An underground steam line for a military base in Alaska, approximately 1.6 km (1 mile) in length, used the units depicted in Figure CS-32, each unit being 9.14 m (30 ft) in length. As shown, the steam line for each unit was enclosed in a cylindrical conduit made of sheet steel and weighed approximately 356 kg (785 lb). The conduit was to serve as the return drain line and to provide insulation. Each length of steam line was supported within the conduit by means of three U-shaped legs, made by cold-bending hot-rolled steel bar stock. See section A-A' for a cross section. These legs were welded to the pipe about 2 m from each end.
The units were fabricated in California and were transported to Alaska by a sequence of truck, barge, and railroad. Because of an early winter, only one-half of the line was installed the first summer. Before work was resumed the following spring, someone decided to test the line and found that there were numerous holes in the conduit. The holes (slits in casing) were located where the edges of the U-shaped supporting legs contacted the outside conduit (casing) on the top of the casing. As the result of these holes, the project was delayed and a costly litigation ensued.
1. Why do you think these holes occurred?
2. Was it a design error? A fabrication error? A service error? A service environment error?
3. What error(s) caused the failures? When did the defects actually develop?
4. What action would you recommend to avoid this problem in the future?
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5. What should be done about the part of the line already installed?
SOLUTION
1. The holes (cuts or slits) in the outer casing occurred where the edges of the U-shaped supporting legs contacted the casing. Essentially the entire weight of the outer casing rested on the upper vertical leg during transportation. During operation, the weight of the inner line and the steam will rest on the two lower legs.
2. A design error as explained below.
3. The designer of this line was not sufficiently familiar with cold forming operations to realize that when flat bar stock is bent to form the U-shaped support legs, a sharp corner will be produced on each outer corner of the bent pieces due to the stretching-contraction of the bars about the center line. This will create a line contact where the legs contact the outer casing, and the weight of the casing will be great enough to cause the load per unit area to exceed the strength of the casing wall in these regions. This was a design error. (The designer never went out to look at the line being fabricated or he might have caught this condition at the plant.)
4. Two design modifications are suggested. Grind off the sharp corners of the legs and shape the contour of the leg to conform to the interior curvature of the outer casing. This will reduce the unit loads to an acceptable level. The second design modification might be to eliminate the legs altogether and let the steam line simply lie on the bottom of the return line with a layer of appropriate insulation material placed between the two. This eliminates the legs and their fabrication and installation altogether.
5. The line must be totally disassembled and these modifications made even though no damage was visible or detectable. An entire new outer casing should be obtained since it appears that the damage was caused during shipping, not installation. Once operational, however, similar damage to the outer casing will be produced by the bottom legs, unless modifications are made.