Page 269 - Centrifugal Pumps Design and Application

240 Centrifugal Pumps: Design and Application

of the real pressure prevailing at the stuffing box, which results in exces-
sive use of gland water and increased maintenance.
Built into some slurry pump designs are methods to reduce pumped
pressure at the stuffing box by hydrodynamic means. (For example, see
Figures 13-7 and 13-8 for diagrams of pump out vanes on impellers and
expellers.) The side-suction-pump configuration is subjected only to
suction pressure and has an advantage over end-suction pumps, one not
fully recognized by users. By proper application of impeller pump out
vanes and expellers, the pressure at the box can be reduced to almost
zero. This is called a dry box arrangement. In these cases, weep-type
seal is satisfactory, with either water or grease being injected into the
cavity formed by the lantern ring.
Sump Design

Many slurry pump problems will be eliminated if proper attention is
given to the sump design. Design considerations are:

• The suction feed box should be placed as close to the pump as possible.
• The slurry level in the feed box above the pump center line should be at
least seven times the pump suction nozzle size.
• The feed box should always have a hopper bottom sloping to the pump
suction as shown. See Figure 13-9.
» The suction pipe should always have a minimum slope of at least 30°;
this particularly important when handling settling-type slurries.
• The feed box should be sized so there is a minimum retention volume
of slurry equal to or greater than two minutes of pump flow. If the
slurry is frothy, then a greater retention time is required (e.g. eight
minutes of pump flow).
• A dump gate should be provided at the bottom of the feed box.
« Turbulence near the feed box walls should be avoided to prevent exces-
sive wear.

Pump Drive

Generally slurry pumps are belt driven because it is almost impossible
to match the pump to the system by trimming the diameter of rubber and
hard metal impellers, due to their design and materials of construction.
Traditionally slurry pumps are driven by V-belts so that pump perfor-
mance can be adjusted to meet actual conditions of service in the field,
thereby saving power and reducing wear. As wear increases, pump out-
put is reduced. This can be easily and inexpensively rectified by increas-