The Corus pipe mill in Hartlepool produces linepipe in diameters of up to 84inches using the world's strongest edge crimping and 'O' press facilities. These pipes are used globally for oil and gas pipelines, conductor and carrier pipes for the energy industries, tubular piling for jetties, berthing dolphins and similar applications. In order to remain a leading supplier in this field, Corus operates a continuous improvement programme to identify areas where productivity can be raised. For example, by working with consultants from Pilz Automation Technology and by upgrading conventional safety relays and interlocked gates to a modular safety controller that monitors crane motion and Pilz light curtains, Corus has achieved a potential 17 per cent increase in throughput on the 'tab attach' process in its 42inch pipe mill at Hartlepool, while still maintaining a safe working environment. Safety is key to all the continuous improvement work being carried out in the mill.
The first step in the production of a steel tube from flat plate is the 'tab attach' process, where tabs are welded to the four corners of the plate as it lies on a roller bed. On the existing plant, access to the plate was prevented by interlocked gates until the crane had returned to the plate carrier; only then could operators at either end of the roller bed actuate the crane immobilisation switches and open the safety gates. Once the tabs had been welded, the operators would return to the safe area, close the gates and re-enable the crane via the switches. However, improvements in downstream processes meant that the tab attach process had become a bottleneck.
As part of the continuous improvement programme, a project team was tasked with seeking ways to improve efficiency and therefore increase the productivity of the tab attach process. It was soon recognised that there was significant waste or 'non-value-added' time associated with the crane's travel back to the plate carrier; if a way could be found to enable the welders to gain access while the crane was at its highest position and moving away from them, then much of this waste could be eliminated.
Tony Brown, the Project Leader, was already aware of the Pilz PNOZmulti software-configurable modular safety controller and thought this could provide the answer. Following discussions with personnel from Pilz, who also performed a risk assessment, it was decided to use a PNOZ m1p base unit with a PNOZ ms2p speed monitoring module and two PNOZ m04p relay output modules. The speed monitor would check that the crane was at its maximum height and moving in the correct direction, the input signals being provided by encoders mounted on the crane.
Now that the system has been commissioned, the project team has confirmed that around 25 seconds are saved by this crane monitoring system. A further five seconds have been gained by replacing the two interlocked gates with Pilz PSEN op4B-4-090 light curtains. These Type 4 light guards are 900mm high with four light beams.
During normal operation the speed/direction monitoring module detects when the crane is in a safe state, whereupon the PNOZmulti disables the crane's hoist motor, mutes the light curtain and lights an indicator lamp so that the welders know they may turn the switches to implement a production immobilisation and then pass through the light curtains. When the welders have completed their task, they return through the light curtains and re-enable the production immobilisation zone.
Should someone pass through the light curtains into the hazardous area before the production immobilisation has been activated, the PNOZmulti initiates a total zone shutdown to ensure that safety is maintained. In addition, several emergency stop buttons are connected to the PNOZmulti controller and, if one of these is activated, a full zone shutdown is initiated. Around 20 safe inputs and 12 safe outputs are used in total, with the safety-related control system designed to meet the requirements of SIL 3 as defined in BS EN 62061:2005, 'Safety of machinery, Functional safety of safety-related electrical, electronic and programmable electronic control systems'.
Tony Brown comments: "The PNOZmulti with the speed monitoring module proved to be ideal for this project; the safety functionality was much easier to implement than with conventional safety relays and speed/standstill monitors, and the system will be quick to modify should we need to upgrade it again. Setting up the PNOZmulti was certainly straightforward. With minimal assistance from Pilz one of our technicians became adept in the software in just a few weeks and was able to configure the safety controller and write the programme himself. This programme was subsequently verified by Pilz before the system was commissioned."
He continues: "Making the changes to the safety control system has cut 30 seconds from the cycle time, giving us the potential to increase the throughput by 17 per cent. A new shift record was achieved just one week after the upgrade, though the true extent of the improvements will not be fully realised until improvements to the downstream processes are complete. The safety upgrade has already resulted in additional throughput which means the payback time for the project is less than three months."