Functional Safety and Tank Overfill Prevention

As global oil and gas production continues to fan out to new areas, the need for storage tanks and terminal facilities as part of the distribution processes increases. Making sure those tanks are not overfilled is paramount for safety, environmental protection, and the ongoing viability of the enterprise should a major accident occur.

Emerson's Carl-Johan RoosTank-Storage-Taking-Overfill-Prevention-to-the-Next-LevelEmerson’s Carl-Johan Roos, whom you may recall from an earlier post on tank radar gauges used in safety applications, has written an article for Tank Storage magazine, Taking overfill prevention to the next level.

Carl-Johan actively participated in the API 2350 tank overfill protection standard. Besides API2350, he has actively participated in numerous product-specific IEC 61508 safety certifications and site-specific IEC 61511 functional safety-related projects, and he often addresses national overfill prevention regulations such as TÜV/DIBt WHG in Germany.

In the article, he describes the technology changes taking place in detecting tank overfill conditions:

…products have emerged that allow for replacement of mechanical and electromechanical point-level switches to new and modern electronic level gauges. Traditional and well-proven tank gauging concepts, such as continuous level measurement, is rapidly becoming the preferred industry choice and the new ‘best practice’ also for overfill prevention sensors.

Unlike continuous level measurement devices, on/off level switches have the fundamental issue:

…it is difficult to know whether they are working or not.

Storage tanks with petroleum products, like other processes requiring risk mitigation, typically have numerous independent protection layers. Dikes and secondary containment around the tanks are passive protection layers. A control system or tank gauging system provides level control and movement of fluids in and out of the tank. An independent safety instrumented system, separate from this system, is often referred to as a HiHi level alarm or overfill prevention system.

Carl-Johan notes:

A common misperception, inevitably caused by the nomenclature, is that the safety layer is the most critical component. This should not be the case in a properly designed system; the tank gauging system is continuously in operation 24/7 and is the operators’ primary tool to prevent overfills from occurring. The overfill system is only to be used in exceptional circumstances, and the more seldom the better.

Given the importance of the tank gauging system in every day operation to prevent overfills, it important to look at replacing mechanical level measurements with modern tank gauging systems with continuously operating diagnostics and temperature-compensated leak detection.

Carl-Johan highlights the similarities of the API 2350 standard with the IEC 61511 global safety standard:

API 2350 has been inspired by IEC 61511’s life cycle approach. The entire journey from requirement specification to commissioning, and from operations to decommissioning is covered. An essential part of this is the risk assessment and management system, which now both have become mandatory parts of the standard.

Modern tank farms, at a minimum, must be equipped with:

  • 1x automatic tank gauge (ATG) and
  • 1x independent overfill prevention system (OPS).

API 2350 requires point-level gauges or switches be tested every 6 months. With continuous measurement devices such as separate guided wave radar devices used for the tank gauging system and for the overfill protection system, the readings can be compared against one another to verify that they are operating properly:

Often a fairly generous deviation alarm (e.g. 5cm) is sufficient to help the operators early detect any problems while at the same time avoiding false alarms.

He concludes:

Although traditional level switches can still be used, the most efficient and future proof solution today appears to be an IEC 61508 (SIL)- certified overfill prevention sensor that measures the level continuously and independently of the automatic tank gauge.

You can join in discussions with Carl-Johan and the tank gauging team in the tank gauging track of the Emerson Exchange 365 community.