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ME Boiler Operator practice questions

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Question 1Boiler and Pressure Vessel Construction, Design, and Fittings

What is the primary function of a safety valve installed on a boiler?

  1. Drain water and sediment from the lowest point of the boiler
  2. Limit internal pressure to the safe working limit Correct answer.
  3. Prevent feedwater from flowing backward through the feed line
  4. Indicate the steam pressure inside the boiler drum at all times

Explanation

A safety valve opens automatically when pressure reaches its set point and discharges steam to prevent the boiler from exceeding its maximum allowable working pressure. It is the final overpressure protection device and must be sized for the boiler's maximum steam output.

Question 2Boiler and Pressure Vessel Construction, Design, and Fittings

Which fitting provides a direct visual indication of the water level inside a boiler drum?

  1. A fusible plug
  2. A safety valve
  3. A try cock
  4. A gauge glass Correct answer.

Explanation

A gauge glass is connected to the boiler drum so that the water level in the glass matches the water level in the drum. Operators use it as the primary visual indication of water level.

Question 3Boiler and Pressure Vessel Construction, Design, and Fittings

What is the purpose of a fusible plug in a boiler?

  1. It melts when the metal overheats Correct answer.
  2. It prevents backflow of feedwater into the boiler
  3. It relieves excess pressure from the steam drum
  4. It measures the steam pressure in the boiler

Explanation

A fusible plug contains a low-melting-point alloy that melts if the plug is exposed to steam or hot gas because the water level has dropped too low. This releases steam and warns operators before the metal overheats and fails.

Question 4Boiler and Pressure Vessel Construction, Design, and Fittings

What is the main function of a water column on a boiler?

  1. To increase the steam storage capacity of the drum
  2. To provide a mounting point for the safety valve
  3. To steady the water level at the glass Correct answer.
  4. To separate moisture from steam before it leaves the boiler

Explanation

A water column is a vertical fitting connected to the boiler drum that reduces turbulence and provides a steadier water level for the gauge glass and level controls. It also provides a convenient mounting location for level-sensing devices.

Question 5Boiler and Pressure Vessel Construction, Design, and Fittings

What does the abbreviation MAWP stand for in boiler design?

  1. Maximum authorized water level
  2. Mean absolute working pressure
  3. Minimum allowable water pressure
  4. Maximum allowable working pressure Correct answer.

Explanation

MAWP is the maximum allowable working pressure, the highest pressure at which a boiler or pressure vessel may be operated under its design conditions. It is established by the applicable construction code and the material thickness.

Question 6Boiler and Pressure Vessel Construction, Design, and Fittings

Which ASME Boiler and Pressure Vessel Code section applies to power boilers?

  1. Section I Correct answer.
  2. Section IV
  3. Section IX
  4. Section VIII

Explanation

ASME Section I covers rules for construction of power boilers, including their materials, design, fabrication, and inspection. Section IV applies to heating boilers, and Section VIII applies to pressure vessels.

Question 7Boiler and Pressure Vessel Construction, Design, and Fittings

What is the purpose of a staybolt in a boiler?

  1. To reinforce the tube-to-tubesheet joint
  2. To direct flue gas through the boiler passes
  3. To connect the steam drum to the mud drum
  4. To brace flat plates against pressure loads Correct answer.

Explanation

Staybolts tie together flat or stayed surfaces so that pressure loads are carried in tension rather than bending the plate. Proper staying allows flat plates to be used with reasonably thin material.

Question 8Boiler and Pressure Vessel Construction, Design, and Fittings

What is the function of a boiler blowdown valve?

  1. To prevent water from leaving the boiler
  2. To remove sludge and reduce dissolved solids Correct answer.
  3. To release steam when pressure is excessive
  4. To add treatment chemicals to the boiler water

Explanation

Blowdown removes water containing suspended sludge and dissolved solids from the lowest point of the boiler. Regular blowdown helps prevent scale, carryover, and foaming.

Question 9Boiler and Pressure Vessel Construction, Design, and Fittings

In boiler shell thickness calculations, what does joint efficiency represent?

  1. The margin added for future corrosion
  2. The seam strength relative to solid plate Correct answer.
  3. The ratio of steam space to water space
  4. The percentage of heat lost through insulation

Explanation

Joint efficiency accounts for the fact that a welded or riveted seam is not as strong as the solid plate. The calculated shell thickness is increased by dividing the strength term by the joint efficiency.

