U bends

There are several methods to determine and compare the resistance to partial discharges. Some tests are done on finished cables, such as the U-bend test, and others are done on laboratory samples molded from the insulation, that are subjected to partial discharges created by sharp objects, such as needles under high voltages. The tests compare either the energy required or the length of time required to erode or fail (short circuit) samples of similar thickness. 

S tandard Practicefor Making and Using U-Bend Stress—Corrosion Test Specimens, ASTM G 30-84, American Society for Testing and Materials, Philadelphia, Pa., 1992. 

Siace dimer acids, monomer acids, and trimer acids are unsaturated, they are susceptible to oxidative and thermal attack, and under certain conditions they are slightly corrosive to metals. Special precautions are necessary, therefore, to prevent product color development and equipment deterioration. Type 304 stainless steel is recommended for storage tanks for dimer acids. Eor heating coils and for agitators 316 stainless steel is preferred (heating coils with about 4s m (50 ft ) of heat transfer surface ia the form of a 5.1 cm schedule-10 U-bend scroU are recommended for a 37. 9-m (10,000-gal) tank. Dimer acid storage tanks should have an iaert gas blanket. 

Cascade coolers are a series of standard pipes, usually manifolded in parallel, and connected in series by vertically or horizontally oriented U-bends. Process fluid flows inside the pipe entering at the bottom and water trickles from the top downward over the external pipe surface. The water is collected from a trough under the pipe sections, cooled, and recirculated over the pipe sections. The pipe material can be any of the metallic and also glass, impeiMous graphite, and ceramics. The tubeside coefficient and pressure drop is as in any circular duct. The water coefficient (with Re number less than 2100) is calculated from the following equation by W.H. McAdams, TB. Drew, and G.S. Bays Jr., from the ASME trans. 62, 627-631 (1940). 

The same end may be achieved by continuous operation at total external reflux with a small U bend in the reflux line for foamate holdup [Rubin and Melech, Can. ]. Chem. Eng., 50, 748 (1972)]. 

Slow Strain-Rate Test In its present state of development, the results from slow strain-rate tests (SSRT) with electrochemical monitoring are not always completely definitive but, for a short-term test, they do provide considerable useful SCC information. Work in our laboratory shows that the SSRT with electrochemical monitoring and the U-bend tests are essentially equivalent in sensitivity in finding SCC. The SSRT is more versatile and faster, providing both mechanical and electrochemical feedback during testing. 

As with other removable-bundle exchangers, the inside of the shell, and to the outside of the tubes. However, unlike the straight-tube exchanger, whose tube internals can be mechanically cleaned, there is no way to physically access the U-bend region inside each tube, so chemical methods are required for tubeside maintenance. 

Model IV Regenerator and reactor at approximately equal elevation and pressure. Catalyst circulates through U-bends, controlled by pressure balance and variable dense-phase riser. 

The common theme of many of these items is that blowdown lines and flare and vent stacks should be kept simple because they are part of the pressure relief system. Avoid flame arrestors, molecular seals, water seals, and U-bends. Avoid steam, which brings with it laist and scale and may freeze. 

Water can be trapped behind heat exchanger baffles and then suddenly vaporized by circulation of hot oil. It can also be trapped in dead-ends and U-bends in pipework (see Section 9.1.1). Such U-bends can form when one end of a horizontal pipe is raised by thermal expansion. The trays in a distillation column were damaged during startup when hot gas met water, from previous steaming, dripping down the column [3J. Section 17.12 describes an incident somewhat similar to a foamover. 

The recommended minimum radius of bend for various tubes is given in Table 10-5. These measurements are for 180° U-bends and represent minimum values. 

TEMA, Par. RGB 2.31 recommends the minimum wall thinning of tubes for U-Bends by the minimum wall thickness in the bent portion before bending, p. 

Figures 10-25A-E are for U-bundle tubes and require a wide blank space across the center of the tubesheet to recognize the U-bend requirement at the far end of the tube bundle, see Figures 10-lE, 10-lAitem U, and 10-lK Figures 10-25F-Kare for fixed tubesheet layouts.

Elimination of one tubesheet reduces initial cost. Tube bundle is removable for inspection and cleaning. Full tube bundle minimizes shell-side bypassing. U-bends permit each tube to expand and contract individually. Tube bundle expansion is independent of shell no expansion diaphragm is required. 

The tube bundle is an assembly of tubes rolled into tubesheets and assembled into headers. See Figures 10-175, 10-176, 10-178, 10-179 and 10-180. The usual headers are plug and cover plate but can accommodate U-bend types if the design so dictates. 

Maraging steel welds are somewhat less resistant than base plate. U -bend exposure of 1 240 MNm strength welds survived for up to 2 years in seawater while at 1 380 MNm failures occurred in 2-18 months. ... 

Studies on welds showed that cracking resistance as indicated in U -bend tests was substantially increased by a post weld anneal (1 h at 815°C) prior... 

Practice for making and using U-bend stress corrosion test specimens Recommended practice for laboratory immersion corrosion testing of metals Method for vibratory cavitation erosion test Practice for recording data from atmospheric corrosion tests of metallic-coated steel specimens... 

Corrosion of metals and alloys stress corrosion testing. Part 2 Bent-beam specimens Corrosion of metals and alloys. Part 3 U-bend specimens... 

The dissipated energy due to flow resistance and turbulence F consists of four parts the energy consumption in the riser, that in the separator tubes, X that in the down-comer, and that due to the flow direction changes of the U-bending channel at the top and bottom of the reactor,/,. Therefore, F can be written as ... 

FIG. 25-17 Specimens for stress-corrosion tests, (a) Bent beam, (b) C ring, (c) U bend, (d) Tensile, (e) Tensile. if) Tensile, (g) Notched C ring, (h) Notched tensile. (0 Precracked, wedge open-loading type, (j) Precracked, cantilever beam. [Chem. Eng., 78,159 (Sept. 20, 1971)1]... 

If U-tubes are used the number of tubes will be slightly less than that given by equation 12.3a, as the spacing between the two centre rows will be determined by the minimum allowable radius for the U-bend. The minimum bend radius will depend on the tube diameter and wall thickness. It will range from 1.5 to 3.0 times the tube outside diameter. The tighter bend radius will lead to some thinning of the tube wall. 

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