Page 50 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Installation, Maintenance, and Repair of Horizontal Pumps 35

the pump (and the accompanying increased power required to pump the
additional flow) seem to many to be a small price to pay, if in fact such
criteria are considered at all. But, from a purely mechanical standpoint,
the stability of the rotor is perhaps safeguarded only as long as normal
running clearances are maintained. Typical consequences of liberally
open clearances are likely to include excessive vibration, overheating and
ultimately pump or driver bearing failure, shaft breakage, driver over-
loading, and possible total pump destruction. Ultimate maintenance costs
can be very high and unit operation can be compromised through prema-
ture and repeated outages.
If two or more pumps are designed for parallel operation and share to-
tal capacity, then unequal running clearances can cause unequal load
sharing by the pumps. One or more of the units can be forced to operate
at significantly more or less than its design flow rate. Efficiency falls off
and brake horsepower requirements increase even beyond those caused
by excessive running clearances.

Running clearances. Greater than normal wear ring clearances at the im-
peller inlet eye increase the flow rate through the impeller (not out the
discharge nozzle of the pump) , increase the effective inlet fluid tempera-

Increased power consumption, bhp
Figure 1-14. Annual costs based on 300 days per year continuous operation.