Fig 1 – Installation of precast access shapes

Precast Shape Technology in Refractory Linings by Paul E. Fisher, P.E. TFL, Incorporated ____________________________________________________________________________

The design and manufacture of precast refractory shapes has become a specialized field

within the refractory industry in recent years. As the demands increase for greater refractory

lining performance and lower maintenance costs, refractory users are finding that one effective

way to achieve those goals is to incorporate a broader use of precast refractory shapes into

their lining systems. Across virtually all industries – petrochemical, steel, power generation,

metal casting and treatment, wood products, minerals processing and others - the applications

for precast shapes are limited only by the imagination, and almost invariably their use will result

in better performance and true cost savings. This article will discuss the design and

manufacture of precast refractory shapes, and the benefits to be gained in both refractory

material performance and installation logistics.

Precast Shape Design and Manufacturing

In order to realize the true benefits to be gained from the use of precast shapes, a thorough

knowledge of how the shape system will be used and

installed in the field is an absolute requirement during the

design phase. The successful design and manufacture

of a high-performance refractory shape system requires

a unique understanding of refractory materials,

manufacturing, anchoring systems, and construction

practice. Dimensional tolerances, construction

sequencing, lifting and handling capabilities at the site,

anchoring facilities, and the actual service demands

Website: www.tflhouston.com Email: tfl@tflhouston.com

Fig 2 – Precast Headwall Shapes for Carbon Bake Furnace

within the refractory lining environment are all factors that must be well known before the shape

is designed.

Precast shape manufacturing inherently requires the use of a mold or pattern to form the

shape. There are several methods for mold-

making which are routinely employed, and the type

of mold construction and materials used depends

on the size, complexity, and dimensional

tolerances required in the shape, and sometimes

the quantity of shapes required. For simplistic

shapes with loose dimensional tolerances (+/-

1/16”), plywood forms or metal fabricated forms

can be used. Toward the other extreme, some shapes may require extremely tight tolerances,

which require the use of a more sophisticated mold made from wood, plastic or metal. These

molds may be of the type made by a foundry pattern maker or machine shop.

Another factor in the design of a precast shape has to do with the schedule and sequencing

of the actual field installation. The shape design must take into account job accessibility, what

other lining components will already be in place when the shapes are to be installed, and how

the shape can be handled physically on the job site. Weight and lifting limitations must be

considered and planned for, as well as the type of access available into the furnace or vessel. If

necessary, lifting lugs or other fixtures can sometimes be incorporated into the shape design.

The design of the anchoring system to be used in the shape is of tremendous importance. In

addition to the normal considerations of alloy type and anchor size, the precast shape design

must also consider all alternatives for attaching the shape to the structure. Numerous methods

Fig 2 – Precast roof panels ready for shipment

can be used, including threaded stud attachments through the wall, welded fixtures, or bolted

assemblies.

Perhaps most importantly, the proper refractory material must be selected to suit the

demands of the application. Factors such as the desired temperature profile through the lining,

expected mechanical stresses, potential chemical attack on the lining, erosion mechanisms, and

expansion allowance must all be understood prior to selecting a material to be used in the

precast shape.

A well-equipped precast manufacturing facility should include high-energy, large capacity

mixers, automated mixing stations with conveyors for material delivery, vibration tables, digitally-

controlled water addition, mixing time controllers, and adequate lifting capabilities for large

shapes. Firing of shapes is accomplished with a digitally-controlled furnace with burners

capable of firing to at least 1300 deg. F. In-house mold/pattern fabrication capabilities and

CAD-generated drawings for design assistance should also be expected.

Benefits from Material Property Enhancement

Regardless of how complex or sophisticated the refractory castable is that is selected for an

application, the physical properties of the material can be drastically reduced if care is not taken

during the mixing, pouring, and curing processes. Particularly with the use of more complex

refractory castables to solve specific wear issues,

installation variables become even more critical to

the performance of a lining. Unfortunately, lining

quality is often compromised by field conditions

during material placement. Project schedules,

crew skill levels, equipment availability, job cost

pressures, or other demands can sometimes have an impact on proper refractory installation.

Improper water addition, mix time variations, over- or under-vibration, and improper curing can

drastically affect material quality. With precast shapes, cast in a controlled shop environment,

the physical properties of a castable will be more fully optimized.

