How PLCs Help to Avoid Hazards and Increase Plant Safety
When SIS is required, one of the following should be determined:
- Level of risk reduction assigned to SIS
- Safety integrity level (SIL) of the SIS
Typically, a determination is made according to the requirements of the ANSI/ISA S84.01 or IEC 61508 standards during a process hazard analysis (PHA). A process demand is defined as the occurrence of a process deviation that causes an SIS to transition a process to safe state. SIL can be considered as the statistical representation of the availability of an SIS at the time of a process demand. It is the litmus test of acceptable SIS design and includes factors such as device integrity, diagnostics, systematic and common cause failures, testing, operation and maintenance.
In modern applications, a fail-safe programmable logic controller (PLC) is used as the core of SIS. Safety integrated PLC system is used for fire and gas applications, and improves plant operations by responding in a pre-determined manner.
The fire and gas logic is implemented in a fault-tolerant PLC control architecture that ensures that no single point of failure will cause an inadvertent action or prevent the fire and gas systems from sounding the alarm and taking appropriate actions. All system failure modes are designed so that loss of any component (CPU, I/O module, power supply, communications, etc) will not only ring an alarm but will also allow the system to isolate the failure and continue providing safe and uninterrupted operations.
National and international standards for safety and critical control systems require that companies document their systems as well as design, maintain, inspect, test and operate them in a safe manner. Fail-safe systems need to comply with the standards; procedures must be written for operating, testing and maintaining the system for the specific application. This requires the services of agencies affiliated to TUV that have the thorough process knowledge for documenting the system.