Subject: Hydrodynamic gas
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
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
- 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
- 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
- You have to have a continuous supply of inert gas on
- Unlike hydrostatic seals, most hydrodynamic designs are
- 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
- 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
- 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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