In this example we will add trunnions onto elbows (some people call these stanchions) in a line of 12"STD piping. If the trunnion is a pipe section users have the option to define and model it as either a structural pipe section which will not be code checked for stresses, or define it as a pipe section which by default will be code checked.
First, we will import an 8"STD section of piping using Define>Pipe sections and Define>Pipe property sets as covered in the CSiPlant tutorial and other sections of this Wiki. This modeling procedure will run a stress code check on the trunnion. When defining a Pipe property set for the trunnion, make sure there are no fluid contents in the pipe property set since trunnions do not carry process fluid.
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Next step is to draw orphan joints/nodes to the location where the centerline of the trunnion will be welded to the elbow. In this example we click the Draw joint icon where we enter -18" in the Z direction and click the two elbow joints as shown below left to add joints where the trunnion will be welded to the elbow. Next, assign a Body constraint to each pair of joints in order to create a zero weight rigid connection. As shown below right, there needs to be a separate Body constraint for each of these pairs of joints.
Next, draw additional 2 orphan joints/nodes below to the base of the trunnion. The offset distance of that newly drawn joint will define the length/height of the trunnion. In this example we'll make the trunnion 16" long. Click Draw Pipe icon, change the pipe property set to the 8"STD section which we imported earlier, clicking the base joint then double click the joint above it in order to draw the trunnions. Draw anchors at the base of the trunnions to complete.
Use a similar modeling technique to add a structural section to the elbow using Define menu>Structural sections. Here we import a Wide flange section from an AISC library of standard steel shapes and assign A992 material to it. You can assign any material which is compatible with the piping material to which the trunnion is attached.
Use the Draw frame icon to draw the frame section. In this example, we will rotate the local axis of the frame elements by 90 degrees. Select the frame elements that we want to rotate and use Assign menu>Frame>Local axes and type 90 degrees as shown below. After the frames are rotated, they display local axis arrows. Red/Green/Blue arrows correspond to local axes 1/2/3. Click 'Show Undeformed shape' icon to clear the display of the local axis arrows. Local 1 axis (red arrow) is axial/longitudinal direction.
Trunnions on elbows can reduce elbow stiffness. Users can reduce elbow flexibilities by selected elbows with a trunnion, then Design menu>Assign Design Properties for Object Type Elbow as shown below left. Users can also select elbows then assign Pipe>Property modifiers to reduce the moment of inertia and stiffness in other directions of selected elbows as shown below right.