Question 10Boiler and Pressure Vessel Construction, Design, and Fittings

Why is a corrosion allowance added to the calculated shell thickness?

  1. Extra thickness to resist internal steam pressure only
  2. Extra thickness for metal loss over time Correct answer.
  3. Extra width added to the tube sheet for rolling
  4. Extra length added to the shell for tube expansion

Explanation

The corrosion allowance provides additional metal thickness so that the boiler can lose some material to corrosion or erosion over its service life while still meeting the required pressure strength. It is added after the pressure-containing thickness has been calculated.

Question 11Boiler and Pressure Vessel Construction, Design, and Fittings

What is the purpose of a non-return valve on a boiler feedwater line?

  1. To measure the quantity of feedwater delivered
  2. To stop reverse flow to the feed line Correct answer.
  3. To relieve pressure if the feed pump stops
  4. To regulate the feedwater temperature entering the drum

Explanation

A non-return or check valve allows feedwater to enter the boiler but prevents boiler water and steam from flowing back into the feed line. This protects the feed system and helps maintain water level if the pump stops.

Question 12Boiler and Pressure Vessel Construction, Design, and Fittings

Under ASME Section I, what is the standard hydrostatic test pressure for a new boiler?

  1. 1.25 times the maximum allowable working pressure
  2. 1.0 times the maximum allowable working pressure
  3. 1.5 times the maximum allowable working pressure Correct answer.
  4. 2.0 times the maximum allowable working pressure

Explanation

ASME Section I requires a hydrostatic test at 1.5 times the maximum allowable working pressure for a new boiler. The test verifies the tightness and structural integrity of the pressure parts.

Question 13Boiler and Pressure Vessel Construction, Design, and Fittings

Why does a boiler have inspection openings such as manholes and handholes?

  1. To serve as the main steam outlet
  2. To admit combustion air into the furnace
  3. To provide a connection for the safety valve
  4. To permit internal inspection and cleaning Correct answer.

Explanation

Manholes and handholes allow inspectors and maintenance personnel to examine internal surfaces, remove scale and sludge, and verify the condition of pressure parts. They are closed with covers and gaskets during normal operation.

Question 14Boiler and Pressure Vessel Construction, Design, and Fittings

Why are boiler tubes expanded or rolled into the tube sheet?

  1. To seal and grip the tube in the sheet Correct answer.
  2. To increase the tube's resistance to high-temperature creep
  3. To reduce the flow area and raise steam velocity
  4. To allow the tube to expand freely when heated

Explanation

Rolling expands the tube ends against the tube sheet holes, creating a tight mechanical seal and a strong grip. The expanded joint prevents leakage between the water side and the gas side.

Question 15Boiler and Pressure Vessel Construction, Design, and Fittings

What is the purpose of a superheater in a boiler?

  1. To increase steam temperature above saturation Correct answer.
  2. To remove moisture from steam before the drum
  3. To heat combustion air before the furnace
  4. To preheat feedwater before it enters the drum

Explanation

A superheater adds heat to saturated steam after it leaves the boiler drum, raising its temperature above the saturation temperature for that pressure. Superheated steam improves cycle efficiency and reduces moisture in the turbine.

Question 16Boiler and Pressure Vessel Construction, Design, and Fittings

In which boiler type do hot combustion gases pass inside tubes that are surrounded by water?

  1. Watertube boiler
  2. Once-through boiler
  3. Firetube boiler Correct answer.
  4. Waste-heat boiler

Explanation

In a firetube boiler, hot gases from combustion travel through tubes that are surrounded by water, transferring heat to the water. In a watertube boiler, the arrangement is reversed: water flows inside tubes that are surrounded by hot gases.

Question 17Boiler and Pressure Vessel Construction, Design, and Fittings

What does ligament efficiency measure in a boiler tube sheet?

  1. The ratio of steam flow to feedwater flow
  2. The ratio of tube length to tube diameter
  3. The metal between tube holes divided by pitch Correct answer.
  4. The ratio of furnace volume to heating surface

Explanation

Ligament efficiency compares the width of solid metal remaining between adjacent tube holes to the pitch, or center-to-center spacing, of the holes. It is used to account for the weakening effect of the holes when calculating tube sheet thickness.

