Experimental and numerical studies on corrosion failure of a three-limb pipe in natural gas field

• The synergistic effect of corrosion and flow erosion evokes the failure incident.
• The change in the flow state leads to the condensation of water vapor.
• Higher inlet velocity or a 90° merging angle can lead to more severe flow erosion.
• Reducing merging angle and inlet velocity can lessen the flow-assisted corrosion.

A bursting incident occurred in a three-limb pipe, having 16Mn steel for the main pipe and 316L + L416 composite metal for the branch pipe, in a natural gas field. The failure analysis was performed by means of inspection, experiments and computational fluid dynamics (CFD) simulation. The CFD results indicated the radical change in the flow characteristics inside the three-limb pipe due to its upright structure and the formation of a low vortex in the downstream near the junction, which indicated the condensation of water vapor containing high salinity. The condensed brine saturated with CO2 adhered to the inner wall surface of the main pipe. In such a corrosive medium, 16Mn steel acts as an anode and is preferentially corroded due to galvanic corrosion. In addition, the downstream area, covered by low vortex, exhibited high shear stress and droplet impingement stress, resulting in an increase in flow erosion. Thus, the failure of the three-limb pipe can be attributed to the synergistic effect of galvanic corrosion and flow erosion.