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Before you can pick the best pump for your pond, you need to determine what flow rate and total dynamic head (TDH) you want. Since the flow rate is affected by the head it is working against, you must know both parameters to properly select a pump.
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Flow –
The exact flow you will need depends on many factors including the size of your pond and waterfall as well as the amount of fish, plants and sunlight. In general, most ponds will operate nicely if you turn the water over approximately once an hour to once every three hours. This means if you have a 4000-gallon pond, you want about 4000 gallons per hour or 67 gallons per minute.
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Head –
Head is a measure of resistance to flow. If a pump has a maximum output of 20 head feet, it means it can pump water 20' straight in the air. If a pump is rated at 50 gallons per minute at 10 feet it means it can overcome 10 feet of head (TDH) and still deliver 50 GPM. As you increase the head, you decrease the flow rate. To maximize your flow, you must minimize your head. For pond applications the 3 main sources of head are:
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Static Head –
This is the vertical distance you raise the water. To determine your static head, measure from the surface of the pond (vertically), to the highest point in the discharge line where the water is discharged to the atmosphere. This is usually the top of your biological filter, or waterfall.
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Friction Head –
As water flows through pipe and fittings there is resistance. The higher the flow and/or the smaller the pipe, the higher the resistance. Determine your overall pipe length, including equivalent length for your fittings. Consult the friction loss charts. Find where the column for your pipe diameter intersects the row for your flow rate and read the friction loss per 100 ft. pipe. Use large enough pipe to minimize friction loss. It is usually best to keep your friction loss (per 100 feet of pipe) to less than 6 feet.
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Pressure Head –
Any additional pressure required by sand filters, spray nozzles, etc. must be calculated. The conversion is 1 PSI = 2.31 head feet.
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TDH –
To determine your TDH, add your static head, friction head and pressure head. Now that you know your flow and head, you can select a pump that provides this performance, and does so efficiently.
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