Dairy basics - Manure |
| Written by Nancy Puck |
As we plan a pumping system to move liquid on a farm, we must know what the capabilities of our pump are, and what the friction losses in our system will be. This is an overview of pump capabilities as detailed on the pump's performance curve.
On one axis of the pump curve, you will find capacity, and on the other, total dynamic head (TDH). Capacity is measured in U.S. gallons per minute or cubic liters per hour.
This tells us what the pump is capable of flowing if we fulfill the other inputs: RPM, horsepower and NPSH required.
These inputs are detailed in Figure 1 above, as well as the pump's efficiencies. Click here or on the image at right to view it at full size in a new window.
TDH is measured in feet or meters. By dividing feet of TDH by the 2.31, we can convert feet of head to pressure as psi (100 feet / 2.31 = 43 psi).
This is useful if you know how much pressure you need, or so you can read the pump's outflow pressure gauge, and in reverse, multiply by 2.31 to estimate the feet of TDH (100 psi x 2.31 = 231 feet).
You can then better understand how the pump is performing through referencing the pump curve. Higher pressure does not equal higher flow.
Why is the measurement on the pump curve in feet or meters and not pressure? Pressure is related to the type of liquid, its specific gravity. Heaver liquids will create more resistance and therefore more pressure.
Head is a fluids term that measures the kinetic energy a pump creates. The pump will move any liquid to the same vertical height (feet) if the pump can be spun at the same rpm.
For liquids with a higher specific gravity than water, more power is required. Pump performance curves are based on clear water at sea-level.
RPM tells us how fast we are rotating the pump. Using the rpm curves, we can find how much TDH the pump will create at a given flow rate at any point on that curve.
Intersecting the rpm curves are horsepower requirements. Horsepower increases with higher flow rates and higher TDH.
Net Positive Suction Head Required (NPSHr) is the required minimum inflow to keep the pump from cavitation and allow it to work properly. This number helps us calculate how much lift a pump has on the intake side.
Pumps create a vacuum at the center of the impeller that draws fluid into the pump. After priming the pump, this vacuum continues to work if the pump is within a close enough distance to the liquid allow atmospheric pressure to fulfill NPSHr.
If not, you may need to provide inflow to ensure this requirement is filled. Cavitation creates massive damage to the pump. We will discuss NPSHr and cavitation in more depth later. PD
Nancy Puck
Puck Custom Enterprises, Inc.
(712) 653-3045
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