Wednesday, July 07, 2010

Overfill Protective Systems - Complex Problem, Simple Solution

Useful paper by Angela E. Summers, available from

It considers the Buncefield, BP Texas City and Longford accidents and the issues related to high levels. The main conclusion is as follows:

"Catastrophic overfills are easily preventable. When overfill can lead to a fatality, follow these 7 simple steps to provide overfill protection:
1. Acknowledge that overfill of any vessel is credible regardless of the time required to overfill.
2. Identify each high level hazard and address the risk in the unit where it is caused rather than allowing it to propagate to downstream equipment.
3. Determine the safe fill limit based on the mechanical limits of the process or vessel, the measurement error, the maximum fill rate, and time required to complete action that stops filling.
4. When operator response can be effective, provide an independent high level alarm at a set point that provides sufficient time for the operator to bring the level back into the normal operating range prior to reaching a trip set point.
5. When the overfill leads to the release of highly hazardous chemicals or to significant equipment damage, design and implement an overfill protection system that provides an automated trip at a set point that allows sufficient time for the action to be completed safely. Risk analysis should be used to determine the safety integrity level (SIL) required to ensure that the overfill risk is adequately addressed. While there are exceptions, the majority of overfill protection systems are designed and managed to achieve SIL 1 or SIL 2.
6. Determine the technology most appropriate for detecting level during abnormal operation. The most appropriate technology may be different from the one applied for level control and custody transfer.
7. Finally, provide means to fully proof test any manual or automated overfill protective systems to demonstrate the ability to detect level at the high set point and to take action on the process in a timely manner.

1 comment:

Anonymous said...

You don't need a Phd. to come the conclusions stated in the article, to most of our Problems the solution seems to be simple if you're sitting in your ivory tower and judge people's decisions in hindsight. The Problem is that these incidents keep occuring and it seems like the solution is not as simple as some people believe. Personally I wouldn't just rely on instrumentation and focus more on the Human factor side