Analyzing the 10 Truths of Safety Instrumented Systems - Emerson Automation Experts

Analyzing the 10 Truths of Safety Instrumented Systems

After reading about the “10 Truths of Safety Instrumented Systems” in a email (Control Magazine’s online website), and getting a copy, I’d spoken to our of our safety experts, Andrew Dennant, and was working up an analysis of the 10 truths…

…and then I read the comments of Nova Chemical and ISA Safety Division Newsletter Editor/Webmaster Brian T. Smith on Walt Boyes’ SOUND OFF!!! blog.

Andrew and I will add just one point to the discussion underway.

When thinking about availability, consider the entire safety loop, not just the logic solvers, since the majority of failures occur outside the logic solver. An analysis of the data sources like the Offshore Reliability Database (OREDA) and exida’s Safety Equipment Reliability Handbook shows that up to 92% of hardware failures happen in the field and only 8% happen in the logic solver. Studies by the Health and Safety Executive in the UK show that less than 15% of all failures are hardware-related. Doing the math, 15% x 8% = 1.2%, puts us in the range of failures that are caused by the logic solver.

The key to high process availability is having a smart SIS which diagnoses the complete safety function including the logic solver, sensor, and final control element, correctly engineered in accordance with globally-agreed best practice safety standards, aka IEC 61511.

UPDATE: The figures cited in the Health and Safety Executive study are from the publication, Out of Control: Why control systems go wrong and how to prevent failure (2nd edition), orderable from the HSE site.

Posted Thursday, May 11th, 2006 under Safety.


  1. Hi Jim,
    What is yuour opinion about use of electric actuators vs pneumatic actuators in a safety system?
    What actuator do you prefer to use?

  2. Jorge, Thanks for your question. I asked a couple of our safety experts and received this reply from Riyaz Ali:

    Use of Pneumatic vs Electrical actuators is application specific, site specific and cost specific decision. Few off shore sites may not have compressed air and would like to use electrical or hydraulic actuators. Once again selection is dictated by various factors like PST (Process Safety Time), Reliability, Type of Process application, Maintenance schedule and others. Few applications are such that output from controller is electrical signal and may be economical to use directly an electrical actuator.

    Generally Pneumatic spring return actuators are simpler in maintenance and operational and variable cost of air is cheaper than electricity. Also pneumatic actuators are easy to understand and have relatively lesser failures compare to other kind of actuators (I do not have figures to substantiate – this is general thinking among industry). Electrical actuators may have failures related to short circuit, open circuit, lightening, surge in electrical power etc. Though, pneumatic actuators may have air related failures but it is relatively less penalizing.

    Overall, if I were to select actuator, my first preference will be to use pneumatic actuators until circumstances or conditions forces to use electrical actuators.

  3. hi sir my name is A.Haribabu and i am from india.sir i want some material related SIS(safety instrumented systems) if it is possible please send me sir.

  4. Hi A. Haribabu. Thanks for your comment. Let me suggest a couple of resources. For background of Emerson’s smart SIS, visit:
    To receive a package of information, visit the SIS order page at:
    Take it easy, Jim

  5. Sudeengg1 says:

    SUDE team is well aware of the latest developments in modernization and automation with stress on economy, efficiency and quality. The products are Pneumatic Actuators etc.


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