News & Events.
Article Published in Chemical Processing Magazine
September 21, 2018
An article titled Proof Test Prudently written by Denise Chastain-Knight and Jim Jenkins from exida was published in the September issue of Chemical Processing.
The functional safety lifecycle covers a safety instrumented system (SIS) from concept to retirement. While important activities occur in each phase of the lifecycle, operation phase activities stand out because they are performed repetitively and are critical to long term reliability.
An SIS is a high reliability system comprised of sensors, logic solver(s) and final elements. It includes a number of safety instrumented functions (SIFs), each designed to provide a specified risk reduction. The necessary risk reduction is assigned as a safety integrity level (SIL) that establishes the reliability requirements for the SIF. A clear understanding of the failure rate, failure mode and failure effects for devices as well as implementation of a management program to effectively identify and correct failures on a routine basis are essential for achieving the needed reliability.
SISs operate in one of three modes: continuous, high demand or low demand. In low-demand systems, proof testing is an effective tool because the SIF components generally are dormant for long periods of time — which provides the opportunity to detect and repair failures and then return the component to service between demands.
In this article, we’ll review failure rate and failure mode basics, discuss proof test frequency and effectiveness, consider the robustness of the maintenance program, identify information to be collected during a proof test, and provide tips for analyzing the data to ensure continued reliability.