Q. Show the corrosion mechanisms?

The following corrosion mechanisms are likely to happen in the Oil Treating Unit:

• Under Deposit Corrosion and MIC

Since cooling emulsion is typically a high fouling application, the inlet coolers must be cleaned regularly to prevent under deposit corrosion. Also maintaining velocity is critical to minimize fouling.

Off Spec Storage and Dilbit Storage Tanks are subject to this type of corrosion due to the sedimentation of scale at the bottom of tanks. Internal coating of al storage tanks is highly recommended.

• CO₂/H₂S Corrosion in the Gas Streams

Normally there should be no gas evolving within the FWKO or Treater, because the vessels operating pressure has been selected to suppress gas flashing. However, in the event of a process upset, it may be corrosive depending on the dissolved CO₂ and H₂S.

A corrosion analysis shall be performed to evaluate the corrosion rate and intensity of the PG from FWKO and Treater during process upset condition as stated earlier.

CO₂/H₂S corrosion shall be categorized into one of three regimes:

o When PCO₂/PH₂S > 5000: CO₂ corrosion dominates. The Modified Deward-Milliams Model or Predict5.0/6.0 software may be employed to predict the CR.

o When 20< PCO₂/PH₂S < 5000: Mixed corrosion regime, i.e. corrosion in slightly sour environments. It is most likely the localized corrosion rate will be considerably lower than the predicted corrosion rate, due to protection by scales.

o PCO₂/PH₂S < 20: H₂S corrosion dominates. Corrosion rate is governed by the effectiveness of the corrosion scales. There are possibly localized corrosion risks in the event of breakdown of the protective sulphide scales.

When modelling corrosion using Predict 5.0/6.0 software, gas shall be in the water dew point condition. One more run shall be performed to simulate corrosion rate under 1% free liquid water hold up as well. This scenario is water upset condition and may happen if FWKO and/or Treater are not operating properly.

For CR > 0.3mm/y, some precautions, including easy to drain and maintain temperature, shall be considered in the design. Corrosion Monitoring and frequent inspection are also an integral part of corrosion management of the gas streams.