Squeezing the most out of your Inspector – Part 2

By Lynne Kaley

In the last blog, we discussed how to use API 571 to define potential mechanisms for inspection planning. Without operating data to more accurately predict the presence of mechanisms and susceptibilities, the inspection plan will be conservative. However, if the information required to fine tune the damage prediction and even calculate the potential corrosion rates (reference API RP 581 Part 2, Annex B to identify operating data needed) are known, optimized inspection plans may be developed.
The overhead system is a little more complex. Potential Damage Mechanisms for column overhead and reflux equipment (per API RP 571):
• 2 – Wet H2S
• 5 – Polythionic Acid Cracking
• 8 – Ammonium Chloride
• 9 – HCl Corrosion
• 20 – Erosion/Erosion Corrosion
• 42 – CO2 Corrosion
• 48 – Ammonia Stress Corrosion Cracking
• 52 – Liquid Metal Embrittlement
• 66 – Organic Acid Corrosion
The difficulty in this corrosion loop is that the potential damage mechanisms that will actually occur are dependent on the materials of construction, operating temperature and other process conditions such as pH. Since that is more than most inspectors are going to try to define let’s do a little bit of screening to get a feel for the advantages of using API RP 571.
Polythionic Acid Cracking and Liquid Metal Embrittlement present potential problems with Type 300 series stainless steels. If Type 300 series stainless steels are not used in the overhead, these mechanisms are not active. Wet H2S damage and Ammonia Stress Corrosion Cracking may occur in carbon steels in alkaline conditions. Most often, atmospheric column overheads are neutral or slightly acidic due to chlorides. For this corrosion loop, the primary concern is internal thinning and corrosion at and below the dew point (where water condenses). The corrosion is likely to be localized due to chlorides forming other corrosive products in aqueous conditions. The damage descriptions for the thinning mechanisms will provide insights into where inspection should concentrate. Finally, cracking is a potential damage type with H2S damage being the most likely. Including some weld related inspection or for possible hydrogen blisters of primarily non-PWHT’d pressure vessels may be included in the plan.
API RP 571 includes a fairly conservative list of possible damage mechanisms, even if they are not highly likely to occur. Having an experienced materials specialist to consult with can help you screen out inactive mechanisms and focus the plan on active mechanisms. If you don’t have a materials specialist available, the most effective method is to concentrate on the materials of construction, operating temperatures and damage types (thinning, cracking, etc.) to identify the active mechanisms, group the mechanisms by damage type for each material of construction and develop the inspection plan based on methods for detecting and sizing those specific types of damage. Most corrosion loops will be relatively simple and not nearly as complex as an atmospheric column overhead. You’ll learn more every time you do it and it will get easier with practice.
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