The service life of a glass furnace is closely related to glass furnace design, refractory selection, daily maintenance, glass composition, and construction quality. High-quality construction not only enhances glass furnace durability but also facilitates later management and maintenance. However, detailed standard specifications for construction are still lacking for some glass furnace constructions; many glass enterprises pay insufficient attention to construction and lack management, leading to frequent hidden quality risks. The relevant standards and specifications for modern industrial glass furnace construction are relatively broad, often resulting in casual construction practices, neglect of quality, arbitrary shortening of construction periods, ultimately increasing the probability of glass furnace abnormalities and directly shortening the glass furnace‘s service life. Therefore, only strict and scientific glass furnace construction can lay a solid foundation for a long glass furnace campaign.
Currently, the quality of glass furnace construction in the domestic glass industry mainly relies on the work attitude and experience of the construction team. If the team is responsible and implements high standards during construction, the construction quality is relatively good; conversely, if the team is careless, rushes progress, or personnel are unprofessional, construction quality cannot be guaranteed. Therefore, formulating construction standards and specifications specifically for the glass industry is crucial for extending glass furnace life.
Currently, common glass furnace types in the glass industry include cross-fired glass furnaces, end-fired glass furnaces, oxy-fuel glass furnaces, and all-electric melters. Among these, all-electric melters and oxy-fuel glass furnaces have simple structures and a small footprint; whereas conventional end-fired glass furnaces have a relatively more complex structure. Mastering their construction essentials allows for easier understanding of other glass furnace types. The following takes a conventional regenerative end-fired glass furnace as an example to introduce key points and precautions in glass furnace construction, covering major stages such as melting tank wall construction, tuck stone block construction, melting tank crown construction, weir construction, and distributor construction.
1. Melting Tank Wall Construction
Melting tank wall blocks are generally laid according to drawing numbers or block layout drawing numbers. Three key points require attention during construction:
(1) The Outer Wall of the Melting Tank Must Be Level
A level outer wall of the tank wall blocks firstly facilitates simultaneous construction of insulation bricks, and secondly creates conditions for later block patching. To extend glass furnace life, block patching is often required later. If patching is performed, the outer wall surface from about 300mm below the top plane of the melting tank wall blocks must remain level. This ensures good tightness after late-stage glass furnace patching. If the patching is not tight, a hollow gap forms between the patch block and the original tank wall block. This hollow layer acts as insulation, preventing sufficient cooling of the original tank wall block, thereby accelerating its erosion and making it prone to glass leakage. Therefore, when constructing tank wall blocks, a leveling rod must be used to check the outer wall, ensuring its flatness. As shown in Figure 1, a worker is using a leveling rod to check the external levelness of the tank wall blocks.
(2) Treatment of Gaps Between Tank Wall Blocks
Expansion gap dimensions for tank wall blocks are sometimes reserved based on the expansion coefficient of the fused cast AZS blocks, leaving an expansion joint every few blocks (at intervals of 1.2–1.5m). The cumulative linear expansion coefficient of fused cast AZS (alumina-zirconia-silica) blocks from 20–1000°C is 6–8 mm/m. Alternatively, based on the expansion coefficient, plastic-steel woven bags of a certain thickness can be inserted between adjacent tank wall blocks. Mechanistically, the latter method for reserving expansion gaps is more reasonable: because under this method, the expansion movement of individual tank wall blocks is smaller, displacement is smoother, significantly reducing the risk of block fracture or upper section tilting.
After constructing the tank wall blocks, the expansion gaps between blocks must also be treated. For small gaps, sealing with tape is necessary, primarily to prevent mortar or sand particles from falling into the expansion gaps during the construction of tuck stone blocks, crown blocks, and tank wall insulation bricks. If impurities enter the gaps, the expansion gaps of the tank wall blocks will not close properly during heat-up, accelerating joint erosion. In later glass furnace operation, excessive cooling of brick joints increases energy consumption; insufficient cooling easily leads to glass leakage from the gaps. For large expansion gaps, foam sealant must first be injected to fill the gap. After the foam cures, excess overflow is scraped level, and then sealed with tape. Figures 2 to 4 show on-site pictures of foam filling for tank wall block expansion gaps, tape sealing of tank wall block gaps, and erosion caused by excessively large tank wall block gaps, respectively.
