Historically, piping flexibility analysis has approached dynamics using the "lumped mass" forumulation. Typically this has been more than adequate.

However:
1) Theoretical (academic) type problems (such as a cantilever) have noticeable error in the eigen solution - especially with minimal nodes.

2) Some "real life" systems if not modeled with at least one node between supports or bends can also be in error.

Rather than try to perform some type of "node insertion" into these systems, we opted instead to go with the exact approach - "the consistent mass matrix". Read the "help text" on this option for details.


Usually the response of a piping system is dominated by the 1st mode. So that is usually what you observe.