Properties of Castable Refractories

Refractory linings are common throughout the refinery and found in many different units including the FCCU, fired heaters, and Claus thermal reactors. In the previous posting of Becht Blogs: Refractory series, we gave a cursory overview of several refractory types and where they are typically applied (Introduction to Refractory). These materials come from the family of aluminosilicate refractories, often taking the form of castables bonded with calcium-aluminate cement (CAC), phosphates, or both. Although produced from a narrow range of chemistries, castable refractory materials exhibit diverse physical properties. These properties have evolved with available technologies driving stronger and stronger performance in refining and chemicals plants. Testing of these properties is the best way to ensure a quality material is manufactured and a high-performance lining is installed.

Four of the distinguishing measures for castable (monolithic) refractories are density, Cold Crushing Strength (CCS), Permanent Linear Change (PLC), and Erosion Loss (EL) (see Figure 1). These are the properties found on compliance datasheets (per API STD 936), QA/QC documentation, and in specifications to monitor and control the applied physical properties. Measurement of these properties is critical because castables are mixed, installed, and fired in-situ. Compare this with a brick, which is mixed, formed, and fired before the product leaves the manufacturer’s plant. If uncontrolled, these added operations for castables can adversely affect the installed properties. These deficiencies will ultimately contribute to reliability difficulties and additional maintenance in the unit. Table 1 lists some of the key ASTM standards used to measure the respective properties of refractory.

Figure 1. Graphical reference for the common methods to measure density,
cold crushing strength (CCS),permanent linear change (PLC) and erosion loss (EL)

Table 1. Distinguishing measures of castable refractory and applicable ASTM standards

Notice the quick reference for permanent linear change indicates a firing temperature. This temperature is standardized for convenience to 1500 °F to provide a basis for comparison. Each of the above tests (CCS, Density, PLC, and EL) are performed on fired material. However, applications exist in the refinery that exceed 1500 °F or may never reach 1500 °F. Thus, it must be noted that the reported properties may differ from the in-situ properties for any given material. Although measured independently, each property is interrelated to each other. A practical approach relies on (fired) density measurements to provide guidelines for intended service. For example, lightweight to indicate applications that prioritize insulating ability or heavyweight to indicate applications where strength is more important.

Cold crushing strength is a measure of the refractory’s strength and durability. As refractory falls within the domain of ceramic materials, it tends to fail in brittle fracture. In addition, castable materials naturally contain a large population of flaws in the form of porosity and surface cracks. These materials tend to be very strong in compression, which acts to close the flaws within the material, and can be capable of 20 ksi strength. However, in tension or bending this is reduced greatly. Lining design emphasizes the compressive stress imposed by hoop stress or by gravity to maximize this effect. The strength of the material increases with its density.

PLC is a measure of a castable’s change in length after initial heating to the standardized 1500 °F. As the material is heated the first time, the final processes of cement conversion will occur leading to a volumetric shrinkage related to the amount of cement within. Refractories in the refinery typically have 0 – 1 % PLC, which can be observed as surface cracks in the lining at ambient. Ideally, the total width of these cracks is like the expected thermal expansion during operation. Thus, when the lining heats up again, the lining expands to close the surface cracks and impose minor compressive stresses to the surface. PLC remains relatively unchanged in value but does have a slight positive correlation with density and CCS.

Erosion loss measures the volume of material removed by direct impinge of SiC abrasive. Both the direct impingement and the material of abrasive are extremely aggressive when compared with typical applications in an FCCU cyclone. In the equipment, the lining is exposed to tangential impingement of softer zeolitic-based catalyst and is further complicated by a distribution of particulate sizes, elevated temperatures, and high velocity. Fundamentals of erosion describe a relationship between the particulate (shape, size, hardness) and the target (profile, impact angle, ductility), but is most influenced by the velocity of their interaction. In short, predicting the amount of erosion is difficult. Nonetheless, industry experience has shown the ASTM C704 erosion loss test is effective for predicting performance in extreme erosion environments. The material’s reported EL tends to positively correlate with density and CCS.

