3.6.4 Jacketed Pipe
Jacketed piping can be idealized as two piping systems running along coincident lines in space. The two piping systems are of two different sizes, so the smaller runs inside of the larger. The internal pipe contains the piped fluid, while the outer pipe (the jacket) is used for protection or to carry a heated fluid to warm that in the inner pipe.
An example of jacketed pipe is shown in Figure 3-121.
Jacketed piping systems are modeled by running the jacket elements directly on top of the core elements where the two are concentric. Internal supports (spiders) offer negligible resistance to relative bending and axial displacement, so rigid restraints should be placed between the inner and outer pipe (for example between node points 15 and 1015) only in the local Y- and Z-directions. The end caps connecting the core to the jacket of the pipe are usually much stiffer than either the core or the jacket. For this reason at end cap locations (node points 10 and 25), the inner and outer pipes should share the same node point (i.e., node points 1010 and 1025 should not be used) — this ensures that the rotations and displacements are identical for the two pipes at these locations.
The +Y support acting on the jacket at node point 1020 does not cause any restraint to be inserted between nodes 20 and 1020. Node 20 is included in the model so that interference with the outside diameter can be checked at the 20-1020 cross section. Should there be concerns about interference, a restraint with a gap equal to the clearance between the inner and outer pipes can be entered. If a load develops at that restraint, this indicates an interference.
The specific modeling process is fairly simple. The inner pipe is modeled first, with the user taking care to place a node point at each location where there is an internal spacer support. Next the inner pipe is duplicated using the element block copy feature (accessed with the List hot key from the input spreadsheet). The entire run of the inner pipe should be copied, with a suitable node increment to ensure that no nodes are duplicated between the two copies.
The second copy becomes the outer pipe. It is necessary to first change the diameter and wall thickness (and possibly the fluid density and temperature) of the pipe on the first screen of the outer pipe; these changes propagate through. Next the user must go through and change the bend radii of each of the elbows in the outer pipe. The first and last node numbers of the outer pipe should then be changed to the same node numbers as those of the first and last point of the inner point — this serves to connect the inner to the outer pipe, a fact that can be confirmed by using CAESAR II's PLOT option.
Finally, the internal supports are modeled by placing guides (and vertical supports on horizontal runs) at each of the support points on the inner pipe, with CNODES to the corresponding points on the outer pipe. Any pipe restraints are then placed on the outer pipe only.
Modeling And Analysis Of The Piping System
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