Emerson is a manufacturer of high accuracy gas analysers that use semi conductor lasers and photo-sensors to measure the concentrations of different species in gaseous mixtures. The instruments are continuously operating, providing the user with a live measurement of a process or an environment.
Emerson CT5800 Continuous Gas AnalyserEmerson asked CRA to assess their CT5800 Continuous Gas Analyser using IEC 61508, an international standard used for functional safety that is used in safety critical industries to ensure that Electrical, Electronic and Programmable Electronic (E/E/PE) devices are sufficiently reliable for the demanding situations in which they are used.
This standard determines the maximum Safety Integrity Level (SIL) level that could be claimed for the equipment and to confirm a suitable level of reliability for its intended application. This can be important to ensure maximum availability of the device in operation and when producing safety cases, to prove the risk associated with an operation is As Low As Reasonably Practicable (ALARP).
CRA’s assessment was performed using a Failure Modes, Effects, and Diagnostic Analysis (FMEDA).
A FMEDA can be used to calculate a Safe Failure Fraction (SFF). This gives the proportion of failures that are expected to lead to a safe state. The hardware fault tolerance of an element is defined based on its level of redundancy. For a continuously operating or high demand element (such as the analyser being considered here) the maximum allowable SIL level can be determined based on the SFF and hardware fault tolerance.
The outcome of this type of project depends largely on the engagement of the design engineers, who are the equipment experts. The assessment team’s role is to help the engineers understand the process and what it intends to achieve, illicit the required information and then analyse the data gathered. Joint workshops are a key part of the assessment process, which is summarised below:
A strong working relationship was developed between CRA and Emerson engineers, which ensured the success of this project. Two joint workshops were held, where a large amount of information was collected and discussed. CRA consultants then used this information to perform the required analysis and calculations.
Based on the Safe Failure Fraction calculated from the FMEDA, it was determined that the maximum SIL level that could be assigned to the analyser was SIL 1. This reflected the complex nature of the device. A Mean Time Between Failure (MTBF) of 77 years was calculated for the CT5800’s laser assemblies which is considerably better than typical industry values.
The design team were very pleased with the level of care and attention CRA put into the assessment and were happy to recommend our services to other clients.
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