When specifying tubing for equipment used in power plant condensers, heat exchangers and desalination plants, finding a material that can safely and efficiently handle a variety of chemical environments is key. Not only must the tubing be able to withstand forming and fabrication requirements unique to your operation, but you also want to understand how the material will operate in specific temperature ranges, water properties, and will withstand corrosive environments. The proper material selection will not only save time and money over the life of the tubing but specifying the correct size increases thermal conductivity which increases efficiency from the beginning of the tubes’ operation.
The corrosive properties of stainless steel make it an obvious choice, but what else do you need to know?
Viable Material Options
Super Austenitic Stainless Steel
AL6XN® is a super austenitic stainless steel with excellent resistance to chloride pitting, crevice corrosion, and stress corrosion cracking. AL6XN was developed for and is used in highly brackish and highly corrosive environments. It has high nickel (24%), molybdenum (6.3%), nitrogen and chromium contents that give it excellent resistance to chloride stress corrosion cracking, chloride pitting, and exceptional general corrosion resistance. AL6XN is primarily used for its improved pitting and crevice corrosion resistance in chlorides. It is a formable and weldable stainless steel. Given its nitrogen content, AL6XN has greater tensile strength than other austentitics, while retaining high ductility and impact strength.
2507 is a super duplex stainless steel and also designed for brackish applications that demand exceptional strength and corrosion resistance. Alloy 2507 has 25% chromium, 4% molybdenum, and 7% nickel. This high molybdenum, chromium and nitrogen content results in excellent resistance to chloride pitting and crevice corrosion attack and the duplex structure provides 2507 with exceptional resistance to chloride stress corrosion cracking. Duplex 2507 is best used in applications below 600° F (316° C) as elevated temperature exposure can reduce both the toughness and corrosion resistance of alloy 2507.
Ferritic Stainless Steel
High performance ferritic stainless tubing is designed to provide superior corrosion resistance and versatility compared to standard stainless steels – at a lower cost than highly austenitic and duplex stainless steels, nickel base, and refractory alloys. The structure of super ferritic stainless steel is specifically intended for applications where chloride-induced pitting, crevice, and stress corrosion cracking may be encountered. If you are working in especially brackish and/or corrosive environments, a more specialized material, such as SEA-CURE® high-performance stainless steel, a tubing exclusive to Plymouth Tube Co, can deliver advanced performance and economics for equipment systems. The chromium and molybdenum protect against general acid corrosion and chloride pitting and the nickel addition further improves mechanical properties. The combined characteristics of high elastic modulus, strength and ductility are advantageous in achieving strong leak-tight joints. The nickel addition produces a tough crack-resistant base metal and weld that can be reduced to over 15% wall reduction or more. SEA-CURE is most often the most cost-effective steel tubing of these 3 materials and has the longest track record with greater footage in the power industry than the other 2 combined
Once you have the material that can withstand your unique environment, you may be wondering, will that material hold up to the welding, bending, and machining that our operation requires?
The bundle of tubes you select will be unique to the configuration of your plant and the environment in which it operates. There will be many variations, and although product decisions will be based foremost on properties such as corrosion resistance and strength, specifiers and buyers should not overlook manufacturing, especially fabricating, when it comes to material selection. It is important that the material selected can easily be formed and bent to your exact specifications and tolerances. The ferritic properties of SEA-CURE stainless steel, for example, provides a high strength/low work hardening material with good ductility, allowing for tube-rolling, flaring, and bending. These properties allow high design stress limits with good fabrication characteristics. Because of the nickel addition, SEA-CURE has a lower ductile-to-brittle transition temperature than similar ferritic steels without nickel additions.
Traditional ferritic stainless steels are sometimes considered difficult to weld, however by reducing carbon and nitrogen contents, weldability, ductility, and toughness are improved. More careful welding conditions can be used to attain good weld corrosion resistance and toughness too. For example, SEA-CURE uses gas tungsten arc welding (GTA) to minimize heat input.
Now that you have selected the material based on environmental and product requirements, and can be machined to your unique specifications, what else do you need to know when working with a supplier to ensure the highest quality?
Additional testing and specifications
Some tests you may want to discuss with your supplier include eddy current testing, Ultrasonic testing, and ASTM G48-C Corrosion Test, which is a critical pitting and crevice corrosion test. Your vendor should test in as-shipped condition and the material must pass at 65 degrees C.
You also want to ensure the tube is chemically passivated for optimal corrosion resistance and is not bright annealed. In summary, the following should be cited for reliable tubing:
- Ability to pass G-48 Method at 65º C for 72 hours in as-shipped condition
- Cold working of weld using both OD and ID tooling
- Air Under Water test at 150 psi
- Available ASTM A803-S1 eddy current testing
- Available ultrasonic testing
- Water quenched to ensure the absence of detrimental secondary phases
There are many things to consider when selecting tubing for your operating plant, particularly when the conditions are especially brackish or corrosive. Engineers must approach the process with an understanding of water conditions, flow rates, layup procedures; and the Buyers must approach the process with an understanding of the intricacies of sizing and machining requirements as well as the total cost of ownership over the life of the tubing.
Contact Plymouth Tube Co. today to speak with a technical specialist to learn more about selecting the best steel tubing for your next project.