How To Adjust The Impellers Of A Vertical Centrifugal Pump
The impellers of any must is very important hence, it must be adjusted before any attempt is made to start the pump. A pump operating in a new well should be run with its impeller in the mid-position, which is approximately 3/16 inch above the lateral bowl wear rings.
This is to minimize the possibility of damage due to sand in the water. When the water is clear of sand, the impellers should be reset to the most efficient pumping position.
The next stage is that when making the impeller adjustment, it is necessary to allow for shaft stretch due to the downward hydraulic force on the impellers known as the hydraulic thrust.
For instance, pumps with settings 500 feet or less, can be determined as explained below. For pumps having settings over 500 feet, additional factors are involved. Such factors can be obtained from the pump dealer.
From the table 15 to 1 of the manufacturer’s handbook manual, select the value for hydraulic thrust of the pump being installed. This is given in pounds per foot of pumping head. Multiply this value by the height in feet for the pumping head to determine the total hydraulic thrust of the impellers.
From the same table, under the shaft diameter and cross from the previously determined downward force, read the value of the elongation i.e. stretch per 100 feet of shaft. Elongation values for forces not given in the table may be found by interpolation. This is explained in the examples provided in the manual.
In order to find the elongation i.e. shaft stretch, multiply the elongation per 100 feet by the total length of the shaft, including the top shaft, divided by 100 feet.
Here is an example of a shaft stretch calculation;
Question: determine the shaft stretch for an 8LB pump which operates against a 350 foot head and is equipped with a total of 250 feet of 1 ½ inch diameter shaft. The bowl unit consists of 12 stages.
- From the handbook table 1 to 15, select 2.6 pounds per foot as the hydraulic thrust of an 8LB pump. Multiply 2.6 by 350 to obtain the total hydraulic thrust of the impellers, 910.
- Consulting the table, you will find that 910 pounds is not listed in the downward force column. However, values are given for 800 pounds and 1000 pounds. From these we can find an in-between value for 910 pounds:
i.e. (0.024-0.019)/(1000-800)=0.00025 in./lb
Elongation at 910 pounds will be: 0.019 = (910-800) x 0.000025 = 0.022 inches
- To find the elongation i.e shaft stretch for 250 feet of shafting: 0.022 x (250/100) = 0.055 inch.
Before any adjustment is made, the impellers will be resting on the lateral bowl wear rings, and considerable resistance due to friction can be felt when turning the shaft by hand. To set the impellers to mid-position, proceed as follows:
Screw down the top shaft nut, while the impellers just clear the seals and the shaft can be turned freely by hand.
Scribe a line on the thread of the top shaft flush with the top of the nut. Continue to turn down the nut until the distance from the scribe line to the top of the nut is 3/16 inch plus the shaft stretch previously determined.
Turn the nut an additional few degrees, if necessary, to align two of the holes in the top shaft nut with the tapped holes in the top drive coupling. Install the lock screws in the top shaft nut.
In conclusion, to set the impellers to the most efficient position, follow the procedures listed in the handbook manual strictly. If the pump is driven by an electric motor, the power output calculations will focus on the specifications of the electric motor.
And when there is need for motor replacement, the exact motor specification should be used so that the work output can be maintained. The same goes to pumps driven by other mechanical means,
Note that when operating a pump, the driver cover must be in place when the pump is in operation. Rotating parts below the cover should not be left open to avoid injury to the operator.
Conclusion
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