Hi Venugopal,
The forces and moments given by the turbine supplier are the "allowables" from piping system. The displacements are the actual movements of the terminal points.
In piping analysis you should, therefore, model the pipe terminal point as an anchor having the thermal displacements as given by the supplier. Your objective would be to design the piping system so that the loads/ moments at that anchor (representing turbine connection) do not exceed the allowables. In most cases, you will struggle to achieve that
If supplier's allowables are less than NEMA 23, you do not have to assume the higher value. If they are, however, higher than NEMA 23 and you can not meet his stipulations, you may try by asking him to review the allowables and give reference of NEMA 23. Ultimately, the supplier's data prevail. You have to assume they have more exact methods (FEA) to determine the safe loads for their equipment and must adhere to that.
Hope this helps.
cheers,
Sanjay