Page 106 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Installation, Maintenance, and Repair of Ertical Pumps 91

no more costly than the removal of case and impeller wear rings and the
installation of new ones. Significant savings can be achieved by reduc-
tion of spare parts inventories.
As is the case with bowl bearings, the wear rings have differential
pressure across them and will act to some degree as bearings provided
that they are not grooved. Grooved wear rings should only be used in
installations where the product contains abrasive material. We acknowl-
edge, however, that some users report acceptable results from the use of
grooved wear rings in nonabrasive services as well.

Replacement Impellers
Comparison testing has shown that replacement impellers which are
not furnished by the pump manufacturer can be very expensive in the
long run. In addition to potential shortening of pump life between re-
pairs, inadequate NPSH performance, improper curve shape and invali-
dation of pump warranties, the cost of additional power can be signifi-
cant. In one well documented case (Reference 5) the power requirement
increased from 134.7 horsepower to 139.8 horsepower resulting in an
estimated additional electric power cost of $3,000 per year. Although the
cost of purchasing the replacement impeller from the pump manufacturer
may be greater, economic analysis will show that purchasing the impeller
from another source is false economy. Some pump users have purchased
impellers from others because delivery time from the pump manufacturer
was too long. The problem of delivery time can be avoided by anticipa-
tion of the requirement and timely placement of the order, or by handling
of the order on a rush basis by the manufacturer.

Impeller Dynamlc Balance

The dynamic balance must be restored prior to reinstallation if any
work has been done on an impeller or if it shows appreciable wear. Im-
pellers which are out of balance can cause vibration and rapid wear of
bearings and wear rings.
Vertical pump impellers normally are balanced individually to quality
grade G-6.3 as indicated on the nomograms Figure 2-15 for impellers
which weigh 50 kilograms (1 10 pounds) or less and Figure 2-16 for im-
pellers which weigh more than 50 kilograms. By alignment of the pump
operating speed in rpm with the impeller weight in kilograms, the toler-
ance or allowable unbalance in gram-millimeters is given by the center
scale on each nomogram. The weight in kilograms equals 0.454 times
weight in lbs, Le., a 100 lb impeller weighs 45.4 kilograms. To convert
the allowance in gram-millimeters to inch-ounces, multiply the value in
gram-millimeters by .00139 to get the value in inch-ounces, Le., an al-