Diagnostics are key to fieldbus adoption

When Roche started construction of a new 25,000 tonne per annum vitamin E production plant in 2001 the decision was made to use fieldbus technology.

The demand for vitamin E for use in foodstuffs has increased, as it is known to prevent the oxidation of unsaturated fatty acids in the metabolic cycle.

In 2001, Roche at Sisseln in Switzerland, part of DSM Nutritional Products, started construction of a new 25,000 tonne per annum vitamin E production plant.

With such a large plant using a continuous process, a high degree of process automation was needed, and the decision was made to use fieldbus technology.

The arguments for this were not only the familiar aspects such as simple wiring, greater accuracy, and centralised control, but particularly the ability of fieldbus devices to use application-specific configuration to pass diagnostic information to the process control system independently and cyclically.

This diagnostic data allows central monitoring of the health of the field devices and the process condition variations around the plant, to initiate action when any faults are detected: this is "event controlled maintenance".

An earlier fieldbus trial in a pilot plant, a section of the wastewater treatment plant on site, had employed 25 Profibus PA devices, and based on this positive experience the new plant automation was also based on Profibus systems.

The Roche production plant occupies several floors of a building in Sisseln, and also includes a tank storage depot.

Around 5000 field devices in total are divided into 27 plant sections, each with a controller (Siemens S7 400) for control purposes.

Typically these controllers use two Profibus DP cards to interface to most of the field devices, but to power the Profibus PA devices in the hazardous area on the plant, Profibus fieldbus barriers were required, powering a maximum of four devices each.

Each section used typically six fieldbus barriers, giving access cycle times of less than 0.4 seconds for all devices.

Various Hart enabled devices were also included in the control systems, which interface via Profibus DP remote I/O into the process control system.

There are also some direct connections to motors, actuators and frequency inverters.

All the 27 section controllers are networked with one another via industrial Ethernet using the standardised Profinet protocol and connected to the higher-level terminal bus via redundant servers.

This terminal bus serves as a databus for interfacing the operator control stations and the engineering station, and as a management communication interface with the office environment.

In the plant operation the major sources of the most useful diagnostic are the sensors and closed-loop control valves.

"Diagnosis" is the interpretation of symptoms either measured or established in some other manner, derived from the devices or the process, the aim being to recognise and remedy malfunctions and their causes in good time.

This diagnosis was regarded as important for operation of the vitamin E plant because the production is operated as a continuous process, without any significant downtime, largely without any changes over its lifetime.

The diagnostics acquire major importance, allowing optimisation of plant operation and event controlled maintenance.

The specific objectives on the part of users for exploiting the diagnostic information were: clearly organised state visualisation for the operator and maintenance staff; logging for statistical fault detection and wear-related maintenance; and development of a standard strategy in the process control system in the event of a device alarm and a maintenance requirement.

In overview, the results of the installation have been positive, and the operation is successful.

The capital cost of the fieldbus based system was about 30% less than for a system using parallel wiring.

The Profibus based system was found highly suitable for automating a complex process engineering plant, not least because with a universal communication protocol a very wide range of field devices can be interfaced and operated.

This covered practically all device types occurring in process engineering production plants, ranging from the Profibus PA field device and the conventional Hart device to motors and simple switches.

The diversity of field devices that needed to be used on this plant highlighted the differences between the diagnostic signals transmitted from different devices, or similar devices from different manufacturers.

This lack of standardisation was addressed on plant by the development of their own I/O driver block to deal with heterogeneous field devices, to develop their own diagnosis evaluation scheme.

This is also being addressed by the standardisation authorities.

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