Page 266 - Centrifugal Pumps Design and Application

238 Centrifugal Pumps: Design and Application

Figure 13-6. Approximation of
flushing water requirements for
"flush-type" slurry pump stuffing
boxes where flush pressure is 15
psi above slurry pressure at the
box. "Weep-type" stuffing boxes
use about 5% of the water that
"flush-types" use.

An alternative method for sealing is shown on Figure 13-5b. Here the
lantern ring is positioned between packing rings. This configuration is
called a "weep" seal. Again, clean liquid should be injected at a pressure
higher than the prevailing slurry pressure near the stuffing box. Product
dilution is significantly reduced compared to the "flush" seal design.
However, the barrier so created is not very effective, causing abrasive
particles to penetrate and cause wear. If the service is only mildly abra-
sive, then grease can be used in lieu of liquid.
An approximation of flushing requirements for a "flush" type packing
arrangement for conventional throat restriction devices where no attempt
has been made to curtail the use of flushing water and where the pressure
differential is 15 psi is displayed in Figure 13-6. Such a restriction will
have an annular radial clearance in the order of .007 times the sleeve di-
ameter. The length of the throat bush will be about the same as the width
of one turn of packing.
It is impossible to predict the exact amount of flushing water required
when the packing is "weep" type, since this is dependent on shaft deflec-
tion and gland maintenance. However, under normal operating condi-
tions, weepage would be in the order of 5% of the values stated in Figure
13-6 for "flush" packing arrangement.
In most cases, seals and flush requirements are provided in ignorance