This is an excerpt from the May/June 2013 InTech by Henry Johnston and Fahad Howimil.  To read the full article, please see the link at the bottom of this post.

International standards for safety instrumented systems (SIS) have had a profound influence on the analysis and design of these protection systems. The old prescriptive or recipe type was changed to a performance approach that designers must satisfy. The first stages of the safety life cycle (SLC) are now well known by a majority of designers and engineers involved in SIS; however, such grade of understanding and influence has not been widely accomplished at the final stages of the SLC as are the operation and maintenance (O&M).

O&M involvement in the engineering of SIS is normally passive, participating in specific analysis when requested. Such approach leaves almost the complete engineering of the protection system under project designer “responsibility.” An early involvement with a proactive approach to complement the designer experience with reliability and maintainability vision is necessary to balance the design and to manage the SIS.

The following are some of the actions for founding the basis for SIS management in the O&M phase:

Establish clear objectives

The most important factor is to establish clear objectives or goals. The standard, IEC-61511, section 16-2-2, helps us with the first objective, which is obviously safety-related: “Maintain the as-designed functional safety of the SIS.” The second objective in many companies is economical: “Maintain the SIS efficiently.”

But what do “as-designed” and “efficiently” mean regarding safety? How are these objectives understood by the organization? The “maintain as designed” goal means to achieve during operation a safety instrumented functions (SIF) probability of failure on demand average (PFD_avg) lower than required in the SIL target during the analysis phase of the SLC. The “maintain SIS efficiently” means to reevaluate tasks, methods, and frequencies to intervene where and when necessary. Once the company agrees on the objectives, resources and action will depend on them.

To read the full article on safety instrumented systems, click here.

About the Authors
Henry Johnston is a principal specialist in functional safety in aeSolutions. He has more than 25 years of experience in engineering, maintenance, and reliability analysis of instrumentation, controls, and safety systems. He has worked in the development of instrument maintenance strategies for large-scale projects. Henry earned master's degree in electrical engineering from Missouri University of Science and Technology. Contact Henry at henry.johnston@aesolns.com.

Fahad Howimil is a senior project engineer. He has executed projects in the field of instrumentation and process control and worked in engineering support for the same field. Fahad works in engineering and project management at SABIC Saudi Arabia. Contact Fahad at fahadlm@hotmail.com.