Continuous distributed temperature sensor
Figure 2: A continuous distributed temperature sensor measures attenuation and backscattering of laser radiation to infer the temperature at any point along the cable's entire length. Source: APsensing
Fiber-optic thermometers can be single-point, multiple-point or continuous. For your application, the continuous design should be used (Figure 2). This design sends a 10-nanosecond laser pulse through the glass core of the optical fiber. As the optical pulse propagates through the fiber, it undergoes scattering (called Raman scattering) due to structural defects in the glass fiber. In this way, the fiber itself is the sensor and the scattered radiation carries the information on both the temperature and the location of the cold spot. Some of the scattered radiation travels forward and some back to the source, and their ratio is a well-defined function of temperature. This ratio reveals the average temperature of each 1-meter section of the fiber, while the time of the round trip flight of the backscattered pulse indicates the location of any cold spots. This method of location detection is called optical time domain reflectometry (OTDR).
Béla Lipták
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A2: This sounds like a completely ill-advised proposition to me.
With a boiling point of -162 °C at atmospheric pressure, how much liquid do you expect to see and for how long if it’s a minor spill? Where would you mount the detector to just be able to catch the transient liquid? This is a job for gas detection.
With a larger spill, a thermal flame detector or thermal imaging camera mounted at some distance would be more useful in detecting the flames. And that’s only useful if people miss noticing the whole plant burning down.
Check out Schlumberger fiber-optic solutions (https://www.slb.com/business-solutions/sensa-fiber-optic-monitoring). They talk specifically about LNG applications including storage and pipelines.
However, I would not want to install a whole lot of temperature elements or a continuous fiber-optic system in a containment area. To me, it's a maintenance nightmare. It will never work effectively. Also, to do this properly would cost a lot of money.
Just because a standard says it should be done doesn’t always mean it’s a practical idea.
There are also various thermal imaging camera solutions (some even have gas concentration measurements via a spectrometer). I would favor non-contact solutions that look at an overall area. Line of sight gas detectors could be a useful addition in this application. You want to be able to cover as much of the area as possible. This will end up being a much more cost effective solution.
In this way, you're not trying to predict where the leak may be.
Simon Luccini
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A3: Measuring the temperature of a liquid that's evaporating quickly may present errors due to uneven cooling or vaporization. Furthermore, when trying to detect a leak, you'd need a lot of RTDs since the location of breaks are unlikely to be predictable. Finally, if I remember correctly, the British Standards you're referencing are probably more than 10 years out of date.
But if you insist on using RTDs, the only way to do so would be to have a small support, and place the tip of the RTD perhaps 2 in. from the ground, so the pooled LNG can cover it. I'd recommend that you space them around 3 meters apart, and perhaps use an RTD MUX box to interconnect them. It may work, though I don't believe it would be a reliable system.
My personal preference would be to use an infrared imaging temperature detector as the safest, non-contact way.
Alejandro (Alex) Varga Meder
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