Refractory Lining: Working, Safety and Insulating Layers, Materials and Design

A refractory lining is the layered refractory system between a high-temperature process and the steel shell: a working lining at the hot face, a safety lining where metal or slag must be contained, and an insulating layer behind them. How thick does each layer need to be, and which check decides it? In a calculated wall with a 1000 °C (1,832 °F) hot face, swapping 100 mm (3.94 in) of a 250 mm (9.84 in) dense castable wall for insulating castable and board lowers the shell from about 211 °C (412 °F) to about 83 °C (181 °F).

In the US, refractories are nonmetallic materials for structures or system components exposed to environments above 538 °C (1,000 °F), ASTM C71-12(2024). The classes of these refractory materials start under refractory materials.

Lining design sets the layer order, the class per layer, the thickness by ASTM C680-23a or ISO 12241:2022, anchors 75 to 85 % through the dense layer and a brick-ring start-up and shutdown limit of 56 to 69 °C/h (100 to 125 °F/h).

Every value names its standard and edition, the product data sheet or the dated source it comes from; see our methodology and fact-checking process.

What Is a Refractory Lining?#

A refractory lining, also called a furnace lining or kiln lining, is the refractory and insulating material inside a furnace, kiln, boiler, reactor or vessel that keeps process heat, metal, slag and gas away from the steel shell. Each layer has its own job, class and temperature limit.

Furnace lining design fixes three things on paper: which layers the vessel needs, how thick each one is, and how the lining stays attached while it expands and contracts.

Design is the first step of refractory practice, and the installation, dry-out, inspection and repair steps that follow it are covered under refractory practice.

Four Functions of a Furnace Lining#

A refractory lining has four functions named in the API RP 982 announcement (2023): thermal protection of the equipment, energy retention and process efficiency, resistance to abrasion and corrosion, and personnel safety. The four functions are listed below.

• Thermal protection of the equipment: shell and internals stay below their design temperature.
• Energy retention and process efficiency: process heat stays inside the vessel.
• Resistance to abrasion and corrosion: the hot face withstands slag, metal, gas and solids.
• Personnel safety: the outer surface stays within the owner's limits.

API RP 982, 1st edition, released 2023-11-01 as "Inspection and Assessment of Refractory Linings", covers equipment from fluid solid and FCC units, fired heaters, incinerators and sulfur recovery units (SRUs) to calciners, cracker furnaces, boilers and flue gas stacks.

Working, Safety and Insulating Layers#

An industrial lining has up to three layers: a working lining that meets the process, a safety (permanent) lining behind it in metal-containing vessels, and an insulating (backup) layer against the shell. Table 1 compares the three layers by job, material class and a published example.

Working Lining: The Hot Face Against Process#

The working lining carries the full process temperature and the chemical and mechanical wear, so its class is chosen for the hot-face conditions first and for insulation last. In the sulfur recovery unit reaction furnace described by Jenkins et al. (2011, conference paper), the working lining is 229 mm (9 in) of 90 % alumina brick, a low-permeability hot face in front of two castable layers.

A working lining is either fired brick laid with joints or a monolithic layer placed wet, which carries its mixing water into the first heat-up.

Safety Lining: Containment Behind the Working Lining#

A safety lining (permanent lining) holds the metal or slag if the working lining is breached, and it stays in place through several working-lining campaigns. In steel ladles the permanent lining is high-alumina brick in most cases, roseki brick where the ladle is not stirred, and magnesia or magnesia-chrome where hot high-basicity slag reaches it, with two or more layers desirable (Nippon Steel Technical Report No. 125, 2020). A tundish carries a permanent lining, an Al2O3-SiO2 castable wear lining and a MgO coating, with optional insulation at the shell (same report).

Ladle safety-lining brick is laid in arch-wedge, semi-universal and book-tile designs, and a 13 mm (0.5 in) dry vibratable layer separates a shotcrete safety lining from the brick working lining (HWI ladle brochure, 2025, producer guidance). The steel ladle is covered zone by zone under steel ladle refractory lining.

Insulating Layer: Backup Between Hot Face and Shell#

The insulating layer cuts heat flow to the shell, and its temperature limit, not its conductivity, decides how close to the hot face it may sit. In the Jenkins et al. (2011) SRU lining, 76 mm (3 in) of dense castable and 76 mm (3 in) of insulating castable sit behind the brick, so the insulating castable is two layers away from the process.

In a producer case study from an Indian steel plant (Morgan Thermal Ceramics, 2021), 7 mm (0.28 in) of WDS microporous insulation plus 13 mm (0.51 in) of board in a ladle backup gave a calculated cold face of 195 to 207 °C (383 to 405 °F) instead of 268 to 297 °C (514 to 567 °F). That result is a calculation for one ladle design, not a class value.