One property that may or may not appear on the compliance data sheet is thermal conductivity. Also known as the “K-value,” this property typically governs lining thickness to suit a desired wall temperature. It can be reported from several different methods including ASTM C177, C201/C417, C1113, and C1421. Each of these has a certain bias on the reported value, but a normalization can be performed according to the data in API PUBL 935 – Thermal Conductivity Measurement Study of Refractory Castables. This measurement is used with the numerical methods in ASTM C680 to model the thermal gradient through the lining to predict shell temperatures (which will be covered in more detail in a future post). For a given composition, thermal conductivity lowers with increasing amounts of porosity. Thermal conductivity is inversely correlated with density and strength.

In the higher temperature applications in the refineries and chemicals plants, refractory linings employed at temperatures >2300 °F can be vulnerable to creep. For these applications, it may be beneficial to request a creep test per ASTM C832. This tests the material to a 25 psi suspended stress at a certain temperature (typically process temperature +300 °F) and monitors for deformation over a 50 to 100 hour duration. A similar alternative method is called Refractoriness Under Load (RUL). Creep testing is essential for load-bearing constructions such as reaction furnace linings in Sulfur Recovery Units (SRUs).

Another test applicable to materials that may be subject to submerged acidic conditions or for services including ash or slag is an acid test. There is not a common ASTM method for this, but most refractory testing labs will be familiar with the crucible test. In this, a crucible is made from the material being tested and the corroding solution is recreated inside the crucible. After exposure, the change in strength can be recorded alongside the depth of the solution’s penetration into the refractory crucible. Although each application has a specific case, a common application in the refinery is for material destined for refractory-lined sulphur pits.

Table 2. Additional measures used to verify refractory performance capabilities

Dual layer linings take advantage of both a high-strength, high-conductivity material as well as a low-strength, low-conductivity material. The “hot-face” or process-facing layer of the high-strength material is designed to resist erosion, vibration, or chemical attack while protecting the “cold-face” or insulating layer underneath. The insulating layer provides the low conductivity required to protect the pipe or vessel’s metal wall but does not have the strength to resist mechanical damage (see Figure 2). Three common variations of the dual-layer system are V in combination with Y anchor, V anchor with hexmesh facing, and the ceramic anchor systems. The Y anchor system can accommodate much thicker linings than V anchors alone. The hexmesh system offers additional anchoring to locations that would be difficult to secure, such as hot-to-cold wall transitions in the FCCU, and the ceramic anchor system can be used in very high temperatures without compromising the anchor’s integrity.

Figure 2. A schematic showing three variants of a dual-layer lining

Another property that will appear on a datasheet and specifications that has not been discussed is chemistry. In comparison to steels, where an ASME standard specifies method, shape, and chemistry – there is no such standard for refractory materials. This can lead to confusion when a brick specified to minimum 60% Al₂O₃ content can be either a mullite-bonded brick suitable for high-temperature arch construction with good creep resistance, or it could be a 60% Al₂O₃ andalusite brick that is more suitable for a burner tile.  Notably, if either of these bricks are not properly hard fired during manufacture, they could simply end up as agglomerations of silica glass bonded, high alumina grains with no useful high-temperature properties.

Chemistry alone is not enough information to determine whether a refractory is suitable. Most aluminosilicate refractories are manufactured by processing blends of raw materials.  Therefore, it is important to keep tramp iron and titanium low as well as limit alkalis that might interfere with the cement.  Consequently, proper control phase content and morphology (microstructure) are key to optimizing the physical properties of the material. For certain applications, it may be important to specify chemistry, such as low SiO₂ in H₂ service. On the other hand, identifying a specific constituent such as fused or vitreous silica helps ensure good thermal shock resistance.

The performance of the refractory lining depends on its properties. Products that seem similar in name, composition, or datasheet can quickly become favorable or unfavorable with basic testing of the properties. Specialized service environments and new process development may require additional testing to validate material selection. The next post will further detail how the properties of castable refractory properties have evolved with improved technology as well as the quality standards such as API STD 936 to assure that those new quality standards are met on a reliable basis.

For questions, comments, and discussion of castable refractories, contact one of Becht’s mechanical experts.

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