Installation of Monolithic Refractory and Resulting Properties
Monolithic refractory linings are a necessity of operating a variety of furnaces, boilers, and reactors in the refinery. Despite their prevalence, the “off-the-shelf” product looks nothing like the final durable and insulating lining that may be familiar. These materials are sold in dry bags of powder, like concrete from the hardware store, and require the purchaser to transform the product into the recognizable monolithic lining.
In broad strokes, installation of monolithic refractory includes the mixing and the placing, which is covered within this article, and the dryout, which will be covered in a later post. This peculiarity allows flexibility of material properties, lining shape, and a rapidity of installation that suits the petrochemical industry. However, the installation steps can vary greatly in required equipment and final result (see Figure 1), which necessitates attention to detail to ensure good quality. When a root cause failure analysis points to a problem with the refractory, the quality of installation and dryout are the common culprits. Problems with the quality can occur without major visual cues on the lining itself.
Figure 1: Sketch of a castable as supplied in a dry powder form (left); photo of an installed castable lining after service (right)
Water Starts the Clock
Installation of monolithic refractory begins with mixing dry materials and a liquid. The liquid serves two primary purposes. First, it activates the setting reaction that leads to the material hardening. Second, the liquid surrounds the solid particles of aggregate and binder, enabling the mixture to be conveyed. Depending on the installation method, the liquid can be added in the mixer either all at once or as only a pre-dampening amount, with the rest of the water added at the gunning nozzle.
For phosphate-bonded monolithic materials, the liquid is supplied by the manufacturer alongside the dry material. However, for cement-bonded monolithic materials, water can be supplied from a variety of sources, and as such it is important to secure a water testing report before use. As one of the primary purposes of the liquid is to start the chemical reaction, it is important that the water be at suitable pH and chlorine levels. Local potable water is likely acceptable, but even this can be dependent on the quality of municipal supply. Too much acidity can lead to flash setting, or premature hardening of the mixture (i.e., hardening still in the mixer, bucket, or hose), and too much chlorine can inhibit completion of the cement reaction. In addition to sensitivity to water chemistry, the cement reaction rate can be very sensitive to temperature, and excellent execution can rely on chilling the water before mixing, mixing in an air-conditioned facility, mixing at the coolest part of the day, and air-conditioning the equipment that the lining is going in to. Consequently, all these factors need to be specified, monitored, and controlled along with many more.
Good Mixing for Consolidation
The mixer needs to generate high shear force to engage each particle of the dry mix with the liquid in just a couple minutes. For any mixer, this requires periodic maintenance and adherence to the mixer manufacturer’s original equipment specification. Horizontal shaft mixers with paddles are the most effective and therefore desirable. Vertical shaft mixers may be suitable but require verification that enough shear force is generated. Sometimes mixers that fail verification can be retrofitted to increase force generation and become suitable.
Drum mixers are not designed for the ultralow water content of refractory and will struggle to consolidate the mix. Planetary mixers are suitable for erosion-resistant refractory but not efficient for the batch sizes of typical installations. The manufacturer’s mixing times should be followed to ensure a balance of working time and material consolidation. The combined factors of temperature, water quality, mixer type, and mixing time are major contributors to unreliable refractory linings. Thus, for any quality refractory installation, a mixing sheet is kept as an agreement between owner, inspector, installer, and manufacturer that conditions and procedure were followed and adequate for installation. See Figure 2 for an example of how the right mixer develops a dry powder into a castable.
Figure 2: Video still shot of a qualified vertical shaft mixer preparing an 80 lbs/ft3 castable for casting. The dry material (already in the mixer) develops into a putty consistency within 60 seconds.
After mixing, the material needs to be conveyed quickly to the installation location. The highest-quality method for installation is casting. The level of quality comes from precisely measured water addition and avoidance of defects created during the conveyance. Fully mixed material is moved from the mixer either by buckets or hydraulically by a swing-tube pump. “Bucket brigading” is versatile and often practical in a pinch, but can be challenged by tall elevations, hot ambient temperatures, and lack of external structure (see Figure 3). Keep in mind that buckets can become contaminated with leftover material from the first carry and cause flash setting during subsequent uses.
Cast Installation
Hydraulically pumping refractory, also called pumpcasting, can be difficult and requires a specifically designed pump rig to be successful. In large installations, especially when the bucket brigade cannot move material fast enough, pumpcasting could become necessary. After just a few minutes of mixing, the countdown for working time begins. Typically about 30 minutes, the working time is the amount of time the installer has to convey the material from the mixer to the point of installation. Exceeding the working time will result in the material setting (hardening) before the installation completes, leading to failed installation.
Large installations make use of several mixers or multiple batches to work around this limitation. In general, denser materials are less sensitive to external factors and therefore easier to install. Most materials are available from the supplier in cast or gun installation variants, except for rammed materials, which are typically installed on hexmesh anchoring and are incompatible with cast or gunned installations.
