Case Studies in Process Plant Accidents

     Accidents happen....that is for sure.  But when the stakes are as high as they are in the chemical industry, every effort must be made to ensure that diasters are avoided.  One of the best ways to avoid accidents is to learn from others.  Let's take a look at some past accidents that were all too real and derive a simple lesson from each one.

A Temporary Modification That Proved Deadly

     Nypro Factory, Flixborough, United Kingdom in 1974

     A series of six reactors were installed side by side, each installed a little lower than the previous one to allow for gravity flow between them.  The reactors were joined by 28 in. diameter pipe with an expansion bellow on the end of each pipe.

     The bellows were installed on the short pipe runs to allow for expansions and small movements that typically accompany operating conditions such as those at Nypo (150 ⁰C and 10 bar).  These reactors were usually cooled via brine to jackets on the outside of the vessels.  The brine system had to be taken offline for repairs so city water was used in the interim.  Reactor #5 collapsed soon after, but no significant leak was experienced.

     A 20 question modification approval form was completed before any modifications were made.  The questions were not answered properly and the form was treated as a formality.  A professionally qualified engineer was not available to review the suggested modification.

     While Reactor #5 was being repaired or replaced, the modification that was approved and installed is shown below:

     Reactor #5 was replaced with a 20 in. pipe with two bends to accomodate the height difference.  The existing bellows were left in place at the ends of the temporary pipe.  The temporary pipe was supported by resting it on scaffolding.

Lack of Concentration of Faulty Design

     An operator was asked to maintain the temperature of a reactor at 60 ⁰C.  The control panel was set up in such a manner that it may have resembled the diagram below:

     Essentially, the set point should have been established by percentage of the temperature range from 0 to 200 ⁰C.  It's not difficult to guess what happened.  The operator simply set the instrument to 60, which corresponded to a reactor temperature of 120 ⁰C!  The result was a runaway reaction, an overpressurized reactor, liquid release, and injured operators.

Understanding Why Testing is Being Done

     A new storage tank was being filled with water to check for leaks and mechanical stability.  While the testing was being performed, two welders were on top of the tank completing the hand rails for the tank.

     The line used to feed the water to the tank was previously used for transporting gasoline.  Some residual gasoline entered the tank with the water.  Once inside the tank, the gasoline immediately floated to the top.   The welders accidentally ignited the gasoline vapor.  Fortunately, neither welder was seriously injured.

Scaling Up Is Not Elementary

     Scaling up of chemical processes has resulted in many accidents throughout history.  Designers sometimes fail to realize that increasing the volume of a reactor does not mean that the surface area and heat transfer will increase proportionally.  Consider a cylindrical reactor:

     After scale up, the volume increased by about 2.7 times the original design.  However, the surface area only increased by 1.9 times the original design.  Consider an example where a reaction was studied in a laboratory.   In the lab, there was no observed rise in temperature so the reaction was deemed "thermally neutral" (which is rare).  When the reaction was moved to the pilot plant level, no cooling supply was added to the reactor.

     A combination of an increased reactor size (2.5 m³) and a decreased heat loss rate (0.5 W/kg ⁰C in the pilot plant) resulted in a runaway reaction.  Fortunately, a relief valve was in place and a serious incident was avoided.

     After reviewing these incidents, its easy to see how knowing what caused past accidents can prevent future accidents from happening.  With that in mind, I would urge everyone to read the reference cited below.  Mr. Trevor Kletz assembled a well-written compilation of process plant accidents in his book.  It's a fantastic resource!

Reference:

Kletz, Trevor, What Went Wrong: Case Histories in Process Plant Disasters, 4th Ed., Gulf Publishing Company, 1998, ISBN 0-88415-920-5
www.gulfpub.com/books.html