TECHNICAL | SAFETY/FIRE
LASER-SUPPORTED MEASUREMENT OF TEMPERATURE PROFILES The DTSX1 signalling unit continuously generates laser pulses
which are coupled into the temperature sensor as excitation signals.
The relevant measurement signal is the light backscattered from the fibre cladding, which is fed back to the signalling unit via modes of the optical fibre. The modes act as individual light paths in this process. The detector unit analyses a part of the low-intensity scattered
light, namely the frequency-shifted, longer-wavelength Stokes and shorter-wavelength anti-Stokes signals. These arise as a
result of the Raman effect, i.e., the interaction of the laser light with the lattice vibrations of the glass fibre molecules. The temperature at any given location on the optical fibre is a result of the intensity ratio of anti-Stokes and Stokes scattered light and the travel time of the light to the measurement location and back to the detector. Up to 32,000 temperature measurement points are possible
per fibre, with a fibre length that can be up to 16km for the DTSX1,” adds Ralf Cymontkowski, DTSX account manager at Yokogawa.
In both tubes, experts simulated a car fire with fire
and smoke in accordance with a standardised test setup. In this way, it was demonstrated that, once broken out, a fire with a 5MW fire load – corresponding to about 20l of burning gasoline on an area of four square meters – would be detected by the fire detection system and localised to within 50m within 18 pre-defined zones. In accordance with fire protection class A1N, fire
detection takes place either when a temperature threshold of 60°C is exceeded at the sensor cable – even briefly, if necessary – or when the temperature rises by more than 25K over 100s. The sensor cable is then connected to the fire alarm system. A measuring cycle with two independent
measurements takes less than ten seconds. This information must then be passed on to the fire alarm control panel via a relay circuit and processed there – within 60s, according to the safety specification. Powerful fans then start up to transport smoke and heat as efficiently as possible to the outside, depending on the location of the source of the fire. In the event of a fire, the emergency services are
notified and the entire tunnel is closed to traffic. Loudspeaker announcements in the tunnel and outside provide instructions on what to do in such a case.
A TECHNOLOGY WITH POTENTIAL Grewe, from SPIE OSMO, summarises his experience from the project: “The detection system has fitted seamlessly into the safety infrastructure and has proven to be efficient and reliable in the fire test. “In addition to the quality of the technical
components, Yokogawa’s expertise also contributed to this – from planning to commissioning.” Yokogawa’s Schwarma commented: “Our fibre
optic temperature measurement systems are very versatile and are constantly opening up further areas of application. “The fire alarm system in the Frankenhain tunnel
was a premiere for us in this application environment and an exciting challenge for that reason alone. It became apparent that the requirements and project handling in the construction industry are in part quite different from what we are familiar with from the process industry.” Grewe adds: “But there were no obstacles that
couldn’t be cleared up quickly after a clarifying discussion.” For future projects, which are already on the horizon,
he would particularly like to see a standard 19-inch built-in version of the DTSX1.
Above left: Smoke emission was measured during the fire test in the tunnel
Above right: Northbound tunnel showing (running along the top) the fibre detection cable PHOTO CREDIT: YOKOGAWA/CARSTEN SCHWARMA
26 | August 2023
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