Page 155 - Centrifugal Pumps Design and Application

Vertical Pumps 133

Figure 9-15. Impeller configurations.

For larger pumps or when handling hot or cold liquids, the following de-
sign practices are recommended:

• The bowl joints should be flanged and bolted. Gaskets, when required,
should be of the "O"-ring type, so that joints are made up metal to
metal, and pump alignment therefore maintained.
• The impellers should be mounted on the shaft with key drives and se-
cured axially with split rings and thrust collars.
• When selecting closed vs. semi-open impellers, the following must be
noted:

• Closed impellers should always be used for handling hot or cry-
ogenic liquids.
• Closed impellers exhibit lower downthrust when in the axially unbal-
anced configuration.
• Closed impellers are easier to assemble for large pumps with more
than three stages.
• Semi-open impellers are more efficient due to elimination of disc
friction from the front shroud.
• Efficiency loss due to wear on the semi-open impeller vanes can be
regained by adjusting the impeller setting at the adjustable pump to
driver coupling, typically in increments of 0.016 inch.
• When semi-open impellers are axially balanced to reduce axial
thrust, downthrust can be maintained over the full operating range.
This prevents shaft whip from upthrust with associated bearing wear.
• Semi-open impellers can readily be hardsurfaced for erosion protec-
tion.
• Semi-open impellers are less likely to seize when handling sand or
foreign material.