Mechanical Seals vs. Packing Rings



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
CENTRIFUGAL-PUMP.ORG
Related topics

Mechanical seals vs. packing rings
Double or tandem mechanical seals
Mechanical seal flush piping plans
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