We talk a lot about flanged and welded connections but not so much about threaded connections. This is mainly based on my limited experience with threaded connections used in HAZMAT processes. But one thing I did learn about threaded connections is a MAJOR prohibition for threaded connections @ vessels when the piping/vessel will contain flammable liquids, gases, or vapors.
Using ASME Section VIII as my RAGAGEP, we can see the limitation that should be caught during the DESIGN phase and any CHANGES to the process via the MOC hazard/RAGAGEP review.
Using UG-43, we see the following: (emphasis and spacing by me)
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UG-43 METHODS OF ATTACHMENT OF PIPE AND NOZZLE NECKS TO VESSEL WALLS (e) Threaded Connections. Pipes, tubes, and other threaded connections that conform to the ASME Standard for Pipe Threads, General Purpose, Inch (ASME B1.20.1) may be screwed into a threaded hole in a vessel wall, provided the pipe engages the minimum number of threads specified in Table UG-43 after allowance has been made for the curvature of the vessel wall. The thread shall be a standard taper pipe thread except that a straight thread of at least equal strength may be used if other sealing means to prevent leakage are provided. A built‐up pad or a properly attached plate or fitting may be used to provide the metal thickness and number of threads required in Table UG-43, or to furnish reinforcement when required. Threaded connections larger than 4 in. pipe size (DN 100) SHALL NOT BE USED in vessels that contain liquids having a flash point BELOW 110°F (43°C), or flammable vapors, or flammable liquids at temperatures above that at which they boil under atmospheric pressure. Threaded connections LARGER than 3 in. pipe size (DN 80) SHALL NOT BE USED when the maximum allowable working pressure EXCEEDS 125 psi (0.8 MPa), except that this 3 in. pipe size (DN 80) restriction does not apply to plug closures used for inspection openings, end closures, or similar purposes, or to integrally forged openings in vessel heads meeting the requirement of UF-43. |