Safe Control System Functions with SIS Controls and Instruments

Abstract

Quantitative safety assessment for the process industry is presently conducted considering EN 61511 and EN 61508. Most of the standard's demands are expressed clearly and can therefore easily be satisfied. However, some sections seem to be conflicting. Despite the fact that EN 61511 prefers the strict separation of the distributed control system and the safety instrumented system it defines specific conditions under which the use of safety equipment for control purposes is possible. This article is intended to provide the pros and cons of such a concept. A methodology is provided that allows the determination of whether integration is permitted for any specific case. It can be applied to the sensor part as well as to the final element part of a safety loop. It is shown that an improvement of the safety related availability is possible under certain circumstances.