Vacuum Pump Considerations for Vacuum Dehydrator Applications

A vacuum dehydrator uses heat and vacuum to remove free, emulsified and dissolved water from industrial oils.  By using vacuum we reduce the vapor pressure of the water so that it boils, or turns to a gas, at a much lower temperature than  if was at atmosphere.  This is very helpful at removing moisture quickly from hydraulic, lubricating, transformer and other types of industrial oils.

The same technology is used to remove harmful gasses from industrial oils, such as hydrogen sulfide from compressor seal oil.  Hydrogen sulfide (H2S) lowers the viscosity of the lubricating / seal oil thus causing rotating equipment failures.  We use vacuum dehydration to remove these harmful gasses, and other light ends, from oils to maintain a healthy viscosity.

Vacuum dehydration is used to remove moisture and gasses from transformer and other types of insulating oils.  Moisture and combustible gasses are common contaminants in transformers and other electrical apparatus.

At first glance, all of these vacuum dehydrator applications may seem similar, but all have differences that require special attention to the vacuum pump selection.  For instance, removing gross amounts of water from paper machine or steam turbine lube oil does not require the same deep vacuum pressure as removing moisture from a transformer. Additionally, removing harmful gasses, that lower oil viscosity, from compressor seal oil can be detrimental to vacuum pump lubrication and thus the health of the pump.

For years we at High Purity Northwest have standardized on an oil sealed rotary vane vacuum pump.  These pumps are extremely reliable and have proven to be an excellent means of pulling vacuum over the years.  Water is condensed and collected in a condensate tank before the pump, but moisture still finds its way into the vacuum pump oil.  For this, we program our PLC to run the pump for 15 minutes during its shutdown procedure to dry the vacuum pump oil out.  This has proven to be successful at prolonging the life of the vacuum pump.  Even so, the vacuum pump manufacturer recommends changing the oil every 750-1000 hours.  We include an hour meter and reminder with our PLC program for the operators’ benefit.

Frequent oil changes are met with some resistance out in the real world.  The introduction of the positive displacement rotary claw pump has been received with open arms by maintenance crews because oil changes are now extended to every 10,000 hours.  This is due to the dry running and non-contacting internal design of the pump, which does not require any lubrication oil.  The oil that needs to be changed is gear oil, hence the extended oil change intervals.  These pumps pull a deep enough vacuum, 28″ Hg., to remove massive amounts of water quickly.  Our vacuum pump manufacture uses a special internal coating for high moisture applications to insure long life.  This pump is more expensive than the rotary vane, but if less maintenance is a consideration then it is well worth the extra dollars spent.

Another popular vacuum pump for steam turbines, oil refineries and other high moisture applications is the liquid ring vacuum pump.  These are popular where cool water is plentiful and are typically once through service.  Water temperature needs to be considered as temperature has a substantial effect of vacuum level.  We do offer recirculation systems with cooling water to help maintain the cooling water temperature.  If the vacuum dehydrator is to be removing dangerous gasses, care should be given to hard pipe the exhaust and water away from people.  Oil can also be used to seal these pumps and coupled with a booster can be a very effective means of removing gasses from compressor seal oil.

Other pumps that work well in removing high ends and other gasses are the Busch Huckepack Two-stage, once through sealing rotary vane vacuum pump.

Screw pumps with booster options are also useful in removing gasses from oil.  Like claw pumps, screw pumps are dry running.

Transformer oil purification applications can be grouped into two categories, low vacuum and high vacuum.  Our single stage rotary vane vacuum pump, with an ultimate vacuum of 0.5 Torr, does an excellent job at removing moisture to below 10 ppm and combustible gasses.

For high vacuum applications, we can add a booster for 0.05 Torr.  We also offer Tuthill/Kinney vacuum packages that typically include their rotary piston backing pump with booster.  We are flexible to the customer’s vacuum pump requirements.