Modern industrial plants, oil & gas production facilities and other process plants all have some instrumentation and automation that ensures safety. These are known as Safety Instrumented Systems (SIS for short). These systems also are known by various other names such as Emergency Shutdown Systems (ESD for short), Safety Shutdown Systems, High Integrity Pressure Protection Systems (HIPPS) and so on. But all of them belong to the class of systems that are referred to as SIS.
Safety Instrumented Systems in the past
In the past, Safety Instrumented Systems were strictly separate from the normal plant control systems (referred to as a BPCS (Basic Process Control System-which most people refer to as the "plant DCS"). This was done for a variety of reasons, but mainly to segregate the safety and control functions and to have higher availability and reliability. In older plants, typically the BPCS was actually a collection of pneumatic panels having pneumatic instruments and controllers. The Safety System was implemented using hardwired relays and contactors.
The situation today
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. Also other kinds of high reliability specialized systems like HIPPS (High Integrity Pressure Protection Systems), BMS (Burner Management Systems) and so on can be considered as belonging to the same class, i.e. a SIS rather than a BPCS.
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. This is referred to as the BPCS by people who use both SIS and DCS in their plants.
Separation of SIS and BPCS
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 was dominated by companies like Emerson, ABB, Honeywell, Yokogawa and so on. 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.
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