiler can cause a violent steam release.

Question 21Boiler construction, design and fittings

A boiler is being prepared for hydrostatic testing after repairs to the pressure boundary. Which statement reflects correct construction and testing practice?

  1. The safety valve is removed or isolated and the test pressure is applied with the boiler properly vented and filled Correct answer.
  2. The test is performed with the burner firing to warm the water
  3. The safety valve is left in place and gagged during the test
  4. The test pressure is held at the maximum allowable working pressure only

Explanation

During a hydrostatic test the safety valve must be removed or isolated so it is not subjected to test pressure above its set point, and the boiler must be filled and vented to remove air. The test pressure is normally a multiple of the working pressure, not the working pressure itself, and firing during the test is not permitted.

Question 22Boiler construction, design and fittings

A boiler's safety valve is set at 150 psi and the boiler nameplate shows a maximum allowable working pressure of 150 psi. During operation the valve begins to simmer and leak at 130 psi. Which conclusion is best supported?

  1. The gauge glass is reading incorrectly
  2. The valve seat may be damaged or the set pressure may have drifted and should be inspected and reset by qualified personnel Correct answer.
  3. The valve is operating correctly because it is below the set pressure
  4. The boiler should be operated at 130 psi from now on

Explanation

A safety valve that simmers or leaks below its set pressure is not performing as designed; the usual causes are a damaged seat, debris, or a change in adjustment. The valve must be inspected and reset by qualified personnel to its correct set pressure, and the boiler should not simply be down-rated without engineering review.

Question 23Boiler construction, design and fittings

A fire-tube boiler shows a slowly rising stack temperature and reduced steam output at constant firing rate. Which construction-related cause is most consistent with these symptoms?

  1. A failed low-water cutoff
  2. A stuck-open safety valve
  3. A misadjusted try cock
  4. Scale buildup on the water side of the tubes Correct answer.

Explanation

Scale on the water side of the tubes acts as insulation, so less heat reaches the water and more leaves with the flue gas, raising stack temperature and reducing steam output. Treatment and blowdown control are the preventive measures.

Question 24Boiler construction, design and fittings

A boiler drum is fitted with a surface blowdown connection near the normal water level. The primary purpose of this connection is to:

  1. Drain the boiler completely for inspection
  2. Skim water containing concentrated dissolved solids from the surface Correct answer.
  3. Provide a mounting point for the safety valve
  4. Remove settled sludge from the bottom of the drum

Explanation

Surface blowdown, also called continuous blowdown, draws water from near the water surface where dissolved solids concentrate, which helps control boiler water chemistry. Bottom blowdown is used to remove settled sludge from the low points.

Question 25Boiler construction, design and fittings

In a horizontal return tubular boiler, the combustion chamber is located directly behind the furnace and its main structural function is to:

  1. Support the rear tube sheet against pressure loading
  2. Collect scale washed from the shell during blowdown
  3. Provide a mounting surface for the safety valve
  4. Complete combustion of unburned gases before they enter the tubes Correct answer.

Explanation

The combustion chamber gives the hot gases a refractory-lined volume and time to finish burning before they turn into the tubes, which improves efficiency and reduces smoke. It is a gas-side component, not a structural support for pressure parts.

Question 26Boiler construction, design and fittings

Which feature distinguishes a Stirling boiler from a simple fire-tube boiler?

  1. It has a single large flue running the length of the shell
  2. It operates only as a low-pressure heating boiler
  3. It uses water tubes arranged in banks with drums and headers Correct answer.
  4. It relies on a crown sheet for heat transfer

Explanation

The Stirling design is a water-tube boiler: water circulates through tube banks connecting upper and lower drums, and the flue gas passes over the outside of those tubes. Fire-tube boilers instead pass hot gas through tubes surrounded by water.

Question 27Boiler construction, design and fittings

A boiler shell course is made from plate rolled into a cylinder and joined by a longitudinal seam. The seam that resists the hoop (circumferential) stress in the shell is the:

  1. Longitudinal seam Correct answer.
  2. Tube-to-tubesheet joint
  3. Staybolt attachment
  4. Circumferential seam

Explanation

Internal pressure tends to split a cylinder along its length, so the longitudinal seam carries the hoop stress and is the more heavily loaded seam. Circumferential seams carry roughly half that stress and join successive courses end to end.

