Page 352 - Boiler_Operators_Handbook,_Second_Edition
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Plants and Equipment 337
or the other is always fitted with a thrust bearing. If the stable. It may be unstable because, for one set differen-
pump is cantilevered off a single bearing it’s also the tial across the pump, you have two possible flow rates.
thrust bearing. As pumps wear the direction of thrust If the system somehow maintains a constant differential
can change so one excellent measure for pump condition for those two flows the pump will not align with one
is the axial position of the shaft when you can get at it or the other, switching back and forth between the two
to measure it. Taking initial measurements of how much points. When a pump does that we call it ‘surging’ and
a shaft shifts along its axis (using a dial micrometer) be- it’s usually accompanied by a lot of fluid noise in the
fore it’s ever operated provides baseline measurements pump and system to inform you it’s going on. Multi-
for bearing wear. Take them anyway if the pump is in stage pumps can oscillate along the axis of the shaft
good shape then compare them every year or two to
check for wear.
Your first clue of potential operating problems with
a pump is the shape of the curve. If the curve has a nega-
tive slope at all times you should not have any operating
problems with it under most circumstances. Slope is a
value equal to the change in differential divided by the
change in flow at any point on a curve, indicated by a
line tangent to the curve at the point you’re looking at.
If the differential is always decreasing the pump is easy
to handle. A lot of pump curves have a positive slope as
the flow approaches zero. The curve will have a hump
in it where the slope is zero (differential doesn’t change)
at the top. The curve will have a positive slope (differen-
tial decreasing) to the left of the hump where flows are
lower.
Anytime you’re operating at a point close to or to
the left of that hump the pump’s operation may be un-
Figure 10-74. Back pressure with wear rings on cen-
trifugal pump
Figure 10-73. Axial forces on centrifugal pump Figure 10-75. Opposing stages of centrifugal pump