Reliability standards require that suppliers of components that will be used as parts in a safety function or a safety instrumented system shall be documented to show full compliance with the reliability requirements. In practice, however, documentation is often severely lacking. In essence, the documentation required for a given component would include:
- A description of how the component will be used in the safety function, the SIS, and which barrier functions it will support
- A description of failure rate data and calculations to show that performance is satisfactory as measured against PFD or PFH requirements for the given SIL requirement on the function the component is a part of.
- A description of systematic capability and under which architectures the component can be used for a given SIL requirement
- A description of software assurance to satisfy relevant requirements
- A description of quality management and how the vendor works to avoid systematic failures
In many cases one or more pieces of this documentation is missing. However, the same component can be part of many deliverables; for example, a pressure transmitter may be part of various packages delivered by multiple package vendors. In some cases, these vendors deliver bits and pieces of relevant reliability documentation, that by chance would cover all of the relevant aspects. In this case, there is enough proof that the component can perform its function as part of the SIS, provided all relevant configurations are covered. In such cases, should we allow such fragmented documentation?
The principle answer would be “NO”. One reason for this is that traceability from requirement to tag number to vendor deliverable and vendor documentation will be lost. In practice, however, we are not left with much choice. If the component is acceptable to use, we should of course use it. Traceability is, however, important in reliability projects. The system integrator should thus make a summary of the documentation with pointers to where each piece of documentation is coming from. This solves the traceability problem. However, we should also take care to educate the entire value chain on the needed documentation, to ensure sufficient traceability and to allow for assurance and verification activities without resorting to hunting for bits and pieces of fragmented information about each component. We should therefore put equal weight into:
- Ensuring our components are of sufficient quality and proven reliability for use in the SIS
- Influencing our value chain to focus on continuous improvement and correct documentation in projects