Incorporate flexibilities at the vessel and nozzle intersection - CAESAR II - Reference Data

CAESAR II Applications Guide

Reference Data

Before any potentially costly piping system modifications are made, you will analyze the model a second time to incorporate WRC 297 nozzle flexibilities. A more thorough and accurate model of the system might show that redesign is not required. To assist in this model update, CAESAR II calculates and inserts these flexibilities into the system. The pump loads caused by expansion decrease if the thermal growth of the three pipe legs A, B, and C deflect the vessel nozzle. Welding Research Council (WRC) Bulletin 297, Local Stresses in Cylindrical Shells Due to External Loadings on Nozzles (Supplement to WRC Bulletin No. 107) defines nozzle flexibilities. WRC 297 supplies curves by which the outer diameters and thicknesses of the vessel and nozzle define local nozzle flexibilities. These curves are limited to specific ratios of nozzle and vessel terms, such as the following:

d/D < 0.5

d/t > 20

20 < D/T < 2500

d/T > 5


d = nozzle OD (8.625 in.)

t = nozzle thickness (0.322 in.)

D = vessel OD (60 in.)

T = vessel thickness (3/16 in wall thickness + 1/4 in. reinforcing pad = 7/16 in.)

Because the vessel is vertical and the nozzle is in the Z direction, the software defines flexibilities at node 40 for translation along the Z-axis and rotation about the X- and Y-axes. The other three degrees-of-freedom (the three local shear terms) remain rigid because the nozzle was modeled as a rigid connection with its thermal deflections.

  1. In the main window ribbon, click Home > Input > Piping Input with Tutor-B as the current model file.

    The Classic Piping Input dialog box and a graphic view display.

  2. Click File > Save As, and change the model file name to Tutor-B2.

  3. Click Next Element repeatedly to advance through the model, or double click the graphic view at the end of the horizontal run to display element 35-40.

  4. Click the Displacements box.

    The Displacements tab of the auxiliary panel displays.

  5. Change the value of Node 1 from 40 to 6000. Do not change the displacement values.

    Node 6000 represents the steam stripper vessel. Because the vessel has thermal growth, the thermal displacements previously assigned to node 40 are reassigned to the new vessel node 6000. Do not define a piping element between nodes 40 and 6000.

  6. Double-click the Nozzle Flex box.

    The Nozzles tab displays on the right.

  7. Specify the following properties on the Nozzles tab:

    • Nozzle Type: WRC 297

    • Nozzle Node: 40

    • Vessel Node (optional): 6000

    • Direction Cosines: VY: 1

      A value of 1 for the Y-direction vector specifies a vertical vessel.

    • Nozzle Details:

      • Outer Diameter: 8.625

      • Wall Thickness: 0.322

      • Distance to Stiffener or Head: 48.000

      • Distance to Opposite Stiffener: 72.000

    • Vessel Details:

      • Outer Diameter: 60.000

      • Wall Thickness: 0.250

      • Pad Thickness: 0.188

    • With the addition of these properties, the software applies the calculated nozzle flexibilities between nodes 40 and 6000.

    • For WRC 297, the nozzle and vessel orientation defines the local coordinate system. With the nozzle in the Z-direction and the vessel in the Y-direction, the new axial stiffness is in the global Z-direction (the nozzle centerline), longitudinal bending is about the global X-axis (bending into the vessel centerline or long axis), and circumferential bending is about the global Y-axis (about the vessel centerline).

    • Because WRC 297 flexibilities are sensitive to the proximity of stiffeners to the nozzle, the software specifies the vessel dimensions. A tray in the vessel is closest to the nozzle and 4 feet above the nozzle. On the other side of the nozzle, the bottom head tangent and skirt connection is 6 feet below.

  8. Click Save to save definition of the WRC 297 vessel nozzle.