Refractory materials and lining reliability are key to increasing the performance of kilns, furnaces, fired heaters, incinerators, and reactors across a range of industries. Not only can the right refractory lining optimize production yield and minimize energy costs, but it can also enable consistent high performance of a furnace over the lifecycle of the refractory lining, in some cases for as long as 20 years.
However, it is not uncommon for a refractory material to fail, resulting in energy waste, reduced performance and, in some cases, complete furnace shutdown. Careful collaboration between end users and refractories material suppliers can minimize the risk of failure and improve reliability.
Fiber Modules Fallen from the Roof
When fiber modules fall from the roof of a kiln, the issue is often material, design or installation related. If the modules and support anchoring are missing, the most likely cause is an installation issue, insufficient stud welding, or excessive corrosion from the shell caused by sulphur or rust. If most of the fiber is missing but the support anchoring is intact, it is more likely to be a result of mechanical abuse (e.g., water placing excess weight on the fiber). Since fiber is 90% porous, it absorbs many times its weight in water.
Make sure you check the fiber to see if it was torn off the anchoring, or has signs of water damage. It is always important to look at the area affected and take note of what it is telling you. Can you see gaps in the fiber? Is a hot spot associated with the gaps? Always check the fiber chemistry and design to ensure you can quarantine fallen fibers before the issue spreads.
Failing Brick Walls
Insulating fire bricks (IFBs) are commonplace in many furnaces and, like any lining, require good materials, design, and installation to give good life in service. Upon examining IFB linings closely in the event of failure, you may be able to determine if they were the root cause. Look for hot spots outside the unit. If the wall is in bad shape, this may indicate issues with the backup lining. To address voids, pump hot spot repair materials in from the outside of an operating unit.
Make sure you also look at the face of the brick. It may have melted or cracked, which indicates higher furnace operating temperatures, possible fuel impurities, or the wrong grade brick being used. If the hot face brick is in good shape but the wall is bowed, this could be due to inadequate thermal expansion provisions, which can also be a result of changing operating conditions and higher outputs or box temperatures.
Bridgewall/Tunnel Wall Issues
It is not uncommon to see walls lean to some extent. However, if they lean too much, which could be a result of the floor not being level, it can cause failure. A lot of wall issues are also due to inadequate expansion provisions (design), particularly if you increase operating conditions to higher temperatures than expected. Increased temperatures can also cause slumping over time.
Remember, not all firebrick are created equal. They differ in formula, firing and high-temperature properties. The key to making the best selection is investigating both the ambient and hot strength properties. Don’t focus too much on cost, but on reliability—the best products for the job are typically not the cheapest.
Castable linings are unique in that they are not in a finished state when they leave a manufacturing facility. This means that the final quality is dependent on the installer.
Materials must be mixed with clean water of the correct temperature range, and installed and cured before water is removed during the dry-out. If dry-out is not done in a slow and controlled manner, the castable can explosively spall. Some shrinkage cracking is normal. If it becomes excessive, however, it could be a consequence of poor installation and may indicate that too much water was used.
Issues with the furnace floor cracking or heaving are common when temperatures increase after the original design. Provisions for reversible thermal expansion or expansion joints in the floor should be protected, as they can easily be filled with debris during normal operation, limiting the gap’s movement capability. It’s good practice to regularly vacuum gaps to avoid debris buildup.
Floor cracking is also common when dissimilar materials are used. If you have a floor-fired unit, you will have castable burner blocks of a certain material grade and a different floor material surrounding the burner. It’s common to see cracks appear at the corner of the burner blocks if adequate expansion joints are not installed.
Convection Castable/Corbel Damage
Castables are prone to damage during the construction and shipping process. This often manifests itself in the form of visual cracks, typically through the entire thickness. You may also notice some pinch spalling at the surface, which indicates directional mechanical flexure of the steel. Corbels may also be susceptible to damage as they protrude from the base lining. Any apparent damage should be quickly repaired, and the affected portion of the lining removed and replaced to avoid damaging surrounding materials.
Mating Dissimilar Materials
Dissimilar refractory materials adjacently located are common, particularly surrounding openings such as doors (fiber and brick), peep sights (IFB, castable, fiber modules), burner blocks and pressure-relief doors. Because dissimilar materials have different refractory properties at elevated temperatures, this makes a homogenous design difficult.
In many cases, the hot effluent gases will make their way through the compromised refractory lining, resulting in hotspots on the outer casing. If these surround peep sights and door openings, it’s possible that the interface designs are inadequate. In the case of the peep sight, you should use similar refractory materials to those surrounding the opening to avoid design issues and to create the best possible seal.
Tube seals also provide personnel protection, encouraging an influx of ambient air into the furnace. For peep sights and walls, always use a high-temperature fiber expansion joint, as this will avoid the issue of having to mate an expanding material (IFB) with a material that expands and shrinks differently (castable).
This article was originally posted on www.ceramicindustry.com.