Question 18Boiler and Pressure Vessel Construction, Design, and Fittings

Why must a boiler safety valve have sufficient relieving capacity?

  1. It must be tested only during the hydrostatic test
  2. It must be larger than the main steam line
  3. It must open only after the boiler is shut down
  4. It must pass full steam output without overpressure Correct answer.

Explanation

The safety valve must be able to discharge all the steam the boiler can generate at the allowable pressure accumulation limit. If capacity is too low, pressure can continue to rise even with the valve fully open.

Question 19Boiler and Pressure Vessel Construction, Design, and Fittings

What is the purpose of a try cock on a boiler?

  1. To drain the water column during blowdown
  2. To sample boiler water for chemical analysis
  3. To verify water level independently of the gauge glass Correct answer.
  4. To vent air from the steam drum at startup

Explanation

A try cock is a small valve located at a known level on the water column or drum. Opening it shows whether water or steam is present, providing an independent check on the gauge glass reading.

Question 20Boiler and Pressure Vessel Construction, Design, and Fittings

Which ASME material specification is commonly used for carbon steel boiler shell plate?

  1. SA-193
  2. SA-105
  3. SA-516 Correct answer.
  4. SA-213

Explanation

SA-516 is a common specification for carbon steel plate used in boilers and pressure vessels. It is available in several strength grades suitable for shell and drum construction.

Question 21Boiler and Pressure Vessel Construction, Design, and Fittings

A boiler shell has a maximum allowable working pressure of 150 psi, an inside radius of 30 in, an allowable stress of 13,800 psi, a joint efficiency of 0.85, and a corrosion allowance of 0.125 in. Using t = PR/(SE - 0.6P) + CA, what thickness is required?

  1. 0.63 in
  2. 0.51 in Correct answer.
  3. 0.39 in
  4. 0.75 in

Explanation

Substituting gives SE = 13,800 × 0.85 = 11,730 psi and 0.6P = 90 psi. The pressure thickness is 150 × 30 / (11,730 - 90) = 0.387 in; adding the 0.125 in corrosion allowance gives about 0.512 in, or 0.51 in.

Question 22Boiler and Pressure Vessel Construction, Design, and Fittings

Why can a flat boiler plate be made thinner when it is properly stayed?

  1. Stays raise the allowable stress of the plate material
  2. Stays carry load and shorten the unsupported span Correct answer.
  3. Stays increase the plate's resistance to corrosion
  4. Stays eliminate the need for a corrosion allowance

Explanation

Stays divide a large flat surface into smaller unsupported areas and carry part of the pressure load in tension. Reducing the span greatly reduces bending stress in the plate, so a thinner plate can safely withstand the pressure.

Question 23Boiler and Pressure Vessel Construction, Design, and Fittings

Under ASME Section I, what maximum pressure accumulation above MAWP is permitted when a safety valve discharges at capacity?

  1. 3 percent
  2. 15 percent
  3. 10 percent Correct answer.
  4. 6 percent

Explanation

ASME Section I limits pressure accumulation to 10 percent above the maximum allowable working pressure when the safety valve is discharging at its rated capacity. This margin allows the valve to relieve the boiler's maximum steam output without exceeding the code limit.

Question 24Boiler and Pressure Vessel Construction, Design, and Fittings

Why does a riveted lap seam generally have a lower joint efficiency than a butt seam?

  1. Lap seams require more staybolts than butt seams
  2. Lap seams are only permitted on firetube boilers
  3. They load the plate eccentrically and weaken it Correct answer.
  4. Lap seams cannot be calked to prevent leakage

Explanation

In a lap seam, the two plates are offset from each other, so the load path is eccentric and bending is induced at the joint. This reduces the strength of the seam compared with a butt seam, which keeps the plates in line.

Question 25Boiler and Pressure Vessel Construction, Design, and Fittings

What is the main purpose of a mud drum in a watertube boiler?

  1. To collect sediment and help distribute feedwater Correct answer.
  2. To preheat combustion air before the furnace
  3. To house the safety valve and pressure gauge
  4. To store superheated steam for the turbine

Explanation

The mud drum is the lower drum of a watertube boiler where sediment and sludge tend to settle, and it also helps distribute feedwater to the generating tubes. It is blown down regularly to remove the collected solids.

