Functional safety control design for an existing industrial process

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Dr. Kiss Bálint
Department of Control Engineering and Information Technology

This document is a thesis, which describes the design of a safety control system used in a carbon-dioxide container park. The design in this paper is in accordance with the current international standards.

The thesis begins with the examination of the stored material and the overview of the European standards used in process control. In order to set up sufficient safety protocols, the analysis of the risks concerning liquid CO2 storage in large quantities is inevitable. This work uses Fault Tree Analysis to find the connections between risk factors. After the description of the possible error sources, it is briefly explained how to determine the safety level of a function using a risk graph. The process of hardware design is also discussed, focusing on Hardware Fault Tolerance and target fault probability of SIL levels.

This paper examines two possible hardware designs: one using discrete logic components, the other is implemented on a PLC. Both versions are based on commercially available, certified or uncertified components. After selecting the suitable components, the target error probabilities of the safety measures have been verified. Any function that failed to reach the levels set in the standards have been redesigned and re-evaluated, until they passed the requirements. In the previous sections fault probability calculations were shown on the examples of a 1oo2 and a 2oo2 hardware. The two structures were compared in multiple fields, like modifiability, development possibilities, maintainability, price, etc... Because of it's better qualities the PLC software was implemented instead of the discrete logic structure.

A SIEMENS 6ES7 315-2FJ14-0AB0 type CPU was used for implementing the control algorithm. The software was developed in ladder diagram, separating the safety and normal sections of the program. This work details the functions used in the algorithm, and also describes the screens of the HMI display.


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