Features
Selecting Instrumentation Equipment for Safety Applications
The end user must carefully choose all instrumentation equipment used in Safety Instrumented Systems (SIS) applications. All such equipment must be carefully justified. The justification must include sufficient information such that the end user is totally confident that the instrumentation will properly perform in the intended application. The instrumentation must be fully capable of performing the functional requirement. The materials used in the instrument must be compatible with process materials if the instrumentation sees wetted service. Process environmental conditions must not exceed the instrumentation ratings. The functional safety of the instrument must be assessed. All justification decisions must be documented as part of project records.
Functional Safety Assessment
IEC 61511, Functional Safety for the Process Industries, requires that equipment used in safety instrumented systems be chosen based on either IEC 61508 certification to the appropriate SIL level or justification based on “prior use” criteria (IEC 61511, Part 1, Section 11.5.3). Unfortunately the IEC 61511 standard does not give specific details as to what the criteria for “prior use” really means. Most agree however that if a user company has many years of documented successful experience (no dangerous failures) with a particular version of a particular instrument this can provide justification for using that instrument even if it is not safety certified. Most agree that prior use requires that a comprehensive system be in place to record all field failures and failure modes at each end user site. Version records of the instrument hardware and software must be kept as significant design changes may void prior use experience. Operating conditions must be recorded and must be similar to the proposed safety application. Of course the problem with the prior use approach is that many process sites do not have that level of record keeping in place.
Many users have asked manufacturers to help with their justification. Different levels of assessment have been done by third party assessors for instrumentation manufacturers. This work can help reduce the burden of documentation when an end user attempts to justify an instrument for use on safety applications. In the marketplace four levels of assessment have been done on instrumentation products:
1. FMEDA of the hardware according to IEC 61508
A hardware analysis called a failure modes effects and diagnostics analysis (FMEDA) is done to determine the failure rates and failure modes of an instrument (Gob01). A FMEDA is a systematic detailed procedure that is an extension of the classic FMEA procedure developed and proven decades ago.