Refractories

A material in solid form which is capable of maintaining its shape at high tempo (furnace tempo as high as 1650^oC) have been recorded.

Purpose

1. To protect blr casing from overheating and distortion and the possible resulting leakage of gasses into the machinery space.
2. To reduce heat loss and ensure acceptable cold faced temperature for operating personnel
3. To protect exposed parts of drum and headers which would otherwise become overheated. Some tubes are similarly protected.
4. Act as a heat reservoir.
5. To be used to form baffles for protective purposes or for directing gas flow.

Properties

1. Must have good insulating properties.
2. Must be able to withstand high tempo's
3. Must have the mechanical strength to resist the forces set up by the adjacent refractory.
4. Must be able to withstand vibration.
5. Must be able to withstand the cutting and abrasive action of the flame and dust
6. Must be able to expand and contract without cracking Note: no one refractory can be used economically throughout the boiler

Types

1. Acid materials- clay, silica, quartz , sandstone etc
2. Neutral materials-chromite, graphite, plumbago, alumina
3. Alkaline or base materials- lime, magnesia, zirconia

Note that acid and alkaline refractories must be sepperated

Forms

1. Firebricks- these are made from natural clay containing alumina , silica and quartz. They are shaped into bricks and fired in a kiln
2. Monolithic refractories- These are supplied in the unfired state, installed in the boiler and fired in situ when the boiler is commissioned.
3. Mouldable refractory- This is used where direct exposure to radiant heat takes place. It must be pounded into place during installation . It is made from natural clay with added calcided fire clay which has been chrushed and graded.
4. Plastic chrome ore- This is bonded with clay and used for studded walls. It has little strength and hence stud provides the support and it is pounded inot place.It is able to resist high temperatures
5. Castable refractory-This is placed over water walls and other parts of the boiler were it is protected from radiant heat . It is installed in a manner similar to concreting in building
6. Insulating materials- Blocks, bricks , sheets and powder are usually second line refractories. I.E. Behind the furnace refractory which is exposed to the flame. Material; asbestos millboard, magnesia , calcined magnesia block, diatomite blocks, vermiculite etc. all having very low heat conductivity.

Furnace linings

Studded tubes

- these are lined with plastic chrome ore The amount of studding and the extent of tube surface covered with chrome ore is varied to suit the heat absorption rate required in the various zones of the boiler furnace.
Floor tubes may be situated beneath a 3" layer of brickwork, the tubes are embedded in chrushed insulating material below which is a layer of solid insulation and then layers of asbestos millboard and magnessia.

PRESENT DAY TYPES

TANGENT WALL.

Membrane wall

Furnace floors

- Two layers of 50 mm firebrick above the tubes and 100 mm slab insulation below. Tubes in castable insulation are covered with crushed firebrick. Note; Before castable insulation applied ,tubes coated with bitumen to allow expansion clearance when tubes are at working tempo

Front walls

- In front fired boilers these need additional insulation (200 mm) made up of 125 mm mouldable refractory backed by 50 mm castable or slab and 25 mm of asbestos millboard.

Burner openings

- These have specially shaped bricks called quarls or have plastic refractory

Brick bolts

There are two basic types;

1. using a hole right through the brick
2. Using a recess in the back of the brick.

A source of weakness is where bricks crack, bolts will be exposed to the direct heat which leads to failure.
Adequate expansion arrangements must be provided. For floor tubes a coating of bitumastic is first applied before the castable refractory is applied. When the boiler is fired the bitumastic is burnt away then a space is left for expansion

Refractory failure

This is one of the major items of maintenance costs in older types of boiler

SPALLING

This is the breaking away of layers of the brick surface. It can be caused by fluctuating temperature under flame impingement or firing a boiler too soon after waterwashing or brick work repair. May also be caused by failure to close off air from register outlet causing cool air to impinge on hot refractory.

SLAGGING

This is the softening of the bricks to a liquid state due to the prescience of vanadium or sodium ( ex sea water ) in the fuel. This acts as fluxes and lowers the melting point of the bricks which run to form a liquid pool in the furnace Eyebrows may form above quarls and attachment arrangements may become exposed Material falling to floor may critically reduce burner clearance and reduce efficiency Flame impingement may lead to carbon penetrating refractory.

SHRINKAGE CRACKING

Refractories are weaker in tension than in compression or shear thus, if compression takes place due to the expansion of the brick at high temperature , if suddenly cooled cracking may occur.

Failure of brick securing devices