Initial drying and firing of a refractory castable is a critical installation variable that can

influence lining performance. Precast shapes are typically fired in a digitally-controlled furnace

prior to shipment, ensuring that the refractory manufacturer’s recommended bake-out schedule

is closely followed. Since the shapes are fired slowly from all sides, the moisture is removed

through the entire thickness of the shape in a controlled manner. Depending on the

temperature to which the shape is fired, this can optimize the physical properties of the material

through the entire thickness of the shape, not just the hot face surface. This results in a truly

homogeneous lining. Micro-cracking within the shape, which is often introduced during field

bake-out but may go unnoticed, may also be reduced since the initial firing is more controlled.

In service, linings comprised of precast shapes often see less stresses and cracking, due to

the independent, “floating” nature of the lining. The performance of the lining can also be more

predictable, resulting in better opportunities to plan for maintenance and repairs.

Benefits from Installation Logistics

Other major benefits to be gained from the use of precast refractory shapes are related to

simplified installation and repair logistics, which can lead directly to reduced costs and shorter

down times. With the use of precast shapes, forming labor, materials, equipment costs, actual

placement time and expense, and associated costs during form removal, curing, and cleanup

are all eliminated. These costs are shifted back to the manufacturer of the shape, who can

absorb them much more efficiently when spread over his overall production capacity.

Fig 3 – Precast Hearth Blocks for a car-bottom furnace

Refractory installation contractors have begun to consider precast refractory shapes much

like they do any other pre-manufactured item such as block insulation, ceramic fiber blanket,

anchors, etc. These items can be bought and then re-sold as a component of their installation

projects.

Whenever any portion of refractory repair work can be completed prior to crews being on

site, costs are automatically reduced. Installation contractors have also found that the use of

precast shapes can often give them a substantial advantage in competitive bid situations. With

the use of precast shapes, crew sizing can be minimized. Speed of installation is another

obvious benefit to both the installer and the owner, resulting in reduced costs due to shorter job

duration. Material usage is also reduced, when compared to other installation methods such as

guniting, where as much as 45% extra material is required to compensate for rebound and other

job losses. Environmental hazards such as dusting and tripping hazards associated with

equipment and hoses are also reduced substantially, if not eliminated.

Future repairs also become much more economical and quicker to accomplish. Repair

areas can often be isolated to just the immediate wear area within the boundaries of a shape.

Anchor attachment points can typically be reused. Replacement shapes, purchased early and

kept as spare parts on site, can be easily installed in a fraction of the time required for

conventional repair methods. Repairs become more of a mechanical maintenance job, rather

than a refractory installation job requiring a specialized crew and equipment.

The initial bake-out of a new refractory lining

on site can be a very expensive and time-

consuming component of a refractory repair

project. The use of precast and prefired

refractory shapes can sometimes reduce or even

eliminate the need for an extensive initial bake-out. If an entire repair is made with a prefired

system, then normal furnace start up schedules can be used, without the fear of steam spalls or

other damage during the initial heating. Bake-out of multi-component linings, which may include

a combination of precast shapes and other materials placed in the field, can often be reduced

by the pre-firing of the castable shapes, particularly if that material would have been the critical

item determining the bake out schedule. This can have a positive impact on not only job costs,

but in reducing down time as well.

Typical precast shape applications

Burner Blocks Pier Blocks Manway Plugs

Checker Wall Blocks Wall Panels Air Grid Tiles

Burner Pipe Covers Divider Walls Curb Blocks

Hearth Shapes Jambs, Lintel & Sills Peep Sites

Delta Sections Well Blocks Nozzles

Impact Pads Flue Caps Flue Walls

Burner Ports End Caps Exhaust Ports

Flare Tips Roof Sections Spouts

Skimmer Blocks Safety Linings Tap Blocks

Troughs Runners Sleeves

Precast refractory shapes will continue to be a growing specialty in the refractory

industry in coming years. With the improved quality that can be achieved through controlled

manufacturing processes, their expanding use will play a major role in improving refractory

lining performance and reducing maintenance costs across all industries.

TFL, Incorporated is a supplier of refractory materials and related services located in Houston, Texas, and is a leading manufacturer of custom-designed precast shapes. Web site: www.tflhouston.com