10 Truths of Safety Instrumented Systems – Truths 7-10

By Control Engineering Staff September 19, 2007
10 Truths of Safety
Instrumented Systems

September 19, 2007

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/10truths to 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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