Refractory Lining Materials by Layer#

Furnace linings use refractory bricks and dense castables, plastics and gunning mixes in the working and safety layers, and insulating firebrick, insulating castables, ceramic fiber, microporous and calcium silicate products in the insulating layer. Table 2 lists each material class by layer with the standard that classifies it.

Shaped Refractories for Working and Safety Linings#

Shaped working linings are classed by chemistry under ISO 836:2001: fireclay 30 to below 45 % Al2O3, high alumina 45 % and above, silica 93 % SiO2 and above, magnesia above 80 % MgO. In the US, ASTM C27-98(2022) classifies fireclay brick by physical properties and high-alumina brick by Al2O3 content, so a lining specification names which of the two systems it uses.

Brick lining thickness follows the brick size. ISO 5019-1:1984 squares measure 230 × 114 × 64 mm (9.06 × 4.49 × 2.52 in) and 230 × 114 × 76 mm (9.06 × 4.49 × 2.99 in), with the 76 mm series strongly recommended, and in North America a square is known as a straight. The working-lining class also follows the slag, and slag chemistry decides between acid, basic and neutral refractories.

Dense Castables and Other Monolithic Linings#

Monolithic refractory linings are cast, pumped, gunned or rammed in place, and a castable counts as dense under ASTM C401-12(2022) above a dried bulk density of 1.68 g/cm³ (105 lb/ft³). ASTM C401-12(2022) separates regular, low-cement, ultra-low-cement and no-cement castables by CaO content, and ISO 836:2001 names the same cement levels as terms.

Placement changes the in-place density that a take-off and a heat-loss calculation must use. In the Allied Mineral Products insulating castable guide (2023), INSULMIX 2350 places at 0.80 g/cm³ (50 lb/ft³) when cast and 1.04 g/cm³ (65 lb/ft³) when gunned, typical values of that product. PUMPLITE 60 from the same guide places at 0.91 g/cm³ (57 lb/ft³) pumped and 1.14 g/cm³ (71 lb/ft³) shotcreted, so a gunned or shotcreted layer weighs 25 to 30 % more than the same layer cast.

A dense castable and a brick working lining can serve the same zone, and the trade-offs are compared under castable vs refractory brick.

Insulating Firebrick, Insulating Castables, Fiber and Microporous Backup#

Insulating refractories are rated by a classification temperature, not a service temperature: ISO 2245:2006 groups insulating firebrick by the test temperature at which permanent change in dimensions stays at 2 % or below. The group number is that test temperature divided by 10 (groups 80 to 180), and ASTM C155-97(2022) classifies insulating firebrick in the US.

Ceramic fiber products carry a classification temperature under EN 1094-1:2008: 24 h permanent linear change at 4 % or below for blankets, felts, mats and papers and 2 % or below for boards, to the nearest 50 °C. For amorphous wools, continuous use sits 100 to 150 °C (180 to 270 °F) below it as a design margin, not an EN 1094-1 requirement.

Refractory ceramic fiber (RCF, aluminosilicate wool) is classified "Carc. 1B H350i (May cause cancer by inhalation), GHS08, Danger (CLP Annex VI 650-017-00-8)", notes A and R, and has been on the REACH Candidate List since 2010-01-13. Exposure limits and alternatives are covered under ceramic fiber health rules.

Thermal Design: Layer Thickness, Heat Flow and Shell Temperature#

Lining thickness is set by a steady-state heat-flow calculation through the layers in series, repeated until every interface stays below the next layer's limit and the shell meets the owner's temperature limit. For a flat wall the heat flux is q = (T_hot − T_amb) / (ΣL/k + 1/h_s), with k taken at each layer's mean temperature and iterated; a cylinder replaces L/k by r_o ln(r_o/r_i)/k.

Table 3 compares three calculated build-ups of the same 250 mm (9.84 in) total thickness at a 1000 °C (1,832 °F) hot face.

k in W/(m·K), with BTU·in/(h·ft²·°F) in parentheses; calculated values, illustrative conductivities.

Calculation Standards: ASTM C680-23a and ISO 12241:2022#

Two standards set the calculation rules: ASTM C680-23a for heat gain or loss and surface temperature of flat, cylindrical and spherical systems, and ISO 12241:2022 (EN ISO 12241:2022), which replaced EN ISO 12241:2008. ASTM C680-23a, last updated 2023-12-13, treats one-dimensional steady or quasi-steady heat transfer, and C680-23 is historical. ISO 12241:2022 is edition 2, dated 2022-06-23; the two are used side by side, not as equivalents.

