Page 265 - Centrifugal Pumps Design and Application

Slurry Pumps 237

There is little to be gained by fitting a rear wear plate, provided the
rear of the hard metal casing extends to the stuffing box. Experience has
shown that the rate of wear in this area is not any greater than in the cas-
ing itself.

Bearing Frames

Usually the bearings are oil lubricated with a calculated life of over
50,000 hours. Slurry pumps are installed in dirty dust-laden atmo-
spheres, and extra precautions have to be taken to seal the bearing covers
and prevent the ingress of liquid and dust. In severe services, taconite
seals are provided (i.e. double-lip type seals with grease cavities).

Sealing

Slurry pumps are often subjected to severe shock loading and shaft
whip due to the presence of solids and system upsets. For these reasons
soft compression packing is still favored as a means of sealing at the
stuffing box.
The preferred method for packing a slurry pump is the "flush" seal
shown in Figure 13-5a. Here the lantern ring is positioned in front of the
packing rings and a copious supply of clean liquid is injected at a pressure
higher than the prevailing slurry pressure in the stuffing box. The clean
liquid acts as a barrier and prevents the ingress of abrasive particles that
cause packing and sleeve wear. The disadvantage of this system is that
large amounts of flushing water are required and the pumped product
will be diluted. This system is recommended for severe abrasive ser-
vices.

Figure 13-5. (A) Typical "flush-type" slurry pump stuffing box. Barrier flush pre-
vents abrasive wear. (B) Typical "weep-type" stuffing box. It uses considerably
less gland water but is much more susceptible to abrasive wear.