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Warut Sintapanon | Dreamstime
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To highlight in depth the most common causes of refractory failure and explain the steps end users should take to avoid an unplanned shutdown, Morgan Advanced Materials and SK Energy collaborated on a report — “Understanding Refractory Failures in Fired Heaters.”

Understand the Common Causes of Refractory Failure

Aug. 12, 2017
It is not uncommon for furnace lining material to fail, resulting in loss of thermal energy, reduced performance, or even a complete outage. Informed collaboration with refractory material suppliers can minimize risk and improve furnace reliability.

Refractory materials and lining reliability are critical factors for increasing the performance of a furnace or thermal chambers used for other industrial processes (e.g., kilns, heaters, incinerators.) Not only can the right refractory lining optimize production yield and minimize energy costs, but it also can establish the basis for consistent high performance of a furnace over the performance life of a refractory lining, in some cases for as long as 20 years. However, it is not uncommon for a refractory material to fail, resulting in wasted energy, reduced performance, and in some cases complete shutdown. Through careful collaboration between furnace operators and refractory material suppliers, the risk of failure can be significantly minimized, and reliability improved. Here are seven of the most common causes of refractory failure, outlining what engineers can do to both fix a problem and avoid it in the future.

1.  Fallen fiber modules. This is often related to material, design, or installation. If the modules and support anchoring are missing, the most likely cause is an installation error, like insufficient stud welding, or excessive corrosion from the shell caused by sulfur or rust. If most of the fiber is missing but the support anchoring is intact, it is more likely to be due to mechanical abuse, for example water placing excess weight on the fiber.

If a section of insulation shows most of the fiber is missing but the support anchoring is intact, it is likely due to mechanical abuse.

 

Because fiber is 90% porous, it absorbs many times its weight in water. Make sure to 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. Are there 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.

2.  Failing brick walls. Insulating firebricks (IFBs) are commonplace in many fired furnaces, and like any lining these require good materials, design, and installation to give good service life. 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 back-up lining. To address voids, pump hot spot repair materials in from the outside of an operating unit.

Insulating firebricks (IFBs) are a common lining material, and close examination of failed IFB linings may reveal hot spots or issues with the back-up lining.

 

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 that also can be a result of changing operating conditions, and higher outputs or box temperatures.

3.  Bridgewall/tunnel wall leaning, or deformation.  It is not uncommon to see walls lean to some extent, but if they lean too much it may cause failure, and it could be a result of the floor not being level. Also, a lot of wall issues are 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 that not all firebricks are created equal; they differ in formulation, firing and high temperature properties. The key to making the best selection is to investigate both the ambient and hot strength properties. Don’t focus too much on cost but on reliability – the best products for the job typically are not the cheapest.

4.  Castable cracking. Castable linings are unique as they are not in a finished state when they leave a manufacturing facility. This means final quality is dependent on the installer.

Refractory may fall from furnace roof sections if the modules and support anchoring are installed improperly, or if there is excessive corrosion from the shell

 

Materials must be mixed with clean water of the correct temperature range, 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 may spall explosively. Shrinkage cracking is normal, but if this becomes excessive it could be a consequence of poor installation, and may indicate that too much water was used.

5.  Floor cracking/heaving. This is a common issue when temperatures increase because of the original design. Provisions for reversible thermal expansion or expansion joints in the floor should be protected, as they can easily become filled with debris during normal operation, limiting the gap’s movement capability. It’s good practice to vacuum gaps of collected debris regularly, to avoid build-up.

Floor cracking also is 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 inadequate expansion joints are not installed.

6.  Convection castable/corbel damage. Castables are prone to damage during the construction and shipping process and this often manifests itself in the form of visual cracks, typically through the entire thickness. You also may notice some pinch spalling at the surface, which indicates directional mechanical flexure of the steel. Corbels also may 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.

7.  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.

Shrinkage cracking is normal in furnace lining, but if it becomes excessive it may indicate too much water was used during installation 

In many cases outlined above, the hot effluent gases will make their way through the compromised refractory lining, resulting in hot spots 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 a peep sight, you should use refractory materials similar to those surrounding the opening to avoid design issues and to create the best possible seal.

Tube seals also will 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).

To highlight in depth the most common causes of refractory failure and explain the steps end users should take to avoid an unplanned shutdown, Morgan Advanced Materials and SK Energy collaborated on a report — “Understanding Refractory Failures in Fired Heaters” — available for download at www.morganadvancedmaterials.com