now, I have some questions:
1.when we have a harmonic analysis for reciprocating compressor discharge pipe, whether the drum should be added to the static model?
2.A static analysis result shows sustained case(w+p1) cann't pass,what shall I do with it.

loking forward to your replies

1) The drum should be included for "weight contributions". CAESAR II isn't going to perform any sort of acoustic or hydraulic analysis, so from CAESAR II's point of view, only the weight is important.

2) Typically if the Sustained case is failing, the system needs additional supports.

Thank you!

But I take the harmonic analysis only to get the natural frequency of the piping system. when I add the drum into the system, the value of frequency shows quite different. for example, a piping system with a drum, the value of the natural frequency(mode 1) is 15.954HZ; but without the drum, the value of mode 1 changes to 0.711HZ, so my question is: should the drum be added to the system.

regards

It appears that "without the drum" you may have left the model "free ended", i.e., no boundary condition such as an anchor or displacement at this location.

A harmonic analysis will not give you the natural frequencies of the system. For this you need to perform a modal analysis.

Yes the drum would affect the natural frequencies, it will alter both the mass and the stiffness. Since the drum is really there in the physical system, it should be included in your analysis.