cpp INSTRUMENTATION, CONTROL, AUTOMATION Fibre-optic temperature measurement in a tight space Timely detection of hotspots A fibre-optic temperature measuring system developed by Siemens enables temperature profiles to be determined in tight spaces, for example in tube or tube bundle reactors. By installing a large number of temperature measuring points inside a single tube reactor, areas with excessive temperatures (hotspots) can be detected at an early stage and action taken to control the flow of heat. Following successful trials at Evonik Resource Efficiency GmbH, this measuring system is now available. In-line measurements of temperature profiles in applications where space is restricted represent a daunting challenge forthe measuring equipment. This is particularly true when it comes to determining the temperature profiles in tube or tube bundle reactors. A large number of temperature measuring points are needed in order to optimise and maintain the catalyst activity; these were difficult to achieve using conventional measurement technology owing to the small diameter of the reactor tubes, the required number of measuring points and the necessary measurement speed. The fibre-optic temperature measuring system developed by Siemens in response to this problem is based on the fibre Bragg principle. Fibre Bragg gratings are created by inscribing optical periodic structures into the core of an optical fibre. Since only a defined wavelength of the incident light is reflected while all others are transmitted, each grating acts like a filter with a narrow bandwidth. Wavelengths determine temperature Sitrans TO500 temperature transmitters use fibre Bragg gratings (FBGs), which are arranged at individually defined points on the sensor probe. The transmitter sends light waves to the fibre-optic sensors and evaluates the reflected portions. In the transmitter, light is generated in the wavelengths from 1500 to 1600 nm and output to the sensor probe by means of a continuously tunable laser. Each fibre Bragg grating reflects light of a defined wavelength, which varies depending on the temperature at the measuring point. Since the measured value transfer (reflection of the light) takes place in the same fibre, no additional cables are necessary, so that a much smaller cross section of the protective tube is possible for this measurement setup. This firstly means that more reaction space is available in the reactor, with a positive impact on the throughput. Secondly, the sensors have faster response times because the damping effect of the air gap between the fibre in which the gratings are inscribed and the tube walls can be reduced to a minimum. The Sitrans TO500 temperature transmitter based on the fibre Bragg principle enables temperature measurements in tight spaces Pictures: Siemens Synchronous measurements The system consists of a transmitter to which up to four fibre-optic probes with up to 48 FBGs each can be connected. 192 temperature measuring points can therefore be processed synchronously by a single measuring system. Siemens tailors the measuring probes – the length, number of sensors and sensor positioning – individually to each specific application. Each probe with a diameter of less than 54 cpp 03-2017
Structure of a fibre Bragg sensor: the transmitter sends light waves to the fibre-optic sensors and evaluates the reflected portions Fibre Bragg gratings are arranged at individually defined points on the sensor probe, which can be inserted into a tube or tube bundle reactor, for example 2 mm measures temperatures in the range from 0 to 400 °C with an accuracy of
