Large Diameter Tubes

The techmque was first employed, in real-world conditions, for monitoring external corrosion in the large diameter steel tubing used for oil well casings. In the late fifties, T.R. Schmidt at Shell Developments, pioneered the technique in those demanding applications, although the technique itself was invented, by W.R. MacLean, (Ref. 1) somewhat earlier. T.R Schmidt has written a history (Ref. 2) of much of the early work in the technology, which contains many references, others which may be of interest are held on the NTIAC database (Ref 3). 

The complete problem with composition gradients as well as a pressure gradient, may be regarded as a "generalized Poiseuille problem", and its Solution would be valuable for comparison with the limiting form of the dusty gas model for small dust concentrations. Indeed, it is the "large diameter" counterpart of the Knudsen solution in tubes of small diameter. 

Fig. 8. Cyclone types commonly used (161) (a) conventional, large diameter, tangential inlet, axial discharge (b) smaller tube, tangential inlet, peripheral concentrated aerosol discharge (c) small tube axial inlet and discharge (d) smaller tube axial inlet, peripheral concentrated aerosol discharge.

Fig. 8. Shell-and-tube heat exchanger A, shell of high strength B, tube sheet C, tubes (normally small diameter tubes are seamless, but large diameter tubes (>1 in.) are welded tubes) D, boimets E, baffles to assure more efficient circulation by providing minimum clearance between tubes and tube holes...

Heat Release and Reactor Stability. Highly exothermic reactions, such as with phthaHc anhydride manufacture or Fischer-Tropsch synthesis, compounded with the low thermal conductivity of catalyst peUets, make fixed-bed reactors vulnerable to temperature excursions and mnaways. The larger fixed-bed reactors are more difficult to control and thus may limit the reactions to jacketed bundles of tubes with diameters under - 5 cm. The concerns may even be sufficiently large to favor the more complex but back-mixed slurry reactors. 

Tubular Fixed-Bed Reactors. Bundles of downflow reactor tubes filled with catalyst and surrounded by heat-transfer media are tubular fixed-bed reactors. Such reactors are used most notably in steam reforming and phthaUc anhydride manufacture. Steam reforming is the reaction of light hydrocarbons, preferably natural gas or naphthas, with steam over a nickel-supported catalyst to form synthesis gas, which is primarily and CO with some CO2 and CH. Additional conversion to the primary products can be obtained by iron oxide-catalyzed water gas shift reactions, but these are carried out ia large-diameter, fixed-bed reactors rather than ia small-diameter tubes (65). The physical arrangement of a multitubular steam reformer ia a box-shaped furnace has been described (1). 

The outer tube limit approaches the inside of the sldrt but is farther removed from the inside of the shell than for any of the previously discussed constructions. Clearances between shell diameter and bundle OTL are 22 mm (% in) for small-diameter pipe shells, 44 mm (1% in) for large-diameter pipe shells, and 58 mm (2M6 in) for moderate-diameter plate shells. 

Figure 3.23 Perforation at a dish-shaped depression on the internal surface of a large-diameter steel pipe. A large tubercle capped the depression but was dislodged during tube sectioning. (Courtesy of National Association of Corrosion Engineers, Corrosion 89 Paper No. 197 by H. M. Herro.)...

In hardwoods, morphological structural elements in longitudinal series comprise the segmented structure termed vessel . Vessels, which are exposed in transverse section, constitute about 10-46% of the stem volume in deciduous hardwoods and are cells of relatively large diameters (50-300 p.m). Vessels have in short the appearance of open vertical tubes within the wood structure because their end walls have partially dissolved. By comparison, the hardwood vessel diameter can be as much a 10 times the diameter of a softwood fiber. 

Tubes with diameters larger than 2 nm usually exhibit defects, kinks, and twists. This is illustrated in the TEM image of several relatively large nanotubes shown in Fig. 3b. The diameter of the tubes seems to vary slightly along the tube axis due to radial defor-... 

Fig. 3b. Large-diameter tubes produced with Co and S present the tubes shown have approximate diameters of 5.7, 3.1, and 2.6 nm.

There are two basic types of packed-bed reactors those in which the solid is a reactant and those in which the solid is a catalyst. Many e.xaniples of the first type can be found in the extractive metallurgical industries. In the chemical process industries, the designer normally meets the second type, catalytic reactors. Industrial packed-bed catalylic reactors range in size from units with small tubes (a few centimeters in diameter) to large-diameter packed beds. Packed-bed reactors are used for gas and gas-liquid reactions. Heat transfer rates in large-diameter packed beds are poor and where high heat transfer rates are required, Jluidized beds should be considered. ... 

Figure 2-38C. Critical Pressure Ratio, r, for compressible flow through nozzles and venturi tubes. By permission, Crane Co., Technical Paper 410, 1957. Also see 1976 edition. See note at Figure 2-18 explaining details of data source for chart. Note P = psia p= ratio of small-to-large diameter in orifices and nozzles, and contractions or enlargements in pipes.

Although zinc has an appreciable vapour pressure at the temperatures of treatment, it is unlikely that zinc vapour plays any significant part in the diffusion process and it is generally accepted that the mechanism relies almost exclusively on intimate contact of hnely divided zinc dust with the steel surface. In spite of this requirement, coatings of even thickness and composition are obtained on the most intricate shapes, on fine threads, inside blind holes, and in the bore of small-diameter tubes. Large articles of uniform section, e.g. rods, tubes, etc. can be coated by this process. 

A convenient apparatus is made by connecting a i-l. round-bottomed flask by means of a bent tube of large diameter (2 cm.) to a second i-l. round-bottomed flask containing 500 cc. of water. The second flask is equipped with an exit tube, and the gases which do not dissolve in the water are led into a hood. 

Tube diameter shows a more regular pattern than mass velocity, as the examples in Fig. 29 indicate. The tendency is for the burn-out flux to rise smoothly as the diameter increases, and to approach a limiting value. A slight drop in burn-out flux is shown by the curves (1) and (2) in Fig. 29, and this feature has been confirmed by experiment. Lee (LI) has even noted that at very large diameters the slight drop may revert to a rise in burn-out flux. 

L2. Lee, D. H An experimental investigation of forced convection burnout in high pressure water. 4. Large diameter tubes at about 1600 psia, AEEW-R 479 (1966). 

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