Also be aware that when you specify a displacement at a node point you are declaring a "boundary condition" (in other words a restraint). Once this DOF (degree of freedom) is restrained, it is restrained for all load cases, even if you don't include the "D1" vector in the load case.

As an example, consider this:

Node 150, dy=+10mm
L1 W+P1+T1 (OPE)
L2 W+P1+T1+D1 (OPE)

In case 2 node 150 will be displaced up 10mm, the other 5 degrees of freedom will be free to do anything necessary. In case 1, node 150 will be fixed in the "DY" direction and have a resulting displacement of zero. This is because the "DY" direction has been fixed by your displacement specification, and in the real world you can't run out and change your boundary conditions between load cases.

Make sure you don't have the "tail wagging the dog here". Don't specify a boundary condition (either a restraint or a displacement) unless you really have the pipe attached to something. If you need to model "real world displacements" at an arbitrary point in the piping system, that is not attached to something, then you should consider applying a concentrated force. Using a force allows the node to move as necessary in all load cases, applying a displacement fixes that node at either zero or the specified displacement.