How to Calculate Water Pump Horsepower
Decide on the desired flow rate., Measure the height the water needs to travel., Estimate friction losses from the pipe., Add the pumping lift and friction loss together., Look up the specific gravity if you are pumping anything besides water...
Step-by-Step Guide
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Step 1: Decide on the desired flow rate.
The needs of your project determine the necessary flow rate of liquid from the pump.
Write this value down in gallons per minute (gpm).
You won't be using this value right away, but it will determine which pumps and pipes you consider.
Example:
A gardener has an irrigation plan that requires a flow rate of 10 gallons per minute. -
Step 2: Measure the height the water needs to travel.
This is the vertical distance from the top of the water table (or the top of the water level in the first tank) to the final destination of the water.
Ignore any horizontal distance.
If the water level changes over time, use the maximum expected distance.
This is the "pumping lift" your pump will need to generate.Example:
When the gardener's water tank is nearly empty (the lowest expected level), its water level is 50 feet below the area of the garden that needs watering. , Besides the minimum pressure needed to move water a certain distance, your pump also needs to overcome the force of friction as the water moves through the pipe.
The amount of friction depends on the pipe's material, internal diameter, and length, as well as the type of bends and fittings you use.
Look up these values on a pipe friction loss chart such as this one.
Write down the total friction loss in feet of head (meaning the number of feet you "lose" from your pumping lift because of friction).
Example:
The gardener decides to use 1" diameter plastic pipes, and needs 75 ft of pipe total (including horizontal lengths).
A pipe friction loss chart tells him that 1" plastic pipes cause a loss of
6.3 ft of head for every 100 ft of pipe length.75ft∗6.3fthead100ft=4.7fthead{\displaystyle 75ft*{\frac {6.3ft_{head}}{100ft}}=4.7ft_{head}}He also looks up the friction loss from each fitting in the pipe.
For 1" plastic, one 90º elbow connector and three threaded fittings contribute a total loss of 15 ft.Adding this all together, the total friction loss is
4.7 + 15 =
19.7 ft., or about 20 ft.
These charts often include an estimate of water velocity as well, based on flow rate and the pipes you use.
It's best to keep velocity below 5 ft / s to prevent "water hammer," the repeated knocking vibration that can damage your equipment., The vertical distance water needs to travel plus the friction losses from the pipe make the "total dynamic head" or TDH.
This is the total pressure load the pump needs to overcome.Example:
TDH = vertical distance + friction loss = 50 ft + 20 ft = 70 ft. , The basic water horsepower formula assumes you are pumping water.
If you are pumping a different fluid, look up its "specific gravity" online or in an engineering reference book.
Fluids with a higher specific density are denser, and require more horsepower to push through the pipe.
Example:
Since the gardener is pumping water, he doesn't need to look anything up.
Water's specific gravity is equal to
1. , The water horsepower, or minimum power required to run the pump, equals TDH∗Q∗SG3960{\displaystyle {\frac {TDH*Q*SG}{3960}}}, where TDH is the total dynamic head in feet, Q is the flow rate in gpm, and SG is the specific gravity (1 for water).
Enter all the values you found into this formula to find the water horsepower for your project.
Example:
The garden pump needs to overcome a TDH of 70 ft and produce a flow rate Q of 10 gpm.
Since it is pumping water, the SG is equal to
1.Water horsepower = TDH∗Q∗SG3960=70∗10∗13960={\displaystyle {\frac {TDH*Q*SG}{3960}}={\frac {70*10*1}{3960}}=} ~0.18 horsepower. , Now you know how much horsepower you need to supply to run your pump.
However, no mechanical device is 100% efficient at transferring power.
Once you have chosen a pump, check the manufacturer's info for the pump's efficiency and write it as a decimal.
Divide the water horsepower by this value to find the actual horsepower of the motor you need for your pump.Example:
To do
0.18 horsepower of work, a pump with a 50% (or
0.5) efficiency rating would actually require
0.180.5={\displaystyle {\frac {0.18}{0.5}}=} a
0.36 hp motor.
Most modern pumps are between 50% and 85% efficient when used as intended.If you cannot find an efficiency rating for your pump, you can assume the actual motor horsepower needed falls between WaterHP0.5{\displaystyle {\frac {WaterHP}{0.5}}} and WaterHP0.85{\displaystyle {\frac {WaterHP}{0.85}}} -
Step 3: Estimate friction losses from the pipe.
