Page 47 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

32 Major Process Equipment Maintenance and Repair

As the close clearances become larger through wear, corrosion, ero-
sion or perhaps questionable maintenance practices, internal leakage
rates increase. The increased leakage must be pumped and repumped
continuously by the impeller, requiring additional input horsepower.
The amount of added power to continuously recirculate excessive in-
ternal leakage is a function of the pump specific speed*. In low specific
speed pumps (low capacity-high head) excessive running clearances re-
sult in larger percentage changes in power requirements than occur in
high specific speed pumps (high capacity-low head). This is reflected in
the empirical data plotted in Figure 1-1 1.

* For an explanation of pump specific speed refer to Figure 1-13.

35

u)
c
30
E
P
.-
2
0-
P
& 25
3
0
n
n
5
E 20
.-
.-
C
E
P
.- 15
P)
0
m
c.
C
8
& 10
n / /
C / / /is-
/
S
0
0 20 40 60 80 100 120 140 160 180 200
Percentage increase in wear ring clearance
Figure 1-1 1. Added power resulting from excessive wear ring clearance for different spe-
cific speeds.