A Fundamental Study of a Stepped Pumping Seal

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If the low pressure side of a seal is filled with liquid, the lubrication equation is a linear function of film pressure. Accordingly, the lubricating and the sealing performances can be evaluated by hydrostatic film force coefficient, hydrodynamic film force coefficient, hydrostatic leakage coefficient and hydrodynamic leakage coefficient, and operating characteristics can be predicted for revolutional velocity and seald pressure.In this seal, noncontacting operation without wear can easily be attained due to Raileigh-step effect under rotation. On the other hand, liquid is pumped from low pressure side to high pressure side of the seal. However, critical speed exists over which net leakage is positive due to increased gap width. Since the stiffness of the lubricating film is very high, a high pressure liquid can be sealed without leakage if its operating gap is set to be very small. A new apparatus was used to verify the theoretical predictions. Continuous and fine adjustment of balance ratio is possible even under rotation due to a new method.If the low pressure side of the seal is surrounded with gas, cavitation appears in the pumping grooves, and it controlls the pumping intensity. Zero leakage operation is thus attained. Cavitation boundary was always stable, hence seal breakdown phenomenon due to air injection was not observed. Over the critical speed, leakage appears with descrete change of gap width. Experimental results suggests that the change is caused by generation or disappearance of cavitation. Design criteria for lubricating and sealing performances are settled.