I am looking for guidance on selecting appropriate damping ratios for above ground reciprocating pump piping. The pipes range in sizes from DN25, DN40, DN100 and DN250.

Any assistance or references as a starting point would be greatly appreciated.

I usually quote US Nuclear Regulatory Commission Regulatory Guide 1.61 which mimics the old Section III (nuclear piping) code. It states that for Operating Basis Earthquakes (OBE), use 1% critical damping for piping up to an including 12 inch nominal OD and 2% above that.

The newer Section III code (Appendix N) says use 5% for all piping but I think that's used more to select the proper seismic load than reflect the true system damping.

(The idea is that with an OBE, no yielding occurs and I would guess that doesn't happen in your system either. The other earthquake listed - the Safe Shutdown Earthquake, or SSE - is twice as strong and yielding occurs. For this they added another percent to the numbers above.)