The One-Legged Stool
The Value of Simplicity
We continue our series of posts on the topic of Safety in Design. The first post was,
Before discussing specific aspects of design safety it is worthwhile to consider the basic design philosophy.
It is now largely taken for granted that process safety is achieved by adding equipment or instrumentation. In other words, we make the system more complex. Yet there are other approaches.
The following historical lesson was shared by Trevor Kletz (1922-2013).
Early in the 20th century a factory in England manufactured the dangerous explosive nitroglycerine. This is a highly unstable compound, and the process for making it was very dangerous. Concentrated acids were mixed with glycerine in huge vats. If too much glycerine was added too quickly the mixture would become unstable. Were this to happen the operator’s job was to open a large valve at the base of the vessel and dump the whole batch into a large vat of water. Failure to do this quickly could have led to a catastrophic explosion.
But mostly the operation was very dull. The operator (shown in the picture) would sit for long hours with not a lot to do. Hence, there was a possibility that he would fall asleep on the job. To make sure that this did not happen he was provided with a one-legged stool. If he dozed off, he would fall off the stool and be forced to pay attention to what he was doing.
This safety system ― the one-legged stool ― worked. In all the years the factory operated they never once had to dump the nitroglycerine mixture. Nor was there ever an explosion.
The one-legged stool safety system would not be allowed in a modern process or energy facility. But consider some of the elements of process safety.
Process Safety Information
Very little information is needed and none of it needs to be written down; stools have been in used for millennia; we know how they work.Operating Procedures
These are short and easy to understand. Once more, they hardly need to be written down. They boil down to, ‘Monitor the temperature and level in the vessel; open the dump valve if things go awry’.
Asset Integrity
The stool is unlikely to break (although the operator does look rather hefty). If there is a problem the workshop can fix it in minutes. If a new stool is needed, it can be purchased or fabricated quickly at minimal cost.
Process Hazards Analysis
The analysis would be short and simple. There would be no need for long, multi-person meetings.
