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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
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