SIS logic solvers: more choices are needed
By Bob Myles, Director of Engineering, Moore Industries International, Inc.
Friday, 21 July, 2023
Industrial processing and manufacturing industries are not new to the concept of safety processes, methods and designs. In fact, since the industrial revolution, layers of security have been developed to safeguard workers in factories and mining regions. Over several decades, more effective safety regulations emerged, and the accessibility of safety-related equipment significantly increased. IEC 61511 Functional safety — Safety instrumented systems for the process industry sector appears to have recently gained acceptance across all significant manufacturing and processing sectors.
At the heart of the IEC 61511 standard is the SIS, or Safety Instrumented System, which is implemented to mitigate and prevent unacceptable risk by an organisation to protect its personnel, facility and surrounding community and environment. Each SIS is made up of one or more SIFs, or Safety Instrumented Functions, that bring a process or loop to a desired safe state.
The logic solver is frequently the most complex of the three primary components included in a SIF. It is responsible for determining whether dangerous conditions have been met and is responsible for the final element’s ultimate effect on the mitigation function or strategy. Historically, a large gap has existed between simple and complex logic solvers, based on point/loop count, complexity requirements and cost.
Traditionally, low point-count logic solver requirements were handled with single loop logic solvers, for example standalone alarm trips, while more complex applications and larger point systems were tackled with larger and costly safety PLCs. Most safety applications can be handled by safety PLCs; however, they are frequently overkill for safety practitioners implementing smaller or more dispersed SIFs, while trip amplifiers and other simple logic solvers struggle to accommodate voting requirements or multiple-loop safety functions in a single device.
Cost of operations and return on investment remain crucial factors for many facilities. When tens or hundreds of thousands of dollars are needed to establish a safety layer, hesitation frequently arises. With safety standards maturing and products now being supplied with safety certifications, the ‘cost reluctance’ has begun to dissipate a bit, but progress needs to continue to remove all barriers from decisions that protect people and the environment.
Innovative steps need to accelerate, with vendors offering safety-related instrumentation useable in SIFs. Finding solutions to narrow the capability-versus-cost gap with logic solvers would be a good start. To fill this void, the market needs less expensive, less complex, multipoint, voting-capable and IEC 61508-certified logic solvers that allow safety practitioners an option that meets the functionality below that of the safety PLC but above the capabilities of the simple single loop logic solver.
The fundamental requirements of a Safety Instrumented System will ultimately determine what type of SIS logic solver best fits its needs. However, with a limited number of logic solver solutions available, safety practitioners may be budgeted out of adding critical layers of safety protection due to the cost/functionality gap. Offering more IEC 61508-certified logic solver alternatives can only help accelerate the adherence and adoption of IEC 61511 in our mining and process facilities.
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