3.5.4 Use of CNODES When Modeling Restraints

As discussed elsewhere in these seminar notes, CNODES (short for connecting nodes) are used with a restraint whenever a point in the piping system is retrained against another point in the system. The CNODE point may be physically on a vessel, a steel structure, or another piece of pipe; or it may be a fictitious node which can be used to model a wide range of system behaviors, such as settlement.

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For example, pipe on pipe supports can only be modeled using CNODES. One example is shown in Figure 3-112. In that figure, node point 30 rests on a saddle, which fits with an arc (unwelded) to the pipe at node point 225. Because the saddle is welded to the branch line and partially wraps around the pipe at node point 225, node point 30 cannot move down or in the Z-direction, but can move up and can slide in the X-direction —relative to node point 225. If this restraint is simply coded as a +Y and Z restraint, with Mu=0.3, at node point 30, this would indicate restraint against a rigid point in space. This is obviously incorrect; the model can be rectified by placing a CNODE of 225 on both the +Y and Z restraints. This will permit node point 30 to move freely relative to rigid points in space, but provides restraint relative to node point 225.

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A further example is shown in Figure 3-113. Node point 65 is supported from the overhead pipe at node point 195. The rod shown can handle tension only. Therefore node point 65 can move freely relative to rigid points in space, but is constrained against moving down relative to node point 195. The restraint is therefore specified as a +Y restraint at node point 65, with a CNODE of 195. Note that it doesn't matter which node is the connecting node as long as the proper relationship between the nodes is maintained; i.e., the restraint could be specified equally correctly as a -Y restraint at node point 195, with a CNODE of 65.

CNODES can also be used to accurately model the effect of settlement. In Figure 3-114, the pipe rests on the ground at node point 95. The ground at this point is expected to settle in the - Y direction; the pipe may or may not settle with it depending on the stiffness of the nearby pipe. This configuration can be modeled with a +Y restraint at node point 95, with a CNODE of 1095 (friction is optional). The settlement displacement of -0.325 inches in the Y-direction is then imposed at node point 1095. This allows node point 95 to either lift-off, or to settle, according to the configuration of the piping system as a whole.

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Modeling And Analysis Of The Piping System

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