Question 26Boiler and Pressure Vessel Construction, Design, and Fittings

Which type of pressure relief device is required to protect a boiler under the ASME Boiler and Pressure Vessel Code?

  1. Pilot-operated relief valve
  2. Rupture disc device
  3. Spring-loaded pop safety valve Correct answer.
  4. Weighted-lever pressure relief valve

Explanation

Boiler code rules require each boiler to be protected by a direct spring-loaded safety valve that opens fully with a rapid pop action at its set pressure. This type reseats tightly and gives the large, quick discharge needed when a boiler overpressures. Other relief devices are used in piping and process service, not as the primary boiler overpressure protection.

Question 27Boiler and Pressure Vessel Construction, Design, and Fittings

Where is a fusible plug normally installed in a firetube boiler?

  1. In the feedwater inlet nozzle
  2. In the crown sheet above the fire Correct answer.
  3. In the main steam line ahead of the stop valve
  4. In the mud drum below the water level

Explanation

A fusible plug has a core of low-melting alloy and is screwed into the crown sheet, the plate directly over the combustion chamber. If the water level falls and uncovers that plate, the alloy melts and releases steam and water to warn the operator and quench the hot metal.

Question 28Boiler and Pressure Vessel Construction, Design, and Fittings

Why must a boiler carry a nameplate showing its design data?

  1. To give the date of the last inspection
  2. To show its MAWP and design limits Correct answer.
  3. To record the manufacturer's shipping address
  4. To list the fuel the burner was designed for

Explanation

The code stamp and nameplate permanently record the maximum allowable working pressure, design temperature, capacity and other limits established by the manufacturer. Inspectors and operators use this information to confirm the boiler is being operated within its design limits and that relief devices are set correctly.

Question 29Boiler and Pressure Vessel Construction, Design, and Fittings

What is the purpose of a dry pipe in a steam drum?

  1. To dry the boiler shell after a hydrostatic test
  2. To remove entrained moisture from the steam Correct answer.
  3. To spread feedwater along the length of the drum
  4. To cool the steam before it reaches the superheater

Explanation

A dry pipe runs along the top of the drum and takes steam through perforations or slots located well above the water surface. Collecting steam away from the turbulent water line reduces the water droplets carried out of the drum, which protects superheaters, turbines and piping from carryover.

Question 30Boiler and Pressure Vessel Construction, Design, and Fittings

Why must an opening cut in a boiler shell for a nozzle be reinforced?

  1. The opening cuts away load-bearing shell metal Correct answer.
  2. Nozzles are always made of weaker material than the shell
  3. Reinforcement keeps the nozzle from turning during welding
  4. Reinforcement prevents steam from leaking past the gasket

Explanation

A circular shell carries pressure load through its metal; a hole removes a section of that metal and concentrates stress around the edge of the opening. Code rules therefore require compensation, normally by excess thickness in the shell, the nozzle neck or the attaching weld, so the lost load-carrying area is replaced within defined limits around the opening.

Question 31Boiler and Pressure Vessel Construction, Design, and Fittings

What is the purpose of baffles in a firetube boiler?

  1. To support the tubes against pressure loads
  2. To direct feedwater toward the mud drum
  3. To shield the shell from radiant heat
  4. To route gases through several passes Correct answer.

Explanation

Baffles force the hot gases to travel back and forth across the tubes instead of going straight to the outlet, giving multiple gas passes. The longer gas path and higher gas velocity raise heat transfer to the water and improve efficiency.

Question 32Boiler and Pressure Vessel Construction, Design, and Fittings

Why is one end of a boiler shell often mounted on sliding supports?

  1. To reduce vibration transmitted to the foundation
  2. To allow the boiler to be moved without a crane
  3. To permit the shell to be rotated during erection
  4. To allow for thermal expansion of the shell Correct answer.

Explanation

A boiler shell expands lengthwise as it heats up and contracts as it cools. Fixing one end and letting the other end slide on rollers or pads accommodates that movement, preventing the shell from imposing large forces on its supports, connected piping or the setting.

Question 33Boiler and Pressure Vessel Construction, Design, and Fittings

Why is postweld heat treatment required after welding thick boiler shell sections?

