Pressure information such as pressure drops through sections of piping is usually related to flow capacity in gallons per minute (gpm). Each piping system is unique due to a variety of factors including smoothness and the way sections join, but generally pressure has to increase as a function of the square of flow increase. Therefore, if flow doubles, pressure has usually had to quadruple to affect the change in a flowing system. Manufacturers of different types of piping publish flow-pressure data about their products. You can use this data to calculate gpm from pounds per square inch (psi).
How to Determine Pipe Flow from Pressure Loss
Step 1: Define the System
Define a pipeline flowing system to begin the calculation. As an example, if water has to be pumped through a 2-inch Schedule 40 polyvinyl chloride (PVC) pipe from a well to a pond 400 feet away, calculate how many gpm the system is delivering. The pressure gauge on the pipe at the well head reads 66 psi and the one at the pond reads 50.8 psi.
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Step 2: Determine the Pressure Loss
Subtract the lower value, 50.8 psi, from 66 psi to determine pressure loss through the pipe. This equals 15.2 psi over the 400 feet of pipe, or 3.8 psi per 100 feet of pipe length (15.2 divided by 4 = 3.8).
Step 3: Determine the Flow Loss
Use a flow vs. pipe-loss data chart to determine the pressure-drop flow correlation. Reading this type of table, the flow rate corresponding to 3.8 psi loss to 100 feet of 2-inch Schedule 40 PVC plastic pipe is 75 gpm.
Step 4: Determine the Pressure Loss
Determine the pressure loss at one-half of 75 gpm, or 37.5 gpm, to confirm the approximate square-power relationship between pressure and flow. Interpolating between 35 gpm and 40 gpm, the pressure loss computes to a value between 0.9 and 1.2 psi loss per 100 feet of pipe. Since 37.5 gpm is halfway between the two flow readings of 35 and 40 gpm, the interpolation will work out to about 0.707 of the way between 0.9 and 1.2 psi loss (again the square function). So the loss would be:
1.2 psi – 0.9 psi = 0.3 psi x 0.707, which is 0.2121 psi greater than the 0.9 psi value, or 1.1121 psi.
Step 5: Cross-Check the Calculation
Multiply 1.1121 psi by 4 to check against twice the flow, yielding a value of 4.45 psi, or only 0.55 psi greater than actually measured. This difference of about 14 percent greater pressure drop would work out to be only about a 6 percent flow difference, which is close enough to support the general square-power relationship between flow and pressure, especially in piping that can vary that much in interior size and smoothness.