(3) The Top Plane of Tank Wall Blocks Must Be Level
The flatness requirement for the top plane of tank wall blocks is not as strict as for other parts. However, if the top plane is uneven, the compression blocks between the tuck stone blocks and the tank wall blocks will be unevenly high and low, preventing them from providing a good seal. This can easily lead to flame penetration at the compression brick joints. In recent years, many glass furnace designs have eliminated compression blocks between tuck stone blocks and tank wall blocks, using ramming mix or other refractory mortar for sealing instead. The more level the top plane of the tank wall blocks, the more conducive it is to sealing the gap between tuck stone blocks and tank wall blocks, reducing susceptibility to burn-through and flame penetration. Therefore, construction of tank wall blocks must follow standardized procedures. Figure 5 is an on-site photo of checking the levelness of the top plane of tank wall blocks.
2. Tuck Stone Block Construction
Tuck stone blocks are located at the lower end of the breast wall and do not contact the glass melt. Therefore, they are often considered not to affect glass furnace operation. However, the flatness of tuck stone blocks actually affects the stress distribution on the upper breast wall. Level tuck stone blocks allow for more balanced stress on the upper breast wall. During heat-up, the breast wall is thus less prone to cracking; during later glass furnace operation, flame penetration in the breast wall is also less likely. Therefore, when constructing tuck stone blocks, their flatness should be checked to ensure overall levelness. Figure 6 shows on-site verification of tuck stone block flatness during construction.
Additionally, reserved expansion joints and joint treatment for tuck stone blocks should be sealed similarly to tank wall blocks to prevent sand or mortar from falling into the gaps, which could prevent the expansion joints from closing.
3. Melting Tank Crown Construction
Among all glass furnace arches, the melting tank crown (main arch) is the most critical and also the most problematic area. Flame penetration and block fall due to poor crown construction quality are the most common issues. Furthermore, if expansion is not properly adjusted during heat-up, or if maintenance is inadequate during operation, flame penetration and block fall can easily occur. Therefore, crown construction is crucial.
3.1 Crown Pre-assembly
Crown blocks need pre-assembly before construction. This is because the dimensions of blocks produced by refractory manufacturers have certain deviations from drawing dimensions. When dozens of rings of arch blocks are aligned, these deviations accumulate and have a significant impact. For example, constructing according to the designed crown span might result in the space for the final ring of arch blocks being either too large or too small, making it difficult to properly complete the last ring. If the space is too large, the entire crown structure becomes loose, blocks cannot press against each other, failing to form an integrated force. If the space is too small, the final ring cannot be properly tapped into place; forcing it in with excessive pressure might crack surrounding blocks or the final ring itself.
Some glass furnace construction teams address the issue of overly large or small spaces for the final ring by temporarily tightening or loosening the crown skewback jacks on both sides. However, while these methods may seem acceptable superficially, such adjustments often cause the lower part of the crown to "gape," meaning the lower block joints open. If the gape is too large during glass furnace operation, hot gases inside the glass furnace will erode upward along the joint, gradually widening the lower gap, eventually leading to block fall and flame penetration. Another incorrect method used by some teams when constructing the final few rings is to increase the mortar thickness of the last few crown blocks if they find the space larger than the block dimensions with the current mortar thickness. Therefore, crown blocks must be pre-assembled before construction, and the construction plan adjusted based on the pre-assembly. Figure 7 shows photos of crown block pre-assembly.
After pre-assembly, the actual crown span can be adjusted to correct deviations between actual block sizes and drawing dimensions, ensuring tight closure of the lower opening and allowing the final ring to be tapped in with moderate force. The space shape at the top for the final ring should match the block shape or be slightly smaller than the actual block size, achieving the effect of tapping into place with appropriate force.
