Predictions for Non-Contacting Mechanical Face Seal Vibration With External Excitation From Pump Vibration: Part II — Flexibly Mounted Rotor

Abstract

Stability and response predictions are presented for a Flexibly Mounted Rotor (FMR) mechanical seal ring using the model developed by Childs in 2018. The seal ring is excited by lateral/pitch vibration from the rotor/housing. The model includes a frequency dependent stiffness and damping model for the O-ring and a frequency independent model for the fluid film. The dynamic coefficients are speed and frequency dependent. The mechanical seal is modeled after a typical FMR mechanical seal. Parameters for radius, fluid film clearance, and O-Ring axial distances are varied. The axial distance between the O-Ring and seal ring inertia center doz is found to couple lateral rotor motion and seal ring pitch vibration. The predictions show a dependency on both excitation frequency and running speed. The analyzed FMR has a critical region with high transmissibility in a region around a speed and excitation frequency of 70 kRPM. Another region of high transmissibility is predicted to be with sub-harmonic excitation frequency. The FMR seal ring also has an unstable region that is sub-harmonic of 1% running speed. Running back on the HQ curve for a pump causes broadband sub-harmonic excitaiton, which can cause rub failures for FMR mechanical seals.