Valve Magazine - Summer 2011

It’s easier to express probability in terms of failure, rather than in terms of proper performance. As published in the aforementioned standards and some product brochures, four levels of SIL are listed, enumerated 1-4—the higher the SIL level, the higher the associated safety level, and the lower probability that a system will fail to perform properly (Figure 1).

These various SIL levels might be correlated to the above mentioned examples of consequences of failure.

For purposes of this article, it is appropriate to think of SIL as “the degree of likelihood that our system will work when we want it to.” (Generally, SIL 4 is beyond the scope of what we see in the process industries.) Again, we might want to think of a “function” as an emergency shutdown valve system, typically consisting of a sensor of some type (pressure, level, temperature) and a logic controller that will send a signal to an automated valve. The automated

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Figure 3. PFDAVG is decreased due to PST combined with full cycle testing

valve package might consist of an actuator (pneumatic, electric, hydraulic, etc.), solenoid valves, quick exhaust

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valves and the final control element, the valve. The “system” may consist of many functions; it might have five emergency shutdown valves protecting a pressure vessel cooking a process. Or, it might have only one function (the SIF) making the entire SIS.

To summarize, the HAZOP team will determine SIL levels based on determined PFD. Michael Young of General Monitors has summed this issue up nicely in his paper, SIL 101: How Safe Do I Need to Be?

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A simple example will help illustrate the concepts of SIS, SIF, and SIL. Consider the installation of a pressure vessel containing flammable liquid. It is maintained at a design operating pressure by the Basic Process Control System (BPCS). If the process control system fails, the vessel will be subjected to an over-pressure condition that could result in a vessel failure, release of the flammable contents and even fire or explosion. If the risk in this scenario is deemed to be intolerable by the facility owner, an SIS will be implemented to further reduce this risk situation to a tolerable risk level.