In high-temperature industrial pipe and equipment systems, expansion joint castables are the key lining materials to protect metal expansion joints from direct washing by high-temperature flue gas and prevent overheat damage. Whether it is furnace door expansion joint of circulating fluidized bed boiler or high temperature flue system such as dry coke quenching and cement kiln, the quality of castables and construction technology directly affect the service life of expansion joint. Once the castable falls off or cracks, the high-temperature flue gas will directly impact the metal bellows, causing the expansion joint to burn through and fail. This paper will systematically explain the professional technical knowledge of expansion joint castables from material selection, construction technology to quality control.

1. Function and working condition requirements of expansion joint castable

Expansion joint castable, as the lining protective layer of metal expansion joint, mainly undertakes the following functions:

• Isolate high temperature: Prevent high temperature flue gas from directly contacting metal bellows and reduce the working temperature of bellows
• Resistance to Scour: Wear-resistant castables can effectively reduce bellows wear when dusty airflow passes through at high speed
• Heat insulation: Reduce the temperature of the external surface of the expansion joint to meet the requirements of safety specifications

Taking the dry quenching expansion joint as an example, its original design adopted a castable structure, which was divided into a working layer and an insulation layer: the working layer was high-aluminum castable (thickness 100mm), and the insulation layer was lightweight castable (thickness 308mm)。 During the production process, the hot and cold environment caused by frequent furnace shutdown and the water vapor generated by boiler tube burst will cause damage to castables. Once the castable is damaged, the metal shell of the expansion joint will burn through and burn out, resulting in the failure of the entire expansion joint。

The selection of castables for expansion joints should be based on the actual operating conditions of the equipment, and the following parameters should be specified：

2. Material selection of expansion joint castable

1. Select according to temperature grade

Select the refractory degree of the expansion joint castable according to the operating temperature:

Thermal shock stability: For batch kilns with frequent temperature rise and cooling, materials with excellent thermal shock stability should be preferred, such as castables with ZrO₂ or sillimanite added, and the number of thermal shocks should be ≥30 times (1100℃ water cooling)。

2. Select according to wear resistance

High-temperature flue gas containing hard particles (such as boiler fly ash and cement kiln clinker dust) should be selected with high wear-resistant castable：

• Corundum (Al₂O₃ ≥85%) or silicon carbide (SiC ≥60%) as aggregate
• Addition of metal fiber to enhance peeling resistance
• Abrasion ≤5cm³/ (kg·h) (ASTM C704 standard)

3. Characteristics of micro-expansion castables

Another important type of expansion joint castable is micro-expansion refractory castable. The medium and high temperature compound expansion agent is introduced into the formula of this kind of product, which can produce slight volume expansion in the use process, effectively prevent the cracking of furnace lining, conserve heat and save energy, and reduce heat peeling。 Its typical performance indicators are as follows：

Suitable for high temperature equipment such as power plant boiler, petrochemical industry, heating furnace in iron and steel industry。

4. Typical configuration of expansion joint of circulating fluidized bed boiler

A typical castable configuration for a circulating fluidized bed boiler door expansion joint is as follows：

This part should have three properties of fire resistance, wear resistance and thermal shock stability at the same time. Because of the inconvenient construction, wear-resistant plastic is often used。

3. Construction Technology of Expansion Joint Castable

1. Pouring area division

During the construction of expansion joint castable, in order to prevent the stress caused by volume change from causing damage to the castable, the liner should be divided into small areas with diagonal size not larger than 1.5m, and the castable should be cast in partitions, and gaps for expansion and contraction should be left at the boundary line of each casting area。

2. Expansion joint reservation method

The width of the expansion joint is related to the working temperature and the length of the control line. The higher the temperature, the expansion joint can be widened appropriately：

Key Notes：

• Expansion joints should be located to avoid holes in stress zones, furnace frame and lining
• For areas of complex shape (corners, convexity), the expansion joint should be set on a curved surface with a smaller radius of curvature
• When two pouring surfaces cross to form concave angles, an L-shaped expansion joint shall be set
• A control groove of about 200mm shall be set at the convex angle so that the shrinkage crack occurs at the preset position

3. Anchor Setup

For pressure-bearing environment (such as boiler flue, pressure> 0.05MPa), it is necessary to increase the density of expansion joint castable (porosity 。

For vibration conditions (such as pipelines close to fan and pump bodies), structural integrity should be enhanced. Castables containing stainless steel fibers (φ 0.2-0.5mm, length 30-50mm) should be selected, with fiber content 2%-3% (volume ratio) and bending strength ≥15MPa。

4. Key points of construction operation

Expansion joint castable construction should follow:

• Castable vibrations on both sides of the expansion joints should not affect the parts that have been poured and are beginning to harden
• When the castable temperature exceeds 1200℃, fiber felt should be inserted into the expansion joint
• The setting of control seams can be achieved by inserting paper, paraffin wax or sheet, leaving gaps after the disappearance of combustible or fusible materials at high temperatures

5. New and old replacement scheme: castable to refractory brick

For repeatedly damaged expansion joint castables, a well-established alternative is to change the castable construction to a refractory brick construction. The reconstruction practice of dry quenching expansion joint shows that the original expansion joint 80mm in the middle and 10mm on both sides of the expansion joint are changed to 45mm wide expansion joints on both sides, and then filled with 60mm thick ceramic fiber blanket, which successfully solves the problem of frequent damage of castable。

4. Key Points of Quality Control and Inspection

1. Control during construction

• High frequency vibrator (2000-3000r/min) is selected for vibrating equipment
• The curing temperature is controlled at 5-35℃
• Avoid construction in low or high temperature environment to prevent material performance deterioration

2. Quality acceptance criteria

Acceptance of expansion joint castables should focus on:

3. Selection misunderstanding reminder

In practical engineering, there are common misunderstandings in the selection of castables for expansion joints：

Myth 1: Only look at refractory and ignore thermal shock. The refractory of corundum castable used in a kiln reaches 1800℃, but due to poor thermal shock (

Myth 2: Excessive pursuit of high parameters. Ordinary hot air duct (1000℃, no wear) does not need to choose corundum castable, high aluminum and low cement castable is enough.

V. Maintenance and replacement cycle

The service life of expansion joint castables is significantly affected by working conditions. The expansion joint of circulating fluidized bed boiler is made of corundum or plastic, and its expected life can reach more than 4 years。

Regular check points:

• Check the surface of castable for cracks and peeling
• Knock to check for hollow drum sound (prompt to debond from metal shell)
• Check that the filling material of the expansion joint is in good condition
• Local damage should be repaired in time to avoid enlargement

sum up

Expansion joint castable is an indispensable lining protection material for high temperature flue expansion joint. Correct type selection and standardized construction are the key to ensure the long-term reliable operation of expansion joints:

The expansion joint castable with reasonable design and standard construction can effectively protect the metal expansion joint under the harsh working conditions of high temperature, wear and thermal shock, and greatly prolong the service life of equipment. It is recommended to entrust professional manufacturers to select castables and calculate expansion joints in the design stage, and strictly control the pouring quality and curing conditions in the construction stage.