What is the spacing for refractory anchors?

Since the development of monolithic refractory products, metal anchoring systems have been widely utilized in supporting the monolithic materials around the insulated vessels.

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The proper design, material selection and installation of refractory anchors is of paramount importance in order to avoid premature failure of the refractory linings. Such failures would be highly costly, both in terms of lost production and the direct costs linked to correctional maintenance activities. This article briefly reviews some of the important considerations for refractory anchor system design and installation.

ANCHOR DEPTH

Many specifications call for refractory anchors to be 66% to 75% of the lining depth. In general practice, however, the anchor should extend no closer than 25mm of the lining surface.

ANCHOR SPACING

To a great extent, anchor spacing depends on the type of material used, the operating conditions and the physical characteristics of the unit to be lined. Spacing from 75mm on centres to 300 or 400mm is quite common. Some specifications call for spacing the anchor on centres three times the lining thickness. General guidelines for the spacing of anchors are as follows:

• The more severe the operating conditions, such as cyclic temperatures, physical abuse, abrasion and vibration, the closer the anchors should be spaced.

• The lower the physical stress imposed on the lining by the physical characteristics of the unit to be lined, the wider the anchor spacing.

ANCHOR PATTERNS

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Diamond, square and staggered anchor patterns have been widely and successfully used. The most important fact is that an irregular pattern is achieved by opposing individual anchors. This increases the overall holding power of the anchor pattern, it also helps prevent cracks from extending dangerously far in any one direction or from linking together in regular networks to cause weakened areas.

ANCHOR ATTACHMENT

The fixing point of a metal anchor is one of the areas of the anchoring system subject to the greatest load. In refractory installation, anchors are attached to the shell in different ways, in addition to welded connections, screw-in, bolt-on, clip-on and plug-in connections are used. The choice of the type of attachment depends on the type of anchor, the alloy, the installation situation and the amount of anchors to be attached. The more common attachment methods are hand welding and stud welding. Hand welding is slower, more costly and requires skilled welders and welding electrodes. Stud welding is much faster, cheaper and only requires semi-skilled welders and no welding electrode is required.

A significant number of refractory lining failures can be traced to either faulty design or improper installation of the anchor system. The tips of anchors in particular need special consideration due to their exposure to the highest temperatures.

In this Technical Tuesday feature for Heat Treat Today, Dan Szynal, Vice President of Engineering and Technical Service for the Plibrico Company, a manufacturer of monolithic refractories, gives 3 important tips for refractory engineers and managers to use in achieving an improved anchor design.

 

 

Anchor Types and Service Temperatures

For refractory linings using metallic anchor systems, refractory engineers and designers almost always use Class III austenitic stainless-steel anchors of various qualities. The typical grades of stainless steel used are AISI 304, 309, and 310. These contain chromium and nickel to provide the best corrosion resistance and ductility at high temperatures. For some applications in which temperatures are more extreme and the use of ceramic tile anchors is not practical for various reasons, AISI 330 and even Inconel 601 is sometimes used. These anchors have higher nickel content for superior oxidation resistance and tensile strength at temperatures of 2100°F or higher. Inconel 601 gives the added advantage of good resistance to both carburization and sulfidation in extreme applications.

Industry Best Anchor Practices

Anchor sizing for a refractory lining depends on the refractory thickness and number of components. Some designers use the practice of sizing the anchor height to be 75-85% through the main dense castable or gunned lining. Other rules of thumb used in the industry dictate that the anchor tip should be no more than two inches from the hot face of the refractory for thicker lining designs greater than 6-7&#;.

For refractory applications, it is useful to know the temperature gradient through the refractory lining&#;from the hot face to the cold face&#;to choose the proper anchor size so that one doesn&#;t exceed the temperature limit of the alloy being used. To help calculate the correct temperatures at different points in the refractory lining, many industry professionals will use a heat loss calculator/estimator. By using a heat loss calculator/estimator, one can choose the proper anchor height by determining the anchor tip temperature it will experience. There are numerous heat loss applications that can estimate the cold face of a furnace lining given the input conditions of a thermal unit. As part of its value-added service as a refractory solutions provider, Plibrico Company, LLC, has a web-based heat loss application that gives a good estimation of the thermal gradient of the refractory lining from hot face to cold face to maximize anchor thermal performance.

For example, look at figure 2.0. You can see a 9&#; side wall of refractory lining using 6&#; of a typical 60% alumina low-cement castable and 3&#; of 2500°F lightweight insulating castable for an application operating at 2100°F with an ambient temperature of 80°F. For this application, we would select 309 SS or 310 SS metallic anchors because the intermediate temperature at about 80% of the main lining thickness is at about 1800°F. Although 304 SS anchors would be more cost effective and are most commonly used in the industry, the anchor tips would oxidize at this temperature and would essentially burn out.

A Word on Anchor Tips

Standard practice for several years now has been to allow for expansion of the anchor tines by covering the anchor tips with plastic caps, dipping them in a wax, or putting tape on them. Metallic anchors expand at about three times the rate of alumino-silicate refractories. The expansion material affixed to the anchor tips burns out at low temperature and allows the anchor space to expand without causing cracks in the refractory.

Best practices in metallic anchor design also must include anchor spacing. Greatly a function of the specific equipment and geometry size, refractory engineers must consider the specific installation area. For example, anchor spacing patterns will be different in a flat wall or roof as compared to a section that has a transition of geometry or a less critical area of a vessel.

Anchor spacing should be based on the features of each specific project, such as mechanical properties of the anchor, and the refractory lining as a function of the temperature. Refractory engineers will use these properties in mathematical models to help create the optimal anchor spacing pattern and plan.

Often, failures commonly attributed to the refractory component can, in fact, be caused by deficiencies in the anchoring system. A strong anchoring system is key to maintaining monolithic refractory lining integrity, even when it is cracked, to prevent a total structural collapse.

To prevent vessel lining failures, increase service life, and maximize refractory performance, incorporate these metallic anchor tips. With these tips, it is possible to design and optimize an anchoring system that will work well with the demanding needs of refractory linings today.

For more information about metallic anchors and refractory anchoring systems, contact the Plibrico Company at

 

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