SEALING VACUUM SS003
Vacuum means less than atmospheric pressure, and vacuum sealing falls into two neat categories:
- Normal vacuum
- Hard vacuum
Normal vacuum. This vacuum is usually measured in inches or millimeters of mercury.
- This is the vacuum found in condensers, evaporators and at the suction side of the pump every time you use the centrifugal pump to lift liquid.
- Hydraulic balanced seal designs can handle this vacuum because vacuum only means one atmosphere of pressure (15 psi. or one bar) coming from the other side of the seal.
- O-rings are preferred for the elastomer design. Continuous O-rings can seal either vacuum or pressure. They also have the ability to flex and roll to compensate for shaft movement.
- Carbon metal composite seal faces are satisfactory as long as the carbon is sealed at the inside diameter to prevent the pressure from penetrating behind the carbon, upsetting the hydraulic face balance and possible blowing the carbon out of its holder.
- Tandem seals with a higher pressure lubricating barrier fluid circulating between the seals are the most common solution to vacuum sealing.
Hard vacuum. This vacuum is measured in microns, micro inches or portions of a Torr (one millimeter of mercury).
- Elastomers are not acceptable for hard vacuums. The vacuum will cause the elastomer to “out gas” increasing the elastomer’s density and reducing the volume to a point where O-ring leakage is possible. All metal bellows seal designs will probably be your first choice.
- Seal face density and self-lubrication can be a real problem in hard vacuum applications because of the lack of moisture to release the graphite from the carbon/ graphite compound. Conventional carbon-graphite designs are seldom satisfactory in these applications. A great many materials exist that can solve the problem, so you will want to contact your seal supplier for the availability of higher density and self lubricating carbons for these special applications.
- Because of the dry running problems we encounter in many of these applications, dual metal bellows seals with a lubricating barrier fluid circulating between the two seals will be your first choice.