Eugene Vogel
EASA Pump & Vibration Specialist
It is common for vertical turbine pumps (VTP) to be designed with multiple mixed flow impellers (sometimes 12 or more) and for the pump rotor to be supported by the vertical pump motor.
Vertical pump motors can be solid shaft or hollow shaft. Solid shaft motors have an annular keyway in the shaft that is engaged by a solid coupling that supports the pump rotor.
Hollow shaft motors support and drive the pump rotor from the top by means of a head shaft fitted through the hollow motor shaft to the pump line shaft. In either case, there is an adjustment that lifts the pump rotor so it is supported by the motor shaft. This adjustment is obviously critical to the proper operation of the pump and motor and can have a significant effect on the motor load (current). Presented here are some of the main concerns for setting this pump lift adjustment.
An important step in assembling a VTP is to ensure that each impeller is located on the pump shaft so that when the pump is set vertically, each impeller is resting on the bottom of the pump bowl. The available space for the lift adjustment is from where the impeller sets on the bowl bottom to where the impeller would rub on the top of the bowl. If during assembly the impellers are not located on the shaft properly, that available space will be reduced. The first step in setting the lift adjustment is to determine that available space. The procedure is to simply measure the line shaft height with the impellers resting on the bottom of the bowls, and then lift the pump rotor until the impellers rub the top of the bowls and note the change in the line shaft height. The operating height must be between these two dimensions.
Open or closed style impellers
The first concern in setting the lift is whether the impellers are open style or closed style. Closed style impellers have a front shroud and may have a suction eye wear ring. Open style impellers have vanes exposed on the suction side.
The lift adjustment is much less critical for closed impellers with suction eye wear rings; it generally does not affect the pump performance. The adjustment for open style impellers has a significant effect on pump performance, since the clearance between the open vanes and the bowl bottom affects the pump efficiency.
Some VTPs have a single, axial flow impeller. For these pumps, the bowl design may provide a very large space above and below the impeller. The lift adjustment for these pumps is much less critical and has no effect on the pump performance.
Short or deep set VTPs
VTPs may be short set or deep set. While there is no specific length designating the difference, a short set pump is considered one which can be handled (installed or removed) as a single piece. Long set pumps would require assembly-disassembly at the job site. Deep set pumps may extend hundreds of feet (meters) below grade. When the depth below grade is significant (generally greater than 100 feet [30 meters]) shaft elongation must be considered in setting the lift adjustment.
Determining the amount of shaft elongation is not a trivial calculation since the down thrust on the impellers varies with the discharge head (or total dynamic head, TDH). This calculation is done by the pump manufacturer when the pump is sized for the application.
The basic objective in setting the lift adjustment is to have the impeller operating at a vertical positon in the bowl that prevents rubbing on the bottom or top. For closed impellers with suction eye wear rings, that is about the only concern. However, for open impellers, or closed impellers without suction eye wear rings, the clearance between the bowl bottom and the impeller also affects the pump efficiency and the motor load (current). Opening up the clearance at the bottom of the impeller decreases the efficiency and also decreases the motor load. It is common practice to address motor overload conditions by increasing the lift adjustment. Note that this does not work for closed impellers with suction eye wear rings.
Summary
From the discussion above, it should be apparent that setting the lift adjustment on a short set VTP with a closed impeller with a suction eye wear ring is a fairly easy task: Just get the impeller so it doesn’t rub top or bottom. But an open impeller on a 250 ft. (75 m) deep pump would require some detailed calculation to avoid a catastrophic failure. And fixing an overloaded motor by increasing the lift clearance results in an efficiency loss, thus increasing operating costs over the long haul. Pump manufacturers can provide step-by-step instructions for making the lift adjustment on a specific pump.
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