Q. What is Expansion Joints?
Expansion joints are used to accommodate thermal growth in piping and pressure vessel applications. The metal bellows, an integral part of an expansion joint, is removed from direct exposure to the process stream and may therefore be at a different temperature. Some general guidelines for bellows design are as follows:
• Select a material based on the actual measured or calculated bellows material temperature (ABMT).
• Allow natural thermal expansion through the use of floating construction, localized flexibility, and contoured sections.
• Avoid abrupt changes in cross section which result in stress concentrations and use of bimetallic joints.
• Properly locate insulation to slow heat flow and reduce thermal stress.
• Annealing after forming should be considered for those metals which will be operating in the high temperature range to reduce the effect of cold work.
Several alloys are often specified for bellows in high temperature applications. Austenitic stainless steels, type 304H, 316H, and 321H show good corrosion resistance to most high temperature applications. Nickel alloys 800 and 800HT have high temperature strength and good corrosion resistance as well as good resistance to oxidation and sulphidation. Alloy 800HT, however, has failed due to creep in high temperature service and should be annealed to improve creep ductility. Alloy 235MA has excellent resistance to oxidation and carburization and has been used successfully for bellows with 1500°F (815°C) ABMT. Alloy 625 may be used up to 1000°F (540°C) with excellent corrosion resistance. Above 1000°F (540°C) the material may suffer brittle fracture due to loss of ductility. Alloy 825 may also be used up to 1000°F (540°C), and has been reported to have superior resistance to carbide precipitation than alloys 800H and 625. Alloys 230 and 617 have been used up to 1300°F (704°C) with excellent resistance to oxidation and carburization, however, there is a possibility of sulphidation.