Question 28Boiler construction, design and fittings

In a boiler, the term 'crown sheet' refers to the:

  1. Upper drum of a water-tube boiler that separates steam from water
  2. Refractory arch that directs flame toward the tubes
  3. Plate that forms the top of the combustion chamber and is dry on both sides
  4. Top plate of the furnace or firebox that has water on its upper side and flame below Correct answer.

Explanation

The crown sheet is the plate forming the top of the firebox, with water above it and hot gases below. Because it is directly exposed to flame and depends on water coverage for cooling, it is a critical area for low-water protection.

Question 29Boiler construction, design and fittings

Compared with a fire-tube boiler of the same output, a water-tube boiler generally:

  1. Cannot be fitted with a superheater
  2. Can be built for higher pressure because the drums are smaller in diameter Correct answer.
  3. Has a lower response speed to load changes
  4. Requires a larger shell diameter to contain the same heating surface

Explanation

Smaller drum diameters mean thinner walls for a given stress, so water-tube boilers are built for much higher pressures and capacities. Their water content is lower, which also makes them respond faster to load changes.

Question 30Boiler construction, design and fittings

A boiler tube is expanded into a tube sheet by rolling. The purpose of the roll is to:

  1. Bend the tube to align with the flue gas path
  2. Reduce the tube wall thickness to increase heat transfer
  3. Cut a thread that engages the tube sheet
  4. Deform the tube end outward so it seals tightly against the tube sheet hole Correct answer.

Explanation

Rolling swages the tube end outward into firm contact with the tube sheet bore, creating a pressure-tight joint through interference fit. Some joints are also seal-welded, but the roll itself does not thin the wall intentionally or cut threads.

Question 31Boiler construction, design and fittings

The main purpose of a boiler's steam drum in a water-tube unit is to:

  1. House the burner assembly
  2. Support the superheater tubes
  3. Separate steam from water and provide storage for both Correct answer.
  4. Preheat feedwater using flue gas

Explanation

The steam drum gives the steam-water mixture a place to slow down so separation can occur, and it holds a water reserve that buffers load swings. Feedwater heating and superheater support are handled by other components.

Question 32Boiler construction, design and fittings

A boiler is described as having a 'dry-back' design. This means the reversal chamber at the rear of the furnace is:

  1. Surrounded by water on all sides
  2. Open to the atmosphere for inspection
  3. Cooled by the feedwater line
  4. Refractory-lined and not water-cooled Correct answer.

Explanation

In a dry-back boiler the rear turnaround chamber is formed by refractory and is not surrounded by water, so it must be maintained carefully against overheating and cracking. A wet-back design surrounds that reversal chamber with water.

Question 33Boiler construction, design and fittings

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

  1. Room-temperature hardness
  2. Electrical conductivity
  3. Surface finish
  4. Creep resistance at the design metal temperature Correct answer.

Explanation

At elevated temperature, steel progressively deforms under sustained stress, so the plate must have adequate creep strength at the design metal temperature. Hardness, conductivity and finish do not govern pressure-boundary integrity at temperature.

Question 34Boiler construction, design and fittings

A boiler is fitted with a manhole and a handhole. The difference between them is that the manhole:

  1. Is fitted only to water-tube boilers
  2. Is a spring-loaded relief opening
  3. Is used only for adding chemicals
  4. Is large enough for a person to pass through for internal inspection and cleaning Correct answer.

Explanation

A manhole is sized to admit a person for internal inspection, cleaning and repair, while a handhole is only large enough for a hand and tools. Both are openings with covers and gaskets in the pressure boundary.

Question 35Boiler construction, design and fittings

Boiler tubes are often given a slight upward incline from front to rear in a horizontal boiler. This incline primarily:

  1. Allows the tubes to be rolled from the outside
  2. Reduces the need for staybolts
  3. Promotes drainage and helps prevent pockets that trap water or gas Correct answer.
  4. Increases the tube sheet thickness required

Explanation

A small slope lets water drain and discourages trapped pockets, and in some designs it assists natural circulation. It has no bearing on tube sheet thickness, staying, or the method of rolling tubes in.

