GENERAL

With the design of high performance furnaces and vessels containing complete linings of monolithics, anchoring the refractory to the vessel structure is an integral part of any successful installation. The anchors that can be specified for a project now are an immense improvement over early models and provide an installer with a wide variety of choices to do the job depending of the service conditions prevailing. The following information gives a broad overview of this subject, but since all projects are different, the information presented is a guide only. For specific information, contact the Thermal Ceramics Australia Head Office or one of our many sales representatives.

ANCHOR TYPES

Wire Anchors

Wire anchors can be fabricated in a multitude of different sizes and shapes as the designs in the figures at the end of this paper show. They can be specified in various grades of stainless steel depending on the atmosphere and temperature they are expected to face.

Wire anchors are the most common anchoring devices used due to their being relatively cheap to purchase and easy to install. They are used in most applications where service (lining) temperatures do not exceed 1100(C. Various grades of stainless steels are used for wire anchors due to mild steel not being suitable for high temperatures. The choice of metal depends on the actual temperature the anchor will face. This is shown below in Table 1:

The maximum temperature can be used as a guide only, since, as the lining temperatures nears the maximum temperature of the anchor, the anchor will begin to suffer plastic deformation, causing problems where there are high loads of support. In this case, an anchor of higher service temperature should be specified.

In all cases, the anchor or holder should be designed to operate as cool as possible and should be installed in ways which allow heat dissipation by conduction and/or circulation. The temperature that the anchor is subjected to is a main determinant in the life expectancy of a wire anchor due to it increasing the rate of oxidation of the metal. The primary condition being a time-temperature relationship to which the metal is subjected to in the furnace lining. Operating at excessive temperatures can create a carbide precipitation in the metal that will change the original properties of the parent metal, causing rapid oxidation and leading to premature failure.

The atmosphere prevailing in the furnace will also affect the maximum temperature that an anchor can handle. Reducing, sulphurous or nitriding environments can severely affect the metallic anchor and care should be taken in the choice of metal alloy for such environments.

Wire Anchor Styles

The styles shown on at the end of this paper are indications of the various styles that anchors can be supplied in for different purposes. The anchor in Figure 1 is a specifically designed anchor for units which will be subjected to mechanical movement during operation, e.g. rotary kilns. The nut is welded to the shell and the anchor only tack welded to the nut. As the lining and anchor are subjected to the stresses of movement the tack weld will break, allowing the anchor to "float" in the nut, yet remain fixed to the shell. A newer, modified version of this system, called a Rotalock anchor is shown in Figure 2. This system has the butt welded to the shell and the anchor is inserted into the hole. When ready, it is 'snapped' into place in the hole. This design also allows the anchor to move when the lining is stressed.

The anchor shown in Figure 3 is sometimes used in multicomponent linings either gunned or cast. The lengths of the different sections can be altered to suite the different thicknesses of the linings. The Rotalock system is also suitable for multicomponent linings.

Anchor Length

Length of wire anchors should be such that the tips of the anchor are set a minimum of 25 mm behind the hot face. Generally this approximates to three quarters of the lining thickness. Allowance must also be made for the expansion of the wire anchor in the refractory mass. This can be achieved by placing plastic caps over the tips of the anchor or coating the complete anchor with a plastic solution. The plastic melts at high temperature and allows the anchor space to move.

Anchor Welding

Wire anchors need at least 15mm of weld fillet on both sides. Tack welding of anchors to the shell is not sufficient. Some heavy rod anchors may require additional welding. Welding is critical to the performance of the lining. If the weld fail, the anchors will not hold the lining in place and a complete lining collapse could quite easily take place.

Check approximately 1 in every 100 anchors by striking with a hammer. If a dull sound is heard or the anchor falls off, then check all anchors for potential failures and replace those that fail. If a ringing sound is heard, it indicates good welding.

Note: For welding dissimilar metals, e.g. 310 s.s. anchors to a mild steel vessel shell, it is preferred that a 309 rod is used rater than a standard or 310 rod, as the 309 gives a better transition from 310 to mild steel and will be less prone to problems such as crystalline fracture etc, (i.e. it will last longer). In most cases, compatible electrode rods should be used to minimise any problems when welding.

Metal Anchors

Metal anchors are used when the installed lining will be too heavy or thick and thus demands more strength than standard wire anchoring can provide. Metal anchors are made from the same grade of steel as wire anchors and thus the same temperature limitations are imposed. There are many other similarities between wire anchors and metal anchors. The same procedures are followed for anchor spacing and anchor length.

Metal anchors are designed either to be hung from suspended steelwork in the roof, or to be sent out from the wall, supported by brackets.

Roof Anchors

These anchors, as shown in Figure 4, have different bases so that they can be hung from a variety of different steelwork, from I-beams to rods. It is important when attaching roof anchors that the portion of the anchor which grips the supporting steelwork remains exposed to ambient air to permit heat dissipation from the anchor shank.

