Managing and properly equipping a plant with the right components for optimal safety is a complex, ever-evolving task. Managers in the chemical processing and related industries have the enormous responsibility of increasing productivity and profit, while at the same time properly monitoring, detecting and preventing hazardous events from occurring.
Ultimately, any technologies employed for these functions should improve the plant's overall design scheme, increase efficiency and minimize the associated costs to maintain the safety systems. However, plant managers are forced to contend with limited and complicated information.
They are not alone. Most industries employing a safety system face the same challenge. In response to this, the International Electrotechnical Commission (IEC) released safety standard IEC 61508 to help ensure the highest safety for electrical/electronic/programmable electronic-based safety-related systems.
Unlike traditional product certifications that certify only the final test data, IEC 61508 is process based and applies to the entire life cycle of the product. The manufacturing company and all of the processes used to develop and manufacture its product are certified for optimal safety. Therefore, all phases of a safety product are certified ," from its earliest concept and design stages through its manufacturing, application, maintenance and final decommissioning.
Although virtually anyone directly or indirectly related to plant safety has at least heard of the standard, very few really understand its impact and benefits. The standard has been well received by manufacturers of safety components and complete systems, but the facilities using these IEC 61508-certified products benefit the most.
IEC 61508 is process based and deals with all of the necessary activities involved in the implementation of safety-related systems.
In addition to the potential for loss of life or limb, one plant shutdown can result in costly repairs and equipment replacements, lost time and production and other added expenses just to mitigate the situation and get the plant back up and running again. By employing IEC 61508-certified components, plants decrease the likelihood such a hazardous situation will occur.
But do not think of IEC 61508 as simply insurance for a hazardous event. Facilitating the use of a "building block" approach using certified components with recognized safety characteristics such as IEC 61508 reduces long-term operational and capital costs. The need for consultants for system verification is decreased or eliminated, and procurement costs can be reduced through avoidance of re-assessment on a product-by-product basis. Plant users can be confident they are employing first-rate technologies ," without having to compromise safety.
In 1998, the Geneva-based IEC began drafting standard IEC 61508 to support companies that use safety instrumented systems (SIS) to protect their personnel and facilities from hazardous events. The standard, formally titled "Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related System," is composed of seven parts that direct proper management of the life cycle and all components of the SIS.
The first three parts of the standard address management, development, deployment and operation of safety system hardware and software. The remaining four parts deal specifically with definitions, applications and informative annexes to the standard.
To fully understand IEC 61508, it is first necessary to discuss two key terms: safety life cycle and safety integrity level (SIL).
Safety life cycle
Product certification deals primarily with the end product, but IEC 61508 is process based and addresses development, manufacturing, maintenance and all of the other necessary activities involved in safety-related system implementation. The standard's process starts at the concept phase of a project and finishes when all the electric, electrical and programmable electronic safety-related systems; other technology safety-related systems; and external risk-reduction facilities no longer are available for use.
In the safety life cycle of a particular process, process risks are evaluated, and the performance requirements of the SIS are established. Layers of protection are generated and fully tested for optimization. The result is a product designed to meet a particular process risk. Because systems are designed per the safety life cycle, they are more likely to meet the actual requirements of the specific application.
IEC 61508 is an umbrella standard in that it can be applied directly to any industrial process that uses electrical, electronic or programmable electronic products and systems for safety. The standard allows the development of industry-sector-specific standards, provided they follow a safety life cycle model similar to the one defined by IEC 61508. All parts of the standard set out a generic approach for all safety life cycle activities for electrical/electronic/programmable electronic components used in safety-related functions.
Safety integrity level
Four SILs are defined by IEC 61508 to statistically represent the integrity of the SIS when a process demand occurs. The SIL takes into account device integrity, architecture, diagnostics, systematic and common cause failures, testing, operation and maintenance.
An SIL establishes order-of-magnitude targets for risk reduction. This target failure measure is the intended probability of dangerous mode failures to be achieved in respect of the safety integrity requirements, specified in terms of either the average probability of failure to perform the safety function on demand (for a low demand of operation), or the probability of a dangerous failure per hour (for a high demand or continuous mode of operation). The higher the SIL number, the greater the impact of a failure and the lower the acceptable failure rate. See the table.