Rockware Glass produces container glass in three furnaces at the Knottingley works. The Company’s policy is to minimise the environmental impact of its manufacturing processes. As part of this policy, Rockware asked John Turner (Engineering Consultants) Limited to investigate the feasibility of increasing the discharge height of the chimneys serving two of the Knottingley furnaces to bring them up to the height of the third, more recent chimney.

Previous dispersion analyses had shown a useful reduction in ground level concentrations of pollutants if the discharge height of each of the older chimneys was increased by approximately ten metres.

John Turner carried out detailed structural analyses of the chimneys and concluded that it would be possible to extend each by up to ten metres.

Rockware commissioned a structural condition survey of the chimneys and this showed both stacks to be in very good condition.

Glass furnaces are very sensitive to the draught performance of the chimney. John Turner carried out draught analyses and predicted that the changes to the chimneys would have no detrimental effect on chimney draught performance.

The additional draught provided by the chimney extensions was used to offset the hydraulic losses associated with the increase in efflux velocity to comply with statutory minimum requirements.

Glass furnaces are run continuously for up to seven years, but Rockware decided that the improvements to the environmental performance of the plant should not wait until the next furnace shutdown. It was therefore decided to install the chimney extensions whilst the chimneys remained on line. Detailed planning was required to avoid any interference with furnace production and to safeguard site access and an adjacent railway line.

The installation method was planned in conjunction with the contractor, Beaumont Chimneys and their steeplejack subcontractor, Churchill Steeplejacks.

Preliminary work included the removal of permanent ladders and helical strakes, and minor strengthening works.

A temporary shield was incorporated at the top of each extension section to deflect flue gases away from the crane. The draught performance of the chimneys in this temporary condition was also checked by John Turner.

The operation required an 850 tonne crane with an operational height of 90 metres under the hook. A period of low wind speed was necessary for the operation of the crane. The wind had also to be in a specific direction so that it would carry flue gases away from the crane. Before lifting commenced, the crane jib was fitted with thermometers to determine the air temperature close to the top of the chimneys and to ensure that lifting operations could be carried out with no heat damage to the crane.

A suitable weather window was identified and the crane scheduled to allow two days for lifting. In the end, both chimney extensions were installed in a single day.

The operation went entirely to plan, thanks to detailed planning and preliminary work, and suitable weather. The whole contract was completed with no adverse effect on the glass production of either furnace. Another successful project for Rockware Glass and John Turner (Engineering Consultants) Limited.