If the structure has been built, one method damping can be measured is by using an accelerometer and an impact hammer using the decay of the transient. See this article: https://en.wikipedia.org/wiki/Logarithmic_decrement
If the structure has not been built, then use values established in the literature. A welded steel structure is going to have different damping parameters than a bolted steel structure or a concrete structure.
Please show the tabular data that is PERMISSIBLE VIBRATION AMPLITUDE 60 microns. Is that a statement about the pump or the pipe?
When I read PERMISSIBLE VIBRATION AMPLITUDE 60 microns, that sounds like a requirement. After you have applied the unbalanced force caused by pump, you would plot a Displacement result and check that no part of the structure had a vibration amplitude exceeding 60 microns.
The pump specification might have a requirement on the maximum unbalance of the rotor calculated from the mass m of the rotor and the radius r of the center of mass from the rotational axis. You can calculate the force F amplitude as a function of rotor angular velocity w using the formula F = mr(w^2).