Centralized, Distributed, and Hybrid Safety Instrumented Systems

Emerson’s Mike Boudreaux alerted me to a great article comparing safety instrumented system architectures. The ISA InTech article, Centralized or distributed process safety compares and contrasts centralized, distributed, and hybrid safety systems. Centralized systems have a single logic solver with all safety I/O routed and hardwired back to it. Distributed systems locate the logic controllers by the safety instrumented functions, such as on a compressor skid. Hybrid systems, “…combine many of the advantages of centralized and distributed safety…”

The author discussed the strengths and issues of each approach. For centralized systems, strengths noted are:

  • Only one system to learn and support
  • Common operator interface
  • No integration required
  • Single point of supplier responsibility

Issues highlighted with centralized systems:

  • Significant wiring requirements for field I/O
  • Higher installed cost
  • Slower speed of execution
  • Failure of central controller brings down entire plant safety system
  • Programming can be very complex

Strengths noted for distributed systems:

  • Independent operation and shutdown
  • Easier to modify and upgrade without disturbing other functions
  • Easier to install and start up for retrofits and expansions
  • Reduced wiring
  • Simpler to design, programs install, and maintain
  • Improved overall speed of execution
  • Easier to provide local operator interfaces and local control
  • Typically less expensive

Issues enumerated for distributed systems:

  • Multiple systems to learn and support
  • Integration among systems required
  • No common engineering interface
  • No single point of overall supplier responsibility

The author noted that hybrid systems address many of the issues, but may be more complex to engineer and install.

In a prior post, The Question of Common, Integrated, and Interfaced Safety Instrumented Systems, Mike highlighted how this issue of complexity was addressed with DeltaV SIS:

…the logic solver components have physical separation, diverse components, and independent SIS logic solver hardware with a different operating system from the DeltaV BPCS [basic process control system]. DeltaV SIS safety communications are on a physically separate bus and network from the DeltaV BPCS communications.

The physical separation, diversity, independence, and common cause elimination required by IEC 61511-1 clauses 9.5 and 11.2.4 are inherently addressed by the DeltaV SIS integrated yet separate architecture. The integration of DeltaV and DeltaV SIS is at the operation, engineering, and maintenance layer where it makes sense from the advantages described in the article [The Question of Integration].

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