Water and Power — 24 February 2013

A pump curve is a graph that shows the amount of water (or other fluid) a pump can provide at different heads (or pressures it has to pump against).  A pump curve is usually displayed with the flow rate (example gallons per minute gpm) on the x-axis and head (ft or m) on the y-axis.

Here is a typical pump curve.

Typical Pump Curve

Typical Pump Curve

The curve shown above is a typical pump curve for a pump (this particular one is from the National Pump Company).  The pump curve is the top curve on the graph. Notice that at a lower flow, the total head is greater.  As the total head decreases, the amount of flow the pump provides becomes greater.  Total Head is the summation of elevation and friction head.

Elevation Head

Elevation head represents the vertical rise in elevation that the fluid must be pumped. So if the pump is at elevation 500 ft and water is pumped to a tank that is at elevation 550 ft, then the elevation head would be 50 ft.  Even if there is more than 50 feet of pipe connecting the pump and tank, the elevation head is still 50 ft.

It takes more effort to pump to a higher elevation than to a lower elevation.  This can be seen on the pump curve by lower flow rates at higher heads.

Friction Head

Friction head is caused by any disturbance to the fluid as it flows.  As water flows in a pipe, there is friction against the interior of the pipe.  For pipe flow applications this amount of friction Head  that is developed can be estimated from different equations.  Perhaps the most commonly used for pipe flow is the Hazen Williams Equation.

Hazen Williams Equation:

Q = AKCR_{h}^{0.63}(\frac{h_f}{L})^{0.53}

Where: Q = Flow rate (cfs or cms)

K = 1.318 for English units (feet and seconds), 0.85 for SI units (meter and seconds)

hf = friction headloss

L = length of pipe

C = Coefficient for pipe smoothness

Rh = Hydraulic Radius (Diameter of pipe/4 for full circular pipes)

Friction against the side of a pipe is referred to as major headloss.  What is referred to as “minor headloss” is other friction head that occurs when the fluid must change directions such as at elbows, valves, contractions, enlargements, entrances, exits, etc..  A common way to estimate the amount of minor headloss is through coefficients multiplied by the velocity head.

The velocity head is:

\frac{V^2}{2g}

Where: V = velocity (ft/sec)

g = gravitational constant (32.2 ft/sec^2)

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