How to Calculate Water Pump Horsepower

A pump is a basic but important mechanical device that supplies the force to move fluid at a specific flow rate. Like any device that does work (transfers energy across a distance), its effectiveness is measured in power. Although watts and kilowatts are more common units of power measurement, horsepower is still commonly used for high-output electrical devices in the United States. In this context, 1 horsepower is equal to 746 watts.

Water horsepower = minimum power required to run water pump TDH = Total Dynamic Head = Vertical distance liquid travels (in feet) friction loss from pipe Q = flow rate of liquid in gallons per minute SG = specific gravity of liquid (this equals 1 if you are pumping water) Water horsepower = TDHQSG3960\displaystyle \frac TDH*Q*SG3960 Actual power required = (water horsepower) / (pump efficiency). Write efficiency as a decimal (50% 0.5).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.

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.

Estimate friction losses from the pipe. 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.

He also looks up the friction loss from each fitting in the pipe. For 1″ plastic, one 90 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.

Look up the specific gravity if you are pumping anything besides water. The basic water horsepower formula assumes you are pumping water. If may bom cong nghiepÂ  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.

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