The transition from roll to roll and from segment to segment must be exactly maintained. There should be no steps that could force the hot strand of metal to change direction. These could in the worst case cause the thinly formed surface of the strand to rupture (beak-out) allowing the liquid metal inside to spew out in to the caster causing expensive capital equipment damage, caster downtime and at least one day's loss of production.
To a lesser degree, the quality of the steel is affected. For example, the value of a £300,000 steel cast may be halved by minor defects, reduced to less than £50,000 if there are major defects, or in the worst case, it may even have to be scrapped altogether.
In less than one hour, the Sarclad SCM passes through the casting machine and, by rapid, accurate measurement of the critical parameters, provides the data which allows the operator to monitor trends and solve problems quickly.
Critical parameters include the distance between the rollers (roll gap), and the curvature of the roller (roll bend). All of these mechanical measurements are made using Penny + Giles SLS190 sensors; up to 24 on any one SCM, each one measuring with a repeatable accuracy of 0.05mm.
SLS190 is a compact, lightweight linear displacement sensor with a short body-to-stroke length ratio. It is based on proven potentiometer track technology. Steve Shillito, Project Manager at Sarclad explains, "Each Strand Condition Monitor is customised for the casting machine it will be used on. This is highly specialised equipment, which becomes an integral part of our customers' processes. Reliability is paramount, and in the harsh conditions of a steel mill, that is not always easy to achieve. We have chosen this Penny + Giles sensor because it is a simple, passive device which is not affected by difficult physical and electrical conditions."
The challenges are the high temperatures, the possibility of water ingress from the cooling spray, and the fact that the accuracy of on electronic measuring device could be compromised through noise and interference from the SCM data acquistion and communications module. Sarclad had tried other technologies and other designs in the past, but there were problems with failures in the field.
Penny + Giles worked with Sarclad to understand the problem and provide a solution. The applications engineering team recommended use of potentiometer technology because it offers reliable, repeatable measurement which is unaffected by adverse electrical interference. They then developed a stainless steel housing sealed to IP 67 which was compact enough for the dimensions of the SCM and able to withstand the heat and water conditions. Importantly, Penny + Giles subjected its design to over 320,000 operations of the sealed shaft over ±12.5mm travel, whilst the sensor was being subjected to a heavy water spray. The test duration was a continuous 92 hours and proved its suitability to the task.
The result is that since it standardised on Penny + Giles sensors, Sarclad has suffered no failures in the field at all since the first of these sensors was used more than two years ago. The company now supplies 14mm and 24mm stroke devices for use on the SCM machines which are built to order and customised to each casting machine. Between six and 24 sensors are used on each machine to provide an overall measurement accuracy of 0.1mm. An on-board computer collects and stores this and all the other measurement data for download via a radio signal in real time or direct to a PC. Sarclad provides Windows software for analysis of trends and maintenance reports.