Dear Experts,

Over the last 1 month we have observed 02 leakages from our CO2 removal unit piping. Request your expert advice on what are the likely cause(s) for these leakages and what can be done to prevent any further leakages. Given below are brief details of the process and respective leakage events.

Plant: Urea Fertilizer Manufacturing Plant
Unit: CO2 Removal Unit
Piping Loop Age: 30 years
Material: Carbon Steel
Service: Rich Solution (Hot Potassium Carbonate)
Temperature: 250 oF
Pressure (Absorber Outlet): 400 psig
Pressure (Stripper Inlet): 15 psig

Corrosion Rate: As per licenser the thickness loss for CS piping without passivation layer is estimated as 0.29 corrosion year/0.1 inch thickness loss. While with a passivation layer intact, it is estimated to be 1000 corrosion years/0.1 inch thickness loss. Hence it will not take more than a few months for our piping to corrode away in absence of a passivation layer.

Passivation Layer: Passivation layer is being achieved via introduction of Vanadium in process steam. Site has a practice of achieving passivation layer on piping loop for 12 hours in case service circulation is stopped for more than a few hours or any new component is added to the system. Additionally post plant turnaround interventions passivation layer is achieved for 24-36 hours. There is no reported instance of site not being able to achieve passivation layer as per aforementioned guideline.

Flow Rate: Piping loop design has been verified to confirm that piping flow rates through different pipe diameters is under maximum allowable flow rate recommended by licenser, with the piping flow rate being maximum of 9.1 ft/s through 20” absorber outlet line.

1st Leakage
1st leakage was observed from an elbow butt weld joint in pressure let down valves bypass line. This is the first elbow in immediate downstream of 8” branch line (bypass line of pressure let down valves). Elbow was removed and replaced with new one. No thickness loss was observed on base metal of elbow and piping, however upon internal inspection of elbow, damaged passivation layer was observed at upstream side of the elbow however passivation layer was found to be intact at downstream side of the elbow. Severe corrosion was also observed on HAZ area of leaking weld joint.

2nd Leakage:
2nd leakage was observed from a thermowell branch connection welding joint. Welding joint is fillet weld type. No thickness loss was observed in base metal adjacent to weld joint. Internal inspection could not be performed as box up was installed and plant was taken back in service.

In light of the above your expert opinion is requested on what could be the possible cause(s) of such leakages from weld joint only, with no detectable thickness loss on base metal. With plant being in service of 9 months since last Turnaround and piping age of 30 years, why is passivation layer only being damaged at weld joint and its HAZ.

Please refer to attachment for leakage pictures and P&ID.

• https://files.engineering.com/getfile.aspx?folder=786ece34-b506-4650-a462-0