To achieve the pre-assembly effect shown in Figure 7, several crown block sizes should be designed to ensure tight fitting of both upper and lower edges during construction. However, many glass furnace designers habitually design only one crown block size. When reaching the final ring and finding it unsuitable, they resort to on-site cutting. But manually cut arch blocks often have uneven sizes and large deviations, easily causing the lower or upper edges to gape, preventing tight integration of each block in the final ring with surrounding blocks, and relying on mortar to compensate. Such crown construction quality is greatly compromised, posing significant hidden risks for later glass furnace operation. For example, one glass factory did not perform pre-assembly during crown construction and excessively adjusted the skewback jacks, causing the lower edges of crown blocks to gape, as shown in Figure 8. The red portions in the image show significant gaping. After two years of operation, the glass furnace roof experienced large-area flame penetration, ultimately leading to multiple block falls, necessitating shutdown and major repair.
Silica bricks have poor resistance to alkaline gases (R₂O) in glass furnace atmosphere. If gaps exist in the crown, R₂O gases enter the joints, accelerating silica brick erosion and forming "mouse holes."
3.2 Crown Centering (Formwork) Fabrication
Crown centering should be fabricated according to the arch rise dimension from pre-assembly and the total span dimension which is the sum of the pre-assembly span and the mortar thickness per brick. Fabricating solely based on drawing span and rise will lead to the aforementioned issues of lower/upper gaping and brick mismatch. Many construction teams merely nail square wooden strips onto the curved main body of the crown, considering the centering complete. Such non-standard centering is shown in Figure 9.
The crown constructed with such centering will have an uneven inner surface. Standard centering fabrication requires laying a 3–5mm thick plywood layer over the square wooden strips to ensure flatness of the lower edges of the crown blocks. Figure 10 shows a photo of standard centering with plywood.
As seen in Figure 10, although adding a plywood layer seems tedious, a level crown centering not only makes construction easier but also more effectively ensures construction quality. Figures 11 and 12 show on-site photos of the inner surfaces of crowns and regenerator arches constructed using centering with plywood. The inner surface of the crown should be overall level and smooth, with uniform gaps between bricks. Regenerator arch construction has the same requirements as the melting tank crown.
3.3 Making Layout Lines and Stretching Guideline Strings
To standardize crown construction and ensure quality, layout lines need to be marked on the centering, and guideline strings must be stretched during construction. Layout lines should be prepared based on each block‘s dimensions and mortar thickness. When constructing crown blocks, guideline strings should be stretched at both ends of each ring. This ensures the constructed crown meets standard requirements. Figures 13 and 14 show a standard crown centering with layout lines and an on-site photo of stretching guideline strings during crown construction, respectively.
When making layout lines, every three rings can be grouped as one unit. Mortar thickness is generally controlled at 1.5mm, and mortar should be mixed to appropriate consistency. If too thin, adhesion is poor; if too thick in joints, cracking is likely. Cracked joints cannot withstand flame scouring, easily causing flame penetration. Stretching guideline strings during construction ensures the upper joint gaps of each ring form a straight line. When bricks protrude beyond the guideline, a wooden mallet can be used to tap the corresponding crown block, squeezing out excess mortar from the joint, thereby maintaining uniform mortar thickness.
3.4 Crown Expansion Joint Treatment
For small glass furnaces, due to limited crown expansion, expansion joints are usually reserved at both ends. For medium and larger glass furnaces, besides both ends, expansion joints are also set at appropriate middle locations based on crown length. Expansion joints for silica crown blocks are generally reserved at 1.5‰ of the crown length. After heat-up, the joints should nearly close. Additionally, to facilitate sealing of incompletely closed joints, the silica bricks at crown expansion joints must be chamfered on the upper side at an angle, wider at top and narrower at bottom, for easy filling of sealing material. However, some simple glass furnace designs omit chamfering. This not only makes sealing incompletely closed joints difficult after heat-up but also leads to flame penetration at crown expansion joints during operation. Figures 15–17 show pictures of chamfered and non-chamfered silica bricks at crown expansion joints, and an on-site photo of flame penetration due to lack of chamfer at both ends of a crown.
