Safety Instrumented Systems: Complying with the ISA 84 Standards
The ISA 84 standard is a performance based standard that gives guidance on how to determine the degree of risk, and design an SIS for hazards in the process industries. Not only does the standard prescribe design approaches, it also requires testing and maintenance to ensure system performance. This process establishes an SIS safety lifecycle to manage the system from cradle to grave.
Each safety risk/safety instrument fuction (SIF) is assessed on its safety integrity level, or SIL. This hazard and risk analysis identifies the required safety functions and risk reduction required for each potentially hazardous event. SIL verification involves multiple calculations to determine the probability of failure and, therefore, the appropriate SIL range for each identified hazard.
The SIS is designed to achieve the required risk reduction for each SIF. Typically, an SIF includes transmitters or sensors that send signals to a logic controller that activates solenoids or other control elements when pre-determined upset conditions are reached in order to bring the process into a safe state. For example, a pressure that exceeds the maximum allowed will cause the SIF to activate an actuator to open a valve.
While a process control system is actively used, the process safety system is passive and only monitors certain parameters. It takes action only when the monitored parameters exceed their limits. Since the SIS is a passive system, testing is vital, either with built-in diagnostics, and often by manual proof testing.
Safety instrumented systems have two types of failure modes to consider:
- Safe failure (nuisance trip) when an incident triggers a shutdown, like a signal transmitter malfunctioning high and causing a trip. There was not a process upset, but production is interrupted.
- Dangerous failure when the monitoring system calls for a response, and nothing happens, such as when a safety valve fails to open. These consequences can be serious.
Therefore, careful testing is needed to ensure the passive monitoring devices (SIS devices) work, and the system equipment will perform as expected. The process requires thorough procedures to properly conduct testing and perform visual inspections. Maintenance schedules should reflect the level of attention needed to keep all safety instrumented systems functioning properly throughout the equipment life cycles.
Documentation is needed for every phase—from SIL calculations to proper installation techniques to appropriate testing methods to adequate maintenance schedules. Operating and maintenance personnel may need training to ensure the system performs properly in the months and years ahead.
Your safety instrumented systems has important standards that must be met. Matrix Technologies has the expertise to help you through every step of the process to achieve compliance with the standards.
Matrix Technologies is one of the largest independent process design, power systems engineering, industrial automation engineering, and manufacturing operations management companies in North America. To learn more about our solutions for your safety instrumented system needs, contact Michael Johnson, PE, Department Manager, Process & Electrical Design Department.
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