Improving efficiency and safety at Europe's largest ammonia plant

Paul Boughton

Environmental, safety and integrity issues are becoming ever more important in the chemical storage industry today. Jerry Worsley reports.

As plants and facilities become larger in scale, more complex and more remote, maximising safety and minimising risk requires an absolute understanding which is provided by accurate monitoring.

With conventional vessel monitoring technology there is a gap between what is thought to be occurring and what is actually happening. Distributed fibre optic sensing technology developed by Sensornet, the global provider of the most advanced digital monitoring solutions and part of the Tendeka family, overcomes the limitations of traditional measurement technologies by closing the monitoring gap.

Distributed fibre optic sensing has several advantages over conventional sensors. As the fibre is the sensor the measurement is totally non intrusive and can be easily strapped to the outside of the vessel. The fibre provides its own communications path so no additional tie cables are required. Most importantly Distributed Temperature Sensing (DTS) using optical fibres enables you to capture data every meter at near real time speeds.

Sensornet recently installed and commissioned two of its Halo DTS systems and sensing cables at the largest ammonia plant in Europe to provide the continuous process monitoring of two reactor vessels.

By providing reliable, real-time data, the company brought a number of significant benefits to the client, Yara, the global company active in industries ranging from food production to emissions control. Yara is the world's largest supplier of mineral fertilisers, the world's largest supplier and trader of ammonia, and a global leader in a wide range of chemical products.

The ammonia plant, at Sluiskil in the Netherlands, is located on the Ghent-Terneuzen canal, a major waterway opening through the Western Scheldt into the North Sea and is a major European fertilizer complex. Approximately 70 per cent of ammonia production on site is consumed in the production of finished fertilizer products and produces 1.7 million tons each year.

Sensornet developed its Halo DTS technology to cope with the thermal and chemical processes that occur within reactor vessels and columns and which can slowly erode and weaken the structure of the vessel. As this weakening occurs, the skin of the vessel is reduced causing a potential point of weakness/failure. This weak spot has a temperature signature, which differs from the rest of the vessel and can result in a hotspot (or cold spot depending on the process). By detecting this hotspot early on, action can be taken to prevent any accidents and minimise any downtime for the plant.

The Halo DTS system allows for continuous monitoring for up to 4km, responds with update intervals of 10 seconds and can detect a temperature change of less than 0.1°C.

It contains an inbuilt multiplexing module (with either two or four channels) enabling up to four single ended measurements or two double-ended measurements. User configurable zones and alarms functionality are also available. The system is packaged in a standalone unit, which contains both the sensing optoelectronics and an onboard PC. It operates with an intuitive software interface and has been tested to some of the industry's most rigorous standards.

By delivering within the tight deadlines imposed by the need to have the work completed within the down periods, Sensornet was able to complete the monitoring process with the minimal disruption to production.

The two ammonia vessels at Yara Sluiskil operate at high temperatures. The vessels are internally insulated to prevent the shell from overheating. Constant monitoring of the skin temperature (approximately 110 to 150°C) is required to detect the location of any cracks in the internal insulation, allowing any hotspots to be identified and managed from an early stage.

The temperature of the vessels can rise up to 350°C when a hotspot occurs. These hotspots create great stress on the vessel and repairs must be completed during a turn-around.

The initial installation of the Halo DTS monitoring cable on Vessel 1 was conducted during a scheduled maintenance shutdown. The Sensornet team worked around the clock with other service companies to ensure the installation was completed within schedule. The sensing cable on Vessel 2 was installed at the manufacturer's facility in The Netherlands before the vessel was relocated to the Yara site. Final connection and commissioning was then conducted in a matter of days. Both projects utilised the Halo DTS complete with the integrated multiplexer for bi-directional testing.

The former monitoring solution deployed in the vessels, which used electrical resistivity, was outdated and was not supported by the original supplier. Temperature readings were presented as percentile figures based on maximum and minimum values, which was not ideal. In addition, temperature readings were also taken with infra-red thermometers. Although these provided valuable data they could not be deployed on a permanent basis and would not provide a fully distributed temperature profile of the vessel.

Sensornet was required to fill the monitoring gap by providing a solution that created a complete temperature profile of the reactor vessels with near real-time refresh rates. Monitoring the entire vessels online enables the process to be managed to ensure they performs within their operational tolerances. This results in longer run times between cleaning and maintenance periods. Moreover, the operator can be safe in the knowledge that the vessels are operating safely.

The Sensornet solution uses high temperature fibre deployed in a mesh topology to provide complete coverage of the vessels. This ensures that hotspots as small as 20cm in diameter can be detected.

The vessels are divided into a series of zones, each containing a minimum of 10m of sensing cable. The Sensornet Halo DTS is positioned within the local control rooms meaning that no active hardware is required in the field. The DTS communicates directly with the site's supervisory control and data acquisition (SCADA) system via OPC computer process control tags. This illustrates the average and maximum temperatures of each zone on a vessel schematic and refreshes the temperature every 10 seconds. In this instance automated alarms are activated by the SCADA system. If required this can be achieved directly from the DTS via contact relays.

For each of the vessels a total of 1km of sensing cable was deployed directly onto the skin. The fact that the DTS system offers measurement times of down to 10 seconds and temperature changes of better than 0.01°C can be detected by the sensing cable meant a total of 1,000 measurement points were established, providing the client with comprehensive thermal modelling of the reactor vessel.

Xavier Herman, Assistant Engineer, Yara, said: "The installed system enables us to follow-up on the state of the refractory. Due to the system we are able to detect changes that give us the opportunity to act at an early stage. Furthermore Yara is satisfied with the cooperation with Sensornet to find a solution and the ability to react and adapt to small changes during the project. Yara is also satisfied with the effort Sensornet made to provide a system in a short period."

Jerry Worsley is vice president, Sensornet Industrial Monitoring, Elstree, Hertfordshire, UK;