Two failures can’t happen at the same time! Right? Wrong!

The assumption that two unrelated failures cannot occur at the same time is called double jeopardy.  Failures are considered independent if there is no common cause due to a mechanical, electrical, or process condition which could lead to them occurring at the same time.  

Examples of common cause failures include:

-Failure of a level transmitter and a level switch on a vessel with a common level bridle that plugs off

-Failure of two control loops tied into the same input card

Generally it is reasonable to ignore double jeopardy scenarios during a HAZOP or PHA; however, there are some cases which need to be addressed, such as hidden failures or latent failures are failures which can go undetected. In this type of scenario, a second, unrelated failure could occur prior to the first failure being addressed and fixed. 

Examples of hidden failures include:

-Check valve leakage

-Failure of a single pressure regulator where there are 2 regulators in series with no pressure detection between the regulators

Another concern where two failures could occur at same time is if equipment is not repaired or replaced promptly. The failure may continue for a prolonged period of time until a second random failure occurs.

In these cases, the argument of double jeopardy cannot be applied and the potential for two simultaneous failures to occur should be considered.

In 2009, there was an incident at Caribbean Petroleum near San Juan, Puerto Rico, which resulted in a fire and explosion after a tank was overfilled.  The blast damaged 17 storage tanks and neighbouring homes and businesses. One of the contributing factors in the incident was frequent failures of level transmitters. Inspections also revealed a history of poor maintenance including leaking transfer valves. This is an example of a case where poor maintenance followed by another failure led to an incident with impact to the community and environment.