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Fibre optic temperature measurement in LNG heat exchangers

Fibre optic temperature measurement in LNG heat exchangers

In August 2009, the world’s first coil-wound heat exchanger made completely of stainless steel for the liquefaction of natural gas (to be used in Stavanger, Norway) was delivered on time to the customer. This heat exchanger employs new temperature measuring technology using fibre optics (glass fibres).

These are capable of measuring temperatures throughout the entire CWHE, along both the length and diameter. The information is graphically displayed using special software. With this technology, it is possible to detect temperature changes and, indirectly, the distribution of liquid cooling agent at the edge of the CWHE. It is now possible, for the first time, to compare the theory with empirical data.

The picture above shows a LNG (Liquefied Natural Gas) plant built at the Norwegian west coast near the city of Stavanger for LNG base-load production with an annual capacity of 300,000 tons of LNG, which started-up for commercial production in 2010.

LIOS Technology has cooperated together with Linde, the leading engineering and industrial gases company, to install fibre optic distributed temperature sensing (DTS) system on the Coil-wound heat exchanger (CWHE), which is considered as the heart of the LNG plant. Its purpose is to liquefy the dried and cleaned natural gas (NG).

Temperature measurement by optical fibres
The load dependent temperature profile in coil- wound heat exchangers is a desired quantity to further refine and improve the design. With knowledge of the temperature profile, pinch points can be analyzed and optimized and energy consumption can be reduced. A fibre based linear temperature measurement system was built into the coil-wound heat exchanger to measure this data. And LIOS fibre measurement system allows remote operational assistance with measurement data backup from Linde head office in Pullach/Germany.

Temperature is probably the quantity that is measured most in LNG plants. Thermocouples or thermistors are mainly applied to piping, vessels and machinery. When looking at the main heat exchanger, special knowledge for installation of temperature measurements has been developed in the past. The aim was to gain insight into temperature distribution and thus liquid distribution inside the bundles of the coil wound heat exchanger (CWHE). However, more detailed information is coupled to number of measurement points and thus to installation effort.







The Stavanger CWHE LNG Plant has been now equipped with a new approach to measure temperature, which is based on optical fibres. The fibre optic distributed temperature sensing (DTS) technique is an innovative way to obtain thousands of temperature measurements with comparatively low effort. A single optical fibre can replace many point sensors because temperature is measured along the fibre with a local resolution of down to half a meter. By co-installation of the fibre with the tubes during fabrication of the CWHE a huge amount of single measurement points is accessible from inside the bundle of the CWHE.

Briefly, the measurement principle is based on the Raman-effect as a result of laser-light transmitted through the fibre and scattering of spectral components in backward direction. This components travel to the starting point of the fibre where they are filtered and detected. The intensities of these spectral components are related to the temperature at the origin of this scattering process. Because the velocity of light propagation in the optical fibre is well known, the location can be determined from the time-of-flight of the returning backscattered light.

The LIOS DTS system enables more than 4000 single measurement points from the bundles of the Stavanger CWHE, which is a huge number compared to the few single point measurements used previously. During start-up of the LNG plant, the system has already proven its usefulness and successfully demonstrated its capabilities even at the low temperatures of natural gas liquefaction.


LNG heat exchangers

The idea was implemented by installing glass fibres with an overall length of approximately 2,400 metres in the Stavanger heat exchanger. "The most difficult thing was to avoid bending the glass fibres (d=0.2 mm) during the manufacturing of the heat exchanger and to avoid damaging them during the welding work which was carried out in the following months. The outstanding cooperation of everyone involved, in particular the care and quality awareness demonstrated by the Linde manufacturing employees during production, was the key to success of this project", said Gerhard Dägling, the Project Leader responsible for installing the optical fibres.

» Linde Small to Mid-Scale LNG Plants 

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