Spurious Trip Level - STL Vs SIL

STL Vs SIL

Today standards only define the safety integrity level (SIL) for safety functions. Standards do not define STL levels because they do in first instance not represent safety but economic loss. Despite this the STL is also a safety attribute, specially for safety functions in the process, oil & gas, chemical and nuclear industry. In those industries an undesired shutdown of the process leads to dangerous situation as the plant needs to be started up again. Startup and shutdown of a process plant are considered the two most dangerous operational modes of the plant and should be limited to the absolute minimum.

In practice the STL and SIL concepts complement each other. Both factors are attributes of the same safety function. The STL level is determined by the average PFS value of the safety function. The SIL level is determined by the average probability of failure on demand. PFD value of the safety function. The STL level expresses the probability of spurious trips by the safety function, i.e., the safety function is executed without a demand from the process. The SIL level expresses the probability that the safety function does not work upon demand from the process. Both parameters are important to end-users in order to achieve safety and asset protection.

In order to calculate the PFS or PFD value of a safety loop it is necessary to have a reliability model and reliability data for each component in the safety loop. The best reliability model to use is a Markov model (see Andrey Markov). Typical data required are:

• Lambda safe detected
• Lambda safe undetected
• Lambda dangerous detected
• Lambda dangerous undetected
• Repair rate
• Proof test coverage
• Proof test interval
• Common cause factors