Case History

A centrifugal induced draft fan was located in a structure with the fan and motor provided on a steel base that was installed as an isolated system from the steel deck on the structure with the base attached to the steel deck using isolation springs. The isolated system was reasonably designed with an isolation frequency of about 3 Hz (~ 1.09” sag on springs).

Static mixers are used to blend two or more streams of liquid for neutralizing acid or other similar services. When this is done, there is normally a lot of flow turbulence and flow noise at or near the mixer. Since there is moderate flow turbulence and possible flashing with two phase flow downstream of the mixer, it is very common to experience high vibration on the mixer or the piping near the mixer. Vibration generated by the mixer will often be broad band “noise” and with occasional jolts or pulses if there is two phase flow at the outlet. When this occurs, the vibration on the associated piping will be at natural frequencies with frequencies up to about 20 Hz. Vibration dampers are particularly effective for this application as verified by ANSYS modeling and field testing. In this case history, the mixer was attached to two inlet and one outlet pipe with moderately violent mixing that was expected to produce some foaming and/or 2 phase flow. Vibration was observed from 2 Hz up to about 12 Hz. The ANSYS modeling confirmed natural frequencies starting at about 2 Hz. The model was used to select and locate 7 dampers in various locations to effect all modes below 20 Hz. 30+% modal damping was achieved for all excited modes below 20 Hz.

Reciprocating pumps and compressors are known to produce moderate dynamic forces due to the pulsation generated by the reciprocating pistons. The typical design guideline for the piping is to have adequate supports and short enough spans between supports to produce piping mechanical natural frequencies that are 20% higher than the highest significant pulsation order.

The application was a large 24” pipe with 2  phase flow downstream of the control valve to a tower.  The flow was flashing at the valve, resulting in “slugs” of gas moving downstream towards the tower.  Finite element analysis (ANSYS) was used to confirm that field measured natural frequencies matched calculations, then the FEA model was used to validate selection of and mounting locations for dampers. In this case there were two natural frequencies responding at 2.5 Hz and 3.7 Hz, with vibration observed up to 1” peak to peak.  This was quite disturbing to see.  The vibration mode shapes were used to position the dampers, which resulted in 2 size “D” dampers installed strategically at one end of the pipe run.  The damping level was increased to near 50% of critical damping for both modes and vibration was eliminated.