Separation versus integration of BPCS and SIS | Advantages and Disadvantages

.....continued

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. Thus 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 & Control System engineer who got a job in the process plant, had to be adept at both systems simultaneously to do his job well. Plant modifications and changes were cumbersome 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.

Integrated SIS and DCS

So which systems are better? The original separate Safety Instrumented Systems where the logic solver (popularly referred to as the Safety PLC or Safety Controller) is totally different or the integrated version, where the same system has two different kinds of controllers/logic solvers-one type for the BPCS and another type (usually certified by third party agencies like TUV) for the SIS? Note that the integrated SIS DCS does not imply that it is one common system, it is just integrated for ease of use an convenience. Thus the configuration software may have different types of logic blocks, some meant exclusively for use in safety functions, whereas other can be used in the normal BPCS functions. If the logic solvers/ controllers need to communicate with other logic solvers, then it has to be over a "safety bus" (a communication bus that is robust enough to carry safety critical data reliably). Thus the integrated system is not really totally integrated, but is much more close knit than the earlier totally standalone systems.

Only time will tell us which system is better. There were fears amongst a section of the community that a single common cause failure could knock out both systems, but these seem to be unfounded for the moment.

Only time will tell which system is better.

To learn all about Safety Instrumented Systems, Emergency Shutdown systems, HIPPS and similar topics, download the e-learning course on Safety Instrumented Systems now.