Marine Boiler Refractory and Its Types
The primary function of refractory material used inside a marine boiler is to contain the heat generated by the fuel burning in the furnace and to minimise heat losses from the furnace. As a result, it is critical that these materials have insulating properties and can withstand high temperatures.
Furthermore, the refractory used inside a boiler furnace must not contaminate the material with which it comes into contact.
The refractory material used in Marine Boilers should have sufficient mechanical strength and be able to withstand a variety of forces such as:
- Weight of neighboring brickwork
- The action of vibration
- frame cutting and abrasive action
- Dust from a chimney
- Extreme heat
- Temperature fluctuations are common.
- Service load
- The operational phases’ chemical and abrasive action
The materials (pure compounds) used to make refractory (such as MgO, SiC, Fireclay, and others) have high melting points in the 1800° to 2800° C range.
Temperature changes should cause the material to expand and contract uniformly without cracking. Given that it is not economically feasible to use a single refractory in the boiler, different types are used together after ensuring that they can withstand the temperatures to which they are subjected.
Important Terms Defining The Marine Boilers Refractory Property
Refractoriness: It is a property of a refractory that determines how much it will deform under its own load. It is generally determined by the material composition used to make a refractory.
Porosity is the ability to withstand chemical attack ( usually by fuel and water). A low porosity value indicates high strength and thermal conductivity.
Refractory Strength: It is the refractory resistance to compressive, tension, and shear stresses.
Spalling: It is a type of refractory defect caused by an excessive thermal and/or mechanical load on the refractory.
Permanent Linear Change (PLC) on Reheating: A permanent change in the property of the refractory caused primarily by high temperature.
Thermal conductivity: It describes the general heat flow characteristics of the refractory.
Thermal expansion: An important factor in determining a refractory product’s ability to expand during high temperatures and contract during cooling.
Bulk density: It is measured as the weight of a given volume of refractory and is related to the apparent porosity of the material used. A refractory with a higher bulk density is of higher quality.
The materials used to make refractories are divided into three categories: acid materials, which include clay, silica, quartz, sandstone, and gamister.
Neutral material composed of chromite, graphite, plumbago, and alumina.
Alkaline or base material composed of lime, magnesia, and zirconia.
Special care must be taken when selecting the refractory material, and it must be ensured that the acid and alkaline materials are kept separate because at high temperatures, the two react with each other to form salt, reducing the refractory’s effectiveness.
The refractory material is available in two forms for installation:
Firebricks
The material is formed into bricks, which are then fired at high temperatures in specially designed kilns.
Advantages:
- No-hassle performance
- High fortitude
- High heat resistance
- Low upkeep costs
Monolithic Refractory
These refractory materials are delivered unfired and installed in the boiler. They are then fired on the spot or when the boiler is turned on. There are several types of refractory.
- a) Moldable Refractory: This type is used when direct radiant heat exposure is required. During installation, it must be pounded into position.
Advantages:
- Formulation of high quality
- Density is high
- Low operating costs
- Effective performance
- b) Castable Refractory: This refractory is used in areas where there is no direct exposure to radiant heat, such as behind water walls. It is installed in the same way that building concrete is.
Advantages:
- Performance that is efficient
- Long working life
- High fortitude
- Low upkeep costs
- Precision in dimensions
- c) Plastic Chrome Core: This type is bonded to clay and is used to build studded water walls. They can withstand extremely high temperatures but have extremely low mechanical strength. These are welded to the tube after being pounded onto the steel studs. These studs add strength and a point of attachment for the refractory.
Advantages:
- High corrosion resistance
- High refractoriness
- High power
- High heat resistance
Precautions to be taken during and after the installation of boiler refractory:
1) To avoid undue stresses in the refractory material, plenty of room for expansion should be provided. It is designed to prevent these spaces from becoming obstructed in any way, causing the refractory to break away from the attachment and bulge out, posing the risk of collapse.
2) The amount of time required to raise steam is determined by the refractory material. As a result, a greater amount of refractory slows down the steam raising process to prevent refractory damage.
3) When the boiler is turned off, air dampers or checks should be closed to prevent cold air impingement on the hot refractory. This impingement results in surface flaking, also known as Spalling. Spalling reduces refractory wall thickness.
4) Avoid flame impingement on the refractory as this causes carbon deposits to form on the surface. Carbon enters the refractory and damages it.
5) Fuel impurities such as vanadium and sodium salts react with refractory material to form molten slag, which falls to the furnace floor. This causes a reduction in wall thickness, and the accumulation of slag interferes with the shape of the flame. Impurities should thus be kept out of the boiler.
Different Causes of Marine Boiler Refractory Damage
- Incorrect refractory installation
- Installation of ferrules of the wrong shape
- Inadequate refractory dry out after installation results in thermal shock.
- Improper shutdown operation results in rapid cooling of the refractory.
- Thermal, structural, or mechanical stresses can cause ceramic cracking.
- High temperatures within the furnace as a result of incorrectly using a higher level of oxygen or a temperature control system failure
- Corrosion failure causes a cold spot, acid formation, and undesirable chemical reactions.
- Improper flame velocity
- Erosion caused by improper high-tension loading
- Vibration caused by faulty design
- Incorrect refractory selection
The refractory inside the boiler furnace is a critical structure for protecting the boiler structure and increasing its efficiency of operation. The ship engineer in charge of boiler maintenance must ensure that the refractory is visually inspected on a regular basis and that the boiler is properly operated.
