Safety Instrumented Systems- Design

Many process plants handle large amounts of hazardous materials (including flammable, explosive or toxic chemicals)  at high temperatures and pressures (or in vacuum conditions)via complex processes. Hence there is a risk that there could be a disaster if something goes wrong and we could have fires, explosions and toxic chemical releases that can harm people and the environment. These incidents also damage assets and cause large scale destruction.

In these plants and facilities, usually we have Safety Instrumented Systems (SIS) of various types, in addition to the normal plant’s Basic Process Control System (such as a DCS or PLC/SCADA combination or similar). These SIS help in bringing the plant to a safe state in case any anomalies are detected in the process.

Safety Instrumented Systems  follow international standards such as IEC 61508 in general and in case of process industries they should conform to IEC 61511 (or ISA S84 in the US).

You can learn everything about Safety Instrumented Systems in the Abhisam SIS e-learning course.

In many cases, the Safety Instrumented Systems  can get quite complicated. We can have a mix of  simple and complex safety instrumented functions in the same plant or equipment. Simple implies that the Safety instrumented function has a simple design and architecture. Say one temperature sensor (say an RTD Pt-100 type) connected to a temperature transmitter that gives an input to a logic solver, which in turn actuates a single on off valve if the measured temperature exceeds a set point.

On the other hand depending on the requirements in the Safety Requirement Specifications (SRS) one can have complex SIFs such as redundancy and voting logic for the sensors, transmitters, logic solvers and actuators.

We can either use completely separate SIS and DCS  or use an integrated SIS DCS system. No matter what, there are certain useful design tips that you must always keep in mind. The Safety Instrumented Systems design video below shows this.