Fire Damaged Pressure Vessel

Following a refinery fire and explosion a pressure vessel had been “refurbished” and was being re-commissioned.  The pressure vessel had passed a 1000 psi hydrostatic pressure test.  During subsequent start-up a workman attempted to tighten a plug which was installed into a “thread-o-let” welded to the pressure vessel.  As the worker “tightened” the blind plug with a wrench the plug “blew out” severely injuring the workman.  The subject plug after the accident is shown in Photograph A (B004).

         
Photograph A  Overall view of subject plug following the accident.

A close-up view of the first few plug threads is shown in Photograph B (B065).

         
Photograph B  Close-up of first few threads of subject blind plug.

Of particular note is the fact that only the first one or two threads could have been engaged as threads three, four, etc. all have thread  debris and paint thereby giving evidence of the extent of thread engagement.

The thread-o-let, which with the blind plug was ejected, is shown in Photograph C (B125).

     
Photograph C  Close-up view of thread-o-let into which blind plug was installed.

As can be seen the thread-o-let, threads are rust filled and possibly even stripped. 

It is assumed that the worker, who had 25 years experience as a pipe fitter, who testified he put on the wrench to tighten the plug did indeed attempt to tighten the plug.

It was proposed that:

a) The plug had not been properly inserted when the 1000 psi hydrostatic test was conducted and water leakage from around the plug threads was not detected.

b) The plug was not properly inserted when the 1000 psi hydrotest was performed and an air pocket existed in the thread-o-let cavity which slowly leaked AIR from the partially threaded plug threads.

c) The plug was properly inserted and properly pressure tested.  Later someone removed the plug (then reinserted same) and/or partially removed the plug.

The thread-o-let evidence in this matter was spoliated when refinery workers re-threaded the subject thread-o-let for a larger diameter threaded plug.

The matter was resolved without destructive testing and/or expert depositions.