Metallurgical failure analysis was requested following the failure of two high-pressure rupture discs at a chemical plant resulting in an atmospheric discharge of chemicals. After extensive metallurgical investigation, it was determined that a steel threaded pipe nipple that attached a pressure gauge through a thread-o-let to a diluent line had failed. A metallographic section showing the corroded threaded pipe connection is shown in Photograph A.
Photograph A Polished section showing reduction of pipe wall thickness.
This pipe nipple failure occurred due to extensive internal corrosion of the internal pipe surface. The pipe, whose wall thickness was already reduced by threading, was corroded fully through the pipe where the nipple threads had already reduced the pipe wall thickness. The polished cross section of the threading on the end of the pipe revealed the cause of the failure.