10 Truths of Safety Instrumented Systems - Truths 7-10
By Control Engineering Staff -- Control Engineering, 9/19/2007
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10 Truths of Safety Instrumented Systems |
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In continuation of the 10 Truths of SIS. This newsletter we address Truths 7-10 Follows is a summary of each Truth, click on a topic for more information or go to www.triconex.com/10truthsto view all Truths of SIS. Truth 7 Dual SIS Technologies do not cost less than TMR; they almost always cost more. Many companies advertise their Dual SIS technology as a lower-cost alternative to Triple Modular Redundant options that offer equivalent performance. This is an unfortunate misrepresentation of the capabilities of Dual SIS architectures. Dual PLCs in a 1oo2 (1 out of 2) configuration were the initial solution of choice for "fail safe" applications, but they cannot overcome an inherent problem with false trips. Truth 8 SIS vendors advertise their TÜV Certification, but rarely tell you about their implementation and operational restrictions. Most safety system vendors focus on how the system performs when it is healthy, but don't talk much about what happens when an internal failure is diagnosed; worst case, the entire system shuts down. Each SIS vendor must provide clear information on factors that might impair system performance, such as the system's implementation, specific programming or configuration requirements, module or architecture choices, and operational restrictions. Truth 9 Given a choice, the implementation and installation of your SIS should not be entrusted to strangers. Choosing an SIS implementor can be as important as choosing the product itself. No matter how well the system is designed or manufactured, failures are likely to occur if the implementation team is not following proper procedures, is not experienced, or lacks adequate technical qualification for the tasks they must perform. Truth 10 Maintaining a SIS is more than pulling data; the system also requires context to make informed decisions. SIS vendors should provide diagnostic tools with clear guidelines or recommendations for maintaining the system to maximize uptime in the context of plant operations.
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