The latest version of the standard for protecting equipment during refinery shutdowns is . While some search for a "2021" version, the 2018 edition remains the current active standard practice.
) reacts with iron in the equipment walls to form iron sulfide ( FeScap F e cap S ) scale. During a shutdown, if ambient oxygen ( O2cap O sub 2 ) and water ( H2Ocap H sub 2 cap O
When the unit cools and opens, oxygen and moisture from the air react with that sulfide scale. This creates Polythionic Acid ( H2SxOycap H sub 2 cap S sub x cap O sub y ) .
To eliminate moisture and oxygen, thus preventing the formation of polythionic acids. 2. Alkaline Washing (Soda Ash Treatment) nace sp0170 pdf 2021
For PTA SCC to occur, three specific factors must align simultaneously:
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No. This is a common point of confusion. deals with "Control of External Corrosion on Underground or Submerged Metallic Piping Systems," focusing on cathodic protection for buried pipelines. NACE SP0170 is entirely different, focusing on internal cracking of refinery equipment (stainless steel alloys) during shutdowns. The latest version of the standard for protecting
If a system cannot be washed (due to catalyst restrictions) and must be opened, is introduced. High-capacity desiccant dryers circulate air through the equipment to keep the relative humidity (RH) below 20%. Without liquid moisture, polythionic acid cannot form, suppressing the cracking mechanism. Comparison of Mitigation Tactics Primary Mechanism Best Used For Key Operational Risk Nitrogen Blanketing O2cap O sub 2 and liquid water No internal work required Confined space asphyxiation hazards Alkaline Wash ( Na2CO3cap N a sub 2 cap C cap O sub 3 ) Neutralizes acids on contact Full equipment turnaround Spent solution disposal; catalyst poisoning Dehumidification ( Eliminates liquid water phase Catalyst dumping/skimming Equipment failure can cause rapid humidity spikes Sourcing Official PDF Documentation
Will you be for catalyst replacement, or keeping them closed?
While the title is a mouthful, the document represents a critical evolution in industrial safety. Here is why the 2021 revision of SP0170 is one of the most important downloads for materials engineers this decade. During a shutdown, if ambient oxygen ( O2cap
NACE SP0170 is specifically designed to prevent . PTA SCC is a particularly dangerous failure mode because it can lead to catastrophic brittle fractures of equipment made of austenitic stainless steel, specifically during plant shutdowns or start-ups.
Prevention starts at the design stage. The standard guides engineers to select materials that are resistant to sensitization. This involves controlling chemistry (low carbon grades like 304L or 316L) or using stabilized grades (like Type 321 or 347) which contain niobium or titanium to prevent chromium carbide precipitation during welding or high-temperature service.
Surfaces must be washed with an alkaline solution to neutralize any acids formed. This typically involves soda ash ( Na2CO3cap N a sub 2 cap C cap O sub 3 ) or soda ash plus ammonia ( NH3cap N cap H sub 3