Safety: The Lifecycle Approach

Feb. 12, 2014
Production safety is generally thought of as a series of steps necessary to ensure safe interaction with industrial equipment.

Production safety is generally thought of as a series of steps necessary to ensure safe interaction with industrial equipment. The process of identifying, agreeing upon and delineating those steps is where things tend to get complicated. That’s why international standards groups play such a significant role, as they set the guidelines for all of industry to follow.

For the process industries, IEC 61511 is probably the most widely used safety standard, as it applies to those industries that base their safety systems upon instrumentation. The goal of safety-system design in IEC 61511 is for the process, whatever it may be, to go to a safe state whenever a process parameter exceeds preset limits.

A New Way of Approaching Safety

Understanding IEC 61511 means that you must know a thing or two about IEC 61508—a functional safety standard that provides the framework for building industry-specific functional standards. IEC 61511 was created from the guidelines established by IEC 61508.

The key point to understand about IEC 61508 is that it is designed to establish an engineering discipline that will generate safer designs and build safer processes. The uniform procedures built on these disciplines are contingent upon appropriate experts within a company contributing to projects. In addition, the standard also makes it easy for outside auditors and governmental agencies to follow the process.

IEC 61508 can seem confusing at first, because its underlying philosophy is new for safety standards. Older, more conventional safety standards stipulated specific rules and specifications for making processes safe. IEC 61508 and its derivative standards, such as IEC 61511, departed from this approach by being more functional, or performance-based.

A principal aspect of this new approach to safety standards is that it leverages two fundamental principles: safety lifecycles and probabilistic failure analysis. Unlike previous standards that claimed to cover the entire lifecycle of a project, IEC 61508 and its offshoots actually do—from project conception to maintenance to decommissioning.

In essence, the standards specify safety lifecycle activities that need to be followed over the entire life of a production system. Safety lifecycle management provides a method or procedure that enables companies to specify, design, implement and maintain safety systems to achieve overall safety in a documented and verified manner.

Four Phases of the Safety Lifecycle

The IEC 61511 standard promulgated by the International Electrotechnical Commission specifies 12 steps in the safety lifecycle. These are segmented into four phases: analysis, realization, maintenance and ongoing functions.

Safety Lifecycle I: Analysis Phase

The analysis phase includes the initial planning, identification and specification of safety functions required for the safe operation of a manufacturing process.

Specific activities include:

  • Perform hazard and risk analysis: Determine hazards and hazardous events, the sequence of events leading to hazardous conditions, the associated process risks, the requirements of risk reduction and the safety functions required.
  • Allocate safety functions to protection layers: Check the available layers of protection. Allocate safety functions to protection layers and safety systems.
  • Specify requirements for safety system: If tolerable risk is still out of limit, then specify the requirements for each safety system and its safety integrity levels.

Safety Lifecycle II: Realization Phase

The realization phase not only includes design, installation and testing of safety systems, but also the design, development and installation of other effective risk reduction methods.

Specific activities include:

  • Design and engineer a safety system: Design system to meet the safety requirements.
  • Design and develop other means of risk reduction: Means of protection other than programmable safety systems include mechanical systems, process control systems and manual systems.
  • Install, commission and validate the safety protections: Install and validate that the safety system meets the all safety requirements to the required safety integrity levels.

Safety Lifecycle III: Maintenance Phase

The maintenance phase begins at the startup of a process and continues until the safety system is decommissioned or redeployed.

Specific activities include:

  • Operate and maintain: Ensure that the safety system functions are maintained during operation and maintenance.
  • Modify and update: Make corrections, enhancements and adaptations to the safety system to ensure that the safety requirements are maintained.
  • Decommissioning: Conduct review and obtain required authorization before decommissioning a safety system. Ensure that the required safety functions remain operational during decommissioning.

Safety Lifecycle IV: Ongoing Functions

Certain functions are ongoing. Examples include managing functional safety, planning and structuring the safety lifecycle, and performing periodic safety system verification and safety audits over the whole lifecycle.

Specific activities include:

  • Manage functional safety, safety assessment, and safety audit: Identify the management activities that are required to ensure that the functional safety objectives are met.
  • Plan and structure safety lifecycle: Define safety lifecycle in terms of inputs, outputs and verification activities.
  • Verify safety system: Demonstrate by review, analysis and/or testing that the required outputs satisfy the defined requirements for each phase of the safety lifecycle.

Activities for Phases I to III are typically carried out consecutively, while Phase IV runs concurrently with the other phases. However, like all models, the safety lifecycle is an approximation.

Bottom Line: A Requirements Definition
Readers should note that the standards define requirements for safety management, rather than system development. Not all safety lifecycle phases will be relevant to every application; management must define which requirements are applicable in each case. The standards do not prescribe exactly what should be done in any particular case, but guide management toward decisions and offer advice.

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