Organic Acid Corrosion | Materials And Corrosion Control
Damage Mechanism |
Organic Acid Corrosion |
Damage Description |
Organic compounds present in crude oil decompose in the crude furnace to form low molecular weight organic acids which condense in distillation tower overhead systems. They may also result from additives used in upstream operations or desalting. These naturally occurring acids may contribute significantly to aqueous corrosion depending on the type and quantity of acids, and the presence of other contaminants. Corrosion is a function of the type and quantity of organic acids, metal temperature, fluid velocity, system pH, and presence of other acids. In general, these light organic acids do not generate the severity of corrosion associated with inorganic acids such as HCl. Corrosion tends to occur where water accumulates or where hydrocarbon flow directs water droplets against metal surfaces. For example, the bottoms of overhead exchangers, separator drums, boots of separator drums, elbows, tees, pumps, and downstream of control valves. Corrosion is also sensitive to flow rate and will tend to be more severe in turbulent areas in piping systems including overhead transfer lines, overhead condensers, separator drums, control valves, pipe elbows and tees, exchanger tubes, etc. |
Affected Materials |
· All grades of carbon steel; most other corrosion resistant alloys used in crude tower overhead systems are generally not affected. |
Control Methodology |
· Corrosion caused by light organic acids in crude unit overhead systems can be minimized through the injection of a chemical neutralizing additive. However, problems may arise when frequent changes in crude blends lead to changes in neutralizer demand.
· The TAN of the crudes being processed can be used as an initial guide to setting the neutralizer by anticipating an increase in the acid concentration in the overhead system. After new crude is processed, reviews of analyses of water samples from the boot of the overhead separator drum can be used to determine how much light organic acid reaches the overhead system optimize future additions. · Filming amines can be used to prevent corrosion if the filming amine selected does not react with the organic acid. However, using filming amines is not as effective as neutralization. · Upgrading to corrosion-resistant alloys will prevent organic acid corrosion but the selection of suitable materials should account for other potential damage mechanisms in the overhead system. |
Monitoring Techniques |
· UT and RT inspection for loss in thickness are the most common methods of inspection.
· Long range UT techniques can also be used for long runs of pipe. · For carbon steel, damage is usually in the form of general thinning but may be highly localized where a water phase is condensing. · Evidence of locally thinned areas can be found using automatic ultrasonic scanning methods or profile radiography. · Process monitoring should include pH measurement and analysis of water in the crude tower overhead drum to analyze for the presence of organic acids. · Strategically placed corrosion probes and/or corrosion coupons can provide additional information on the rate and extent of damage. |
Inspection Frequency |
· At every T&I. |
KPIs |
· #TMLs with Corrosion Rate > 5MPY (Target = 0) |
References |
· API RP 571 (DM # 66) |
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