  1. To relieve residual welding stresses Correct answer.
  2. To increase the yield strength of the weld metal
  3. To prevent distortion of the shell during fit-up
  4. To remove slag from the weld bead surface

Explanation

Welding thick sections leaves high residual stresses and a hardened heat-affected zone that can promote cracking. Postweld heat treatment soaks the completed joint at a controlled elevated temperature and cools it slowly, relieving those stresses and softening the heat-affected zone.

Question 34Boiler and Pressure Vessel Construction, Design, and Fittings

Why are furnace waterwall tubes often joined by welded fins, forming membrane walls?

  1. To form a gas-tight wall with high heat absorption Correct answer.
  2. To allow the furnace to be cooled by air instead of water
  3. To make the tubes easier to replace individually
  4. To reduce the boiler's water inventory

Explanation

Welding fins between adjacent tubes creates a continuous, gas-tight furnace enclosure. Air cannot leak into the furnace and hot gas cannot escape, and the membrane panel absorbs radiant heat over the full wall area rather than only at the bare tubes.

Question 35Combustion, Fuels, and Burner Operation

The minimum amount of air that supplies exactly enough oxygen to burn all the fuel completely, with no oxygen left over, is called:

  1. Secondary air
  2. Primary air
  3. Excess air
  4. Theoretical air Correct answer.

Explanation

Theoretical, or stoichiometric, air is the exact quantity that supplies just enough oxygen for complete combustion with no oxygen and no unburned fuel remaining. Burners always operate above this value because perfect mixing is impossible.

Question 36Combustion, Fuels, and Burner Operation

Under proper firing conditions, the flame from a gas burner should be:

  1. Short and blue with a distinct inner cone Correct answer.
  2. Long, luminous and orange
  3. Bright white with yellow tips
  4. Smoky at the tip

Explanation

A blue flame with a well-defined inner cone shows that gas and air are properly proportioned and burning completely. Yellow, luminous or smoky flames point to a shortage of air or to overfiring.

Question 37Combustion, Fuels, and Burner Operation

The flash point of a fuel oil is the temperature at which the oil:

  1. Will burn steadily on its own once lit
  2. Gives off enough vapor to flash when a flame is applied Correct answer.
  3. Is thin enough to be atomized by the nozzle
  4. Begins to boil and vaporize completely

Explanation

The flash point is where vapor above the liquid forms a flammable mixture that flashes momentarily when an ignition source is present. It is a safety property used to classify the fire hazard of the fuel.

Question 38Combustion, Fuels, and Burner Operation

The purpose of the pre-purge before burner light-off is to:

  1. Heat the fuel oil to atomizing temperature
  2. Verify the flame scanner is working
  3. Warm the refractory to prevent thermal shock
  4. Clear combustible gases from the furnace Correct answer.

Explanation

Purge air sweeps the furnace and flue passages so that any accumulated fuel gas or vapor is removed before an ignition source is introduced. Light-off without a proven purge risks a furnace explosion.

Question 39Combustion, Fuels, and Burner Operation

Compared with air, natural gas is:

  1. Of nearly the same density as air
  2. Heavier, with a specific gravity close to 1.5
  3. Lighter, with a specific gravity close to 0.6 Correct answer.
  4. Lighter only when it is burning

Explanation

Natural gas is mostly methane and has a specific gravity of about 0.6 relative to air, so leaks rise and can collect at the ceiling or in roof spaces. This is why gas detectors are mounted high in the boiler room.

Question 40Combustion, Fuels, and Burner Operation

Furnace draft refers to:

  1. The pressure drop across the air heater
  2. A slight negative pressure in the furnace Correct answer.
  3. The speed of flue gas in the stack
  4. The pressure of atomizing steam at the nozzle

Explanation

Draft is the small pressure difference that draws combustion air in and flue gas out. The furnace is normally held slightly below atmospheric so gases leak in rather than out.

Question 41Combustion, Fuels, and Burner Operation

Persistent dark smoke at the boiler stack is a reliable indication of:

  1. Incomplete combustion from too little air Correct answer.
  2. Too much excess air
  3. Clean heat transfer surfaces
  4. Water carryover in the steam

Explanation

Smoke is made up of unburned carbon particles, so it appears when oxygen is short or air and fuel are poorly mixed. It also occurs when oil is not atomized properly.

