REFRACTORY MATERIAL IN BOILER- TYPE, PURPOSE, FAILURES

REQUIREMENTS OF BOILER REFRACTORY MATERIAL-:

1. It should contain the heat generated in the furnace.

2. It must  have good insulating properties 

3. Must able to withstand the high temperatures to which they will be

exposed . 

4.It should  have sufficient mechanical strength to resist the forces set up

by the weight of adjacent brickwork.

5. It should withstand vibration and the cutting and abrasive action of the flame and flue dust. 

6. The materials must also be able to expand and contract uniformly without cracking.

The material from which these refractories are manufactured can be grouped into -:

 Three main types:

* Acid materials: Clay, silica, quartz, sandstone, gamister.

* Neutral materials: Chromite, graphite, plumbago, alumina.

* Alkaline or base materials: Lime, magnesia, zirconia.

PURPOSE OF REFRACTORY 

1. They give protection to the furnace casing from overheating, burning    and possible escape  of gases into the boiler room. 

2. They reduce heat loss from the boiler by enveloping the combustion chamber or furnace. 

3. This assists in maintaining the high temperature necessary for complete combustion under all conditions. 

4. They give protection to furnace tubes against overheating. 

5. They act as a reservoir (sink) of heat and ensure more even distribution of heat  throughout the furnace.

TYPES OF REFRACTORIES 

1. FIREBRICK -:  It is made of naturally occurring clays which contain alumina and  silica, the clay is shaped into brick form and then fired in a furnace. 

2. PLASTIC REFRACTORY -:  This type of  refractory can be moulded into position hence Mouldable. A plastic refractory material is generally found inside the boiler furnace and is designed to resist higher temperatures and chemical or abrasive attack

3. INSULATING MATERIAL -:

- Insulating blocks, bricks, sheets and powder are usually second line refractory .

- They are behind the furnace refractory which is exposed to the burner flame. 

-The material  could be asbestos millboard, magnesia asbestos, calcined magnesia blocks or diatomite  blocks, diatomite is porous and siliceous.

- It may be used as a powder, bonded with clay and  made into bricks.

THESE REFRACTORY MATERIALS ARE AVAILABLE FOR INSTALLATION IN ONE OF TWO BASIC FORMS:

FIREBRICKS

These are formed into bricks and then fired at high temperatures in special furnace.

MONOLITHIC REFRACTORIES

 This form of refractory can be subdivided into:

a) Mouldable refractory-: This is used where direct exposure to radiant heat takes place. It must be pounded into place during installation.

b) Castable refractory -:Placed behind water walls and other parts of the boiler where it is protected from direct exposure to radiant heat. .

c) Plastic chrome ore-: This bonded with clay , is used in the construction of studded water walls. It can resist high temperatures, but has little mechanical strength, and is pounded onto steel studs welded to the tubes. These studs provide both strength and means of attachment for tile refractory.

TYPES OF REFRACTORIES FAILURE 

SPALLING  -: Spalling is the name given to the breaking away in layers of  the surface of firebrick . Spalling could be caused by: 

 - Fluctuation in temperature resulting in cracking 

 - Mechanical straining due to furnace pressure variation 

 - Defective construction .

SLAGGING  -:It is  a softening, even up to the liquid state, of the surface of the  firebricks. Slagging could be caused by :

 - Sodium in the ash or in fuel as sea water contamination.

 - sodium being present at the high temperature brick                  surfaces, acts as a flux and lowers the melting points of the      fire brick.

 - It build-up of semi-molten material on the floor, which              results in failure of the bricks.

FAILURE OF BRICK SECURING DEVICE  -:Bricks which are secured by bolts have, due to the restraining action of the bolt, a  tendency to   crack, and once this occurs the bolts are exposed to the high  temperature gases, which rapidly cause bolt failure through  overheating. 

SHRINKAGE CRACKING -: Refractory are generally weaker in tension than in compression . Casing, will be the subject of high tensile  stresses and shrinkage cracking occur when suddenly  cooled.