The design of high performance, reliable furnaces and pyrometallurgical vessels is incomplete without inclusion of monolithic refractory linings and anchoring.
Anchors and monolithic refractories are an integral part of any successful vessel design, insulation, heat transfer management and installation. Despite anchors playing an integral part in the performance of a furnace, they are not always given due consideration when designing a refractory lining. It is important for a plant manager or engineer to fully understand the use and selection of refractory anchors in their plants. Below are a few critical things to note about Monolithic Refractory Anchors.
ALLOY GRADE
Low grade anchors will lead to premature failure and will cost thousands in repairs as such cheap or inferior quality anchors should never be considered, the anchors are a fraction of the cost of your refractory materials and you need the best support possible. Correct choice of monolithic refractory material is also vitally important in optimizing the process and expected life campaign of the refractory material. Typical maximum operating temperatures of different steel grades are shown in the table below:
Typical Maximum Temperature Grade of Steel
450°C Carbon Steel
900°C 304 Stainless Steel
1000°C 310 & 310S Stainless Steel
1200°C Incalloy 601
MECHANICAL PROPERTIES
Bending can affect the mechanical structure of on anchor. A good practice is to “soft form” the anchor in a CNC wire bending machine rather than the old method of hammering out the desired shape in a press, furthermore when using a CNC machine many different designs can be done without having to manufacture costly dies used in a press. Anchors can be custom manufactured to suit any application.
ANCHOR LENGTHS
Anchor lengths should be a minimum of 25mm below the hot face lining, tests have shown that the anchor tip may be 150°C to 350°C below the hot face refractory temperature, however in general it is accepted to use the higher grade anchor material to that of the hot face temperature.
ANCHOR SPACING
Anchor Spacing tables should be used only as a guide. Each application should be considered according to the vessel being lined, the operating temperatures involved, the anticipated stress on different areas and the position (side wall or roof application). On large side walls support plates (wall seats) should be used to take the load off the anchors, this must also be used to separate monolithic and brickwork applications, it also makes it easier for future repairs. Typical Anchor Spacing Table:
Lining Thickness Anchor Centres
50 to 75mm 150mm = 44/m²
100 to 125mm 250mm = 16/m²
150 to 350mm 300mm = 11/m²
The pattern is also important, anchors must never be placed in straight lines as this will cause severe cracking, were possible they should be placed at 90° to each other.
THERMAL EXPANSION
The coefficient of thermal expansion of the anchor material should always be considered as it is always higher than that the refractory material, thus the resulting stresses can lead to lining cracking and eventual failure of the refractory material and the anchor. It is suggested that all anchors be coated with a material (bitumen) that will burn out at low temperature and allow very small movement between the anchor and castable lining to accommodate the differential expansion. Plastic caps only cover the anchor tips and are not adequate, the whole anchor should be coated. It is also said that the plastic caps help with the longitudinal expansion were most of the damage is done to the refractory material, full anchor coating helps with the radial expansion. It is good manufacturing practice to not produce anchors with the legs being the same length, this cause a shear plain across the anchor tips and results in the hot face of the refractory lining breaking off exposing the anchor tips to the hot face and shortening the life of the refractory lining.
COMMON FAILURES
The most common refractory anchor failures are the interface between the hot face and insulation layers, wrong alloy choice, incorrect welding methods and anchor spacing. The anchors used to retain refractory materials on roofs and vertical walls are more critical as they must remain able to support the weight of refractory even at the elevated temperatures and operating conditions.
The Anchor Division at Dickinson Group has been designing, manufacturing, delivering and welding standard and custom-made high-quality refractory anchors for almost 30 years. It manufactures and distributes a comprehensive range of products for standard industrial use, through to specialty applications in the mining, metals smelting and mineral processing industries. The product range includes Refractory Anchors, Hex Mesh and Tabs, Reinforcing Fibers, Stud welding equipment, CD Pins and Clips, Arc Studs, Wear Studs & Shear Connectors.