Why integrate control and safety?

22 March 2011

Control Engineering Europe finds out more about the benefits of integrating control and safety systems and finds out how a Spanish energy company achieved its goal.

Hazards can arise from the material being processed or from the process itself. When dealing with hazardous processes the safety of personnel, plant equipment and the environment are of utmost importance but it is also important that the systems put in place to ensure safety do not themselves compromise the production process through spurious trips.
In order to achieve this combination of safety and fault tolerance a reliable Safety Instrumented System (SIS) is required, which can bring the plant to a safe state when necessary, but which can also meet the high availability requirements of the process industries.

Spanish energy utility company, Endesa, made the decision to implement an integrated control and safety system at its Teruel Power Station to allow it to standardise on a single platform which would address issues of support and maintenance as well as enabling the company to meet the requirements of the Spanish electricity transmission grid.

The plant comprises of three similar units of 367 MW with a total installed capacity of 1.101 MW. Initially, it was decided to update one of the units at the plant. The project created an integrated control system for the unit - a single control system covering the turbine, boiler, water/steam/air/gas cycle, electrical system, desulfurization plant, package and auxiliary plants, and protection relays.

The upgrading of the control system was completed in less than two months, as planned. All pre-existing field cabling has been retained and re-utilised – a key factor in enabling the project to meeting the tight deadlines. The system comprises approximately 11,000 I/O points (8,000 wired + 3,000 communicated signals) with around 120 PID loops. A total of five dedicated Operator Stations have been used to provide visualisation and these are connected via two redundant operator station servers.

The boiler protection system uses an integrated redundant Siemens Simatic S7 417FH controller pair, along with dedicated failsafe I/O. Turbine protection used a dedicated 315F controller.

Flexible modular redundancy
A useful feature of Siemens PCS 7 with Safety Integrated is its Flexible Modular Redundancy (FMR) concept. Depending on the requirements of automation or safety it is possible to define individual redundancy levels for the various layers in the automation hierarchy giving flexibility at the controller, fieldbus and IO levels.

This approach leads to high plant availability – a key consideration for all power plant operators. Also, when considering the safety aspects of the overall system, the FMR concept means that all the differing voting requirements for Safety Instrumented Functions can be easily accommodated at the same time as minimising downtime from spurious trips.
Integrated approach to safety.
The Simatic PCS 7 system has an integrated approach to safety so that safety applications such as boiler protection systems (BPS) and turbine protection systems can be implemented within the same overall system. This brings a number of benefits such as savings as a result of common engineering tools, reduced training, reduced spares and simplified maintenance. In addition, the cost of engineering is reduced because data can be more readily passed between the control and Safety Instrumented System without additional hardware or configuration of communications gateways and time synchronization and SOE becomes much easier.

Integrated diagnostics and system-wide asset management tools give increased scope for improved asset optimisation of SIS assets. In addition to cost savings, integration often offers the potential for use of more integrated tools which in turn can help reduce the possibility of systematic errors within systems. By removing the need to interface disparate systems the need for data mapping is reduced and, with it, the risk of errors. The ease with which appropriate and consistent information can be displayed to the operator also offers the potential for increased safety.

Operators were able to quickly adapt to the new operating philosophy of the PCS7 environment. The intuitive nature of the system helped to ease this transition. The flexible alarm handling and the integrated nature of the system – including the ability to customize where necessary - easily addressed this issue. The ability to integrate the information from the safety controllers and the sequence of events data from the controllers was also a significant benefit.

The control system has improved the control of the plant substantially, reducing the time needed to start the plant, by use of burner start sequences, mills, and turbine and synchronization systems. The implementation of advanced algorithms of control has given more stability to the control variables in transitory states, minimizing the deviation of the main process variables improving process efficiency.

At the information level the system for handling and locating alarms has been improved - detecting the different alarm sources and importance and adding sequence of events recording for analysis of fast perturbations.

Subsequently, the Simatic PCS7 control system has also shown itself to be flexible and has allowed optimisation and improvement to control strategies since installation using its online change capability.

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