Holistic Approach Reliable Communications Ensure Safety on Valve Actuator Automation Projects at Tank Farms
Valve actuator automation is a typical field bus application. Personnel in the control room must be able to operate valves at any time to keep the systems running, prevent supply interruptions and avoid serious accidents. The communications link between the control system and the actuators on the valves must be extremely reliable.
In the conventional sales model, a valve producer buys a suitable product from an actuator manufacturer. A service provider then connects the valve subsystem to the control system on site. The introduction of field bus technology and more stringent communications security requirements have significantly increased the complexity of the actuator-control system link. To an increasing extent, customers are placing orders for value automation solutions rather than actuators only. The specifications may stipulate a particular control system or field bus system.
However, there are a number of different field bus and control systems out there, and many combinations are possible. In order to deliver complete automation solutions, actuator manufacturers need in-depth knowledge of field bus technology, and they have to take a close look at the various control systems on the market. Not every control system supports the full functionality of a field bus. There are other issues to consider besides excellent communications security and compatibility with a specific control system and/or field bus. Projects often involve modernization of existing production systems rather than installation of new systems. Customers are clearly interested in linking the new actuators to the existing control system and minimizing disruption to ongoing operations. Customers may also specify how fast the actuators must react in response to activation commands and the speed at which status information must be sent to the control system.
Different solutions for different scenarios
Three solutions are described below.
1. Tank farm modernization
A project was launched to optimize material flows at the Europoort tank farm in Rotterdam as part of an efficiency improvement program. More than 1000 actuators were replaced along with the entire control system. The key deliverable was fast reaction time without of course neglecting the importance of communications security. The equipment had to be compatible with a Yokogawa control system and Profibus DP. Furthermore, the control system had to be able to retrieve status information from the drive, a function which the control system did not support. The actuator manufacturer had to come up with some creative solutions, and the company eventually engineered the entire communications link between the control system and the actuators.
2. Refinery expansion
The main requirements on an expansion project at the Bío-Bío refinery in Chile were to ensure simple integration of the new equipment into existing systems and offload any data from the control system which is not relevant to ongoing operations. A total of 270 new actuators were involved. The solution included five master stations provided by the actuator manufacturer, which link in the new segments. The master station, which the actuator manufacturer developed, is designed specifically for actuator commissioning and operation, but it also supports open field bus protocols on the field bus and control system links. The master station acts as a data concentrator, sifting through actuator data and only sending key information back to the control system. All events are logged, and diagnostic information is saved for later access if necessary. If for example a drive is replaced, the master station automatically establishes communication with the new device.
A redundant Modbus RTU Ring guarantees data security in this application. If the ring is broken at any point due to a loose or broken cable, the master station continues to operate the two ring segments in a field bus line configuration, and all of the drives remain in the communications loop. With the Modbus Ring, conventional copper wiring can be used to bridge distances which would otherwise require the use of fiber optics.
3. New tank farm
A very sophisticated solution was needed for an automation project at a new GALP tank farm in Lisbon to meet very stringent customer requirements which were generally similar to the requirements on the bio refinery project which was discussed above. Full redundancy had to be provided for the entire communications system including the master stations. As a result, the master stations had to be kept in sync with each other as well as manage the redundant channels to the drives and control system. An additional emergency PLC was also installed per segment to provide an actuator control path based on conventional parallel communications during emergency operation. This meant that the actuators needed to have both a field bus and a parallel interface, and clear rules had to be established to define exactly when activation commands are valid from the different control paths.
Summary: There is no standard solution. In today’s world, actuator manufacturers are expected to do more than produce and sell actuators. Suppliers that do not take a close look at the associated peripherals will find it difficult to become involved in projects like those which were discussed above. Moreover, a holistic approach will give actuator manufacturers valuable insights which will help them during the product development process.