Corrosion damage at an injection point where two process streams mix is a fairly common occurrence.
Following a leak at a T-joint between a liquefied petroleum gas (LPG) line and a naptha line, staff from TWI's Materials Department were asked to conduct a failure investigation by a Member company.
The conclusions drawn were that the attack was the result of the formation of a weak solution of hydrochloric acid (HCl). The LPG line contained low levels of hydrogen chloride which, when mixed with water from the naptha line, formed HCl.
The corroded area was restricted to a relatively small area downstream of the T-joint suggesting that the acid was quickly diluted in the main line. Smooth grooves and rounded holes were found indicating that, at this turbulent location, the damage caused by the corrosive acid was probably enhanced by an erosion-corrosion mechanism.
To prevent this type of failure, either corrosion resistant alloys should be considered for the T-joint or an alternative design used. Injection into the centre of the line usually ensures better mixing although occasionally the problem may be moved further downstream.
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