...
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. When defining a Pipe property set for the trunnion, make sure there are no contents as the trunnion support does not carry fluids. 
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.
...