Figure 3: Video still shot of the bucket brigade technique demonstrated for a casting mockup
Cast installations require formwork to make the interior shape of the lining. The formwork must be waterproof to avoid drawing water out of the castable and interfering with the curing. The form itself may be high-density Styrofoam or plywood, which allows the form to be “burned in” or left in place to be burned by process start-up. Alternatively, the form can be assembled from steel and removed after casting. Caution should be exercised during large installations where the hydrostatic force generated by the refractory can challenge the integrity of the forms. For new construction, casting of refractory-lined pipe should be in the vertical position in pipe sections of 15-20 feet per pour. This length may be reduced based on other constraints such as internal access, multi-piece fit-up, or post-weld heat treatment.
Casting without formwork is defined as “hand casting” or sometimes “hand packing.” This method is often used in combination with gunning, where gunning is limited to installation from the waist up and hand casting is used from the waist down. Consider a relining of a horizontal duct with gunned refractory specified, where hand casting is used in conjunction with gunning to complete the job (see Figure 4). This leads to cold joints parallel to the length of the pipe that will be vulnerabilities in the lining. Hand casting can vary in quality and is inconsistent, but can perform as well as the adjacent gunned lining when carefully controlled.
Figure 4: Sketch of repairs to a horizontal pipe showing the use of gunning and hand casting to complete the installation
Gunned Installation
Gunning refractory is a much more rapid installation than casting, as formwork is not required and the pneumatic conveying of material is much quicker than buckets. These advantages come with a slight degradation of properties (see Table 1), a limitation that installation cannot be below the waist, and an increased likelihood of installation defects such as trapped rebound or laminations. Gunned refractory requires gunning machines, of which the pressurized dual chamber is preferred for optimal control of the final product. Other machines further increase the likelihood of defects during installation, and flywheel-fed machines are especially prone to this.
Application of the material to the wall is done with small circles starting at the bottom of the installation (demonstrated in Figure 4). This technique builds up the material with a solid base, and the continual circling of the nozzle has been proven to reduce rebound entrapment. The quality of the gunned refractory is dependent upon the air pressure supplied by the compressor, which is tuned by the mixing crew, and the final water injection at the gunning nozzle, which is tuned by the gunner. Final setting of these parameters depends on many factors, so instead of specifying every input parameter, API STD 936 recommends testing the output results in a “mockup.” This activity tests and confirms that the overall gunning activity can meet the specified properties with the specific equipment and installation crew.
Figure 5. Video still shot of gunning of castable refractory. Note that there are three hoses. The largest-diameter hose conveys mixed material, the middle-diameter hose supplies the air to blast out the nozzle, and the smallest-diameter hose with the needle valve controls the final water addition.
Table 1: Qualitative comparison of refractory installation methods
Rammed Installation
Although the terms are used interchangeably, hand packing or “ramming” most often refers to installation of thin-layer erosion resistant refractory into a special anchoring system of hexmesh (this is a distinct method from hand casting; see Figure 4). The welding of the hexmesh has been automated by cyclone vendors and others redesigned the anchors for stud welding, but most installations rely on stick welding. The required pattern of 1″ fillet welds will be relied upon to hold the hexmesh together, hold the lining to the equipment, and not let go of either side through thermal cycles, coke ratcheting, and coke jacking.
Figure 6. Ramming in progress with a pneumatic tamper on a large-diameter expansion joint. The completed section has been trimmed flush (appears smooth) and the work in progress has the appearance of a rough texture. Hexmesh where installation has not begun can be seen just below the installer’s knees.
After welding of the hexmesh, the refractory is mixed to a stiff moldable consistency for installation. First, a small ball of material is taken from the mix and placed in an open cell of hexmesh. Next, the installer uses his or her finger to “thumb” the material into the crevices. More material is then added and this is repeated to build up ¾” to 2” of lining after pounding the refractory into the hexmesh cells with hammer or pneumatic tamper. In addition to typical refractory qualification activities, it is important to qualify installers for both welding and ramming of hexmesh.
Although most installations of monolithic refractory are successful, poor installation quality is the most common problem. When things start to go wrong, the identification of the weakest link in the system can help prevent future instances of failure. For complex jobs, such as FCCU turnarounds, the main three installation methods—casting, gunning, and handpacking—are likely to be employed at various locations throughout the unit. The resulting refractory will vary in quality based on installation method, and for certain recurring problem components, consideration of alternative installation methods could be as valuable as alternative materials.
Conclusion
The good practices discussed here set the landscape for best practices during refractory installation and repairs. Every piece of equipment is faced with its own unique challenges and that’s where Becht’s expertise is best applied. Some frequent inquiries are installations that need to be completed without shutting down the equipment, without internal entry, or within a tight execution window.
It is convenient to assume the refractory specified by an equipment drawing was installed with utmost quality and procedure. However, time and time again Becht finds the refractory lining is not doing its job and impeding operation of the equipment. Sometimes selecting another material is enough to mitigate the problem. Yet, when the problem reoccurs, an expert needs to conduct a detailed review of the refractory mixing and installation to ensure the lining—and the equipment it is installed into—has the best possible chance of success.
For questions, comments, and discussion of castable refractories, contact one of Becht’s experts.
