Generally, in refinery distillation units, naphthenic acid corrosion and high temperature crude corrosivity is a reliability issue; and the presence of naphthenic acid and sulfur compounds considerably increases the corrosion in the high temperature parts of the distillation units.

• Naphthenic acid corrosion occurs primarily in high velocity areas of crude distillation units in the 220 to 400°C or 430 to 750°F temperature range. No corrosion damage is usually found at temperatures above 400°C or 750°F, most probably because of the decomposition of naphthenic acids or protection from the coke formed at the metal surface.
  

The difference in process conditions, materials of construction and blend processed in each refinery and especially the frequent variation in crude diet increases the problem of correlating corrosion of a unit to a certain type of crude oil. Crude oil composition from the same field can change with time. When steam flooding or other recovery methods begin in an oil field, specific gravity and the organic and sulfur content of the crude may change. As for example, fire flooding, when used in some fields, tends to increase the naphthenic acid content.

Here three mechanisms of naphthenic acid corrosion are described below, where each one is predominant in specific areas of  the distillation unit.

Vacuum Column

• The corrosion mechanism is mainly a condensate corrosion and is directly related to content, molecular weight and boiling point of the naphthenic acid.
• Preferential vaporization and condensation of naphthenic acids increase TAN of condensates. The corrosion is similar to corrosion in very high TAN cuts and velocity has virtually no effect on the process.
• The naphthenic acids are most active at their boiling point but the most severe corrosion generally occurs on condensation. Corrosion is typically severe at the condensing point corresponding to high TAN and temperature.

Furnace Tubes and Transfer Lines

• The influence of temperature, velocity and degree of vaporization is very large.
• Process conditions such as load and steam rate and especially turbulence affect corrosivity.
• Areas of high turbulence such as thermowells and pumps, is most likely an accelerated corrosion due to the velocity and the two-phase flow.
• The presence of any naphthenic acid most likely increases sulfidic corrosion.

Side Cut Piping

• Conditions are low vaporization and medium fluid velocity exist. An increase in velocity, increases corrosion rates up to the point where impingement starts and corrosion is accelerated dramatically.
• Sulfur has been shown to inhibit somehow the corrosion.

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