A leak occurred at a refinery on a line (NPS 40) conveying caustic soda in the gas scrubbing section of the gas plant: 6 m³ of soda solution and 1 to 2 m³ of propane escaped from the pipe without consequences.

Opening the line resulted in internal corrosion by the soda solution caused by a combination of several factors. Firstly, the line, made of carbon steel was heavily traced (½” diameter steam tracer in contact with the line) to prevent the solution from freezing in the event of extremely cold periods, with the result that the temperature of the metal exceeded the limit permitted for this type of service. In addition, the line was used intermittently (6 hours twice a year) whereas the tracing was maintained continuously.

Following this event, the operator planned to identify the lines conveying caustic solutions (soda) which are traced and lagged in its refineries, and check that the tracing is not likely to raise the temperature of the metal pipework beyond the limits stipulated for the concentration of soda that is used. It also planned to identify the most critical lines: those with an intermittent circulation of caustic solution and in which tracing is likely to remain constantly in operation and those from which hydrocarbons could potentially leak in the event of line failure.

In the case of a new or repaired installation, it recommends the use of self-regulated electrical heat tracing, which prevents the creation of hot spots on the piping whatever the circumstances. It should be noted that this phenomenon may also occur on pipes conveying amines for which the temperature of the metal in contact with the amine must not exceed 127 °C according to the operator’s specifications.

Furthermore, the operator requested that the ancillary utility circuits that come into contact with LPG and lead to a loss of containment (LPG/liquid interfaces, LPG/steam heat exchangers in which the LPG pressure is greater than that of the steam) be identified on its sites.