Shaft seals packed stuffing boxes

Shaft seal requirements and two common types of seals, packed stuffing boxes and simple mechanical seals, are described and discussed in this section. A packed box typically is used on slow- to moderate-speed machinery where a slight amount of leakage is permissible. A mechanical seal is used on centrifugal pumps or other type of fluid handling equipment where shaft sealing is critical. 

Figure 7.13. Types of seals for pump shafts, (a) Packed stuffing box the sealing liquid may be from the pump discharge or from an independent source, (b) Water cooled stuffing box. (c) Internal assembly mechanical seal the rotating and fixed surfaces are held together by the pressure of the pump liquid which also serves as lubricant a slight leakage occurs, (d) Double mechanical seal with independent sealing liquid for handling toxic or inflammable liquids.

STUFFING BOX - That portion of the pump which houses the packing or mechanical seal. The stuffing box is usually referred to as the dry portion of the pump, and is located in back of the impeller and around the shaft. 

Part Purpose Shaft sleeve Protects the shaft where it passes through the stuffing box. Usually used in pumps with packing but often eliminated if mechanical seals are employed. 

The bearings and gears are mounted externally to the gas chamber and are separated by stuffing box seals to prevent gas leakage along the shaft. The packing is usually vented to the suction, so that it is necessary to pack only against suction pressure. 

The simplest, and oldest, form of seal is the packed gland, or stuffing box, Figure 5.12. Its applications range from sealing the stems of the water taps in every home, to proving the seal on industrial pumps, agitator and valve shafts. 

Cast iron is the standard material used to construct the cylindrical housing, but other materials may be used if corrosive conditions exist. The rotor is usually a continuous piece of steel that Includes the shaft and is made from bar stock. Special materials can be selected for corrosive applications. Occasionally, the rotor may be a separate iron casting keyed to a shaft. On most standard air compressors, the rotor-shaft seals are semimetallic packing in a stuffing box. Commercial mechanical rotary seals can be supplied... 

It is not always possible to use a standard stuffing box to seal the shaft of a centrifugal pump. The pump suction may be under a vacuum so that outward leakage is impossible, or the fluid may be too hot to provide adequate cooling of the packing. These conditions require a modification to the standard stuffing box. 

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