B-4A Sealing

The proper selection of a seal is critical to the success of every pump application. For maximum pump reliability, choices must be made between the type of seal and the seal environment. In addition, a sealless pump is an alternative, which would eliminate the need for a dynamic type seal entirely.

Sealing Basics
There are two basic kinds of seals: static and dynamic. Static seals are employed where no movement occurs at the Juncture to be sealed. Gaskets and O-rings are typical static seals.

Dynamic seals are used where surfaces move relative to one another. Dynamic seals are used, for example, where a rotating shaft transmits power through the wall of a tank (Fig. 1), through the casing of a pump (Fig. 2), or through the housing of other rotating equipment such as a filter or screen.

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Fig. 1 Cross Section of Tank and Mixer

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Fig. 2 Typical Centrifugal Pump
A common application of sealing devices is to seal the rotating shaft of a centrifugal pump. To best understand how such a seal functions a quick review of pump fundamentals is in order.

In a centrifugal pump, the liquid enters the suction of the pump at the center (eye) of the rotating impeller (Figures 3 and 4).

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Fig. 3 Centrifugal Pump, Liguid End

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Fig. 4 Fluid Flow in Centrifugal Pump
As the impeller vanes rotate, they transmit motion to the incoming product, which then leaves the impeller, collects in the pump casing, and leaves the pump under pressure through the pump discharge.

Discharge pressure will force some product down behind the impeller to the drive shaft, where it attempts to escape along the rotating drive shaft. Pump manufacturers use various design techniques to reduce the pressure of the product trying to escape. Such techniques include: 1) the addition of balance holes through the impeller to permit most of the pressure to escape into the suction side of the impeller, or 2) the addition of back pump-out vanes on the back side of the impeller.

However, as there is no way to eliminate this pressure completely, sealing devices are necessary to limit the escape of the product to the atmosphere. Such sealing devices are typically either compression packing or end-face mechanical seals.