Question 36Boiler construction, design and fittings

The furnace of a packaged fire-tube boiler is typically the:

  1. Refractory-lined chamber behind the tubes
  2. Water space between the shell and the tubes
  3. Large central corrugated or plain cylindrical shell section where combustion occurs Correct answer.
  4. Bank of small tubes that carry flue gas to the stack

Explanation

In a packaged fire-tube boiler the furnace is the large central cylinder, often corrugated for strength and flexibility, in which the flame develops. The smaller tubes carry the gases away after combustion.

Question 37Boiler construction, design and fittings

A boiler shell is fitted with a 'belly' or 'girth' stay in a flat area. Its purpose is to:

  1. Guide the tubes during expansion
  2. Reinforce the longitudinal seam against hoop stress
  3. Tie the flat plate to an opposing plate so pressure does not bow it outward Correct answer.
  4. Support the boiler on its foundation

Explanation

Stays in flat or stayed areas carry the pressure load that would otherwise bend unsupported plate, tying it to an opposing surface. They are not seam reinforcement, foundation supports, or tube guides.

Question 38Boiler construction, design and fittings

A boiler is rated for 250 psi and the operator finds the shell plate is thinner than the nameplate design calls for. The most appropriate action is to:

  1. Reduce the firing rate and continue at the same pressure
  2. Increase blowdown frequency to compensate
  3. Remove the boiler from service and have the pressure boundary evaluated before further operation Correct answer.
  4. Lower the safety valve setting and keep operating

Explanation

Thinner-than-design plate means the pressure boundary may not have the required strength, so the boiler must be taken out of service and evaluated by a qualified authority. Operating at reduced firing or altered valve settings does not restore lost wall thickness.

Question 39Boiler construction, design and fittings

The purpose of a boiler's furnace front plate in a packaged unit is to:

  1. Mount the burner and seal the front of the furnace Correct answer.
  2. Carry the weight of the rear tube sheet
  3. Support the superheater headers
  4. Provide the main steam outlet connection

Explanation

The front plate closes the furnace opening and provides the mounting for the burner and its controls. It does not carry the rear tube sheet or the main steam outlet, which are located elsewhere.

Question 40Boiler construction, design and fittings

A boiler is to be fitted with a superheater and the designer must choose between a radiant and a convection type. A radiant superheater is placed:

  1. In the flue gas path after the generating tubes
  2. Inside the steam drum
  3. In the furnace where it receives heat mainly by radiation from the flame Correct answer.
  4. In the economizer outlet duct

Explanation

A radiant superheater is located in the furnace and absorbs heat mostly by radiation, which makes it respond strongly to firing changes. A convection superheater sits in the gas path downstream and absorbs heat mainly by convection.

Question 41Boiler construction, design and fittings

The function of a boiler's air preheater is to:

  1. Heat feedwater before it enters the drum
  2. Warm combustion air using heat recovered from the flue gas Correct answer.
  3. Remove moisture from the steam
  4. Cool the flue gas below its dew point before the stack

Explanation

An air preheater transfers heat from the leaving flue gas to the incoming combustion air, improving efficiency and combustion stability. It does not heat feedwater and should not drive the gas below its acid dew point.

Question 42Boiler construction, design and fittings

A boiler carries a nameplate showing a heating surface and a maximum allowable working pressure. The heating surface is best described as the:

  1. Area of the furnace floor
  2. Total internal volume of the boiler
  3. Area of surface exposed to hot gases on the water side of the pressure parts Correct answer.
  4. Outer surface area of the insulation

Explanation

Heating surface is the area of the pressure parts that has water on one side and hot gas on the other, so heat can pass into the water. Volume, furnace floor area and insulation area are not heating surface.