Each layer needs a conductivity, and a data-sheet conductivity belongs to one named product, not to a class. The test methods behind these values are explained under thermal conductivity of refractories.

The two data-sheet values below show how far a dense and an insulating product sit apart.

Refractory Lining Thickness: Three Worked Wall Examples#

At a 1000 °C (1,832 °F) hot face, a 250 mm (9.84 in) dense castable wall runs a shell of about 211 °C (412 °F); a three-layer wall of the same thickness runs about 83 °C (181 °F). Heat flux falls more than fivefold, from about 3,790 to about 710 W/m², because 50 mm (1.97 in) of board at k 0.05 W/(m·K) adds a thermal resistance of 1.0 m²·K/W, eight times the 0.125 m²·K/W of 150 mm (5.91 in) of dense castable.

Insulation lowers the shell temperature and raises the interface temperatures. In the three-layer wall, the face between dense and insulating castable reaches 911 °C (1,672 °F) instead of 766 °C (1,411 °F) in the two-layer wall, so the insulating castable's temperature limit has to cover it, or the dense layer gets thicker.

The inputs in this worked example are illustrative, not product data: ambient 25 °C (77 °F), emissivity 0.9 and still air. Your own layers and limits go into the refractory heat loss calculator.

Interface, Anchor-Tip and Shell Temperature Checks#

Every lining design passes three temperature checks: each interface against the next layer's limit, each anchor tip against its alloy limit, and the shell against personnel-protection and acid dew-point limits set by the owner. An anchor tip runs 150 to 350 °C (a difference of 270 to 630 °F) below the hot face in practitioner guidance by Trevor Dickinson, which gives tip limits of 900 °C (1,652 °F) for AISI 304, 1000 °C (1,832 °F) for 310/310S and 1200 °C (2,192 °F) for alloy 601. Other sources give different limits, so each limit carries its source and is never a hot-face rating.

The shell limit comes from the owner's specification, which covers personnel protection and the acid dew point of the flue gas; in service, shell thermography and embedded thermocouples check it.

Mechanical Design: Anchoring, Expansion and Brick Rings#

A lining must stay attached and intact while it expands: monolithic layers hang on metallic or ceramic anchors, joints absorb expansion, and brick rings in shells rely on a controlled interference fit. Anchor values in this section come from a Plibrico producer article (2022) unless another source is named.

Anchoring Monolithic Layers to the Shell#

Anchors for dense linings reach about 75 to 85 % through the dense layer, with the tip no more than 51 mm (2 in) from the hot face in linings over 152 to 178 mm (6 to 7 in). Metallic anchors expand about 3 times as much as alumino-silicate refractory, and caps, wax or tape burn out on the first heat to leave room; Dickinson adds that the whole anchor is coated (bitumen), since tip caps alone are not adequate.

The same producer claims that up to 40 % of lining failures trace back to the anchor system. Spacing tables and alloy choice for refractory anchors are set out under refractory anchors.

Refractory Expansion Joints and Anchor Clearance#

Expansion joints give the lining room to grow on heating, and anchors stay at least 152 mm (6 in) away from them (RAI, producer guidance). The threefold expansion of metallic anchors is the reason for both the joint clearance and the full coating.

Joint width follows the measured expansion of the installed product, and three methods measure it: ASTM C832-21(2026) for thermal expansion and creep under load, ISO 16835:2014 for thermal expansion, and EN 993-19:2004 for the differential method up to 1700 °C (3,092 °F). These methods report expansion of a specimen; they are listed side by side, not as equivalents.

Brick Rings in Cylindrical Shells: Interference Fit#

A brick ring in a cylindrical shell is held by interference fit: the brick expands against the cooler shell and the ring stays in compression. Too fast a heat-up overstresses the brick, and too fast a cool-down loosens the ring.

After the initial dry-out, start-ups and shutdowns of brick-lined vessels stay at or below 56 to 69 °C/h (100 to 125 °F/h) (Thorpe, cited by Jenkins et al. 2011). How brick expansion is measured, reversible and permanent, is covered under thermal expansion and permanent linear change.

Designing for Installation and Dry-Out#

A lining design fixes the water that later has to be driven out: in the Jenkins et al. (2011) SRU lining, about 9.5 m³ (2,500 US gal) of water in two 76 mm (3 in) castable layers behind low-permeability brick governed the dry-out. Values in this section come from that paper and from HWI dry-out guidance (2020 and Schedule 2 PLUS) unless another source is named.

