Industry Trends: The Alloy You Need for REACs
With an increased demand for cleaner fuels, refineries are taking a new approach to a variety of feedstock–from sweet to sour crude–that exposes their process equipment to harsher environments.
In turn, these harsh environments increase the demand for the equipment’s performance, which ultimately leads to design and metallurgical upgrades. One refinery process that has recently been experiencing such needs for a new standard of performance is the hydroprocessing unit.
Hydroprocessing improves feedstock by removing impurities, such as sulphur and nitrogen, and ensuring the output meets the increasing demand of low sulphur fuels around the world. One of the key systems of a hydroprocessing unit is the Reactor Effluent Air Cooler (REAC). This is the step used to cool and/or condense process streams using ambient air as the cooling medium. As the reactor effluent stream cools down through a series of heat exchangers and REACs, solid ammonium bisulfide (NH4HS) can crystallize out of the vapor phase, plugging the exchanger and causing material failure due to sour water corrosion. It’s critical to use high quality tubes in hydroprocessing not only for optimal equipment performance but to extend its lifecycle.
Historically, duplex stainless steels have been an economical alternative to more expensive nickel alloys for corrosion resistance in ammonium bisulfide environments. However, the refining industry has been experiencing in-service failure of REACs constructed with duplex stainless steels. These failures have been attributed to the high ferrite content found in the imbalanced microstructure of the heat-affected zone left behind after welding, which is a product of poor fabrication practices.
Due to the lack of reliable non-destructive testing and inspection techniques that would allow to check for proper ferrite-austenite balance in the heat-affected zone of thicker duplex sections, and the high risks of failures associated with this issue, the use of duplexes for this application has been on the decline.
Nickel-based alloys, such as alloy 825, have now become the material of choice for these applications. Alloy 825 is a traditional nickel-based alloy suitable for sour water corrosion applications. It has good weldability and excellent resistance to various types of corrosion in both chloride and sour environments.
Alloy 825’s chemical composition allows it to have PRE numbers between 28 and 36. And because it’s a nickel-based alloy, the nickel content makes it resistant to stress corrosion cracking. A joint industry study even ranked alloy 825 at the same level of ammonium bisulfide corrosion resistance as alloy 625.
Controlled Carbon Content
Our controlled, fully traceable production process at our melt shop in Sweden allows us to optimize the chemical composition of our alloy 825, Sanicro® 41, consistently limiting the carbon content to less than 0.01%. Having such a low carbon content helps us ensure this material will not sensitize in service and the extra step of stabilizing anneal is not necessary.
At Sandvik, we have a long history of working with alloy 825 in various applications globally, including critical applications like hydro-processing in refineries. Sanicro® 41 passes the most stringent customer specifications and intergranular corrosion testing.
Want to learn more about material selection for high temperature corrosion? Watch our recent Materials Expertise webinar on High-Temperature Corrosion and Material Selection.
Jennifer Svensson, Product Manager, Heat Exchanger Tubes