Page 274 - Centrifugal Pumps Design and Application

Slurry Pumps 243

ing the pump speed by changing the sheave ratios. Three to five percent
should be added to the motor BMP to compensate for belt losses. It is
always good practice to add one more belt than is normally calculated to
cover upset conditions and belt breakage.
Motors must be rated with an adequate margin to cover upset condi-
tions such as high flow due to lower-than-expected system losses, higher
concentrations, and start-up. At start-up, the concentration is often
higher, and if the pump was not flushed out during the previous shut-
down, the pump could be plugged with solids requiring high breakaway
torques to get the impeller rotating. This undesirable condition happens
all too frequently and can cause pump damage, excessive wear, and mo-
tor overload.
Under well-controlled systems, free from upset, the motor could be
rated at 20% above the motor shaft BMP; however, this percentage could

Figure 13-10. Typical belt-driven, overhead-motor-mounted slurry pump.