I am working on a project where the steam turbine manufacturer has given nozzle displacements plus allowable forces and moments (which are very low). I have modelled the nozzles as displacements with the linear displacements DX, DY and DZ as given by the manufacturer but with rotational displacements RX, RY and RZ set to 0. In some of the operating cases the resulting moments exceed the manufacturers allowables. This is especially true in the OCC case for seismic loading. The piping system is largely suspended on spring hangers with appropriate seismic stops. Stresses in all cases are not a problem. My questions are as follows:

1. Is my modelling method correct with the nozzle rotations set to zero? Is there a better method so that the stiffness of the nozzles can be relaxed? Is the WRC method of modelling nozzles at vessels applicable here?

2. My operating case is (OPE)W+D1+T1+P1+F1
My sustatined case is(SUS)W+P1+F1
My occasional case is(OCC)UNI1 for seismic
My occasional case is(OCC)= SUS+UNI1
My thermal case is (EXP)= OPE-SUS
Are these cases correct for obtaining the forces and moments for the sustained case (turbine cold) and the operating case (turbine hot)?

3. The turbine manufacturers do not state if their allowable forces and moments are still applicable during a seismic event or do they allow some leeway here? What has been your experience?

1a) Is your modeling correct, with the rotation set to zero? Most likely "yes". Since you are at a nozzle, you can't leave the rotations "free" (unspecified). There shouldn't be any rotation due to the expansion of the turbine, but only the vendor can answer that one for sure.

1b) Is there a better method so that the stiffnesses can be relaxed? Again check with the vendor. Perhaps he can give you nozzle stiffnesses.

1c) Is WRC applicable? Maybe, it depends on the various geometric ratios. If you exceed the limitations of WRC you're better off using the displacements and being conservative. There are a number of posts in this forum on WRC limitations. Use the [search] option above to locate and review these.

2) Are the load cases correct? Yes your load cases are correct for the "cold" and "hot" cases. However, you should also check the "hot seismic" event. You can do this with:

L1 + L3 (algebraic)
L1 - L3 (algebraic)

3) Is there any force/moment leeway during the seismic event? Again, you need to get this sort of information from the vendor.