Comparing conventional and sustainable safety instrumented systems

A sustainable safety instrumented system (SIS) is more open than a conventional SIS and are more accessible to process plant personnel while providing opportunities for continuous feedback and improvement.

By Hidehito Shiratsu, Yokogawa Electric Corp. July 10, 2018

A conventional process plant safety instrumented system (SIS) is often comprised of a set of software black boxes, accessible only by a very limited number of highly technical and specialized personnel. A sustainable SIS with embedded functional safety management (FSM) and safety performance monitoring is just as capable for executing plant safety strategies, but it is much easier to work with and understand by a wider range of plant personnel, including control room operators.

This leads to overall process safety improvements and maintains the safety integrity of the SIS throughout the plant’s lifecycle. This is done by automatically capturing process failures and demands and analyzing them against the safety performance indicators (SPIs) as shown in Figure 1, which indicates safety performance continuously through the plant’s lifecycle.

Instrumentation and safety component vendors, recognizing the limitations of a conventional SIS, use various methods for automating data gathering and analysis functions to create a sustainable SIS. 

Sustainable SIS elements

A sustainable SIS provides a holistic approach and allows end users to retake ownership of the process safety environment by making it comprehensible, manageable, compliant, and secure-thereby allowing the process plant to focus on its core business. A sustainable SIS helps achieve optimum plant safety and provides peace of mind during the project realization and operational phases. A sustainable SIS consists of several elements designed to improve safety on the plant floor including a safety application securing solution, safety performance monitoring solution, and safety logic solver (Figure 2).

  • Safety application securing solution: This allows the safety application to be easily maintained at the required level throughout the plant lifecycle in accordance with functional safety standards International Electrotechnical Commission (IEC) 61508: Functional Safety and IEC 61511: Safety instrumented systems for the process industry sector.
  • Safety performance monitoring solution: This provides SPIs for a SIS and other independent protection layers connected to the distributed control system (DCS). It also provides compliance with current safety standards, which periodically require an assessment of the actual safety performance of a process against the designed performance target, and verification of plant safety when a safety function is bypassed.
  • Safety logic solver: This detects hazards in safety equipment. For example, TÜV Rheinland for can certify some safety logic solvers up to safety integrity level (SIL) 3 safety functions in accordance with IEC 61508, and by Exida for ISASecure EDSA Level 1 cybersecurity. 

Conventional SIS issues for plant personnel

A conventional SIS requires plant personnel to perform laborious work processes to maintain safety integrity throughout the plant lifecycle. Other challenges and issues with conventional SIS include:

  • Plant maintenance personnel face barriers to understand the SIS due to the low visibility of implementation.
  • There may be gaps between the safety requirement specification and implemented logic, creating challenges when analyzing performance of safety systems during the plant operational phase.
  • Extensive training is required to understand how a conventional SIS operates.
  • A lack of transparency makes it difficult for a wider range of personnel to understand the SIS design objectives and to efficiently recover from incidents. Manually generating and analyzing safety status reports is a time-consuming task as the interpretation of raw safety data is labor intensive.
  • Safety status reports from a conventional SIS records past events and alarms in a format that isn’t user friendly or formatted for easy interpretation.
  • The quantity of data to be analyzed increases with time, and in cases where the safety data has to be stored throughout the plant’s lifecycle for auditing purposes, data management and storage become a major challenge.

Managing a conventional SIS is cumbersome. Plant maintenance personnel sometimes find a conventional SIS does not reflect the SIS’ present status accurately due to documenting changes improperly and/or incomplete handover procedures. Documenting changes and complying with FSM is often a manual process, which makes it difficult to chronologically trace and compile modifications. These manual processes to handle modifications and manage FSM compliance with a conventional SIS are not sustainable for complex environments.

Advantages of a sustainable SIS for plant personnel

A sustainable SIS provides a better process by addressing the issues found when using conventional SIS (Figure 3). A sustainable SIS provides automation to manage safety applications and process data.

A sustainable SIS provides more accessible and user-friendly interfaces that can be understood, configured, and managed by a wider group of personnel—thereby reducing dependency on a few well-trained and experienced engineers. The key features are the improved visualization of the process behavior in the event of a fault (Figure 4), automated trip analysis, and risk mitigation actions. The sustainable SIS’ offline simulation feature enables functional checking of the safety functions prior to deployment, empowering SIS operators [or designers] to verify designs, and assess the consequences arising from overriding safety instrumented functions (SIFs).

A sustainable SIS automatically collects safety statistics for SIFs to facilitate improvement, and automatically captures and records the required evidence of safety performance and SIFs availability for audit by regulatory authorities. The sustainable SIS, enabling continuous enhancement of the plant’s safety features by dynamically optimizing the SIFs, analyzes the differences between the expected and measured SPIs.

A digitized database management system ensures consistent updates that are documented and synchronized. This form of automated change management helps to uphold the overall consistency of safety system information and allows smooth project execution. Easy access to historical safety records simplifies the regulatory auditing process. Being able to track, identify, and restrict unauthorized changes on the SIS is critical to mitigate cybersecurity threats. 

Improving plant safety with a sustainable SIS

A sustainable SIS represents process safety functions in the form of design documents, cause and effect matrices, and state/transition diagrams. It makes functionality easy to understand by all departments so operators, maintenance, and process engineers can assist application engineers by interactively supporting design and problem solving.

Design documents can be dynamically simulated, allowing designs and modifications to be extensively tested with offline simulation before deployment. In addition, a sustainable SIS helps to assess the impact of applying safety function bypasses before being implemented, including any impact they may have on other equipment. A sustainable SIS improves visibility of potential unsafe situations and increases safety compliance by aiding policy enforcement and traceability.

A sustainable SIS automatically collects safety statistics for SIF improvements, and records evidence of safety performance and SIF availability for auditing by regulatory authorities. The designed safety performance is compared against the actual operational safety performance to highlight issues, validate safety design, optimize test scheduling, and help users improve plant safety and availability.

All SIS information is recorded in the sustainable SIS database, making it is easy to recover historical activities relating to engineering and changes that have been implemented. The latest design documents can be automatically generated at any time to ensure there is no inconsistency with the application being implemented. Modifications can be planned on a design document basis without any apprehension.

A sustainable SIS simplifies the design, operation, and maintenance of process plant safety systems. This concept and its related software elements can be applied to existing and new designs.

Some existing process plants may have staff available to implement a sustainable SIS, while others may require assistance from initial design to implementation, including ongoing support throughout the life of the sustainable SIS.

Hidehito Shiratsu is an ICSS marketing specialist at Yokogawa Electric Corporation. Edited by Emily Guenther, associate content manager, Control Engineering, CFE Media,


KEYWORDS: safety instrumented system (SIS), safety instrumented functions (SIFs)

  • Sustainable SIS benefits
  • Conventional SIS issues
  • How plant facilities can improve processes with a sustainable SIS.

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How can a sustainable SIS improve plant safety? 

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Hidehito Shiratsu, ICSS marketing specialist at Yokogawa Electric Corporation, is responsible for the company’s safety control system business and product planning. Prior to his current position, he was in charge of programmable logic controller (PLC) and real-time operating system (RTOS) controller planning and development. Shiratsu holds a Bachelor of Electronic Engineering degree.