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:

• A growing market for fugitive emission sealing.
• The increasing use of two seals in a pump opens the possibility of contaminating the process fluid with the barrier fluid circulating between the dual seals.
• In many applications there is no flushing water available for face cooling and lubrication.
• Non-contacting gas seal have the potential to generate less heat than conventional face seals.
• Some pumps experience dry running periods that might damage lapped seal faces.
• Air and gas compressors do not have fluid available for cooling between dual seals.
• Nitrogen is the most popular gas used in these applications, but in some instances both shop air and steam have been used.
• The gas leak rate is proportional to the cube of the gap between the sealing faces. This gap is normally in the order of less than one helium light band (0.0000116 inches or 0,3 microns) creating a leak rate of less than one standard cubic foot per minute.
• In those applications where the system temperature must be maintained above 200°F. (100°C) steam is normally selected as the gas barrier fluid.
• Hydrodynamic gas seals work best when there is gas on both sides of the seal faces. When sealing slurries, or those applications where the fluid is sensitive to a change in temperature, conventional environmental controls will be needed in addition to the gas barrier fluid.

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

• You have to have a continuous supply of inert gas on hand.
• Unlike hydrostatic seals, most hydrodynamic designs are unidirectional.
  • There are some bi-directional design available. Check them out if you have to seal double ended pumps, where the ends of the shaft are turning in opposite directions.
• The shaft has to be tuning at a reasonable rpm to provide the proper dynamic lifting forces. Many turbine driven pumps are rolled or rotated at a slow speed to keep the turbine and piping warm. This can cause destructive wear to the seal face geometry.
• The dimensions required are very critical. You need seal face materials that do not distort over a wide range of temperature and pressure. This can be a serious problem with most conventional seal face materials.
• Any gas that gets into the system could cause cavitation problems within the pump if the gas volume exceeds 3%.
• There should be some facility available to remove any excess gas that might leak into the system.
• Some consumers complain of excessive noise in the gas lines.
• In some dual seal applications, the barrier or buffer fluid is used to regulate the temperature at the seal faces. Gas doesn't do this very well because of its poor thermal conductivity.

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