Question 43Boiler construction, design and fittings

A fire-tube boiler is fitted with a 'Morrison' type furnace or corrugated furnace. The corrugations are mainly provided to:

  1. Increase the flame temperature
  2. Provide a surface for scale to collect
  3. Give the furnace flexibility and strength against external pressure Correct answer.
  4. Allow the furnace to be rolled into the shell

Explanation

Corrugations stiffen a large-diameter furnace against external pressure and let it flex as the boiler expands and contracts. They do not raise flame temperature or serve as a scale collector.

Question 44Boiler construction, design and fittings

A boiler is built with a riveted longitudinal seam. Compared with a welded seam, the riveted seam:

  1. Requires a higher plate thickness or added seam efficiency allowance Correct answer.
  2. Is stronger in hoop stress than a welded seam
  3. Eliminates the need for stays in the shell
  4. Has no effect on required plate thickness

Explanation

A riveted seam has lower joint efficiency because of the rivet holes and the way load is transferred, so the plate must be thicker for the same pressure. Welded seams generally achieve higher efficiency.

Question 45Boiler construction, design and fittings

The purpose of a boiler's generating tubes in a water-tube unit is to:

  1. Support the furnace refractory
  2. Carry feedwater to the burner
  3. Provide a path for flue gas to the stack
  4. Carry the steam-water mixture from the heat-absorbing surface to the drum Correct answer.

Explanation

Generating tubes absorb heat from the furnace and gas path and carry the resulting steam-water mixture to the drum by natural circulation. They are water-side components, not gas paths or supports.

Question 46Boiler construction, design and fittings

A boiler is being designed with a water wall in the furnace. The water wall's main benefit is to:

  1. Absorb radiant heat and protect the furnace structure from high temperature Correct answer.
  2. Increase the flue gas exit temperature
  3. Replace the superheater
  4. Reduce the need for a steam drum

Explanation

Water walls are tubes carrying water that line the furnace, absorbing radiant heat and shielding the refractory and structure from the flame. They increase heat absorption rather than raise the exit gas temperature.

Question 47Boiler construction, design and fittings

A boiler's shell is fitted with an expansion allowance such as a slip joint or flexible support. This is necessary because:

  1. The shell must be able to rotate during firing
  2. The shell grows in length as it heats and must be free to move Correct answer.
  3. The safety valve requires a moving seat
  4. The feedwater line must be able to flex

Explanation

A boiler shell expands when heated, and if it were rigidly restrained the resulting stress could damage the shell or its supports. Sliding supports and expansion joints allow that movement.

Question 48Boiler construction, design and fittings

A boiler is described as 'packaged'. This means the unit:

  1. Is factory-assembled with burner and controls on a common base for shipment Correct answer.
  2. Operates without a safety valve
  3. Has no pressure-retaining shell
  4. Is built on site from individual plates

Explanation

A packaged boiler is assembled at the factory with its burner, controls and trim mounted on a base, so it can be shipped and set in place as a unit. Field-erected boilers are built from individual parts on site.

Question 49Boiler construction, design and fittings

A boiler's tubes are found to be bowed or sagging between their supports. The most likely cause is:

  1. A safety valve set too low
  2. Excess blowdown frequency
  3. Overheating from poor circulation or excessive firing Correct answer.
  4. Too much water in the drum

Explanation

Tubes sag when they are overheated, often because circulation is poor or firing is excessive, weakening the metal. Water level, valve setting and blowdown frequency do not cause mechanical bowing of tubes.

Question 50Boiler construction, design and fittings

A fire-tube boiler is fitted with a fusible plug in the crown sheet of the furnace. The plug is intended to melt and release steam into the furnace when the:

  1. feedwater temperature drops below the dew point of the flue gas
  2. furnace gas temperature exceeds the design temperature of the tube sheet
  3. water level falls low enough to uncover the crown sheet, so the metal overheats Correct answer.
  4. steam pressure rises above the set point of the safety valve

Explanation

The fusible plug contains a low-melting alloy that is held at a safe temperature only while it is covered by water. If the water level drops and uncovers the crown sheet, the metal temperature rises, the alloy melts, and steam escapes into the furnace as a warning and to cool the plate. It is a low-water safeguard, not a pressure relief device.