-
Step 4: Add the pumping lift and friction loss together.
-
Step 5: Look up the specific gravity if you are pumping anything besides water.
-
Step 6: Enter these values into the water horsepower formula.
-
Step 7: Divide horsepower by pump efficiency.
Detailed Guide
The needs of your project determine the necessary flow rate of liquid from the pump.
Write this value down in gallons per minute (gpm).
You won't be using this value right away, but it will determine which pumps and pipes you consider.
Example:
A gardener has an irrigation plan that requires a flow rate of 10 gallons per minute.
This is the vertical distance from the top of the water table (or the top of the water level in the first tank) to the final destination of the water.
Ignore any horizontal distance.
If the water level changes over time, use the maximum expected distance.
This is the "pumping lift" your pump will need to generate.Example:
When the gardener's water tank is nearly empty (the lowest expected level), its water level is 50 feet below the area of the garden that needs watering. , Besides the minimum pressure needed to move water a certain distance, your pump also needs to overcome the force of friction as the water moves through the pipe.
The amount of friction depends on the pipe's material, internal diameter, and length, as well as the type of bends and fittings you use.
Look up these values on a pipe friction loss chart such as this one.
Write down the total friction loss in feet of head (meaning the number of feet you "lose" from your pumping lift because of friction).
Example:
The gardener decides to use 1" diameter plastic pipes, and needs 75 ft of pipe total (including horizontal lengths).
A pipe friction loss chart tells him that 1" plastic pipes cause a loss of
6.3 ft of head for every 100 ft of pipe length.75ft∗6.3fthead100ft=4.7fthead{\displaystyle 75ft*{\frac {6.3ft_{head}}{100ft}}=4.7ft_{head}}He also looks up the friction loss from each fitting in the pipe.
For 1" plastic, one 90º elbow connector and three threaded fittings contribute a total loss of 15 ft.Adding this all together, the total friction loss is
4.7 + 15 =
19.7 ft., or about 20 ft.
These charts often include an estimate of water velocity as well, based on flow rate and the pipes you use.
It's best to keep velocity below 5 ft / s to prevent "water hammer," the repeated knocking vibration that can damage your equipment., The vertical distance water needs to travel plus the friction losses from the pipe make the "total dynamic head" or TDH.
This is the total pressure load the pump needs to overcome.Example:
TDH = vertical distance + friction loss = 50 ft + 20 ft = 70 ft. , The basic water horsepower formula assumes you are pumping water.
If you are pumping a different fluid, look up its "specific gravity" online or in an engineering reference book.
Fluids with a higher specific density are denser, and require more horsepower to push through the pipe.
Example:
Since the gardener is pumping water, he doesn't need to look anything up.
Water's specific gravity is equal to
1. , The water horsepower, or minimum power required to run the pump, equals TDH∗Q∗SG3960{\displaystyle {\frac {TDH*Q*SG}{3960}}}, where TDH is the total dynamic head in feet, Q is the flow rate in gpm, and SG is the specific gravity (1 for water).
Enter all the values you found into this formula to find the water horsepower for your project.
Example:
The garden pump needs to overcome a TDH of 70 ft and produce a flow rate Q of 10 gpm.
Since it is pumping water, the SG is equal to
1.Water horsepower = TDH∗Q∗SG3960=70∗10∗13960={\displaystyle {\frac {TDH*Q*SG}{3960}}={\frac {70*10*1}{3960}}=} ~0.18 horsepower. , Now you know how much horsepower you need to supply to run your pump.
However, no mechanical device is 100% efficient at transferring power.
Once you have chosen a pump, check the manufacturer's info for the pump's efficiency and write it as a decimal.
Divide the water horsepower by this value to find the actual horsepower of the motor you need for your pump.Example:
To do
0.18 horsepower of work, a pump with a 50% (or
0.5) efficiency rating would actually require
0.180.5={\displaystyle {\frac {0.18}{0.5}}=} a
0.36 hp motor.
Most modern pumps are between 50% and 85% efficient when used as intended.If you cannot find an efficiency rating for your pump, you can assume the actual motor horsepower needed falls between WaterHP0.5{\displaystyle {\frac {WaterHP}{0.5}}} and WaterHP0.85{\displaystyle {\frac {WaterHP}{0.85}}}
About the Author
Elizabeth Wells
Experienced content creator specializing in home improvement guides and tutorials.
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