3.5 Other Detail Treatments
After crown construction, two additional details require attention:
(1) Top Sealing Treatment
After crown block construction, a layer of hot patching mix is generally applied on top, followed by insulation brick construction. Alternatively, dry silica mortar or quartz sand of the same material as the crown blocks can be used, also sealing the crown block joints. A space about 500mm wide should be left in the middle of the crown for applying hot patching mix and insulation after heat-up completion. This method is generally called "cold insulation" of the crown. Another method is "hot insulation," where overall crown insulation is applied after glass furnace heat-up. As long as heat-up is standardized, well-executed, and expansion adjustments are proper, both methods can be used. Figure 18 shows on-site cold insulation of a crown.
(2) Crown Centering Removal
After completing the top insulation layer, crown centering removal can begin. Since the centering is firmly pressed by the crown blocks, separation is difficult. Forcible removal poses safety risks. The correct method is to set a height gauge on the crown top, then tighten the skewback jacks on both sides to lift the crown by 1–2mm. The centering will slightly separate from the bricks, allowing easy removal.
4. Weir Construction
The most important part of tank bottom block construction is the weir blocks. Although weir construction seems like routine block laying without special requirements, deeper analysis reveals potential issues. Conventional weir design and construction place expansion joints between the weir and tank wall blocks. This design is based on the consideration that if the weir expansion joint does not close after heat-up, the overall impact is minimal because glass flow near the tank wall is weak during normal production, and the amount of glass leakage from an unclosed joint is small, having little effect on glass quality. However, after several years of operation, glass quality gradually deteriorates. Later major glass furnace repair shutdowns revealed that the originally reserved weir side expansion joints were significantly enlarged by glass erosion. Analysis indicated that the weir expansion joint was one cause of quality degradation. Practice proves that during operation, if weir side expansion joints do not close well, glass flow exists there. As erosion enlarges the joints, glass leakage increases, later affecting glass quality. Figures 19 and 20 show a conventional weir expansion joint design and a photo of weir expansion joint erosion after shutdown, respectively.
To avoid such issues, staggered joint reservation can be adopted during weir construction: stagger the expansion joints of front and rear weir bricks on both sides. This method enhances weir durability and minimizes impact on glass quality in later operation. Figure 21 shows an on-site photo of staggered weir expansion joint construction.
5. Feeder Construction
Feeder construction is the final stage of glass furnace construction, usually performed after completing the melting tank and working end. The forehearth is as important as the melting tank; its construction quality is crucial because many glass product defects originate from the forehearth. Although often overlooked, as the saying goes, "forming accounts for 30%, feeding accounts for 70%," highlighting its importance. Forehearth construction also requires attention.
5.1 Construction Air Tightness
Many glass manufacturers now change forehearth heating from traditional electric to gas. During gas combustion, internal mixed gases generate certain pressure. If the forehearth cover plate sealing is poor, flame penetration can occur. Insulation bricks over cover plate joints may melt, and molten material can drip into the forehearth glass, causing cords and stones (seeds/stones) in products. To avoid such defects, during forehearth construction, joints between cover plate bricks must be filled with brick material of quality not lower than the cover plate bricks; ordinary fireclay bricks should not be used for joint filling. However, in reality, many manufacturers producing ordinary quality glass do not follow this. If forehearth design and construction are standardized, defects from the forehearth will significantly reduce. Some designers also adopt interlocking structures for cover plates, achieving better sealing. Figure 22 shows an on-site photo of joint treatment between forehearth cover plate bricks.
