A Transient Dynamic Analysis of Mechanical Seals Including Asperity Contact and Face Deformation

Face seals are typically designed to be in contact at standstill. However, as speed and pressure build up, the seal faces deform from their factory flat conditions because of viscous and dry friction heating, as well as mechanical and centrifugal effects. It is imperative that such deformations form a converging gap for radial flow to ensure stable operation and to promote favorable dynamic tracking between stator and rotor. A numerical simulation is presented for the transient response of a face seal that is subjected to forcing misalignments while speeds and pressures are ramped up and down. Asperity contact forces and transient face deformation caused by viscous heating are included. A new closed-form solution is obtained for the elastoplastic contact model, which allows seamless transition between contacting and noncontacting modes of operation. The model is then used to calculate face contact forces that occur predominantly during startup and shutdown. The viscous heating model shows that the time-dependent deformation (coning) is hereditary and that it lags behind the instantaneous heat generation. The dynamic analysis provides a numerical solution for the seal motion in axial and angular modes. The eventual build up of hydrostatic pressure and coning during startup generates opening forces and moments that separate the seal faces, resulting in noncontacting operation. The reverse occurs during shutdown; however, because of the thermal time constant a seal may continue to leak even after it returns to standstill. The analysis and simulation results compare very well with a closed-form solution that predicts a critical speed of separation of contacting seals.