Subject: Hydrodynamic gas seals.13-01

The idea is very simple. Let the seal faces ride on a film of gas either pumped to, or flowing between the seal faces. Unlike hydrostatic seals that create a balanced opening and closing force to maintain just the right amount of seal face separation, the hydrodynamic seal depends upon the generation of a lifting force to separate the seal faces. Take a look at Paper 12-02 in this series for a description of hydrostatic sealing.

Please take a look at the following illustration:

 

The hydrodynamic lifting force is created by the seal face geometry (shape or configuration).

 

The shaft must be rotating at a reasonable rpm to provide the proper lifting force.

 

Hydrodynamic forces are generated by the viscous shear of the gas film when the smooth face is rotating, so unlike the hydrostatic version these seals operate effectively only while the pump shaft is turning. You experience this same phenomena when you trap water in the tread of your automobile tire causing the car to hydroplane and lift off the road surface.

Unlike liquids, gases are compressible, but you can generate a similar lifting force if the face geometry is designed and built correctly. The idea is to direct the gas into a some narrow channels that will increase the gas pressure causing the face separation.

Gas seals have become very popular in recent years for a variety of reasons:

Hydrodynamic gas seals also present a few problems to the user:

 

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