Page 195 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

The packing lubricant

The use of the appropriate lubricant is an important consideration.
Generally Graphite, moly grease, and oils are good lubricants
depending on the application. Many pumps are set-up to use cold water
flushed into the stuffing box as a coolant, and lubrication for the
packings.
Graphite is a common lubricant for general service. Molybdenum
disulfide (moly grease) is an excellent lubricant because it creates a
lubricating film barrier between the shaft and the packings, protecting
both. It also works well with high temperatures. Mica is also a good
lubricant for very high temperatures. However, it tends to contaminate
the pumped liquid with particles. It certainly wouldn’t be indicated in a
milk pump. Any oil compatible with the pumped liquid is adequate as a
general lubricant.

Be aware that the packing is destined to fail the moment it is installed.
It has to resist every basic operating tendency of the pump. The
packing consumes energy decreasing the pump’s efficiency. It generates
frictional heat by grabbing and abrading the shaft. The packing itself
suffers from abrasion and corrosion originating in the liquid moving
through the pump.
Repacking the pump is a difficult and dirty job and falls down to the
bottom of the mechanics favorite ‘to do’ list. Extreme care should be
taken to assure a proper installation. It’s a job usually put off until the
very last moment. Repacking a pump correctly takes a lot of time. Not
following the correct repacking and start-up procedures results in short
packing life.

Stages in the life of packing

The packing rings must totally fill and occupy all free space inside the
stuffing box. The rings are composed of active and passive fibers (the
passive fibers act as carriers to string and braid the active fibers). The
packing also is impregnated with internal lubricants and bathed in
surface lubricants. When the gland is tightened, the packing begins
compressing and heating. The lubricants will eventually extrude out of
the packings and the passive fibers will normally burn and carbonize
into ashes. As the packing wears, it loses volume. There are generally
four stages in the life of a packing ring:
1. Stage one is when the packing rings occupy the full space inside the
stuffing box.