Water hammer or natural freqency.

Petera,

You wrote the following which left me totally confused.
#1- "I've heard that one should choose the first natural frequency higher than 4-5Hz to reduce the risk of damage to the piping system."
Yes that is true if you are talking about piping around Reciprocating Compressors (Recip.)and some (not all) Positive Displacement Pumps. In the 1960's when I did my actual Compressor Piping Layout, the compressor vendor gave us the natural frequency data on the compressor. We had company guidelines for support spacing, location and type of Hold-downs and Snubbers. After the Design was complete and ready for the internal checking we sent a complete compressor layout package to an outside company for an Analog Study. We would get back a report and if required their recommendations for modifications. These recommendations were normally few and minor in nature. The changes (if any) were made and then the Compressor package was checked, corrected and issued AFC.
Your post does not define any type of equipment and this has nothing to do with Drainage so, as I said I am confused.

#2- "You must add dampers."
Again this is true with Recip. Compressors and some PD Pumps but not all. This also has nothing to do with Drainage of Storm Water or other fluids

#3- "We live in an area without risk of earthquake"
I do not understand this. What does an earthquake have to do with your question?

#4- "but we do have bad draining and therefore big risk for water hammer."
Bad draining of what? If it is surface Drainage then you would not have Water Hammer. Water Hammer would only happen in a closed and pressured system (some not all). Again I am confused.

In your post you have "suggested" a number of scenarios (Possible- the natural frequency in piping for Recip compressors, Positive Displacement Pumps, Drainage which could be surface, atmospheric gravity or pressure system) but nothing definitive.
My bottom line here is this, I have no idea what you are asking or what you need.