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II. Pump Lift

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.

Figure 1, on the next page, shows a simplified schematic of a Service Water vertical pump at Cooper Nuclear Station.

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.

Figure 1. Schematic of Vertical 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 high point or asperity between the bowl and impeller. When this occurs, the effect is detectable by:

  1. a sudden increase in amperage, a decrease in pump output pressure, or both, and
  2. 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.

 

Table 1. Mixed Flow Impeller Clearances

Bowl diameter Impeller Clearance

6 to 12 inches 15 to 20 mils

13 to 24 inches 20 to 30 mils

25 to 36 inches 30 to 50 mils

37 to 48 inches 50 to 80 mils

 

At Cooper Nuclear Station, the current Service Water Pump Lift Adjustment Procedure, CNS Maintenance Procedure 7.2.45, indicates that the lift for the 28 inch Service Water Pumps should be between 40 to 60 mils when the lift adjustment procedure has been completed. The manufacturer of the Service Water Pumps, the Byron-Jackson Pump Company, recommends a lift adjustment of 56 mils.

A pump lift is accomplished by actually lifting the impeller upwards such that itís measured vertical gap between the impeller and bowl is within the range of acceptable values provided by the manufacturer or by the engineer in charge.

Since the pump impeller and bowl themselves are normally immersed in water and are inaccessible, this is done at the top of the pump by loosening the pump shaft from the motor shaft at their coupling and allowing the impeller and shaft to rest on the bowl. This is the "zero" clearance position. The shaft is then lifted upwards, usually by tightening coupling bolts or adjustment plates, from the zero clearance position. The amount of lift is the amount of upward displacement of the shaft and impeller created by tightening the coupling bolts or adjustment plates from the zero position. The clearance measurement is usually made with feeler gages or dial indicators.


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