Wall Anchors

In this case, the mounting bracket is generally welded to the wall and the anchor bolted to the bracket. Other forms allow the anchor to directly bolt to the steel shell. This allows for easier dissipation from the anchor.

To reduce the load on wall anchors, wall seat assemblies can be fixed to the wall. These consist of flat plate and a mounting bracket. The bracket is welded to the shell and the plate affixed to the bracket, perpendicular to the wall. These anchors should be used where extremely heavy vertical and/or sloping loads are encountered. Wall seats should be used when using a dense monolithic over 150mm thick and/or 1 metre high. They are available in a variety of lengths for different lining thicknesses. See Figure 5 for an example.

In some plastic and castable installations where vessel geometry, loading or other considerations make the use of ceramic or wire anchors impractical, metal anchors can be used. An example of this usage is the bull nose configuration illustrated in Figure 6.

Ceramic Anchors

For dense monolithic linings with thick cross-sections (greater than 250 mm), use of pre-fired refractory anchors is the preferred method of anchoring the structure. Ceramic anchors have several advantages over other types of anchoring systems. More holding power than metal anchors is achieved due to their design and greater surface area. They also extend through to the hot face, providing extra retention of the lining. Also, being ceramic, they can withstand much greater temperatures and tougher atmospheric conditions than standard wire or metal anchors.

Thermal Ceramics Australia's 876 Ceramic Anchor is shown in Figure 7.

The 876 Series of ceramic anchors is a 16" long, cut to size, brick anchor, manufactured from MORAL( 85 BP, a high alumina, burnt phosphate brick mix, which shows excellent thermal shock resistance, and is capable of maintaining its integrity to 1800(C. The shape with its grooves and rises provides excellent retention power over the monolithic lining. The head of this anchor is designed to accept slip-over castings (commonly known as C-Clips). In this manner, the brick is excellent in roof applications, and can also be used in wall construction with the C-Clip. A diagram of the C-Clip is also shown. A certain amount of movement can occur between the refractory anchor and the C-Clip to accommodate expansion and contraction of the lining. The C-Clip can be hung from steelwork in the roof, or cantilevered out from the vessel wall. They are available in various length and steel grades (generally 310 s.s.) to suit different applications.

ANCHOR LENGTH AND SPACING

Wire and Metal Anchors

The distance between anchors needs careful consideration. Obviously edges, roofs and bullnoses, and areas where vibration, mechanical movement or gravity impose loads on the lining need more anchoring than a straight wall or floor. Standard spacing for various areas is suggested below. Anchors are usually welded in a square pattern (as near as possible in some cases), but alternative patterns such as diamond are also suitable in many installations. The tines are rotated 90( from neighbouring anchors.

Where X = anchor spacing

Ceramic Anchors

The total length of the refractory anchoring equals the lining thickness.

Suggested anchor spacing is shown in Table 2 overleaf.

SPECIAL ANCHORING SYSTEMS

Hex-Mesh Anchoring

Hex-mesh systems are used when maximum abrasion resistance is required with moderate temperature thin linings. Generally, the mesh stands away from the shell on studs or bars allowing a layer of insulation behind the hot face. Where heat loss is not a problem, the mesh can be directly welded to the shell. Sometimes, mesh is supplied with extended legs, allowing refractory to flow underneath and between the cells, causing bonding between cells, increasing the strength of the complete surface.

Hex-mesh needs to be installed correctly for optimum performance. A continuous full penetration fillet weld should be used to attach the mesh to the shell. If the mesh is not tight against the steel shell, under operating conditions it can "chatter" from vibration or movement. This can cause the refractory to crack or dislodge from the cells. An example of hex-mesh anchoring is shown in Figure 8.

S-Bar Anchoring

S-bars were originally developed as an alternative to hex-mesh systems. The fundamental design requirements for S-bars (or Stop bars) is to:

supply an anchorage system of a design to hold ultra-thin refractory linings in place

reduce wear of refractory by abrasion of particles.

S-bars are directly welded to the vessel shell. Under operating conditions, refractory wears in the direction of the abrasive medium. This is usually the direction of gas flow. The S-bars are arranged such that the head of the bar acts as a barrier to these particles, thus shielding the refractory down-stream. The S-bars are always placed at right angles to the direction of air flow, and since they overlap each other, there is not easy path for the abrasive particles to take. They have proved successful in many application where extreme abrasion resistance needs to be coupled with very thin linings.

Similarly to hex-mesh systems, S-bars can be designed on extended legs to allow an insulating layer beneath dense hot face. S-Bar anchoring is shown in Figure 9.

CONCLUSION

Due to the almost infinite number of different needs for refractory and refractory anchors, the proceeding pages are presented only as a general guide to the selection and installation of refractory anchors. For specific applications and information, contact the Thermal Ceramics Head Office or one of our many sales representatives around the country.

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