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For several years, a debate has been underway regarding benefits of duplex systems versus triplex systems, especially for use in process control (nonsafety) systems. Previous attempts at quantifying the differences have generally taken a narrow view by
focusing on control and safety systems as stand-alone hardware
platforms, often with overly simplified reliability models. This
fails to account for many real-world issues, including the impact
of software diagnostic coverage, field-device reliability and compensating
fallback-control strategies. This article presents a comprehensive
analysis of the benefits associated with duplex and
triplex systems, consistent with an actual plant environment.

D. PERKINS and A. SHELDON, Compressor Controls Corp., Des Moines, Iowa

For several years, a debate has been underway regarding benefits
of duplex systems versus triplex systems, especially for use
in process control (nonsafety) systems. Previous attempts at
quantifying the differences have generally taken a narrow view by
focusing on control and safety systems as stand-alone hardware
platforms, often with overly simplified reliability models. This
fails to account for many real-world issues, including the impact
of software diagnostic coverage, field-device reliability and compensating
fallback-control strategies. This article presents a comprehensive
analysis of the benefits associated with duplex and
triplex systems, consistent with an actual plant environment.

Historical perspective. The tendency to ignore other devices
in the control system can be attributed, in part, to an outdated
view of control system boundaries. At the risk of blurring the line
between a control system and a safety system, this tendency can perhaps
best be illustrated by looking at advancements in process
safety analysis.
In the early 1980s, German standard VDE0801/A11 established
boundaries for the control system as that of the programmable
logic controller (PLC). While it was a notable effort that
helped launch the functional safety movement in the process control
industry, it narrowly defined the system by focusing almost
entirely on the PLC.
Later efforts in this area defined system boundaries to include
field devices such as sensors and final control elements. The most
recent standards (including ISA S84.01,2 1996; EN61508,3 2001;
and IEC 615114, 2003) employ this newer and more appropriate
definition of system boundaries. In fact, they dictate considering
topics like overall system design, operation and maintenance in
evaluating system suitability and effectiveness. This comprehensive
approach is obviously more appropriate than one focused
solely on the controller.

Terminology and definitions.Inconsistent use of various
terms is common, and can easily lead to confusion and misunderstandings.
This section describes several terms, as they are used
in the remainder of this article, so that the intended meaning will
be clear.

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