Step Clearance Seals: An Analysis to Demonstrate Their Unique Performance

Hydraulic turbines and centrifugal pumps at times show low frequency vibrations when installed with upstream step (band) clearance seals with a narrow clearance facing the incoming external flow. When implementing a downstream step clearance seal, one with the narrow clearance located at the seal exit, the same machine does not show the same problem. This paper presents both theoretical and experimental analysis on the leakage and dynamic force coefficients of both upstream and downstream step clearance seals. The predicted and measured results show that an upstream step clearance seal produces a significant negative direct stiffness (K < 0) that could cause a static instability. On the other hand, a downstream step clearance seal generates a positive direct stiffness (K > 0) that is beneficial to a rotor system. Both the upstream and downstream step clearance seals show positive direct damping and virtual mass coefficients.