Sealing a pump casing around the shaft periphery to prevent the pumped liquid from leaking into atmosphere is a major concern. Effective sealing can be accomplished by providing the pump with mechanical seal or packing rings.
That part of the pump casing that contains the mechanical seal or packing rings is called the stuffing box (or seal chamber). Horizontal end-suction pumps and vertical pumps have only one stuffing box, whereas horizontal between-bearing pumps are provided with two stuffing boxes on both sides of the pump casing.
Advantages of using mechanical seals
lower frictional drag than traditional packing means improved pump efficiency
a mechanical seal will not wear out a shaft, or sleeve, as fast as packing rings
near zero leakage is possible with mechanical seal; packing requires some leakage (usually visible) for proper lubrication
properly applied mechanical seals require less periodic maintenance than packing
specially designed mechanical seals can be applied to higher pressures and speeds than traditional packing.
Disadvantages of using mechanical seals
less tolerant to shaft deflection and misalignment
less tolerant to dirty or contaminated liquid; will require cyclone separator to clean the liquid
require expensive seal piping to flush and quench
mechanical seals are more expensive than packing rings
Advantages of packing rings
lower upfront cost
very simple selection, installation, maintenance, and trouble-shooting
Disadvantages of packing rings
require small amount of leakage for lubrication and cooling - therefore packing rings are unsuitable for use in liquids that are toxic, flammable, or otherwise hazardous or polluting
usually require a stack of several packing rings for effective sealing; need longer stuffing box, which in turn results in higher shaft deflection
easily wear out resulting in more frequent replacement of packing rings