i have read some previous topic regarding this matter but i really cannot get the simple answer.

please can somebody clarify if seismic force need to be added to operating load (OPE+SEIS) to compare to nozzle allowables of any equipments (i.e. API610)?

and also, is it necessary to add seismic x and z by SRSS combination method before adding to load combinations?

your reply is greatly appreciated.

regards!

Load combinations for evaluating stress or equipment are subject to project criteria.
That is it will depend on local regulation and governing piping code.

Either you take X/Z and X/Y (an look for the maximum force an moments) or you calculated X/Y/Z !! SRSS is ok.
Then absolut combination with static load.
The dynamic force/moments answer have + and - sign !
Correctly you must have finally 2 combinated load cases. +HS and -HS.
HS ist the load declaration for static und dynamic load sum.