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SIL - Safety Integrity Level

Plant planners and manufacturers

SIL – Verifications for safety systems in the process industry

For the proper technical implementation of safety systems (Safety instrumented Systems: SIS) in the process industry, verification of compliance with the requisite safety integrity level (SIL) needs to be documented as part of the safety lifecycle for safety functions. Safety functions (Safety instrumented Function: SIF) are determined and classified with a SIL in the hazard analysis and risk assessment. The SIL implemented with the safety system for each safety function can be determined, which needs to be identical or better than the classified SIL. The engineers from the weyer group determine the SIL for a safety function from the circuit design and the reliability characteristics that the manufacturers provide for the safety-related components and check whether the classified SIL is complied with.


The weyer group checks the SIL implemented based on the hardware fault tolerance (HFT) and safe failure fraction (SFF) of subsystems. The check results in a qualitative assessment of the protective circuit for a safety function. Depending on the requirement, improvements can be achieved through redundant design or suitable device selection or diversity.

Random hardware failures are assessed by calculating the mean probability of failure on demand (PFDavg) or mean probability of failure per hour (PFH). The reliability data from the manufacturers concerning the failure rate of the devices from the SIL declaration of conformity are used for this purpose.

In order to demonstrate the SIL for a safety function, the sensor technology, logic and actuators subsystems in the safety-related functional chain (loop) are taken into consideration and summarised.

By optimising the structural design through redundancy and/or selecting suitable devices, the SIL implemented can be improved and test periods determined or extended.

In order to assess the control-related safety functions in a plant, the engineers from the weyer group use the DIN EN 61508, DIN EN 61511 and VDI/VDE 2180standards series, which describe the requirements for safety-related functions in process technology implemented with safety systems. In this way, the plant operator can be sure that the currently valid safety standards are met.

Before designing the control technology, a HAZOP and risk assessment determine the safety functions (SIF) and classifies the SIL required. The subsystems sensor(s), logic system and actuator(s) are assigned to the safety system (SIS) with the necessary instrumentation for each safety function as a loop. The engineers at the weyer group determine the architecture and fault tolerance of the safety system. Subsequently, suitable devices (sensors and actuators) and their operating conditions and the safety-related control (logic system) need to be specified (e.g. explosion protection). The device manufacturers need to supply their reliability data (HFT, SFF and failure rates) and the documentation for their devices. As soon as this data is complete, the interval for the recurring test can be determined. If necessary, alternative devices or redundant structures need be selected in order to improve the SIL or extend the test intervals. Test intervals for subsystems can be compared and used to determine a standard maintenance date.

weyer special: SIL - Regulations for safety systems in the process industry

For the professional implementation of safety instrumented systems (SIS) in the process industry, proof of compliance with the required safety integrity level (SIL) must be documented as part of the safety life cycle. During hazard analysis and risk assessment, safety instrumented functions (SIF) are determined and evaluated with a required SIL. The achievable SIL can be determined for each safety function performed with a safety system for systematic and random failures.

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