exida : Resources : Features

Development of a Mechanical Component Failure Database

Posted By exida on August 25th, 2010

In this article, we present a methodology to derive component failure rate and failure mode data for mechanical components used in automation systems based on warranty and field failure data as well as expert opinion. We describe a process for incorporating new component information into the database as it becomes available. The method emphasizes random mechanical component failures of importance in the world of safety analysis as opposed to the wear-out and aging mechanical failures that have dominated mechanical reliability analysis. The method provides a level of accuracy significantly better than warranty failure data analysis alone. The derived database has the same form as that for electrical/electronics databases used in FMEDA analyses used to show compliance with international performance-based safety standards. Thus, the mechanical database can be used in conjunction with existing electrical/electronics databases to perform required probabilistic safety analysis on automation systems comprised of both electrical and mechanical components.

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Using Simulation to Characterize Common Cause

Posted By exida on August 25th, 2010

Fault tolerant systems have been designed for safety critical applications including the protection of potentially dangerous industrial processes. These systems are typically evaluated and certified to functional safety standards with IEC 61508 [1] by agencies like exida Certification or one of the TUV companies. Many factors are taken into account during the certification process including hardware diagnostic capability, level of hardware redundancy, design processes used, software diagnostics and general equipment strength. It has clearly become recognized that common cause failures can have a major negative impact on the safety and availability of a fault tolerant system [2]. The whole value of redundancy may be ruined. Common cause is recognized as an important factor but there is disagreement regarding how to account for common cause in the quantitative modeling. Part 7 of IEC 61508 at least provides a list of questions with a point scoring system [3].

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Selecting Instrumentation Equipment for Safety Applications

Posted By exida on August 24th, 2010

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.

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ALARM MANAGEMENT AND ISA-18 – A JOURNEY, NOT A DESTINATION

Posted By exida on August 24th, 2010

Poor alarm management is one of the leading causes of unplanned downtime, contributing to over $20B in lost production every year, and of major industrial incidents such as the one in Texas City. Developing good alarm management practices is not a discrete activity, but more of a continuous process (i.e., it is more of a journey than a destination). This paper will describe the new ISA-18.2 standard -“Management of Alarm Systems for the Process Industries”[1]. This standard provides a framework and methodology for the successful design, implementation, operation and management of alarm systems and will allow end-users to address one of the fundamental conclusions of Bransby and Jenkinson that “Poor performance costs money in lost production and plant damage and weakens a very important line of defense against hazards to people.” [3] Following a lifecycle model will help users systematically address all phases of the journey to good alarm management. This paper will provide an overview of the new standard and the key activities that are contained in each step of the lifecycle.

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Accurate Modeling of Shared Components in High Reliability Applications

Posted By exida on August 24th, 2010

This article addresses how to model and evaluate the risk reduction factor (RRF) of safety instrumented systems (SIS) when one or more of the components in the SIS can cause the dangerous condition or hazard that the SIS is designed to protect against. Generally a failure that can cause a hazard is referred to as an initiating event (IE). International standards for SIS safety evaluation require that shared components either be prohibited or accurately modeled. Current practice generally falls into one of two extremes, ignoring any degradation of system reliability due to shared components or completely discounting any improvements in reliability as a result of redundancy created by the shared component.

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What does Proven In Use imply?

Posted By exida on August 24th, 2010

The functional safety standards, IEC 61508 [1], IEC 61511 [2], and ANSI/ISA 84.01 [3] each specify the Safety Integrity Level performance parameter for Safety Instrumented Functions. For a Safety Instrumented Function to meet a specific Safety Integrity Level the sum of the average Probability of Failure on Demand (PFDavg) of all components, part of that Safety Instrumented Function, needs to fall in the PFDavg bandwidth related to that Safety Integrity Level.

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What is PFDavg?

Posted By exida on August 24th, 2010

Introduction

IEC61508 requires probabilistic evaluation of each set of equipment used to reduce risk in a safety related system. Different order of magnitude risk reduction levels are achieved depending on the average probability of failure on demand (often called average probability of dangerous failure). In practice, a number of different methods have been used to calculate this probability. Among the most popular are fault tree analysis, reliability block diagrams, simplified equations (derived using a number of different ways) and Markov models. For those who use Markov models, different solution techniques are used. A debate has existed in various circles about the appropriateness of various methods. A tutorial of the different methods is available in a text by Goble, reference 1. A good comparison of the different methods is stated by Rouvroye in reference 2.

The fundamental problem is that these different methods give results that vary by 2X+ for same set of input parameters.

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61508 and 61511; What Is an Operations Company Supposed to Do?

Posted By exida on August 24th, 2010

The typical first reaction from the process operations side of the table when confronted with a new standard is, “How much will this cost and how much extra paperwork will it involve?” Depending on the organisation, the answers to these questions can vary dramatically. Unfortunately, the further question, “How can this save money?” is rarely asked, if ever. Even if it is asked, the hope of implementing a new regulation and actually saving money immediately is dismissed as an impossible dream. IEC/AS 61508 and 61511, the standards covering the design and use of safety instrumented systems to reduce process plant accidents, are no exception to this initial reaction.

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You Asked: Alarm Management

Posted By exida on August 24th, 2010

Setting a new Standard for Performance, Safety, and Reliability with ISA-18.2

Alarm Management affects both the bottom line and plant safety. A well- functioning alarm system can help a process run closer to its ideal operating point – leading to higher yields, reduced production costs, increased throughput, and higher quality, all of which add up to higher profits. Poor alarm management, on the other hand, is one of the leading causes of unplanned downtime and has been a major contributor to some of the worst industrial safety accidents on record.

Changing the practices and procedures used in the plant has become easier and more important with the June 2009 re- lease of a new ISA standard on alarm management. The ISA-18.2 standard, which provides a blueprint for creating a safer and more productive plant, is ex- pected to be adopted by regulatory agencies (such as OSHA in the U.S. and the la- bour ministries in Canada) and insurance agencies as “good engineering practice”. This article will provide an introduction to the new standard and discuss how it will affect the process industry.

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