Different Types of Refractory Materials Used in Boilers
The main purpose of refractory material that is used inside a marine boiler is to contain the heat generated by burning of the fuel in the furnace. It is therefore important that these materials have insulating properties and are able to withstand high temperatures.
Boiler refractory material should have sufficient mechanical strength and should be able to withstand various forces like
1) Weight of adjacent brickwork.
2) Vibration action.
3) Cutting & abrasive action of frame.
4) Flue dust.
The material should be able to expand and contract uniformly with tempearture change without cracking. Economically, it is not possible to use a single refractory in the boiler and thus different types are used to together after ascertaining that they are able to withstand the temperatures they are subjected to.
The materials from which the refractories are made are classified into three groups :
1) Acid materials which consists of clay, silica, quartz, sandstone, gamister.
2) Neutral material which consists of chromite, graphite, plumbago, alumina.
3) Alkaline or Base material which consists of lime, magnesia, zirconia.
Special care is to be taken while choosing the refractory material and it must be insured that the acid and alkaline material are kept apart as under high temperatures the two react with each other to form salt, which reduces the effectiveness of refractory.
For installation, the refractory material is available in two forms:-
1) Firebricks
The material is formed into bricks and these bricks are then fired at high temperature in special kilns.
2) Monolithic Refractory
These refractory are supplied in unfired state and installed in the boiler. They are then fired in place or when the boiler is put into service. This type of refractory can be divided into :-
a) Mouldable Refractory
This type is used when direct exposure to radiant heat takes place. It must be pounded in place during installation.
b) Castable Refractory
This type of refractory is placed where there is no direct exposure to radiant heat for e.g behind water walls. It is installed in manner similar to building concrete.
c) Plastic Chrome Core
This type is bonded with the clay and is used in contruction of studded water walls. They can withstand very high temperature but have a very low mechanical strength. These are pounded onto the steel studs and welded to the tube. These studs provide addition strength and means of attachment for the refractory.
Precautions to be taken during and after installation of refractory
1) To prevent undue stresses in the refractory material ample space should be provided for expansion. It is made to ensure that the these spaces does not get blocked in any way and cause refractory to break off from attachment and bulge out, with danger of possible collapse.
2) Refractory material determines the time required for raising steam. So greater amount of refractory slows down the steam raising process to prevent damage to refractory.
3) Air dampers or checks should be closed as soon as boiler is shut down preventing cold air impingement on the hot refractory. This impingement causes surface flaking which is commonly known as Spalling. Spalling causes reduction in wall thickness of the refractory.
4) Flame impingement should be avoided on the refractory as this causes build up of carbon deposit on the surface. The carbon penetrates into the refractory and damages the same.
5) Impurities in fuel like vanadium and sodium salts react with refractory material to form molten slag, which runs down to the furnace floor. This causes reduction of wall thickness and building up of this slag interferes with the shape of the flame. Thus impurities should be prevented from entering the boiler.
3 December 2021 at 13:05
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