Safety Moment #12: The (Process Safety) Two Second Rule

Two second rule applied to process safety

The Two Second Rule is used in driver training. The rule is that a driver should ideally stay at least two seconds behind any vehicle that is directly in front of his or her vehicle. The rule suggests that most people require at least two seconds before they respond to a problem. The same idea can be applied to emergency response to process safety situations. .It gives a person 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.