1. pumping rate in gallons per minute (gpm). This is the maximum demand of the irrigation circuits in you design.
2. total dynamic head (TDH) in feet. TDH is the total pressure the pump must generate to overcome elevation and friction loss and to provide the operating pressure to the heads. TDH is expressed in feet of "head" (1 psi, or pound per square inch = 2.31 feet of head).
The theoretical power needed for pumping water is called water horsepower (whp) and is calculated by:
(equation 1)
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whp =
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gpm x TDH (ft) |
| 3,960 |
Since no pump is 100 percent efficient, the horsepower output of the power unit must be higher than that calculated with equation 1. This horsepower, referred to as brake horsepower (bhp), is calculated by:
(equation 2)
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bhp =
|
whp |
|
(pump efficiency) |
Total Dynamic Head (TDH)
(equation 3)
TDH = (static head) + (friction loss) + (operating pressure) + (elevation change)
The elements of equation 3 are defined below.
Static Head
Static head is the vertical distance from the water level in the pond or well to the pump outlet during pumping. In areas of falling water table or where the water level in the pond varies, the maximum depth to the water table or pond surface during the pumping season is used.
Friction loss
Water flowing past the rough walls in a pipe creates friction which causes a loss in pressure. Friction loss through pipe fitting is generally minor and will be ignored in our work. Friction loss through water main line and valve are determined using friction loss tables and manufacturer's data. The units from these sources are usually PSI. You must convert PSI to feet of head fro these calculations.
Operating pressure requirements
:Manufacturers provide recommended operating pressures for specific irrigation sprinklers. For example, a Rainbird 1800 series sprinkler requires 30 psi to operate properly. Operating pressure in psi is converted to feet of head by the relationship: 1 psi = 2.31 ft. So in our example 30 psi x 2.31 = 69.3 feet of head.
Elevation change
Use the total change in elevation from the pump to the point of discharge, such as the sprinkler heads. This elevation change may be positive (when the irrigation system is uphill from the pump) or negative (when it is downhill from the pump). Use only the difference in elevation between these two points, not the sum of each uphill or downhill section.
Example
Assume that you have determined the following needs for an irrigation design. Sprinkler operation pressure of 30 psi and maximum circuit size of 50 GPM. Before we can calculate the water horse power we need to know the Total Dynamic Head. TDH = (static head) + (friction loss) + (operating pressure) + (elevation change).
Make the assumptions for the diagram below. The static head = 6', friction loss in the suction pipe, main line, and other equipment = 20 feet of head, operating pressure = 69.3 feet of head, elevation change = 5 feet
Therefore, TDH= 6+20+69.3+5=100.3
 |
Now solve equation 1 for water horse power :
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whp =
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gpm x TDH (ft) |
|
3,960 |
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whp =
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5015 (ft) |
| 3,960 |
whp = 1.7
Next solve for equation 2 to find the brake horse power. Assume an efficiency of 70%
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bhp =
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whp |
|
(pump efficiency) |
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bhp =
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1.7 |
|
.7 |
bhp = 2.4 brake horse power