New SCADA system brings immediate benefits to concrete block manufacturer

Equipment supplied by Procon Engineering is helping Tarmac Concrete Products maintain their position as one of the UK's top manufacturers of high strength blocks for the building industry. From concrete blocks to railway sleepers, Tarmac Concrete Products is one of the leading UK companies supplying concrete products under its strong brands Tarmac Topblock, Tarmac Precast Concrete, Tarmac Topfloor and Tarmac Toppave. The Topblock division has built an enviable reputation for quality in the UK and claims to be the only supplier to offer both aggregate and aircrete (aerated) high strength blocks.

The company's Ford plant near Arundel, West Sussex specialises in the production of their Toplite range of products. These aerated blocks offer excellent thermal insulation properties, combined with high load bearing strength and resistance to moisture, chemicals and fire. Management at the Ford plant had been aware for some time that their existing batching and mixing system, based on an Allen Bradley PLC5 and DOS based ControlView SCADA package, was nearing the end of its useful life. Understandably, with the plant producing over 1.4 million MTS blocks a year and operating five 24 hour days a week, 50 weeks of the year, they were reluctant to make any changes without being sure that any new system could be installed and commissioned with the minimum of disruption.

Following their extensive site survey and detailed proposal, Procon Engineering were chosen to supply the new system. Procon have extensive experience in the Buildings Materials Industry and were fully appreciative of Tarmac's concerns. The new system is based around a Rockwell SLC504PLC and a new PC control desk running Rockwell's RSView SCADA package under Windows 2000. The old weighing instrumentation has been replaced with Procon's EP50 Proweigher batch controllers using the Modbus interface to the Control System to ensure optimum accuracy and reliability. The batching system consists of four main weigh vessels supported on load cells. The existing load cells were thoroughly checked out and found to be in good working order.

One of the criteria set by Tarmac was that the new system should be easy to calibrate. Using conventional methods of applying test weights had shown to be unsatisfactory in the past and therefore Procon modified the support legs on the vessels to accommodate their Force Calibration system for calibration. The system works by placing specially calibrated load cells and hydraulic jacks in parallel with the load cells supporting the vessel by means of special mounting brackets welded to the structure. By applying load to the calibration load cells through the jacks, the weighing system output can be compared with that of the Force Calibration system at several points over the weighing range. The calibration load cells are traceable to NAMAS standards and this method provides a high accuracy, non intrusive and expedient method of calibrating weighing systems.

The main mixing screen shows the four vessels used to weigh out the main ingredients of water and slurry, cement and lime, PFA (ACA) and aluminium.

A typical batch size is approximately 1000-1300 kg and when the correct amount of each material for a particular recipe has been weighed out, the materials are Distributor into the mixer for a predetermined time. Finally the mixture is released into the moulds in cars running underneath the mixer. The small amount of aluminium added to the batch can be compared with the yeast used to make bread rise. It causes an exothermic reaction causing the blocks to rise in their moulds trapping air inside to give the blocks their thermal properties.

The new control system provides Tarmac with a number of significant improvements in weighing performance, overall control, reporting and traceability. The project teams of both companies have worked very closely together and the software specifications ensure that future planned plant I/O extensions can be readily accommodated. The RSView system is capable of storing up to 999 mix recipes or designs and each mix design stores a text description for display and reporting purposes. Full password and key switch protection prevents unauthorised operation and the system maintains an activity log recording significant events such as who logs on and any changes they make. The SCADA system produces printed batch reports in line with Tarmac's requirements and these are automatically archived to CD, with an individual CD storing one month's reports. The system gives continuous alarm monitoring for all important values. All alarms are logged to disk with a new log file started daily. An alarm banner continually displays the most recent alarm as well as the current number of active alarms and unacknowledged alarms. On an alarm condition, the relevant PLC sequence holds the batching cycle until the operator request a restart, at which point the sequence will continue provided the alarm condition is not present.

Tarmac's plant manager, Andy Johnson, is very upbeat about the new system: "We have been impressed with Procon's professional approach and attention to detail throughout the project. We set them exacting demands with regard to cost, system functionality and installation time. The system had to be installed before Christmas and commissioned by 6th January this year. We were concerned that this might be difficult to achieve especially as we had no time to supervise the work. We had no need to worry. Procon complied with all our stringent safety requirements and caused no disruption to the normal operation of the plant. In the first few weeks of operation the system has operated smoothly and we are already seeing the benefits of more consistent batch quality, with rejects down to almost zero. We now have full batch traceability which is vital as customers become more discerning. We can monitor our raw material usage much more closely and plan deliveries so that we don't run out of product. Procon's ongoing service support gives us peace of mind and the new calibration method can be implemented very quickly and easily."