GLASS PLANT AUDITS - THREE CASE STUDIES IN GLASS PRODUCTION PROBLEMS AND THEIR SOLUTIONS

J. M. Uhlik

Toledo Engineering Company, Inc.

Toledo, OH

ABSTRACT

Over the multi-year length of a glass plant campaign, problems arise with root causes traceable to design, engineering, construction and the operational parameters which can change over time. This presentation's intent is to illustrate real-world production problems arising from those changing needs, practical solutions, and the value of non-resident process reviews such as performed by the member companies of the TECO Group. It often takes an experienced or non-routine study of the problem(s) to first determine the root cause, and then engineer how to best resolve it. Problems and solutions can include: refractory design/selection for maintenance, wear issues experienced during the campaign, hot repairs and temporary engineering solutions and operational process adjustments. This can typically result in glass quality improvements and campaign life extension through applying principles of operation optimization and improving maintenance techniques. The results are often significant improvements in glass quality, pack yields and the plant's bottom line. This presentation will discuss three problem instances - in a throated furnace, the waist area of a float furnace, and sidewall refractory replacement maintenance activities.

INTRODUCTION

Ask anyone involved in the day to day operation of making glass - sometimes it seems as if their plant is a living, breathing entity. And sometimes, they become ill.

GLASS PROBLEM ONE - FURNACE WITH A SORE THROAT

TECO was asked to assist with an increasingly evident non-conforming glass attribute from a throated furnace in Europe. The problem was a distortion line in the rolled glass sheet being called a "water mark" by plant personnel, which tested as a high density alumina-zirconia layer approximately 60-75 microns thick, shown below in Figure 1.

Figure 1. Optical inhomogeneity in the Ribbon

When first detected preliminary thinking was that it was a lamination problem (mechanical action on the glass), such as roller mark, lip issue, roller cooling problems, etc. Many initial actions were undertaken to find the root cause and eliminate this defect. These actions included:

• The cover of the lamination area was adjusted.
• Various machine positions were instituted.
• Several machine changes with different rollers were tested, smaller rollers with different cooling, etc.
• A bottom roller with chrome coating was used.
• Refractory lip was changed.

This is a typical operational progression, where the urgency of continuing glass losses force increasingly costly (in terms of lost production and/or equipment replacement) adjustments to the process in a search for improvement. Meanwhile a sample of the distortion line was sent out for laboratory analysis. The results are shown in Figure 2.

Figure 2. Analysis Results of the Glass Inhomogeneity.

Based on the analysis report, an average of five composition measurements yielded higher levels of alumina and zircon content than what was normally found in the base glass. Therefore, increased focus was placed on the batch, the glass furnace and the forehearth operation and structures, which had been previously been operated consistently and at steady state for some period of time.

Technical service personnel from Toledo Engineering Co., Inc. (TECO) and Zedtec, Ltd. were invited to the facility to help the customer assess the situation. Together, the combined team completed several problem solving exercises and developed an evaluation plan. During this investigation, the physical inspection of the furnace interior was performed, as the viewing ports allowed. Figure 3 shows the interior of the Zedtec glass conditioning forehearth - the inspection of the forehearth provided assurance that there was no undo wear, the structure was intact and the glass level was as per the design of 50 mm below top of block.

Figure 3. Forehearth Inspection Port and Forehearth Glass Level Estimate

Finally, the inspection of the furnace interior provided that while the structure and superstructure refractory appeared to be in proper condition, the glass level as observed did not appear to be at the design level of 50 mm below top of block - there appeared to be much less glass freeboard, as shown in Figure 4. To check this observation, first a simple length of tubing was used as a water level, and when checked, showed that the furnace construction was correct, with both the furnace and forehearth top of block set to the same elevation. The actual glass level observation did not make sense, so not only was the water level used several more times, but an optical engineering level measurement was contracted locally, and these readings also verified the correct construction. Engineering 101 teaches us that liquids seek their own level, yet the visual observations appeared contrary to this. The team assembled and discussed the next steps.

Figure 4. Furnace Glass Level Visual Estimate

Although seemingly improbable, a theory developed that perhaps there was restriction in the throat, possibly a buildup of denser glass that was 'wicking off' and presenting in the final product as the aforementioned watermark. The throat became the focus of the discussion, and a plan was developed to retrofit a drain onto the throat bottom, to remove a possible accumulated buildup of denser glass:

Plant management acted quickly to institute this solution. The results after draining the throat for a few hours, during which periods of inhomogeneous glass streaming were evident, was that the furnace glass level returned to the designed 50 mm below top of block. While seemingly improbable, an accumulation of denser glass in the throat area had slightly restricted the glass flow, requiring a higher furnace glass level and head pressure to maintain the operating glass level in the forehearth.

The distortion line in the glass was the presentation of this problem - a buildup of denser glass which restricted glass flow - and was solved by installation of a periodic drain capability in the bottom of the furnace throat, as shown in Figure 5.

Figure 5. Representation of a Sunken Throat Bottom Drain, such as by KTG Engineering.

GLASS PROBLEM TWO - FURNACE WITH A SAGGING WAISTLINE

TECO was asked to assist a float glass manufacturer who had recently changed a large refractory structure in the waist area of their float furnace, to relieve a possible source of refractory contamination in their glass ribbon. In normal circumstances, this should be a straightforward procedure, the replacement of the A arch (see Figures 6 & 7 below).

Figure 6. Layout of Waist Arches A through D

Figure 7. View of Waist, Right Side

The A arch, as can be seen in the Fig. 1, is a high and narrow design that helps shield the downstream area of the waist during normal openings of the upstream access area, in front of A, for routine maintenance in that area. The old A arch, replaced by the customer, is shown in Figure 8.

Figure 8. Old Replaced A Arch

However, during the replacement of the A arch, the support structure of the B arch was exposed to higher temperatures and radiant heat from the open A arch area. This is normally acceptable for the short period of the A arch replacement procedure, in that the B waist arch support steel is designed to be water cooled. Unfortunately, the steel assembly provided by a local supplier had developed water leaks when originally placed in service, and the B arch support beam was necessarily switched over to compressed air cooling to avoid leaking water damage to the refractory structure. Periodic inspections of the B arch had shown only slight sagging (Figures 9 and 10) while being cooled with compressed air, and it had remained stable for several years.

Figure 9. View from Right Side

Figure 10. View from Left Side

When the A arch was replaced, the B arch support was exposed, became overheated, and sagged severely during the replacement work, as shown in Figure 11. The B arch became a possible risk to the safe and efficient operation of the float glass process line going forward.

Figure 11. Views of the B Arch Maximum Sag during Replacement of the A Arch

With the discovery of the damaged B arch, TECO was asked to provide its expertise and participate in the emergency plan for the replacement of the B arch, which also supports an equipment access walkway above the arch. In general, this waist area is a fairly crowded space (see Figure 7 above). The new B arch refractory assembly had to be carefully preheated in order to sustain its introduction into the elevated temperature of its position in the furnace waist area. The procedure which was developed by the team was to transfer the new, preheated B arch into position simultaneously with the removal of the old B arch, and with the reintroduction of water cooling of the new B arch steel support structure. Therefore, the team carefully considered all aspects of personnel safety and staffing, mechanical structures, refractory heating...