Vertical
Turbine Pumps
Pump
lift is an adjustment procedure generally associated with vertical, mixed flow
type pumps. These pumps also go by the descriptors vertical turbine pumps,
irrigation pumps, barrel pumps, fire pumps, or propeller pumps.
In
general, this type of pump takes water from a reservoir and pumps it vertically
through a riser, called a flow tube, to a higher elevation. The flow tube also
contains the shaft which connects the motor which is located on top to the pump
impeller which is immersed in the reservoir. Water from the reservoir enters
the impeller axially at the bottom of the pump and is discharged both axially
and radially into a volute type casing located just
above the impeller.
Mixed
flow, vertical pumps typically are for medium head applications where the
specific speed of the pump ranges from 4000 to 9000. At Cooper Nuclear Station,
there are several systems which contain vertical, mixed flow pumps. The two
systems which contain relatively large vertical pumps of this type are the
Service Water System and the Circulating Water System. The Service Water System
has four vertical pumps, each rated at 8000 gpm, and
the Circulating Water System has four vertical pumps rated at 159,000 gpm each.
In
a vertical pump, the pump impeller "sits" in a casing or bowl. The
outer diameter contours of the impeller vanes match (that is, they are supposed
to match) those of the bowl so that the tips of the impeller vanes are always
parallel to the surface of the bowl. The parallel gap between the impeller
vanes and the bowl, that is, the clearance, significantly influences the
efficiency of the pump.
If
the clearance is too large, water can re-circulate from the high pressure
portion
downstream
of the impeller (above the impeller) to the low pressure portion
upstream
of the impeller (below the impeller). This not only causes a loss in efficiency
of the pump, but it can also lead to accelerated erosion of the bowl.
On
the other hand, if the clearance is too close, the surface hydraulic boundary
layers of the impeller and bowl may interfere with each other. This causes the
hydraulic friction due to viscous shear between the two boundary layers to
increase, which decreases pump efficiency.
Further,
if the clearance is much too close, the impeller and bowl may directly
interfere and scrape on each other. This causes a significant decrease in pump
efficiency. Energy intended for pumping water is diverted and consumed by the
impeller grinding itself into the pump bowl. This contact causes permanent
damage to both the impeller and bowl and shortens the service life of the pump.
Pump
impellers and pump bowls are never perfectly round. A pump bowl about 30 inches
in diameter may have a diametric tolerance of perhaps +/- 5 mils (1 mil = 0.001
inches). Likewise, the outside diameter of the pump impeller that matches the
bowl has a similar tolerance. If the clearance between the pump bowl and
impeller is too tight, one or more of the impeller vanes will impinge on a
common
a sudden
increase in amperage, a decrease in pump output pressure, or both, and
an
increase in pump vibrations that have a frequency of the shaft speed times the
number of vanes on the impeller contacting the bowl. (Note: when there are two
symmetric high spots in the bowl, as would occur if it were elliptical, the
frequency might be two times the shaft speed times the number of vanes making
contact.)
Between
the two extremes of too tight and too loose, there is a "just right"
clearance dimension. This "just right" clearance dimension allows the
boundary layers of the pump impeller and bowl to slide over each other with
minimal shear, but is not so large as to allow excessive re-circulation between
the upstream and downstream sides of the impeller. At the "just
right" clearance, pump efficiency will be maximum.
Impeller
clearances are usually specified by the manufacturer. To provide a
"feel" for the magnitude of typical impeller clearances, Table 1 is
provided.