Jacket Input (1st Half) - CAESAR II - Reference Data

CAESAR II Applications Guide

Reference Data

There are several ways to obtain the jacket model. The most obvious method is to continue using the Classic Piping Input dialog box to build the jacket. A second method is to duplicate the core pipe input file, and then use the CAESAR II include feature to combine the two models. Another method is to use the List processor and duplicate the necessary elements from within the preprocessor. The latter method is the one used for this example.

To begin modeling the jacket, click Edit > List. The software opens the List Option dialog box, which displays all of the applicable input data. Click the Elements tab to view a list of elements and their associated delta coordinates as shown below:

For the first half of the jacket, the core piping is duplicated using the steps below. The duplicated region starts at the pump and ends at the valve.

  1. Click in the row for the element from 1105 to 1110.

  2. Press Shift and click the row for the element from 1580 to 1590. This is the element just before the valve.

    The software highlights all rows between the two selections.

  3. Right-click the highlighted list and select Duplicate.

  4. In the Block Duplicate dialog box, click Identical.

  5. Click At End of Input to place the duplicate block.

  6. Specify 1000 for the node increment, and then click OK.

    CAESAR II duplicates the block and increments all of the node numbers by 1000. This results in a section of pipe identical to the pipe from 1105 to 1590 with node numbers from 2105 to 2590.

Three changes must be made to the new section of pipe to obtain the jacket piping. First, the diameter and wall thickness must be changed to 12-inches, schedule 40. You can use the List Editor to find the element from 2105 to 2110, and then type new values. Next, you must also specify the following values: jacket temperature, jacket pressure, jacket insulation, and jacket fluid weight. Finally, you must change all the jacket bend radii from long to short. The most straightforward way to do this is to click the Bends tab on the List Options dialog box to open the Bends list. Then, starting with the bend at node 2120, change the radius from Long to 12.0-inches. You must make this change to all of the remaining bends.

After the above changes have been made, the first half of the jacket is finished. A volume plot of the system now shows the core piping overlaid by the jacket piping.

Although the two models are correctly positioned with respect to one another, they are not connected. All you have done so far is duplicate several pipes. From the standpoint of CAESAR II, there are simply two discontinuous systems in the same input file. The graphics module plots discontinuous systems such that they all start from the same point, which is why the jacket and core line up properly in this case.

The next step is to correctly connect the jacket to the core and apply any external restraints. The connection between the jacket and the core piping must model the spiders that align the two in the real system. These connections can be modeled in CAESAR II using restraints with connecting nodes (CNodes).

A CNode associates degrees-of-freedom. If a CNode connects two nodes in the Y-direction, they experience identical displacements in the Y-direction. Use CNodes to restrain two nodes to each other without restraining them to the "outside world."

The modeling of the connection between the jacket and the core begins at the pump. In the Classic Piping Input dialog box, enter the restraint field. Then add a restraint at node 1105 with a CNode at 2105 of type anchor. This associates all six degrees-of-freedom between nodes 1105 and 2105.

Next, add two restraints at node 1110. Both of these restraints have a CNode at 2110, one in the Y-direction and one in the Z-direction. These two restraints model the spider between the core and the jacket.

The spider was not modeled using gaps. The actual clearance between the spider and the pipes is very small, and attempting to numerically model this clearance using restraints with gaps causes the job to be highly non-linear. Models with gaps at each spider have convergence problems and are unlikely to reach a solution.

Nodes 1110 to 1120 define the first elbow. Add four restraints to this spreadsheet as follows:

At node 1115, put a CNode of 2115 with Y- and Z-direction restraints

At node 1120, put a CNode of 2120 with X- and Z-direction restraints. These restraints are perpendicular to the axis of the pipe. Also, at node 2115 there is a positive Y external restraint. Add this support to the system on the spreadsheet containing node 2115.

Similarly, add the remaining spiders to the model.

An example of the completed input file, JACKET._A, is delivered to the [CAESAR II Directory]\Examples folder.

When node 1590 is reached, the CNode at 2590 is connected with an anchor. Also add the spring hangers at nodes 2525 and 2575.

Aside from the two anchors at the pump and the valve, all of the spider connections between the jacket and the core are modeled using two perpendicular restraints with connecting nodes. How are the other four degrees-of-freedom restrained? What keeps this model from undergoing rigid body motion? These questions can be resolved by considering two points. First, the jacket is continuous over the core from the pump to the valve. At both of these points, you have connected all six degrees-of-freedom. Second, the translational restraints prevent motion in the three translational directions. Additionally, these restraints also prevent rotation because the jacket is continuous.

Whenever a model is constructed, you must insure that the model, or parts of the model, cannot undergo rigid body motion. Such a model produces a singular stiffness matrix, and the solution cannot be attained. An example of such a poor model is a cantilever beam with a hinge at mid span.

At this point in the data input process, we recommend that you click Error Check to run the model through interactive error checking. CAESAR II saves the input and reports the results in the Error and Warnings dialog box. All reported errors should be corrected before modeling the second half of the piping jacket.