Over the first few years of supplying its stirrer system, the company fine-tuned both the stirrer location and the stirrer configuration and soon real...
Over the first few years of supplying its stirrer system, the company fine-tuned both the stirrer location and the stirrer configuration and soon realized that if it had the correct initial set-up details and tailor made the system to suit each application it could be sure beyond reasonable doubt that it could eliminate most if not all of the “cat scratch” cord problem. The subsequent offer of a money-back guarantee if it couldn’t achieve this was a measure of the company’s confidence in the system and is still offered with every system sold today.
PSR does, of course, have a unique advantage in being both a refractory manufacturer and an engineering company, which means that it does not only design, manufacture and supply every aspect of the system but also takes full control and responsibility for the installation, operation and performance of the equipment. PSR engineers supervise and assist with the installation and commissioning of the system and train the client’s operating and maintenance personnel to ensure that the systems are installed, operated and maintained correctly.
The advantage of a complete package supply was clearly illustrated by one customer using refractory stirrers manufactured by PSR on three stirrer mechanisms supplied by another company. The problem was that the stirrers could not be operated for any significant period of time without breakage – often a matter of only hours. In other circumstances, the design or quality of the refractory stirrers might be called into question, although in this particular case the customer already had four complete PSR Cord Dispersal Systems operating with the same design of PSR refractory stirrers on the forehearths of another furnace without any problem. These had been operating for two years with a typical stirrer life of nine months. It was clear therefore that the refractory stirrers themselves were not the reason for the breakage and the problem was only finally solved when the customer replaced the existing stirrer mechanisms with three stirrer mechanisms from PSR.
Although this immediately solved the stirrer breakage problem, a re-design of the stirrers to incorporate wider paddles more suited to the specific channel block design on those forehearths was also found to be necessary before the “cat scratch” cord was reduced to an acceptable level for the customer.
Quantifying “cat-scratch” cord
It seems likely that most container and tableware glassmakers have a “cat scratch” cord problem to a greater or lesser extent at some time because the extensive use of fused cast AZS refractory blocks in the melting-end of the furnace is still the only feasible way of obtaining an acceptable furnace life. With typically 20 per cent vitreous (glassy) phase present in fused cast AZS blocks, the exudation of this vitreous phase on initial heating of the furnace and dissolution during operation is without doubt the principal source of the “cat scratch” cord. It is, however, mostly a cosmetic problem as the strength and physical characteristics of the glass container are largely unaffected. It is often tolerated in normal quality glass bottles but may be unacceptable if it becomes extreme or is present in premium quality bottles. It can be a particular problem in thin blown tableware because the surface cord contributes to a greater percentage of the overall glass thickness and results in a highly visible defect unacceptable for good quality tableware. The lines can often be felt with the fingernail as ridges on the glass surface and can cause problems with labelling or other surface decoration of the article.
The quantitative evaluation of “cat scratch” cord is a subjective issue and in most cases PSR relies on its customers to evaluate the before and after situation and to confirm that they are satisfied with the results following an installation. One of PSR’s more important customers, however, regularly evaluates its products for “cat scratch” cord and has developed a grading system rating the degree of “cat scratch” cord from a grading of 5 down to 1. Grade #5 is the most severe situation where the “cat scratch” is greater than 2-3 millimetres wide, easily visible, and can be felt with a finger tip or nail. Grade #2 is where the “cat scratch” is less than 2-3 millimetres wide, just visible, but cannot be felt with a finger tip or nail. Grade #1 is where there is no visible “cat scratch” cord.
Of the 14 systems installed with this client, all but one achieved Grade #1. The one installation that didn’t achieve Grade #1 was on a line manufacturing premium containers but even that was not considered Grade #2 so the client introduced an intermediate Grade #1.5 to describe a very widely spread, almost invisible “cat scratch” cord.
Stirrers versus drains
The amount of glass wasted from the continuous use of a drain often seems to be overlooked and the fact is that once installed a drain must be operated continuously to prevent the cord building up, overflowing the special drain block (sump) and contaminating the glass flowing into the spout bowl. Many companies install these drain blocks in the forehearth during a furnace rebuild in case they should encounter a future “cat scratch” cord problem. However, as the purpose of the drain block is to collect the “cat scratch” cord material, this action can actually precipitate a “cat scratch” problem during the campaign making it then necessary to use the drain.
Neither can the installation of a drain be guaranteed to eliminate the “cat scratch” cord, because the drain block has a limited capacity to contain the “cat scratch” cord material. It also has a limited maximum flow capacity of approximately 1.5 tonnes per day because at higher flow rates the hotter surface glass can be preferentially pulled down through the drain instead of the cord material. These factors can result in the excess cord material flowing past the sump rather than being collected and drained.
On one installation, two “F–shaped” tandem forehearths supplied two tandem IS machines producing green glass, and drains were installed on each of the four forehearths to combat “cat scratch” cord. Up to 1.5 tonnes per day was being drained from each forehearth and not only did this fail to eliminate the “cat scratch” cord but also the 6 tonnes per day being drained was limiting the maximum production capacity of the furnace. A PSR Cord Dispersal System was therefore initially installed on one of the forehearths as a trial and this eliminated the “cat scratch” cord, and draining of the glass was stopped. Subsequently, three further PSR Cord Dispersal Systems were installed on the other three forehearths with exactly the same results and all the forehearth drains were stopped allowing the furnace to achieve its maximum production capacity.
