Are integrated SIS/DCS systems better than standalone SIS DCS systems?

Safety Instrumented Systems
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Lately, there have been many launches of new “integrated” control systems, that have both DCS and SIS systems in the same package. For those of you are not familiar with these terms, an SIS is short for “Safety Instrumented System“, which is a special kind of control system that is used for the safety critical parts of process plants, turbomachinery, boilers and so on. Emergency Shutdown Systems (ESD for short), can be considered a subset of the SIS category of control systems.

On the other hand DCS (Distributed Control Systems) are those control systems that are used for normal control and monitoring operations of process plants, oil refineries, oil & gas production platforms, power plants and so on. The DCS is the main system that measures, monitors and controls various process parameters like flow, temperature, pressure and so on.


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In the view of the standards bodies (like IEC and ISA), these two systems have to be separate, as the safety systems have to be dedicated to only the safety critical parts of the plant and the garden-variety DCS cannot be said to be robust, fail-safe and sure to operate the safety critical instruments at all times. This distinction between the DCS and SIS, led to separate markets for both types of systems with separate suppliers for both, initially. Thus suppliers like HIMA, ICS Triplex, Triconex, PILZ and so on were the suppliers of these Safety Instrumented Systems, whereas the DCS market belonged to the companies like Emerson, ABB, Honeywell, Yokogawa and so on.

This obviously, meant that now, if a plant had safety critical instruments and controls, it necessarily required a separate SIS, the DCS would not do. This now,  one control room had two control systems, totally different as chalk and cheese. The SIS had  separate power supplies, panels, monitoring stations, separate programming software and of course totally separate hardware. The same Instrument engineer who got allotted to the process plant, had to be adept at both systems simultaneously to do his job well. Plant modifications and changes were a nightmare as any change had to be implemented in both systems.

Making the two systems communicate to each other also proved not so simple.

Hence, all these instrument engineers started wishing for a new deal, whereby both systems could talk to each other seamlessly (even while remaining separate to conform to the standards) and what if they could share a common engineering /programming platform as well?  That would be great!

The DCS vendors sensed this fervent desire and many of them came out with “integrated” systems, where the DCS and SIS controllers are different but part of the same overall system.  Some examples are Emerson’s Delta V with SIS and Siemens Safety Integrated.

What does this mean for the traditional SIS-only vendors like HIMA and Triconex? Have users started switching over to the new integrated systems? Or is it a passing fad? As a system designer, safety manager, instrument engineer or plant manager, which option is better? Separate or integrated? Your comments on this will be welcome.