We will start with the basic formulas to calculate the velocity through the pipe of a given inside diameter D, and flow, Q. Proper coefficients are provided to account for either U.S. or metric units:
Vft/sec = Qgpm x 0.321 / ( x Din2 / 4) ( π = 3.14)
Vm/sec = Qm3/hr x 277.8 / ( x Dmm2 / 4) = Qm3/hr x 0.43 / ( x Dmm2 / 4)
For clean water, the discharge pipe should be sized for under 15 to 20 ft/sec, and the suction pipe should be sized for under 5 to 10 ft/sec.
The August 2009 Pumping Prescriptions featured an ANSI pump 1x1.5-6, with BEP flow of 100 gpm.
Vft/sec = Qgpm x 0.321 / ( x Din2 / 4) = [ 100 x 0.321 ] / [ 3.14 x 1.52 /4 ] = 18.2 ft/sec
Vft/sec = Qgpm x 0.321 / ( x Din2 / 4) = [ 100 x 0.321 ] / [ 3.14 x 1.02 /4 ] = 40.9 ft/sec
Both suction and discharge dimensions are too tight for this flow, but a typical pump performance coverage chart (see Figure 1) shows a 1x1.5-6 pump at 3,500 rpm at 100 gpm. Why? A common practice is to increase the piping size away from the pump to the next size, because that is where the effect of high velocity (friction losses over a length of pipe) shows up. So, if the suction of the pump were connected to a 1.5 in pipe and discharge to 1.5 in, the velocities would be:
Vft/sec = Qgpm x 0.321 / ( x Din2 / 4) = [ 100 x 0.321 ] / [ 3.14 x 2.02 /4 ] = 10.2 ft/sec
V ft/sec = Qgpm x 0.321 / ( x Din2 / 4) = [ 100 x 0.321 ] / [ 3.14 x 1.52 /4 ] = 18.2 ft/sec
Those numbers are better. Although the discharge velocity is still high, it will be within the limits at lower pump speeds (3,500 rpm was the maximum speed in Figure 1).
Using this logic, you can verify if these rules hold for other pump sizes by reviewing any pump brochure, plugging the approximate BEP flow conditions for various pump sizes and seeing what happens to the suction and discharge side velocities. Do not forget to upsize the actual piping dimension, as we did above.
One final note-the above rules were applied for clean water. For more viscous or abrasive fluids, the velocities should be decreased further. Special formulas exist for such special applications, such as pulp and paper, slurries, etc. Conversely, in some instances the velocities may actually be increased, such as settling mixtures, to avoid settled bridging and similar problems.
Pumps & Systems, December 2009