Systems of tabs for security applications

The high quality of the components is a condition for the prevention of risks, especially in critical processes in the chemical industry. The necessary characteristics of the applied components are defined in the standards IEC 61508 (functional safety in general), as well as IEC 61511 (functional safety in the process industry). This directive defines inter alia the concept of the SIL (Safety Integrity Level). The user has rates of errors in each component, provided by the manufacturers. FMEDA (Failure Modes, Effects and Diagnostic Analysis) is a fundamental tool in this context. This analysis evaluates jointly the statistical values of several components and their functional interrelationship to get information quantified on the likelihood of error and the reliability of the components. To carry out a detailed analysis you should consider in detail the principles of operation of a system of instrumentation with separator.

To perform an adaptation of instrumentation fits the conditions of service applies a membrane of a special material as well as a liquid of specific transmission in the interior of the instrument. The flexible membrane makes the separation between the environment and the instrument. The pressure exerted on the flexible membrane is transferred through the transmission fluid to the sensitive element of the gauge, transmitter of pressure or pressure switch for the reading of values. The mounting of the separator can be made directly to the environment or through flexible hair. A cooling tower applies to withstand high temperatures.

Standard material is steel CrNi 316 L (1.4404/1.4435) resistant to corrosion. All parts in contact with the environment with special materials for example PTFE, ECTFE etc. can be covered for further resistance to aggressive media Another option is Wikaramic ceramic coating to protect the instrument against abrasion. In highly critical applications it uses exotic materials e.g. Tántalo or Hastelloy C that can resist temperatures continuing to + 400 ° c depending on the fluid transmission or special material. The user can choose from more than 20 special materials, being the most common in the chemical and petrochemical industry Hastelloy C4, Hastelloy C276 and Monel 400.

IEC 61508 covers all the applications of electronic systems that can lead, in the event of failures, serious consequences for people, environment and facilities. The demands on security are determined with the probability of the failure and its possible consequences. Highest potential harm and its likely to happen more high is the classification in the nomenclature SIL 1 to SIL4. To assign the potential risk to an appropriate classification the user applies a risk chart.

According to IEC 61508 must observe the complete circle of security including all the components in function (sensors, processing logic, actors). This calculation and a consistent risk assessment requires good knowledge of the relevant components in the case of the separator. The standard construction, for example, composed of a body and membrane, has only a connection between membrane and body and the other between instrument and tab. This can be a relatively small failure probability.

In the case of more complex constructions, for example, tabs with protruding membranes, the probability of failure increases with each component and each connection. In the annexed table are available a very simplified categorization of a system to carry out an evaluation of a system of critical application. There are also more detailed indicative values to consult if necessary.

With indicative security values user must make a calculation of the full system considering all the operational components to ensure the maximum reduction in the risk.