5.2 Maintaining Consistent Glass Level Height Across All Feeders
Glass furnaces producing glassware and containers usually have multiple forehearths. Ensuring consistent glass level height across all feeders is crucial. However, many glass factories have differences in glass depth among feeders. Forehearths with shallower glass levels have correspondingly less glass in the spout. When producing slightly heavier products, it‘s often impossible to adjust to the required gob shape, leading to many defects during gob delivery and forming. To address inconsistent glass levels, some manufacturers adjust the height adjustment screws under the forehearth steel structure individually during production. This method requires professional operation; improper handling can cause glass leakage, leading to safety incidents, which have occurred in some factories. Therefore, during forehearth construction, the overall glass level line must be strictly calibrated. Each feeder‘s level line should be based on the working end glass level. However, many construction teams use improper level marking methods, such as referencing adjacent feeders. This leads to significant level differences among feeders, with glass depth errors up to 5mm, sometimes exceeding 10mm. Excessive errors affect forming operations.
Tools for marking level height include laser levels, water levels, and optical levels, each with limitations. Laser levels produce errors over long distances, increasing with distance. Water levels have similar issues due to fluid resistance in the tube over distance. Optical levels have slight errors due to operator viewing angle. Therefore, when marking feeder glass levels, two tools can be used simultaneously. If the discrepancy exceeds 5mm, there‘s a problem requiring re-calibration; if within 2–3mm, correction is unnecessary. The amount of stagnant glass in the feeder should be minimized. The feeder bottom is cooler, glass viscosity higher. Prolonged stagnation leads to devitrification. Two common feeder structures are:
(1) Conventional Horizontal Feeder
Feeder channel blocks must be laid level to minimize stagnant glass. During construction, not only should a spirit level be used to check each channel block‘s levelness, but a laser level should also assist to ensure overall feeder horizontality, preventing overall unevenness, especially upward slope towards the spout. Figure 23 shows on-site feeder construction and leveling.
If the feeder is not level, the stagnant bottom layer increases. When feeder temperature changes or glass flow balance is disrupted, devitrification products from this stagnant layer become mobile, causing defects in glass products. These defects vary: some appear as transparent jelly-like substances, some as white small particle stones (seeds), others as cat‘s paw or broom streaks. Some factories habitually call the transparent jelly-like substance "dirty glass" (believing it‘s unmelted batch or impurities), but many don‘t know its origin. Actually, this is devitrified glass. Therefore, feeder channel block construction must be meticulous and strict, not approximate, otherwise the aforementioned defects easily occur during operation.
(2) Special-Shaped Feeders
Many glass furnace designers and constructors lacking practical experience design and build feeders with steps, grooves, or upward slopes. Such feeders more easily generate stagnant layers and accumulate poor-quality glass. When feeder temperature or pull rate changes, devitrified material and severe cords appear in large quantities. Figures 24–26 show design drawings and schematic diagrams of these feeder types.
The initial design intent of these feeders is to block so-called "dirty glass" generated in the feeder. In early glass furnace operation, such structures might block some devitrified material or high-viscosity glass from the bottom. However, over time, more stagnant layers accumulate in grooved, stepped, or sloping feeders. Under specific conditions and suitable temperatures, these layers gradually devitrify, accumulating until the entire feeder bottom is covered with poor glass. When feeder temperature or pull rate changes, this poor glass surges out, severely affecting product quality.
6. Conclusion
In summary, tank wall outer blocks must be constructed level to facilitate subsequent insulation brick construction and patching, extending glass furnace life. Expansion joints between tank wall blocks must be reserved with appropriate dimensions based on fused cast AZS block expansion coefficients, avoiding block damage from overly small gaps or glass leakage from overly large gaps. Breast wall Tuck stone blocks must be constructed level to ensure balanced stress distribution on the entire breast wall. Melting tank crown construction requires pre-assembly to avoid deviations between actual block sizes and design dimensions, preventing lower crown gaping. Crown centering must be fabricated to standard, ensuring levelness of the lower plane.
During tank bottom construction, attention to detail in weir construction, using staggered joint methods, yields better performance. Feeder construction must ensure air tightness of flame space brick joints and horizontal channel bricks to guarantee stable glass delivery and quality control. In summary, all the above construction stages must strictly control quality and undergo inspection, ensuring safe and stable glass furnace operation.
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