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Overcoming single-point temperature sensing limitations

21 November 2016

Suzanne Gill reports on the potential uses for distributed temperature sensing technology in a host of new application areas.

Distributed temperature sensing (DTS) solutions measure temperature distribution over the length of an optical fibre cable, using the fibre as the sensing element in place of more traditional single-point electrical temperature measurement techniques, which rely on the use of thermocouples. 

When you fire a laser pulse into a molecule within a fibre optic cable most of the light will bounce back unchanged. However, a small amount of light will change. The change from the light source is called Raman scatter. Because Raman scatter is thermally influenced by temperature, its intensity will depend on temperature. DTS captures the change from the returning light pulse and measures the intensities between the different signals. Temperature is determined by measuring the respective intensities of the stokes and anti-stokes signals. The position of a temperature reading is determined by measuring the time taken for the signal to return to the source. 
It is a technology can offer huge cost-saving benefits in applications where hundreds or thousands of traditional single-point measurement sensors would be needed to measure temperature stratification or temperature profile across a large area. DTS can easily be deployed over long distances to provide accurate and precise temperature measurement at 1m points along the length of a fibre optic cable. Its optical characteristics also make it immune to the effects of electromagnetic noise. 

Yokogawa’s DTS offering is DTSX. In combination with the company’s production control systems, the DTSX solution’s dedicated software is able to translate laser pulses into temperature data, using Modbus outputs to communicate this to other control systems to enable the data to be represented on operator screens.

New application areas
Traditional applications for DTS include site safety and facilities diagnosis. Cordova Cordova, product manager for DTSX and transmitters at Yokogawa Europe, explained how the company is expanding these traditional applications with its DTSX fibre optic sensing solutions. For example, the technique is now able to show temperature stratification in downhole applications and can also offer benefits for leak detection applications in pipelines too. 

“DTSX is able to utilise two different types of existing fibre optic cables – single mode (typical diameter 9 µm) or multi mode (typical diameter 50 µm). The mode is related to diameter of one fibre strand which influences the behaviour of how the laser will propagate along the fibre,” said Cordova. “There will usually already be a fibre optic cable attached to a pipeline for communication purposes so it is possible to use this cable to measure the temperature along many pipelines. In some instances the system can work using just a single fibre optic strand inside the cable, so in some applications the cable could continue to be used for its original purpose while additionally offering useful temperature data too.

“Currently pipeline leak detection is most often measured via the different upstream and downstream pressures within the pipeline. If there is a leak there will be a difference between these two pressures. However, this does not tell the operator where in the pipeline the leak is. Because most leaks will cause a temperature change between normal operation and the leak, with DTSX it is possible to identify where in the pipe the leak is, to an accuracy of 1m.”

Power cables
Another exciting potential area of application for DTSX is for measuring temperature within power cables. “Power transmission cables generally already have fibre optics within the cable jacket for communication,” explained Cordova. “This cable can be used to measure the temperature of the copper core inside the power cable to provide a better understanding of how much power is being delivered through the cable gives power companies useful data to help optimise their power cable operations.”

In process applications DTSX can be used to compliment existing point temperature measurement techniques. “One thermocouple is only able to send data relating to a single point on the process,” said Cordova. “Sometimes, however, there is requirement to see details about temperature distribution across a vessel. This would require multiple temperature sensors, which is uneconomical and difficult to implement. With DTS, however, it is simply a case of attaching a fibre optic cable along the vessel to gain temperature data at 1m intervals to provide a complete temperature profile of the vessel.”

Challenges
Challenges to implementing a DTS system lay mostly in issues relating to the fibre optic cable itself. “If, for example, you want to undertake temperature monitoring in a hazardous area it would be necessary to protect the cable with a mechanical tube,” said Cordova. “Fibre optic cable also does not respond well to hydrogenation. They cannot tolerate hydrogen ingress which reacts with the silicone causing darkening and degrades the performance of measurements. Applications requiring temperature measurements above 1000°C are also a challenge for DTS.”

For basic DTS measurements all that is required is a DTSX unit, DTSX software and a fibre optic cable. The DTSX unit is based on a Yokogawa Stardom PLC design so it is small and compact requiring minimal power consumption which allows it to be situated either in a control room or placed in outdoor cabinets where it can be adequately powered via a solar cell.

“We can see huge potential for the DTS technology in the power market, for optimising power cable distribution, as well as for pipeline leak detection applications or for temperature stratification monitoring in a wide variety of processes,” said Cordova.

Another interesting area for the technology is for predictive maintenance applications across all industry sectors. Conveyor belts, for example, employ many bearings on their rollers. When a bearing starts to wear out it creates heat. Running a fibre optic along the belt across the bearings will allow this heat to be detected early, alerting engineers to a potential bearing problem and reducing downtime due to conveyor belt failures. “We already have the system working successfully on such applications in Japan,” said Cordova.

It must be expected that DTS will find increasing applications in the future as enterprises demand more plant floor data to help improve their efficiency and profitability. Keys for diffusion of the technology include an ability to be integrated with various monitoring or production control systems. Yokogawa has this covered. The DTSX provides communication functions that can combine with either production control systems such as DCS or SCADA. A dedicated function for Yokogawa’s FAST/TOOLS, for example, has been developed to transfer temperature data by file transfer, making the technology simple to incorporate into an existing control system.


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