Taken over one year, the amount of glass recycled through these four drains would previously have amounted to 2,190 tonnes. PSR does not have access to the client’s detailed manufacturing costs and margins but taking into account the cost of mixing, re-melting and conditioning this quantity of glass, and adding the lost profit on the sale of the bottles, the company believes that annual saving following the installation of these four Cord Dispersal Systems would have been a significant number and that the pay-back period could easily have been measured in months. This does not consider improvements in glass quality, reduction in rejects or increased customer satisfaction due to the elimination of “cat-scratch” cord.
Over the many installations carried out to date by PSR, some beneficial side effects have also been noticed, sometimes of equal significance to the dispersal of the “cat scratch” cord.
PSR’s Cord Dispersal Systems have been installed on forehearths producing white flint, amber and many shades of green glass from the furnace, as well as on colourant forehearths producing standard and special furnace colours. When used on colourant forehearths, they also help to disperse the colour streaks that can occur following colour changes, reducing the time required to obtain an acceptable glass colour.
One client producing premium quality containers in black glass achieved a significant increase in production efficiency due to the elimination of “cat scratch” cord in addition to a significant improvement in glass thermal homogeneity following installation of a PSR Cord Dispersal System. As measured by the three tri-level thermocouples installed in the equalizing section across the spout entrance the 9-point thermal homogeneity value increased from 88.9 to 96.3 per cent whilst the average of the nine temperatures reduced by just 2°C.
Another client installed a PSR Cord Dispersal System to eliminate “cat scratch” cord on one of four forehearths on a furnace producing green glass. Due to the success of this installation the client budgeted to install systems on the other three forehearths the following year. In the meantime, however, a further problem was experienced with a black streak being present in the glass of the forehearths without the Cord Dispersal System installed. This accelerated the supply of the three additional systems which, on installation, eliminated both the “cat scratch” cord and the black streak.
The highest thermal homogeneity value that has been achieved on PSR forehearths producing green glass was 99.0 per cent at an installation in which Cord Dispersal Systems were installed to guard against the possible appearance of “cat scratch” cord. In this situation, PSR estimated that the thermal homogeneity value was increased from 97.0 to 99.0 per cent by the action of the stirrers.
Cord Dispersal Systems have been installed on different forehearth types from different forehearth manufacturers and different forehearth sizes, including all standard equalizing section channel widths ranging from 16 inches (406 millimetres) to 48 inches (1,220 millimetres) of both profile and classic design. They have also been installed on forehearths equipped with electric boosting in the equalizing sections. Whilst electric boosting systems installed in the forehearth equalizing sections can improve the thermal homogeneity of the glass at the spout entrance, “cat scratch” cord can still be present as the boost systems can mask deficiencies in the forehearth design and/or operation allowing the conditions necessary for the viscous “cat scratch” cord material to settle out in colder areas of the forehearth upstream from the equalizing section. Systems have also been installed on forehearths supplying gathering bays operating with robot ball gatherers.
The success of the system is best illustrated by the number of times the installation of an initial system has lead to subsequent installations for the same customer and PSR has three customers who currently each have 10 or more systems installed.
Alongside the on-going success of PSR’s Cord Dispersal System, the company has continued to develop and significantly improve the system over the years. The latest systems now feature a shaft drive rather than the original chain drive. This eliminates several components, including the chain drive shaft and bearing and the chain tensioner, as well as the maintenance associated with the chain in terms of the need for re-tensioning due to stretching and lubrication. The shaft drive mechanism drives the stirrer shafts via shaft mounted bevel gears instead of via a chain driven gear on the stirrer shaft nearest the motor. This provides a smoother operation which is not reliant on chain tension and does not produce any side thrust on the stirrer shaft nearest the motor. This reduces the potential for carbon bearing wear. A telescopic drive shaft connection allows easier setting up and replacement of the drive motor/gearbox, as well as initial positioning of the stirrer back plate. The original horizontal drive shaft bevel gear trantorque couplings have also been replaced with adjustable shaft mounted bevel gears to allow easier and more reliable setting of the relative stirrer paddle positions during replacement. A new back plate guard allows access to the bevel gears during stirrer replacement and set-up via a lockable access door without having to remove and replace the entire back plate guard. Remote grease points are provided for the shaft mounted high temperature pillow block bearings.
Upgrades of existing systems to incorporate these new design features are available.
The majority of installations to date have been made on non-PSR forehearths although every forehearth supplied by PSR is pre-configured for future easy on-the-run installation of a PSR Cord Dispersal Stirrer System should this ever be necessary. Furthermore, every PSR Cord Dispersal System is supplied with a money-back guarantee such that the equipment will be removed and its capital cost refunded if the system fails to improve the glass quality to the customer’s satisfaction. To date this has not been necessary.
So for customers that have reached the end of the road in their search for a fix to this perennial problem, PSR has the perfect solution.