How to Calculate Pipe Flow Rates

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Schedule 40-steel pipe is available in a range of sizes.
Schedule 40-steel pipe is available in a range of sizes. (Image: pipe fitting image by Joann Cooper from Fotolia.com)

Piping systems are designed by selecting piping systems and materials that are compatible with the flowing media and selecting the pipe size that provides the best overall economy for the application. While a larger pipe size may cost more initially, it provides better operating efficiency, which can represent a quick payback. One of the best ways to optimize a pipe flow investment is to calculate the actual pipe flow rates on the basis of pressure losses that might needlessly inflate operating costs.

Things You'll Need

  • Calculator
  • Pipe flow-pressure loss data for the specific piping system

Schedule 40-Steel Pipe Water Pumping

Define the steel pipe application. An industrial well application requires pumping 425 gallons per minute (gpm) of water 575 feet away into an in-ground reservoir that is 80 feet higher than the pump at the well. If a 60-psi (pounds per square inch) pump is available, you can calculate the minimum Schedule 40-steel pipe size that would handle the flow and pressure constraints.

This steel pipe can be welded end to end.
This steel pipe can be welded end to end. (Image: Pipes image by Sergei M. Kharitonov from Fotolia.com)

Determine the pressure available to provide the 425-gpm flow. Because the reservoir is 80 feet higher than the pump, some of the 60-psi pumping pressure will be lost pushing the water uphill. Dividing 80 feet/2.31 feet/psi yields a static pressure loss of 34.63 psi, which, when subtracted from the 60-psi pump pressure, leaves 25.37 psi to push the 425-gpm flow through the 575-foot pipe.

Scale the pressure drop per 100 feet of pipe, since this is how published pipe data is presented. Substituting values yields 25.37 psi/575 feet/100 feet = 4.41-psi drop per 100 feet at 425 gpm.

Consult published pipe pressure and flow data for Schedule 40-steel pipe to select a potential pipe size. The 4-inch Schedule 40-steel pipe pressure-loss to flow chart shows a loss of 5.5 psi for a 476-psi flow.

Calculate the flow with a 4.41-psi loss based on the 5.5-psi loss flow at 476 gpm. Since flow varies proportionately to the square root of pressure loss difference, extract the square root of (4.41 psi/5.5 psi) = square root of 0.8018 = 0.895 and multiply by the 476-gpm flow cited to yield 426.23 gpm, which would just work for 425 gpm.

Schedule 40-Steel Pipe Steam Flow

Define the steam flow application. Saturated low pressure steam at 12 psi is flowing through 3-inch Schedule 40-steel pipe. If it loses 1.5 psi pressure at the end of 100 feet of the pipe, you can calculate the flow rate in pounds/hour (lb/hr) of steam.

Steam flow rates are carefully monitored in power plants.
Steam flow rates are carefully monitored in power plants. (Image: power plant image by Andrei Merkulov from Fotolia.com)

Consult the Steam Flow Rate and Pressure Drop for Schedule 40 Pipe chart (see Resources) to find the reference points for the 3-inch pipe. The 1-psi drop flow value reads 1,670 lb/hr and the 2-psi value reads 2,400 lb/hr.

Calculate your flow at the 1.5-psi drop by taking the square root of (1.5/2.0) = 0.866 and multiplying by the 2,400 lb/hr at the higher pressure drop = 0.866 X 2,400 = 2,078.4 lb/hr with a 1.5-psi drop.

Compare against the calculation using the lower 1-psi drop value to interpolate the actual flow value. Square root (1.5/1) = 2,045.32 lb/hr. Adding half the difference to the lower value or (2,078.4 minus 2,045.32 = 33.08/2 + 2,045.32 = 2,061.86 lb/hr, which would be a reasonable interpolation result in light of the tabular data.

Steam headers in plants are sized according to flow rate.
Steam headers in plants are sized according to flow rate. (Image: Pipes inside energy plant image by Andrei Merkulov from Fotolia.com)

Tips & Warnings

  • Conservative design with flow piping may mean moving up one pipe size to require less energy to move the volume.
  • Steam and superheated steam can cause serious injury or death from third-degree burns. Always have pressurized designs checked by a professional.

References

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