PSR Cord Dispersal - Parkinson-Spencer Refractories

Technical Services Division
Cord Dispersal Stirrer
System
PSR’s Cord Dispersal Stirrer System is a proven
solution to the specific problem of ‘Cat Scratch’
cord.
The system has also been found to be effective
for thermal homogenisation and could be used
for other types of chemical homogenisation.
Our Technical Services Division is available to
advise on specific applications.
Cat scratch cord
'Cat scratch' cord is the name given to a defect that occurs
on the surface of glass articles, particularly containers
and thin blown tableware, where the appearance is as
if a cat has scratched its claws along the surface of
the glass. It is generally the result of a high density
viscous glass enriched in alumina and zirconia which
originates from the exudation on initial heating or
subsequent corrosion of the vitreous (glassy) phase
present in fusion cast AZS refractories used in the
melting end tank and superstructure. This glass is more
viscous than the glass being melted and can settle out in
the bottom of the furnace, the distributor and forehearths,
eventually being present on the surface of the gob and appearing as a
line or series of lines on the surface of the glass product.
One of the most confusing features of this problem is the way in
which it can come and go and can appear on specific production lines
and not on others. This can be explained by the different flow rates
of the glass passing through the furnace, distributor and forehearths,
together with the forehearth layout and changing operating conditions,
allowing the dense viscous glass to settle out at different locations
as the glass flows towards the spout. The most affected areas
are deeper areas where the glass is colder and areas where the relative
glass throughput is low resulting in a longer residence time. At higher
temperatures it becomes more mobile whilst at lower temperatures it
is more likely to settle out and accumulate on the bottom of distributors
and forehearths.
Typical cat
scratch cord
Cord is denser
and more
viscous than the
parent glass and
will find its way
to the bottom of
the glass flow
Cat Scratch Cord
The location of the cordy glass as it passes along the forehearth may
not be exactly as illustrated above, but by the time it reaches the
spout bowl its density and viscosity ensures that it will be at the bottom
of the glass flow and will appear on the surface of the gob. In a
multi gob operation the gob that will be the most affected is the one
first in line to receive the bottom glass flow, either with centre line
shearing as illustrated or as determined by the rotation of the tube.
Accumulation
of cordy glass
(illustrated
green)

The stirrer system
The purpose of the PSR Cord Dispersal Stirrer System is to lift the dense, viscous cordy glass off
the bottom of the forehearth and to mix it into the body of the glass so that it is no longer present
on the surface of the gob during the forming operation and then no longer readily visible in the
final article. In order to achieve this we use twin counter-rotating paddle type stirrers that have
a wide sweeping area and are effective at stirring and lifting material that is located at or near
the bottom of the glass flow.
The stirrers are located in the equalising section close to the spout, their exact location and
configuration depending upon channel width, depth and profile.
Although the 'cord' forming material is still present in the glass, after stirring it is no longer
concentrated on the gob surface and is distributed throughout the base and sidewalls of the
article during the forming process so that it is no longer readily visible in the finished product.
Features of the PSR Cord Dispersal Stirrer System
• Can be fitted to any design of forehearth.
• Suitable for forehearth widths of 26” (660mm) and above. For forehearth widths less than 26”
(660mm) twin counter-rotating helical type stirrers are specified.
• Different stirrer configurations are available to suit specific cord problems and installations.
• Can be installed with pre-existing drains. In some instances the drains have then been stopped.
• Can be installed with pre-existing submersed electric boost electrodes.
• With a suitable pre-configured access slot the system can be installed on-the-run. If an access
slot is not available equalising section superstructure refractories can be replaced to provide one
during job change or other machine stoppage with typically a maximum of 6 hours production
interruption.
• Cat scratch cord appearance closely and predictably controlled by adjusting stirrer speed.
Stirrer speed can be reduced to minimum as cord condition improves.
• Stirrer life typically 6-12 months (based on PSR stirrers and subject to service conditions).
• Stirrer configuration provides high stirring efficiency at low stirrer speeds (typically 2 to 5
revolutions per minute) to minimise stirrer wear. Stirring efficiency can be maintained as stirrers
wear by increasing stirrer speed.
• Stirrer operation does not interfere with thermocouple readings or gob weight control.
Fluctuations in downstream infra-red thermometer readings due to disturbance of the glass
surface can be overcome by filtering of the temperature signal.
• Stirrer operation can assist thermal homogeneity at spout entrance.
• Stirrer operation does not contribute to glass head loss and can reduce glass head loss at high
forehearth loads.
• Increased channel block corrosion at the stirrer position is minimal. Any local additional
long term wear can be accommodated by stirrer operating height adjustment. This can in fact
be beneficial to the stirring operation as the stirrers then operate at a height lower than the
adjacent upstream channel block base to better lift the cordy material.
• Stirrer mechanism mounted to casing top flange provides ease of installation. Can be mounted
to support steelwork or forehearth platform if casing support is inadequate.
• System design allows stirrer mechanism to be raised up, pulled out to the side and then
lowered to forehearth platform level for easy stirrer replacement and mechanism maintenance.
• Stirrer variable speed drive motor and gear box mounted on the stirrer cross beam for easy
access. Cable management system allows stirrers to be operated outboard at the side of the
forehearth, above the forehearth or with the stirrers above glass level to ensure correct stirrer
operation before final positioning.
• Stirrer system control panel houses the inverter drive which controls the stirrer drive motor
speed and provides indication and control of the stirrer rotational speed as well as local and
remote alarming of stirrer faults.
• Access to speed control and inverter drive locked to prevent changes by unauthorised personnel.
• 5 year inverter drive warranty.
• Control panel can be mounted in control room or supplied with air-conditioning unit for
mounting on machine shop floor.
• Air-cooled carbon bearings and stainless steel stirrer chucks ensure accurate and concentric
stirrer rotation, reliability of operation and minimal maintenance.
• Adjustable shaft mounted bevel gears enable reliable setting of relative stirrer positions during
installation.
• Designed, manufactured and supplied by PSR. System performance guaranteed.
• PSR engineer ensures correct on-site installation. Original PSR spares and refractory parts
ensure long term satisfaction.
A PSR Cord Dispersal Stirrer System in service
Stirrer Control Panel
The occurrence of ‘cat scratch’ cord is now becoming
so prevalent that the equalising section of every new
forehearth from PSR is pre-configured for the subsequent
installation of stirrers. This includes the provision of cutouts
in the roof blocks with cover tiles to close the position,
together with pre-configuration of the superstructure
steelwork to enable a stirrer support frame to be retrofitted
quickly and easily if required.
Typical equalising section superstructure pre-configured
for a Cord Dispersal Stirrer System
Summary of benefits of the PSR
Cord Dispersal Stirrer System
• Guaranteed to eliminate or reduce any typical cat scratch
cord defect to acceptable levels (subject to prior technical
evaluation).
• Can be installed on-the-run.
• Does not require special ‘drain channel block’. Where ‘drain
channel block’ is installed continuous operation of the drain is
required to prevent build-up of cordy glass and “cat scratch”
problem.
• Stirrers can be reduced to minimum speed, stopped and raised
above the glass or removed completely when no cord is present.
• No reduction in production output associated with a drain.
• No cost of waste glass associated with a drain.
Parkinson-Spencer Refractories Limited, Holmfield, Halifax, West Yorkshire, UK. HX3 6SX
Tel: (44) (0) 1422 254472 • Fax: (44) (0) 1422 254473 • Email: admin@parkinson-spencer.co.uk • www.parkinson-spencer.co.uk