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Maintenance of Water-Cooled Bubblers in Glass Furnaces

Original Paper | Published:27th March 2018.
Author: kakugy Guo
12k Accesses |159Citations |869Altmetric | Metrics

Abstract

Bubbling technology is widely used in large glass furnaces. It involves injecting clean gas with a certain pressure and flow rate into the bottom layer of molten glass through bubblers in the furnace. The gas expands at high temperatures, forming bubbles in the molten glass, which rise and eventually burst at the glass surface.

During the ascent and growth of the bubbles, they enhance the flow of molten glass around them, absorbing some small bubbles in the process. Thus, the bubbling process improves glass melting efficiency, enhances glass clarification, and promotes uniformity.

Additionally, the bubbling process can bring low-temperature molten glass from the bottom of the furnace to the surface, facilitating heat exchange with the flame space and improving overall energy utilization. However, the working environment of bubblers involves high temperatures, especially at points in contact with glass, requiring materials that can withstand temperatures above 1100 ℃. This involves using either special high-temperature-resistant materials or employing dedicated cooling systems.

This article uses the example of water-cooled bubblers and analyzes their failures in a specific float glass furnace. It aims to provide insights into the manufacturing, cooling water system design, and routine maintenance of such bubblers.

Application of Bubblers in a Float Glass Furnace

The examined company’s float glass furnace is a cross-fired regenerative pool furnace with six pairs of small melting units. The melting pool measures 40.7m in length,12.2m in width, and 1.4m in depth. The melting zone’s area is 273.28 square meters, designed for a discharge capacity of 600 t/d, with a melting rate of 2.2 t/(d·m2). The furnace employs bubblers from the UK-based F.I.C company.

Bubbler Structure

The schematic diagram of the bubbler structure is shown in Figure 1. The design includes an outer water-cooling jacket added to the bubbler gas pipe to protect it. The water-cooling jacket is made of carbon steel, with a working end diameter of 40mm and a wall thickness of 4.25mm.

The outer jacket is created by drilling water-cooling holes directly through the drill bit, with no welding around the pipe wall in contact with the glass. This ensures good thermal conductivity throughout and prevents cracking and water leakage from welding points.

Under normal circumstances, the bubbler uses industrial softened water as the cooling water. The water supply combines a water pump and a safety water tower to enhance water supply safety. The water supply mainline is also connected to the firefighting water pipeline through a bridge valve. In emergencies, firefighting water can be used temporarily to cool the system, minimizing the risk of water shortages.

Each bubbler has relatively independent cooling water supply and drainage pipes, with shut-off valves on their respective inlet and outlet pipelines. Additionally, each bubbler’s return water pipeline is equipped with a thermal resistor to measure the return water temperature, which is remotely transmitted to the control room for daily monitoring.

Bubbler Failures and Production Impact

The bubbler was put into operation on September 25, 2011, and its first failure occurred on May 21, 2012. Subsequently, multiple bubblers experienced failures, totaling 11 failures by September 11, 2012. The lifespan of these bubblers was less than one year, significantly deviating from the design requirement of one furnace life (approximately 8 years).

3.1 Phenomena Before and After Bubbler Failures

Comparing various phenomena before and after the 11 bubbler failures, two common characteristics emerged.

(1) Gradual Increase in Return Water Temperature

After a period of use, the return water temperature of each bubbler gradually increased. Individual bubblers experienced a significant deviation from the general trend, with higher return water temperatures corresponding to a higher likelihood of failure. Therefore, elevated return water temperatures above 50 ℃ increased the probability of failure, with higher temperatures associated with a greater risk of failure.

(2) Sudden Increase in Bubble Diameter

Bubblers operating at high return water temperatures experienced a sudden increase in bubble diameter after some time. The bubble diameter became several times larger than normal, indicating a malfunction. Upon inspection of removed bubblers, burnt-through holes were found on their outer walls. The increased bubble diameter was due to the vaporization of leaked cooling water heated by high-temperature molten glass. This phenomenon is a typical characteristic of bubbler burn-through, and the severity of water leakage corresponds to the increased bubble diameter. Figure 2 shows an industrial TV image captured during a bubbler failure.

3.2 Impact of Bubbler Failures on Production

The impact of bubbler failures on production can be divided into two aspects. First, after the increase in bubble diameter, the violent churning of molten glass may lift the stagnant layer at the bottom of the furnace, leading to the formation of devitrified stones. Second, during the downtime of bubblers, their suspended functions are temporarily interrupted, affecting glass clarification and uniformity. After the first bubbler burn-through in this furnace, due to insufficient experience and delayed detection, the churning of the bubbler continued for an extended period (about 1 hour), lifting a considerable amount of stagnant layer at the furnace bottom, resulting in a high occurrence of devitrified stones. The daily quantity of stones increased from around 60 during normal times to several thousand, serving as a significant lesson. Subsequent detections of bubbler failures were more timely, and their cooling water was immediately shut off. The bubblers were then withdrawn to a safe position, and the rear temperature of the pool furnace was increased during the downtime, strengthening glass clarification. No further significant occurrences of devitrified stones were observed, but there was an increase in internal bubbles during the corresponding faulty periods. Figure 3 shows the stone statistics curve.

