These are nearly identical spectra that I get long term on a 2 cylinder horizontally opposed recip. Here is a section from my monthly report. 1 ips rms = 25.4 mm/s rms.
Machine internals are ok.
The HP (v8V), cylinder head vert vib slowly varying around 13 mm/s. LP head vib 3.5 mm/s never changes (v7V).
Recent offshore work suggests this is a long term resonance problem, corresponding closely with 9x shaft speed, with the mass of the cylinder & head bouncing on the stiffness of the concrete column. The levels are 3 to 4 times the LP head, and cause the foundation bolts to pull loose every few years. The LP head and foundation is bigger than the HP head so modal characteristics will be different.
Ideally, modal analysis and an FEA model are needed to tune it to 8.5 or 9.5x and the level will drop. It’s not essential as it has been doing this since 4/01, but not previous to that.
There is another possibility which is an acoustic resonance of the inlet or outlet bottle and pipe work which can be reduced with orifice plates in the nozzles.
Both acoustic AND structural resonance could exist together.
Pressure measurements Vs crankshaft angle in the pipes and bottles should not be difficult, to prove the fault is accoustic, which appears to have a simple solution, and can avoid a final failure of the concrete foundation.