Implementing SIS (Safety Instrumented System)
A number of technologies are available for use in safety instrumented systems: pneumatic, electro-mechanical relays, solid state, and microprocessor/PLC.
There is no one “overall” best system; each has advantages and disadvantages. The decision over which system may be best suited for an application will depend upon many factors, e.g. budget, size, level of risk, flexibility, maintenance, interface and communication requirements, security, etc.
Pneumatic systems are still in use and are perfectly appropriate under certain circumstances, e.g. offshore operations where electrical power is not available. Pneumatic systems are relatively simple and relatively fail safe.
The most common application is to use a pneumatic controller, normally field mounted, to compare a process variable with a set point. The output signal goes to a pressure switch that drives a final element to execute a trip.
Relay systems are relatively simple (at least when they are small). They are relatively inexpensive, and are immune to most forms of electro-magnetic interference (EMI) and can be built for many different voltage ranges.
However, they are prone to nuisance trips, and they can become unwieldy as the system gets larger. Any time logic changes are required, wiring must be changed and drawings must be updated. Relay systems are based on discrete (on/off) logic signals. Traditional discrete input sensors (switches) were used. About the only time relays are used is for very small systems, typically those with less than about 15 inputs and outputs (I/O).
Solid State Systems
The configuration of a typical solid state SIS with its input signal processing stage, the logic solver function performed by standardized electronic function blocks mainly AND gates, OR gates, logic inverters and timers.
Solid state systems are hardwired, much like relays. They typically have about 50:50 failure mode characteristics. Those that were built for safety generally include features for testing and performing bypasses. Most systems also offer some form of serial communications to external computer-based systems.
Considering the merits and demerits of solid state systems they have essentially the same characteristics as relay based systems with the advantage of using purpose built components such as multi-channel input signal processing boards and logic solver blocks. The modules must be wired into the logic configuration that is required for the system.
The modules of the logic solver are operated in a continuous switching mode transmitting a square wave signal through each gate or circuit. Diagnostic circuits on board each module then immediately detect if the unit stops passing the pulses. The detectors in turn link to a common diagnostic communication module that reports the defect to the maintenance interface. Normally the detection of a failed unit will lead to an alarm and sometimes a trip of the plant.
Solid state systems offer several significant benefits over PLC-based systems. The most obvious is that they do not use softwares. Wiring is relatively easy to test and check. These systems can also respond faster than software-based systems. Like relays any changes made to system logic requires changes in wiring and updates of drawing. These systems are also expensive.
Microprocessor/Programmable Logic Controller (PLC) Systems
Computer-based systems appear to be the system of choice for many applications today. Such systems offer low cost, the ability to make changes easily and flexibly, serial reporting capabilities, graphical operator interfaces, etc.
PLCs were originally designed to replace relays, yet their application as shutdown systems requires close scrutiny. Conventional PLCs were not designed for use in critical safety applications, as they lack extensive diagnostic capabilities, fail safe characteristics, etc. A specail class of PLCs, known as Safety PLCs, are used instead.