Performing a SIL Verification calculation on preliminary design information can be a very useful tool to dial in the Safety Instrumented Function (SIF), Safety Requirements Specification (SRS) content , and define critical elements for the field component purchase specifications. Conceptual verification is typically performed quickly with general design information. The objective is to gain confidence that the planned design will meet SIL target after detailed specification and procurement. Generalized suppositions are adequate for screening in many situations; however, there are circumstances where preliminary assumptions are not “close enough” to avoid last minute rework in order to achieve final verification. A savvy engineer learns how to adjust assumption to evade the “not so much” moments.

Conceptual verification assumptions often include statement like: “We use ball valves in most applications so assume generic ball for all valve final elements.” The verifier will add to assumption basis by anticipating air actuation and assuming a type of seat. In most cases, a global assumption provides adequate conservatism. The trick is to understand where exception assumptions are better predictors of final design conditions. The following table is a simple comparison of SIF risk reduction factors (RRF) for typical 1oo1 and 1oo2 architecture, comparing generic valve options (data from exSILentia® SERH database). 

The comparison exercise demonstrates that a ball valve is indeed a conservative option for the screening verification exercise in most cases. The assumption of hard or soft seat is of minor significance in the preliminary verification.  Data also demonstrates that generic hydraulic and air actuated ball valves share similar performance characteristics. Two situations present as non-conservative: butterfly valves and motor actuation.

In a SIL 1 situation, butterfly valve performance is not significantly different than a ball valve, and conceptual assumptions can be adequate to assure equipment procured will be serviceable with only minor adjustments to proof testing procedure or frequency, or mission time. However in a SIL 2 application, a butterfly valve failure characteristics lead to a significantly lower PFDavg and fails to meet target.  The valves have a comparable safe undetected failure rate, but the butterfly valve has a much higher dangerous undetected failure rate.   Butterfly valve is commonly used for economic reasons in 6” or higher line sizes. The valve body selection is not significant in SIL 1 service however SIL 2 and higher service utilizing butterfly valves can be problematic.

Process conditions sometimes dictate use of a motor operated valve. Safe undetected failures are somewhat higher, and dangerous undetected failures are nearly 3X higher than an air actuated ball valve. In addition, electrical faults are not easily predicted so proof test coverage is lower than for other valves. Motor operated valves can be marginal in SIL 1 service, and can be near impossible in higher SIL services.

Another consideration is the process needs of the SIF application. Special circumstances such as high temperature, high pressure, tight closure, severe service and line size can constrain valve selection. In some applications, certified valves are not available. Economics are always of concern in making the final valve selection. To make the most of the conceptual verification, screen SIFs for special circumstances where general assumptions may not be applicable, and invest additional time in refining those individually.