SIS Component Selection – Safety Instrumented System

Safety Instrumented System (SIS) component selection is to be done to suit the plant safety requirements and must follow all the specifications as per the below International standards.

SIS Component Selection

SIS components selection has to be done rigorously with experienced, qualified design Engineers under the supervision of Safety experts.

  • Verify SIS component suitability for the operating environment.
  • All the device subsystems shall meet the fault tolerance requirements.
  • Overall SIS should achieve the target probability of failure on demand (PFD) for a demand mode system and meet the SIL.
  • Use the measurement instruments selection tool and identify the proper instrument for a given service.
  • Each plant owner and their approved Engineering/Technology provider shall develop best practices for instrumentation selection.
  • Experts shall be used to select measurement principles and perform proper instrument selection and instrument design work.
Safety Instrumented System SIS Component Selection
Image Courtesy: Emerson

Safety Instrumented System General Requirements

Components selected for use in an SIS must either be certified for use in SIS applications or selected based upon prior user experience. Of course, the components and the overall design must meet the requirements for the SIF as defined in Safety Requirements Specification.

Prior use is based on the installation of similar instruments under an identical environment in multiple facilities and the instruments are proved for use for several years.

Whether to go for Certified Instruments or not?

  • Certified instruments for SIL applications are not a must. However, certified instruments can be specified if
    • Additional instrument cost with SIL certification does not exceed the cost of the proof test effort.
    • New Technology or new model
  • Certificates can be issued by
    • Commercial Organizations (Exida/ TUV)

Find below the pressure transmitter (Emerson) used in SIS approved with safety agency – Exida.

Advantages of Certified Instruments

  • Data provided for
    • Failure Rates
    • Safe Failure Fraction
    • Diagnostic Coverage
  • The supplier takes control of software and hardware modifications
  • Safety manual provided

Certification Flaws

  • Some certificates issued by lesser reputable institutes are based on hardly anything more than manufacturers’ claims and a simple failure mode analysis. 
  • Some instrument manufacturers are busy having their products certified to be in accordance with IEC 61508 in urgency without completing the complete test.

Prior use Instruments

  • Definition
    • when a documented assessment has shown that there is appropriate evidence, based on the previous use of the component, that the component is suitable for use in a safety instrumented system
  • The standards and tools implemented by various manufacturers enable to provide appropriate evidence that the sensors and final elements are suitable for use in the safety instrumented system based on prior use.


End users to select the SIS instruments with careful evaluation of certifications or the Prior use category of those instruments that are well proven in many plants.

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