Water demand follows the castable family: low-cement 4 to 8 %, conventional 8 to 12 % and insulating 20 to 40 %, with the lowest permeability in low-cement dense types. Water turns into steam of about 1,600 times its volume.

The producer's standard schedules apply to single-component linings of 229 mm (9 in) or less; above 305 mm (12 in) the producer is consulted, and a multi-layer lining counts by total thickness. The SRU lining above, 381 mm (15 in) in three components, falls outside a standard schedule on both counts.

Placement belongs in the design: the cast or gunned density of each layer enters both the take-off and the heat-loss calculation. Casting, gunning and bricklaying are compared under refractory installation methods.

Water needs a path out. Weep holes or wicking are required where the castable has no permeable, unobstructed path through the cold face, and schedule temperatures refer to the hot gas in contact with the lining, not to the lining itself.

Standards for Lining Design and Installation Quality Control#

No single standard designs a refractory lining: ASTM C680-23a and ISO 12241:2022 set the heat-flow arithmetic, and API Std 936, 975 and 976 set installation quality control for monolithic, brick and fiber linings. The current editions are API Std 936, 5th ed. March 2024, API Std 975, 2nd ed. April 2025, and API Std 976, 2nd ed. October 2024; API RP 982, 1st ed. 2023, adds in-service inspection and assessment.

ASTM and ISO or EN documents on one row of Table 4 are listed side by side, not as equivalents. Material qualification and as-installed testing are covered under API 936 refractory installation quality control.

Table 4 lists the standards a lining design and its installation draw on, with US and ISO or EN documents in parallel columns.

Common Lining Design Errors and the Failures They Cause#

Six design errors lead to lining failures that trace back to the drawing, and each one is set on paper before the first brick is laid. The six errors and the failures they cause are listed below.

1. Anchor design errors: wrong length, alloy or joint clearance; Plibrico (2022, producer claim) traces up to 40 % of lining failures to the anchor system.
2. Insulation too close to the hot face: an interface above the insulating layer's classification temperature.
3. Cast density used for a gunned layer: take-off and heat loss both run on the wrong density.
4. No water path or an uncontrolled heat-up: in the Jenkins et al. (2011) case, an uncontrolled main-burner dry-out raised the reaction-furnace temperature by about 633 °C/h (1,140 °F/h), and the castable separated 25 to 51 mm (1 to 2 in) from the tube sheet; see refractory dry-out and heat-up schedules.
5. Brick rings cooled too fast: the ring loosens in the shell, one of the refractory failure modes.
6. Cooling a lining just to look for cracks: ceramic materials fail in tension, and Hotwork (Cobane, 2002, producer view) discourages it; see refractory inspection and quality control.

Contractor Match and Quotes for Lining Design and Relining#

Lining design, relining and dry-out are quoted per job, and a request that names the equipment, layer build-up, area and service temperature gets comparable quotes. Quote factors are lining area and thickness, access and scaffolding, anchors (alloy and count per m²), material tonnage, placement method, crews and shifts, curing environment, dry-out (burners, fuel, thermocouples, days), tear-out and waste class, inspection and test panels, and outage cost per day. No public per-tonne installation price list was found on 2026-10-27.

How Does Lining Design Connect to Industry Linings and Insulation?#

The layer logic on this page is the same in every industry; what changes is the working-lining class, which follows the process, slag and temperature of each vessel. Boiler refractory is the lining of boiler water walls and of the grate chambers of waste-to-energy units, built from the same working and insulating layers. Boiler linings from the package boiler to waste-to-energy units are covered under boiler refractory.

Furnace insulation is made of insulating firebrick, insulating castables, ceramic fiber, microporous insulation and calcium silicate, the insulating-layer rows of Table 2.

Which Industries Use Which Lining Systems?#

Seven industry pages apply these layers to their own vessels, listed below.

• refractories by industry: steel, cement, glass, non-ferrous, petrochemical and power.
• refractories for iron and steel: blast furnace, converter, EAF, ladle and tundish.
• cement kiln refractory: cement and lime kilns by zone.
• glass furnace refractories: fused cast, silica, zircon and regenerators.
• refractories for aluminium: melting and holding furnaces, anode baking and reduction cells.
• refinery and petrochemical refractories: FCC, fired heaters, reformers, SRU and gasifiers.
• induction furnace lining: ramming mass selection, sintering and lining life.

Is a Kiln Lining Designed Differently From a Furnace Lining?#

No: a kiln lining follows the same working, safety and insulating layer logic, with the working-lining class set by the kiln's process zones. Zoning of rotary cement and lime kilns is covered by the cement kiln page in the list above.