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A pilot that burns without turndown throughout the entire time the boiler is in service.

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The length of time fuel is permitted to be delivered to a proved pilot before the flame-sensing device is required to detect pilot flame.


Caustic corrosion or corrosion resulting from the presence of sodium hydroxide can occur in boilers. It was previously stated that maintaining elevated hydroxyl ion concentrations is desirable for both steel and copper surfaces. This is true, but there is a range of conditions (pH values) above which the presence of free hydroxyl ions can be damaging to these materials.

Sodium hydroxide (NaOH) is one of the major chemical additives used in the treatment of boiler water. The purpose of sodium hydroxide is to maintain the hydroxyl ion concentration in the optimum range for the formation of good protective magnetite on steel surfaces. Its other role is to help in the formation of non-adherent sludge instead of scale when hardness enters the boiler water.

Excessive amounts of sodium hydroxide can, however, lead to corrosion. This is particularly true of ultra-high pressure boilers. If there is too much sodium hydroxide present at a steel surface, this chemical can react with the steel to form a soluble material which can then precipitate as a loose, porous magnetite deposit. The following reactions illustrate the manner in which it can occur:

Fe+2NaOH — Na2FeO2+H2

3Na2FeO2+4H2O — 6NaOH+Fe3O4

The normal concentrations of sodium hydroxide maintained in boilers are not harmful. However, it is possible for sodium hydroxide to concentrate in localized areas of boilers, thereby leading to localized corrosion. This occurs when heavy layers of deposits form on the boiler tubing, causing sodium hydroxide to be concentrated under the deposits at the metal surface.

Caustic corrosion can also occur when boiler tube surfaces become steam blanketed because of either excessive boiling or separation of steam and water in horizontal or inclined tubes.

Excessive boiling can result from very high heat transfer rates which can occur when burners are misaligned in a furnace and the flames impinge on the boiler tubing. In this case, boiler water containing sodium hydroxide can splash onto the steam blanketed surface and as the water boils off the sodium hydroxide concentrations can become excessive.

A similar effect sometimes occurs in the horizontal or inclined tubing if there is insufficient mass velocity in the tubing to keep the steam and water well mixed. If the water separates and flows along the bottom of the tube and the steam along the top of the tube, water can splash onto the hot dry upper surface. As the liquid boils from the droplet on the hot upper surface, excessive sodium hydroxide concentrations can occur.

Because caustic corrosion is a very significant problem in ultra-high pressure boilers, the free hydroxide must be eliminated from the boiler water by using the co-ordinated phosphate method of treatment.


To burn oil the temperature must be raised to vaporisation temperature, this can not be done in heaters due to gassing but is done by radiant heat in the flame. The lighter hydrocarbons in the atomised spray are rapidly heated and burnt in the primary flame. The heavier fractions pass through this achieving their vaporisation temperature. The primary flame is essential to good combustion. By design the primary flame exists where it receives maximum reflected heat from the shape of the quarl. The size of the primary flame ( shown smaller than actual in drawing) just fills the quarl space. Too large and impingement leads to carbon deposits building up. Too small unheated secondary air reduces combustion efficiency. The tip plate creates vortices reducing the mixing time for the air/fuel and reduces the forward speed of the flame.


This is the name given to an assembly of vane air swirler plates etc fitted within the boiler casing in association with each burner ,its functions is to divide air into primary and secondary streams and to direct them such as to give the correct air flow pattern. The air must pass through the air check to enter the register . In some cases the check can be formed by the swirl vanes themselves by rotating them about their axis, in other cases a sliding sleeve is used. The inner primary air flows until it reaches the tip plate ( stabiliser ) then spills over to form a series of vortices which reduces the forward velocity of the air. This retains the primary flame within the quarl . The outer , secondary air passes over the swirler vanes which causes the air to rotate thus assisting the mixing of air and fuel. The secondary air shapes the flame, short and fat for side fired, longer and thinner for roof fired. A small amount of cooling air is allowed to flow to the tip plate and atomiser tip. It is important that the air check forms a tight seal otherwise thermal shock can damage the quarls when the burner is not in use The front plate is usually insulated , the complexity of the air control is related to the TDR .The steam jet types have the steam providing additional energy for the mixture of air and fuel.


A small burner which is used to light the main burner.


A hot water boiler fitting that controls the starting and stopping of the burner by sensing the temperature of the water.


A device to stop the burner on unsafe water conditions in the boiler.


Here the heavier fractions are burnt in the boiler furnace. The velocity of the air and fuel must be matched to the required flame propagation rate.


The lowest water level in a boiler drum at which the burner may safely operate.


A safety device that will shut off the boiler burner operation if the water level in the boiler becomes too low.

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