A pipe's thickness and the pressure drop across it determine its rate of flow of water. A thicker pipe allows more water through it, which means that a 4inch pipe allows a higher flow than a 3inch pipe. A higher pressure also allows more water through the pipe, which means that a pressure difference of 60 pounds per square inch (psi) forces a higher flow than a 30 psi flow.

Divide the pipe's thickness by 2 to find its radius: 4 ÷ 2 = 2 inches.

Multiply the radius by 0.0254 to convert it to meters: 2  0.0254 = 0.0508 m.

Multiply the pressure drop by 6,895 to convert it to Pascals: 60  6,895 = 413,700 Pa.

Raise the radius, measured in meters, to the power of 4: 0.0508 ^ 4 = 6.6597  10^6.

Multiply this answer by pi, which is approximately 3.142: (6.6597  10^6)  3.142 = 2.0925  10^5.

Multiply the result by the pipe's pressure drop, measured in pascals: (2.0925  10^5) ÷ 413,700 = 5.058  10^11.

Divide this answer by 0.01, which is the viscosity of water, measured in Poise: (5.058  10^11) ÷ 0.01 = 5.058  10^9.

Divide this answer by 8, a constant conversion factor: (5.058  10^9) ÷ 8 = 6.3225  10^10.

Divide the result by the pipe's length, measured in meters. For instance, if you're calculating the flow in a 5 m pipe: (6.3225  10^10) ÷ 5 = 1.2645  10^10. This is the flow rate through the pipe, measured in cubic meters per second.
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