Michael Herbstritt looks at the growing confidence in fieldbus demonstrated by tank farm operators.
As with most new technologies, the operational safety of fieldbus when it was introduced in the 1990s was challenged. A degree of insecurity was also created by the abstract presentation of signals as bits and bytes that replaced the widely spread values given in volt and ampere. Suppliers of fieldbus solutions, including actuator manufacturers, therefore had to provide suitable reassurances regarding safety. In recent years, the same solution providers have also had to support a widespread move toward more sophisticated safety standards in process technology plants which has intensified the considerations for developing innovative solutions that respond to concerns relating to communication safety.
These issues have been addressed and today, fieldbus systems are widely implemented including installation in safety-relevant areas such as tank farms.
A fieldbus is an industrial communication system connecting a variety of field devices such as transducers (sensors), motor operated valves (MOVs) and actuators (drives) with a control system. Fieldbus technology was developed in the 1990s to replace parallel wiring of binary signals which was common at that time.
Confidence in fieldbus technology grew and this supported a new level of sophistication for actuation technology. New generation bus systems have removed the need for hard wiring and have provided a simpler and more effective option which offers many benefits, including cost savings. Traditionally, a 20-core multi cable has been connected to each actuator at the plant.
However, with the digital option, a simple two-core cable for a number of actuators is used resulting in substantially less wiring which is reflected in significant cost reductions. With just two wires instead of 20, installation becomes considerably easier with less chance of wiring errors.
In conjunction with the growth of fieldbus installations, the necessary supporting specialist infrastructure has been established.
Knowledge levels have developed and today there is wide ranging expertise available in all aspects of bus technology.
A typical fieldbus application is valve automation. To ensure the plant or farm operation runs smoothly, and to avoid accidents, the valves have to be operable at any time from a distributed control system (DCS). Consequently, substantial communication safety between the DCS and the actuators, mounted on the valves, is required.
The traditional distribution channel for actuators is that the valve manufacturer sources actuators from a supplier that provides products that complement the valve. These actuators are connected to the DCS of the plant by the service provider.
The introduction of fieldbus technology, and the rising demand for communication safety, has made the integration of actuators increasingly complex. Today's actuator manufacturer needs to move beyond merely supplying actuators to the provision of complete automation solutions for valves with options that accommodate any predefined DCS or fieldbus system. And the actuator provider has to bear in mind that there is not just one fieldbus system or DCS, but a number of combinations is feasible.
Actuation supports automation
As a result, the actuator manufacturer needs to offer comprehensive fieldbus system expertise to allow the development of complete automation solutions. Additionally, the DCS of various manufacturers has to be analysed, as not every DCS will support the functionality provided by the fieldbus.
To add to the challenges, the actuator supplier is frequently briefed to support the upgrade of a plant or farm and rapid integration of systems is essential to avoid any downtime. On these occasions, additional specification requirements are often outlined regarding the actuator response times on operation commands or the speed of receiving feedback signals within the DCS.
Three case-study examples follow that illustrate AUMA actuators' work supporting fieldbus solutions for tank farm operators. The first confirms how the challenges of an upgrade requirement have been met, the second illustrates how the actuator supplier has supported a farm extension and finally, work at a new farm is detailed.
- BP Refinery, Rotterdam. Within the framework of an efficiency enhancement program, fluid flows were to be optimised within an existing infrastructure at BP Refinery Rotterdam. To this end, not only 1000 actuators were replaced but the complete DCS was renewed. The demand on short response times, without impairing communication safety, was the main issue of efficiency enhancement that had to be addressed. The project plan defined a Yokogawa DCS and Profibus DP as standard. In addition, it was required that the DCS could request actuator status information. BP's brief was met by an innovative solution from AUMA that included a projection of the complete communication structure between DCS and actuators.
- Bio Bio refinery, Chile: At the Bio Bio refinery in Chile, AUMA supported the integration of new plant components into the existing host system, discharging additional data from the DCS that extended beyond normal operational requirements. AUMA's solution included the supply of over 250 new actuators and five master stations to link the new refinery facilities. The master station developed by the actuator manufacturer, designed to commission and operate actuators, works with open fieldbus protocols in both directions ie towards the field and the host level. In this application, the master station is working as data concentrator processing the actuator signals and only sending the most important feedback signals to the DCS. All events are recorded and diagnostic information is saved and, as required, the data can be accessed. If an actuator's controls need to be replaced, the master station develops and maintains communication with the new device. In this example, data reliability is achieved by a redundant Modbus RTU loop: if the loop is interrupted at one point, for example, by a loose or broken cable, the master station treats both loop segments as fieldbus line structures and all actuators can still be addressed. The Modbus loop allows the coverage of ranges using conventional copper cables, achieving distances which are normally achieved by fibre optic cables.
- GALP oil tank farm, Lisbon, Portugal: For automating the GALP oil tank farm, a particularly sophisticated concept was required due to the prerequisites of the plant user. In principle, the requirements were similar to the procedure for the Bio Bio refinery, previously described. Additionally, all components and consequently the master stations participating in the communication process had to be provided in redundant versions. Therefore, the master stations were required to not merely manage the redundant channels to the actuators and the DCS but, in addition, they all needed to synchronise. Moreover, an additional emergency PLC per segment was installed for emergency operation; this PLC is used to control the actuators via conventional parallel communication.
For the actuators this means that they are equipped with both a fieldbus and a parallel interface. Additionally, clear rules were defined regarding which operation commands have to be executed and by which interface.
Confidence in fieldbus technology has been confirmed by tank farm operators around the globe, as detailed in the case-studies cited in this report
Flexibility and innovation are essential for the successful actuator supplier: there is no standard solution and the role of today's actuator manufacture extends beyond merely the manufacturer and sale of actuators.
The holistic approach required to adequately support operators in the tank farm sector encompasses addressing the demands of communication safety, DCS requirements and fieldbus technology.
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Michael Herbstrittis an electronic engineer and technical author with AUMA Riester GmbH & Co, Muellheim Germany. www.auma.com