The price vs capability matrix for SIS logic solvers

For situations with higher point counts than a single loop logic solver can handle, the SLA Multiloop Logic Solver and Alarm may be a better fit for many SIFs.

The pricing and capabilities for different types of logic solvers for Safety Instrumented Systems (SIS) can vary widely. This not only pertains to the initial expense of purchase, but also for the implementation of and lifecycle costs in maintenance and programming. The most common determining factors in choosing which logic solver best fits a safety application are loop and point counts, communication requirements, and the complexity of logic required for safety mitigation.

When dealing with high-density point and loop counts in a centralised location where there is interdependency on various points and loops, a larger more complex logic solver such as a safety PLC may be the best choice. Larger systems like these more easily afford themselves to more complex control logic and voting schemes with high reliability across a multitude of loops with several points. If points are not centralised, many of these larger safety systems can be networked together for flexibility in cross-communication, as well as for future expansion. The tradeoff is that the initial costs of purchase, programming, and installation for a larger safety system or PLC can be very expensive — tens of thousand of dollars and much more, not to mention the high lifecycle costs. Most larger safety systems often require programming modifications by a highly trained and skilled safety system programmer.

For scenarios where only one or a few points need to be monitored in a single Safety Instrumented Function (SIF), a much less expensive and complex single loop logic solver can be a very effective choice. An example of this type of application would be simple on/off functionality for pump/valve control when filling, emptying, or preventing overflow in a container or tank. Single loop logic solvers — also known as alarm trips — have made significant strides in their capabilities since they were first installed in SIS applications. These advanced capabilities include programmable inputs, local configuration using onboard controls, safe password protection, process display and comprehensive internal, input and sensor diagnostics. Single loop logic solver output relays can also be wired in series to provide voting architectures, but this requires extra field wiring and comprehensive wiring schematics. And while that may not be an overly complex task, if the voting logic changes, meticulous rewiring of relays and rewriting of proof test procedures will follow. Standalone single loop logic solvers are of course less expensive and easy to program but are quite limited in their ability to handle multiple loops, accept multiple inputs, perform logic or internal voting, or provide digital communication with a process control system or other host system.

SLA Multiloop Logic Solver and Alarm. For a larger image click here.

In the wide space that exists for applications that require higher density loop/point counts with more advanced logic and loop monitoring than a single loop logic solver can handle, the SLA Multiloop Logic Solver and Alarm may be a more effective and better-sized fit for many SIFs. Like the larger safety systems and PLCs, the standalone SLA includes 17 I/O points, handles one to three SIF loops, performs logic and math equations, and offers significant flexibility for voting architectures at a fraction of the cost and complexity of larger safety systems and PLCs. The SLA harnesses the cost and configuration simplicity of single loop logic solvers, but also offers much of the advanced functionality of the larger safety systems and PLCs, albeit at smaller loop and point counts. The SLA can potentially meet many of the SIS applications that once required safety PLCs, but also offer an advancement in functionality and capability at an attractive price point.

Image credit: iStock.com/Weerapong Khodsom