When allowables are given for equipment, such as a HRSG, which occasional loads are required to be combined with deadweight and thermal to be compared to the basic allowable? If wind and seismic analyses are both required should both these loads be added to DWT + THERMAL in order to qualify the nozzle per the basic allowable given?
The combination of all loads together can result in drastic solutions such as using snubbers, which I have not seen too frequently today.
I'm wondering is this a more question for the client. e.g does he want to see snubbers on his system?

For the sake of simplicity consider the allowables for API 610 that everyone is familiar with. Do these allowables consider wind and or seismic loads being added on to DWT + THERMAL?

The equipment vendor should be asked the question about the allowable loads.
If the allowable loads are part of the client's design criteria they should be discussed with both parties.

The client should specify applicable "occasional" events that need to be considered in the design of the specific plant.

Use of the snubbers should be avoided IF POSSIBLE. Decision about the use of snubbers should come from the competent design engineer and based on all of the above (design criteria, criticality of the piping system, operating practices and expected climatic / seismic conditions).