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

Two second rule applied to process safety

The material shown here has been extracted from the ebook 52 Process Safety Moments.


  • Introduction
  • Polystyrene Runaway Reaction
  • Piper Alpha
  • Conclusion
  • Further Information


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. This is because most drivers 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

A chemical plant manufactured polystyrene in a large, batch reactor that was around 5 meters tall. Styrene monomer and other chemicals were added to the reactor, which was then slowly heated. The styrene polymerized to form polystyrene, which was a viscous, prepolymer liquid within the reactor. Once the reaction was complete the liquid was drained from the reactor, and the next batch started.

In order to ensure that all the materials were well mixed, and in order to maintain an even temperature throughout the body of the reactor the viscous prepolymer was mixed with a slowly-turning ribbon stirrer.

A well-known hazard to do with this process was that the reaction could run away. This resulted in the prepolymer polymerizing all the way to solid, polystyrene. If this happened, a “lollipop” of solid polystyrene would form around the stirrer. If this happened the lollipop had to be pulled out the reactor using the ribbon stirrer axle. The lollipop was then removed to a piece of waste ground where the polystyrene would be blown off the ribbon with dynamite. This whole exercise presented obvious environmental and safety problems, and always led to serious economic losses.

To avoid this highly undesirable result the operators were instructed in their “Two Second Rule”. If the reaction showed signs of running away they added elemental sulfur to the reaction. This would kill the reaction and allow the prepolymer to be drained in a controlled manner. This action meant that the batch would be lost. But there were no significant safety issues and the economic losses were far less than those incurred if a lollipop were to be formed.

This example shows that management had developed their “two second rule”, they had instructed their operating staff in its use, and had implicitly accepted responsibility for the economic losses.

Piper Alpha

Process Safety Emergency Response Piper Alpha The 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).

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