Analysis of Bubbleizer Malfunctions

The glass liquid temperature in which the bubbleizer operates ranges from 1100 to 1350 ℃, and the outer pipe material of the bubbleizer directly in contact with the glass liquid is carbon steel. Carbon steel oxidizes rapidly at high temperatures and is quickly eroded in high-temperature glass liquid. The higher the temperature, the faster the erosion rate. The purpose of using cooling water is to lower the temperature of the bubbleizer through the flow of water. Industrial softened water is used for cooling the bubbleizer, with a daily monitoring hardness of 0.4 to 0.6 mg·N/L, thereby preliminarily excluding the possibility of scaling. During the inspection of the bubbleizer return water, it was found that there were a significant number of suspended solids in the cooling water, primarily sedimenting as mud and other debris. The supply water system of the bubbleizer is a semi-closed loop supply system, as illustrated in the process flow diagram in Figure 4.

Although the return water of the bubbleizer is closed-loop, the return water tanks of other water equipment in the melting furnace and tin bath are mostly open-type collection tanks, which are susceptible to dust falling due to environmental influences.

Upon further inspection of the cooling water supply system, it was found that the cooling towers in the supply system are open-type cooling towers, situated near factory roads and broken glass yards. Dust and airborne debris from the surrounding air enter the tower with the cooling airflow and then enter the cold water tank with the descending water flow. Although some sedimentation of mud and sand occurs in the water tank, a large amount of suspended mud and sand enters the pump. Moreover, the original filtration method for cooling water was “side filtration,” with filters installed in parallel with the supply pipeline, filtering the water before sending it to the cold water tank. Thus, it only gradually purifies the water quality inside the tank. Once the water in the cold water tank is contaminated, the poor-quality cooling water directly enters the supply pipeline, and the filter cannot block pollutants in this portion of the water. As a result, when cooling water with a large amount of suspended matter enters the bubbleizer, some will gradually deposit and adsorb in the pipeline. With an increase in sediment, the flow rate inside the pipe will decrease, making debris more prone to deposition, creating a vicious cycle that eventually leads to severe blockage of the pipeline. However, since the internal cavities of other water cooling equipment in the melting furnace and tin bath are larger, the impact of suspended solids in the water on them is minimal.

Countermeasures and Effects

5.1 Countermeasures

Through the above discussion, the root cause of the bubbleizer burn-through has been clarified, which is the presence of suspended solids in the cooling water causing blockage of the cooling water pipeline. Therefore, the focus of countermeasures is to improve the cleanliness of the cooling water, with specific measures outlined in 4 items.

(1) Install covers on the collection tanks of each return water on-site to prevent dust from falling.

(2) Install two layers of filter screens at the air intake of the cooling tower, with an outer filter precision of 10 mesh and an inner filter precision of 40 mesh. Regularly flush them to block external dust and debris from entering the collection tank through the cooling tower.

(3) Install spray pipes next to the broken glass yard near the cooling tower. Regularly moisten the broken glass to reduce the airborne dust, thereby reducing dust entering the cooling tower at the source.

(4) Improve the purification measures for cooling water, transforming from the original circulating water side filtration to full filtration. Install high-pressure backwash filters at the outlets of each pump, with a filtration precision of 50 μm, fundamentally ensuring the cleanliness of the cooling water.

5.2 Tracking the Effectiveness of Countermeasures

5.2.1 Improvement in Water Quality

Regarding the suspended solids in the cooling water, the “turbidity” index measurement was added.

Turbidity refers to the degree of obstruction that occurs when suspended solids in water block the passage of light. Suspended solids in water generally include soil, sand, fine organic and inorganic matter, plankton, microorganisms, and colloidal substances. Turbidity in water is not only related to the content of suspended matter in the water but also to their size, shape, and refractive index. Countermeasure measures were implemented step by step based on difficulty, and by February 2013, all measures were completed, and the turbidity of the water gradually decreased.

In severe cases, turbidity was 14.2 NTU. After the implementation of the first two measures, turbidity was 10.1 NTU, after the implementation of the third measure, turbidity was 7.5 NTU, after the implementation of the fourth measure, turbidity was 3.6 NTU, and under stable conditions, turbidity was 3.3 NTU. In early March 2013, on-site inspection of the return water of each bubbleizer revealed clear water quality, and suspended particles were practically invisible to the naked eye.

5.2.2 Usage Status of the Bubbleizer

With the gradual implementation of improvement measures for the supply water system, water quality has been gradually improving, and the return water flow rate has consistently been maintained at 55-65L/min. There has been no significant deviation in the return water temperature of each bubbleizer.

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