Logic solver for tank overflow protection (December 2013)

Moore Industries white paper shows complex nature of seemingly simple process safety task.

A new white paper from Moore Industries illustrates the complexity of an apparently simple piece of equipment, a tank overflow protection device. Those familiar with the Buncefield, UK tank farm explosion and fire will appreciate the importance of overflow protection.

Reducing such risks relies on accurate and reliable safety instrumented systems (SIS) whose levels of functional integrity are set out in IEC 61511-1. This has been adopted as the worldwide standard for such systems in the process industry. SIS integrity requirements span sensors, controllers, logic solvers, actuators and valves. A key part of the loop is the logic solver, which actually takes the final decision as to whether to activate the overfill protection.

The white paper explores some of the different approaches to logic solver design, along with some examples of system topologies and the associated safety integrity level calculations. Insights from the paper include the fact that the instrumentation used in the SIS should be independent from the tank gauging system to avoid interference and/or common failure modes. Because sensors degrade over time it is also judicious to deploy multiple devices with a different technology from the tank gauges. Register and download the white paper.

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