When people think of fiber optic cables, they most likely think of their role in the transmission of telecommunication data, carrying light pulses over great distances at the highest possible speed. And yet, thanks to the unique characteristics of light itself, they also serve a very different purpose: detecting minute temperature changes along the length of a heated sulphur or crude oil pipeline. What is the biggest difference between fiber optic and traditional sensors used in this field, and what are the advantages? Find out in this article.
The Main Difference Between Traditional and Optical Sensors
Traditional sensors include mechanical devices such as thermostats and electronic sensors such as thermocouples, thermistors and resistance temperature detectors (RTDs). While these devices are all different in terms of the technology used, they all have one thing in common: they are point-sensing devices. They can only sense the temperature at the discrete location where they are installed. With mechanical sensors placed at individual points along the pipeline, the pipeline temperature is only known at those discrete points. It is not cost effective to place many sensors on the pipe, so cold or hot spots may exist that are not detected using point sensors.
With fiber optic sensing, however, it is different. A fiber optic cable is installed under the insulation on the full length of the pipeline. A laser is used to send pulses through the fiber optic cable, generating backscatter that travels back through the cable toward the source. By combining the intensity of the backscattered light — which varies according to temperature — and the time between the launch of the pulse and its backscatter detection, the system can detect both the exact temperatures and their locations. These features allow for distinct advantages over traditional sensors.
Distributed temperature sensing is highly effective in combination with STS technology to improve the safety and performance of cross-country liquid sulphur transmission lines. It also can be utilized to monitor the transportation of organic compounds such as phenol in conjunction with polymer-insulated series heating cables. This heating technology provides the operator with the added advantage of field terminations and increased flexibility in heating complex piping applications for shorter transmission lines.
Phenol is a solid at usual ambient temperatures and has a melting or solidification point of 105.8°F (41°C). It is, however, customarily transported and stored as a liquid at temperatures above its solidification point. In order to keep the phenol in its liquid state, the piping must typically be heated and insulated at a consistent temperature range to avoid heat losses.
During the transport of phenol, there is always the risk of freezing due to a potential failure of the auxiliary heating or thermal insulation. The addition of fiber optic DTS technology provides a means to continuously monitor every point along these phenol transmission lines, which allows the operator to identify and troubleshoot any compromised heating and insulation sections. This actionable information ultimately keeps the pipeline operating safely and efficiently, while minimizing the potential for costly operational interruptions.
1. Continuous Sensing Along the Entire Length of the Pipeline
This is the first advantage for fiber optic sensing and it is most important because it touches on the core benefit of the technology. Instead of sensing at discrete locations, fiber optic monitoring systems can continuously monitor the entire pipeline. With a temperature measurement at intervals of approximately three to six feet along the pipeline, its full temperature profile is now known rather than just a few discrete points. This is referred to as distributed temperature sensing (DTS).
2. High Levels of Sensitivity
Fiber optic sensing not only provides continuous monitoring along the entire length of the pipeline, it also delivers an unprecedented level of accuracy. Distributed temperature-sensing systems that are installed today can determine the temperature along the length of the pipeline with an accuracy of ±1.8°F (1°C).
3. Making Temperature Sensing More Actionable
Accurately measuring temperature is one thing, putting that data to good use is another. In relation to the transportation of sulphur specifically, it is imperative to always keep an eye on the tight operational temperature range and to respond quickly to any unusual events. As the data comes in, the distributed temperature-sensing system produces profiles of temperature versus distance, allowing the user to know the exact location of any unusual drops or spikes. This is particularly useful during the sulphur remelt or heatup processes.
Alternatively, thermal intelligence provided by distributed temperature-sensing systems can also assist in troubleshooting and preventive maintenance. It can help to locate sections where the thermal insulation may be damaged or compromised.
4. More than Temperature Sensing Alone
Apart from changes in temperature, fiber optic cables also can be employed to detect pipeline strain and unwanted, third-party interference. Strain monitoring, a newer tool being used by pipeline operators, can play a significant role in preventing pipeline mechanical failures through accurate information on pipeline movements (mostly expansion caused by high temperatures). This capability has been integrated with distributed temperature sensing to form distributed temperature- and strain-sensing systems (DTSS).
5. Relevant Throughout the Operational Lifecycle
Because of its continuous sensing, high accuracy and immediate, actionable data, distributed temperature-sensing systems have been successfully used in conjunction with skin-effect heat-tracing systems (STS) to monitor temperature in all stages of the pipeline’s lifecycle. During startup, the DTS and STS systems can be coordinated to ensure that the empty pipeline is not overheated.
When the molten sulphur is introduced in the pipeline and temperature profiles stabilize, operators can still detect drops in temperature. In the event of a remelt procedure, fiber optic distributed temperature sensing is used to continuously monitor the real-time temperature profile to ensure the pipeline temperatures remain within operational limits while the skin-effect heat-tracing system is used to melt the sulphur plug.
In conclusion, taking into account these five advantages, it is safe to say that fiber optic technology can improved the safety and performance of heated liquid sulphur pipelines. In combination with skin-effect heat-tracing systems, these advancements have resulted in efficient, integrated heat management solutions for sulphur pipelines.