Consulting engineering and project implementation firm Hatch reports it has developed a Tapblock Fibre Optic Temperature Measurement System that provides accurate temperature readings from previously inaccessible locations inside a furnace.

Hatch global director of control technologies Terry Gerritsen says the system makes use of fibre-optic sensing technology to measure temperatures on the hot face of copper coolers and in the tapping channel of a water-cooled matte tapblock. The temperature measurement system is currently being used on several furnaces in South Africa and Australia, and will soon be in use in Canada.

“On most applications, the fibre-optic sensors and associated basic monitoring software provide data from areas of the tapblock that have never been accurately measured on an operating furnace. “Compared to thermocouples, which have traditionally been used, the fibre-optic sensors provide superior temperature measurements, in terms of accuracy, multiposition measurement density and the ability to be installed in various locations,” explains Gerritsen.

The Tapblock Fibre Optic Measurement System’s ability to obtain dense multiposition temperature coverage on the hot face of copper coolers and tapblocks was not previously achievable. Gerritsen says this important advancement allows the operator to gain continuous feedback on the refractory condition, as increasing temperatures are indicative of refractory degradation.

Key Features
When the fibre-optic sensor technology was first discovered in the 1990s, scientists and engineers realised that, by strategically intro- ducing defects or sensors into the fibre, light transmitted down the length of the fibre would be partially reflected back. “By analysing the light that is reflected back, one can establish the temperature at each of the sensor points,” adds Gerritsen.

In addition to the temperature range, a key consideration is the small size of the fibre-optic sensor cable. A single optical fibre including protective cladding and buffer layers is still only about the same diameter as a strand of human hair, a couple of hundred micrometres. The actual length of the optical fibre varies, depending on application, while the number of sensors varies depending on the density of mapping required.

The size of the optical fibre allows for complete temperature mapping, which, in turn, allows plant engineers to accurately predict when a controlled shutdown will be necessary, which results in greater efficiencies in terms of costs and timing.

“Having to shut down a furnace a few days early could equate to millions of rands being saved. “Predictability is a key issue here in that the Tapblock Fibre Optic Temperature Measurement System enables one to detect the advance of refractory damage much sooner than has been possible with thermocouples. You can see problems developing several weeks in advance,” says Gerritsen.

A further disadvantage of using thermocouples is the limited number of sensors or measurements possible owing to their large size in spatially confined applications. This, in turn, does not allow for an accurate temperature reading of the entire tapblock. Readings taken from a thermocouple could indicate that the tapblock is functioning as it should, when, in fact, it could be in danger of failing in unmeasured areas.

Gerritsen notes that, in quantities greater than about 100, the cost difference between a fibre- optic sensor and a thermocouple is minor. “Ultimately, it is a negligible cost to the client when compared with the effects of shutting down early or waiting too long for the furnace to shut itself down and suffer damage,” he adds.

Meanwhile, Gerritsen points out that the surface temperature map (data received from the sensors), enables the detection of refractory loss and is monitored and analysed by hand and by software currently under development. “The aim is for computers to assist with the evaluation using software that should be ready in several years,” he explains.

Meanwhile, Hatch is currently looking to establish a Control Technologies Group in South Africa to build capacity locally. The Hatch Africa Control Technologies Group is currently managed by Warren Braun.

The Control Technologies Group falls under the technologies division within Hatch.

“It is somewhat different to the consulting engineering function of Hatch in that the Control Technologies Group delivers Hatch products. “We specialise in process control systems, including the design and supply of control technology for Hatch furnaces. “The control technology is also available as a retrofit to other furnaces. “The Tapblock Fibre Optic Measurement System is a smelter differentiator technology for Hatch,” Gerritsen concludes.