Honeywell Temperature Controller

For those of you who have been entrusted with the task of selecting a safety instrumented system for use in your plant, often face a tough question. Should I go for an integrated safety instrumented system, which will be totally different from my distributed control system (DCS or for short)? Or should I choose a new "integrated systems", which include both the distributed control system and safety instrumented system in one package?

A safety instrumented system (SIS) is a special type of control system that is used for safety critical parts of treatment plants, turbo machinery, boilers and other machinery or equipment to be controlled with high degree of reliability to do to operate safely, and bring it to a safe state in case of trouble. Emergency shutdown systems (ESD for short), can be considered a subset of the category of SIS control systems.

On the other hand DCS (Distributed Control Systems) are control systems that are used to control and normal monitoring of treatment plants, oil refineries, oil platforms and gas production, power electrical and so on. The DCS is the primary system that measures, monitors and controls the various process parameters such as flow, temperature, pressure, etc.. In the literature on standards, this system is also called the BPCS (Basic Process Control abbreviation System).

In the opinion of the standards bodies (such as IEC and ISA), these two systems are completely separate, because the safety instrumented systems must be dedicated to control only the safety critical parts of the plant. The DCS can not usually be said to be robust, fail-safe and secure use security key instruments at any time. The distinction between DCS and SIS, has led to different markets for both types of systems with separate suppliers for both, first. Thus companies such as Triconex, HIMA, Pilz and ICS Triplex are traditional suppliers of safety instrumented systems, while the market BPCS or DCS belonged to companies like Emerson, ABB, Honeywell, Yokogawa and Invensys.

This obviously meant that now, if a plant instrumentation and controls that have been designated as critical to safety, it necessarily require a separate SIS, DCS would not at all because of what standards state. So, now, a control room has two systems of control, completely different as chalk and cheese. The SIS has a separate power supply (or UPS), separate panels, monitoring stations, separate programming software and course material quite distinct from the DCS. The same engineer who has kept the instruments and instrument controls that processing plant, should now be able to two systems simultaneously. Plant modifications and changes have been a pain as any changes would be implemented in both systems, with many tests to ensure that the locking loops of origin and has worked as intended to.

Do both systems to communicate with each other, also proved not so simple, because even after DCS vendors were left with open systems, vendors of safety instrumented system were reluctant to migrate to such systems, because of the large trial evidence should ensure the "open" systems are free of bugs.

Thus, the engineers of the instrument the most in these situations began to wish for a new agreement by which both the SIS and DCS could talk to the other seamlessly (while remaining separate to comply with standards) . Also, if they could share a common engineering / programming platform as well? This would be the icing on the cake!

DCS vendors felt this mood of users and thousands of them came out with "integrated" DCS / SIS systems, where the DCS and SIS controllers are different, but part of the overall system ..