Question 42Combustion, Fuels, and Burner Operation

Fuel builds up in the furnace after a failed pilot and then ignites with a loud rumble. This event is called:

  1. Pre-ignition
  2. A proper purge
  3. Delayed ignition Correct answer.
  4. Flame lift-off

Explanation

Delayed ignition occurs when a fuel-air mixture accumulates before a spark or hot surface ignites it, producing a pressure wave. Repeated delayed ignition points to a weak pilot or faulty ignition timing.

Question 43Combustion, Fuels, and Burner Operation

Raising excess air from 10% to 50% in a balanced-draft boiler will normally:

  1. Reduce the oxygen in the flue gas
  2. Have no measurable effect on stack loss
  3. Raise the stack temperature and reduce efficiency Correct answer.
  4. Lower the stack temperature and raise efficiency

Explanation

Extra air passes through the boiler and leaves at flue gas temperature without giving up heat to the water, so stack loss and stack temperature rise while efficiency falls. Excess air also increases fan power.

Question 44Combustion, Fuels, and Burner Operation

Fuel oil is atomized before burning mainly because atomization:

  1. Raises the heating value of the oil
  2. Exposes far more surface area to the air Correct answer.
  3. Lowers the flash point of the oil
  4. Removes sulfur from the oil

Explanation

Oil burns as a vapor, so it must be broken into fine droplets that evaporate quickly and mix with combustion air. Poor atomization causes smoke, carbon monoxide and flame impingement.

Question 45Combustion, Fuels, and Burner Operation

#6 fuel oil is heated before it reaches the burner because heating:

  1. Raises the heating value of the oil
  2. Reduces viscosity so the oil atomizes properly Correct answer.
  3. Raises the flash point above furnace temperature
  4. Removes water from the oil

Explanation

Heavy oil is too viscous to pump and atomize at ambient temperature, so it is held at a temperature that gives the viscosity the burner needs. Overheating can cause coking and vapor lock at the nozzle.

Question 46Combustion, Fuels, and Burner Operation

A flue gas analyzer shows 0% O2 and 4% CO. The most likely cause is:

  1. An oversized burner nozzle
  2. Too much excess air
  3. Insufficient combustion air Correct answer.
  4. Excessive furnace draft

Explanation

Zero oxygen means all the supplied oxygen has been consumed, and the carbon monoxide shows that carbon was left partly unburned. That combination is the classic signature of air-starved combustion.

Question 47Combustion, Fuels, and Burner Operation

Burning sulfur-bearing oil can corrode the cold end of the boiler because:

  1. Sulfur oxides form acids that condense on cold metal Correct answer.
  2. Sulfur particles plug the burner nozzle
  3. Sulfur lowers the flame temperature
  4. Sulfur raises the dew point of the steam

Explanation

Sulfur in the fuel oxidizes to SO2 and some SO3, which combines with water vapor to form sulfurous and sulfuric acid. If the metal surface drops below the acid dew point, the acid condenses and attacks the economizer and stack.

Question 48Combustion, Fuels, and Burner Operation

A burner rated 12,000,000 Btu/h maximum with a 4:1 turndown has a minimum firing rate of:

  1. 3,000,000 Btu/h Correct answer.
  2. 4,000,000 Btu/h
  3. 6,000,000 Btu/h
  4. 1,000,000 Btu/h

Explanation

Turndown is the ratio of maximum to minimum firing rate, so the minimum is the maximum divided by the turndown. Here 12,000,000 divided by 4 equals 3,000,000 Btu/h.

Question 49Combustion, Fuels, and Burner Operation

If the flame scanner loses its signal while the burner is firing, the flame safeguard control must:

  1. Shut off the fuel within a few seconds Correct answer.
  2. Drive the burner to high fire to steady the flame
  3. Sound an alarm only and keep firing
  4. Open the forced draft damper fully

Explanation

The flame safeguard must treat loss of signal as a flame failure and close the fuel valves quickly so unburned fuel cannot accumulate. An alarm alone, or continued firing, defeats the purpose of the interlock.

Question 50Combustion, Fuels, and Burner Operation

The swirl vanes in a burner air register improve combustion by:

  1. Reducing the amount of air entering the furnace
  2. Promoting thorough mixing of air and fuel Correct answer.
  3. Cooling the burner throat
  4. Increasing the fuel pressure at the nozzle

Explanation

Swirling the air creates turbulence and recirculation that mixes oxygen with fuel vapor and stabilizes the flame. The register setting also controls how much air enters the burner.