Safety Moment #12: Process Safety Emergency Response

Process Safety Emergency Response Two Second Rule

The Two Second Rule

Process Safety Emergency Response is a key part of any Process Safety Management (PSM) program. No matter how effective preventive measures may be, there is always a chance that something will go badly awry. In which case an emergency response program is needed.

The Two Second Rule is taught in driver's education. The basic idea is simple: stay two seconds behind the vehicle that you are following in order to ensure sufficient stopping distance. In other words, in an emergency situation you know what to do and you have sufficient time to take the appropriate action.

The concept of a “two second rule” can also be used in process safety management programs. But here the idea is that within a short period of time, everyone knows what to do if a catastrophe is looming. Two examples come to mind — one which worked and one which went wrong.

Polystyrene Runaway Reaction

Process safety emergency respone styrene runaway reaction
The Styrene Molecule

Early in my career I worked on a chemical plant that manufactured polystyrene. An intermediate reactor that was around 5 meters tall contained a viscous prepolymer that was mixed with a slowly turning ribbon stirrer. There was a possibility that we could lose control of the reaction and that the prepolymer would quickly turn into polystyrene, thus creating a "lollipop" around the stirrer.

If this happened we had to remove the lollipop by lifting it from the reactor, transport it to a waste ground and blow the polystyrene off the ribbon with dynamite. Clearly this runaway reaction posed some obvious safety and environmental hazards.

But, if the reaction showed signs of running away, all we had to do was add elemental sulfur. This would kill the reaction and allow us to drain the prepolymer in a controlled manner. It was an expensive decision because we lost the batch, but we did not have to wonder what to do. Our Two Second Rule was clearly defined; we knew what to do and had sufficient time to do it.

Piper Alpha

Process Safety Emergency Response Piper AlphaThe failure to have a two-second rule was a major factor in the well known Piper Alpha catastrophe in the year 1974.

The inventory of oil and gas on a production platform such as Piper is quite low; in the event of a major fire the gaseous hydrocarbons are blown down to flare in just a few minutes and the oil fire should die down within 20 minutes or less. But Piper burned for hours, the reason being that other platforms kept feeding condensate and gas to Piper (which was a hub). The Two Second Rule here (which was not followed) is: in the event of a major fire on a production platform close the subsea riser valves.


In many cases the correct response to a major event has been identified by management and the risk analysts, but (a) the operating personnel may not have been properly trained in what to do, and/or (b) they need to assured that they are doing the right thing, even if it leads to economic loss.

Process Safety Emergency Response and Black Swans

Process Safety Emergency Response Black Swan
Nicolas Taleb

Of course, not all catastrophic events are anticipated — there is always the chance of a Black Swan, as discussed in our Safety Moment Black Swans and Bow Ties. But even here, it should be possible to put in place process safety management systems that help prevent the situation from getting out of hand.




Process Safety Emergency ResponseFor the latest information to do with Process Safety Emergency Management  please check out our Process Risk and Reliability Management blog.



Reference Material

Process Safety Emergency ResponseAdditional information to do with Process Safety Emergency Management  is available in our book Process Risk and Reliability Management.