Saving energy and wear
May 1st 2006 Stripping out energy inefficiency at Corus: Siemens has recently supplied four new systems featuring new motors and SIMOVERT MV VSD Systems; saving both energy and water
Corus Strip Products UK, at Llanwern near Newport is part of the world leading Corus Group of companies. Yet, by its own admission a review of the energy efficiency of its strip mills was long overdue. Partnering the project to improve efficiency at Llanwern is Siemens Automation & Drives, which is supplying a drive solution involving the retrofit of four variable speed drives to achieve electricity savings in the realm of £17 000 a week.
Approximately 40 years ago, the steel plant at Llanwern was fitted with four 1.4MW, 11 kV fixed speed descaling pump motors. According to Corus electrical engineer, Gareth Hudson, the amount of energy being wasted by these ageing units was "Excessive and typical for 1960's vintage". With the old fixed speed scheme, the pumps run at nearly full power all the time, whether water is needed or not. As a result the company began to look at ways of reducing energy costs, concluding the most effective method to be the introduction of variable speed drives which would allow the pumps to be slowed, and hence energy to be saved, when descaling water is not needed. A scheme was produced and sanctioned, and three global drive system providers were asked to tender for the contract, which was worth in approximately £760 000.
The tenders were subjected to an evaluation process that considered important factors such as MTBF (mean time between failures), reliability, availability, back up and cost of installation. The proposal also had to be based on a medium voltage solution, which was preferred by Corus.
"Siemens won out because they provided the best overall technical solution," explains Mr Hudson, "and also because they had previously supplied us with fixed speed solutions for four other descaling machines. These had proved reliable and we were aware of the benefits Siemens could provide."
The winning Siemens proposal was based on the supply of four identical systems featuring new motors and four Siemens SIMOVERT MV medium voltage variable-speed drive systems. As well as providing large energy savings, the solution would also be far kinder on the mechanical train of the units. The ageing descaling pumps featured fixed-speed motors that ran continuously, whether water was required or not. If descaling output was not required a control valve was closed causing the pump to 'deadhead' - the result is energy wastage and frequently-worn parts, many of which were becoming increasingly difficult to source.
The Hot Strip Mill at Llanwern takes steel slabs up to 27 tonnes and reduces them into high quality strip between 1.4mm and 12mm thick, in a variety of widths. At each of the roughing mills (six in total) and immediately prior to the finishing mills, the descaling pumps blast the hot steel slabs with water at high pressure (120 bar) to break and remove the surface scale. The Corus hot strip mill at Llanwern consists of a roughing mill train consisting of six 'stands', and a finishing train of seven close-coupled 'stands'. Each of the processes feature dedicated descaling stations of four descaling pumps. Today, the mill runs continuously for 49 weeks of the year, with each one serviced by two fixed speed pumps and two variable speed pumps, all automated by Siemens.
"Siemens supplied a complete turnkey variable speed solution," says Mr Hudson.
"It includes a new control regime that monitors pressure requirements: Siemens pressure switches and a PLC help provide a constant output of 120 bar."
The use of variable speed drives at Llanwern offer significant benefits. The primary reason is because of the cube relationship between power and speed when descaling water is not needed (e.g.
when there is no metal in the mill). This means that power drawn drops away much faster than speed, when speed is reduced. For instance, at 80% speed, the motor only uses 50% of the power.
All four variable speed pumps have now been commissioned at Llanwern and the estimated savings are phenomenal, as senior electrical engineer Alistair Ritchie verifies: "From post-commissioning studies we have calculated the annual anticipated energy savings resulting from this project to be £820 000 in electricity alone, which will provide payback in 0.88 years. In addition, as the new system is considerable kinder to the various mechanical components, wear rates will be less. This will result in an estimated annual reduction in the cost of maintenance of around £125 000." More articles from Siemens Moore Process Automation: | 
