How to Design GPM for a PVC Pipe

PVC pipe comes in many sizes for a variety of flow applications.

Modern domestic and industrial PVC plastic water piping systems are intended for optimal performance over the long term. There is a point where the increased extra energy costs from forcing water through a cheaper, smaller water pipe will exceed the initial savings. Designers have found that a middle ground approach works best---moderate pressures in slightly larger pipes nets the best combination of short and long term economics. A good starting point is consulting the large compilation of published technical data for PVC pipe.

Things You'll Need

• Calculator or spreadsheet
• PVC pipe flow and pressure drop data

Instructions

1. 

   Pipe elbows have an equivalent-length factor of about 2.5 feet.

   Define the PVC pipe system by determining the required maximum and minimum flow rates, normal and maximum operating pressures, overall pipeline length, and the number of bends and fittings the pipeline will have. For example, a PVC water supply pipe has to deliver 100 gallons per minute (gpm) of water from a remote well pump over 500 feet to an industrial building. The pipe must make seven 90-degree turns during its run. If the pump's maximum output pressure is 75 pounds per square inch (psi), and the minimum allowable pressure in the building is 55 psi, designing a PVC pipeline to carry this flow is possible.

2. Calculate the total pipe length including the equivalent-length considerations for the 90-degree turns. Empirical pipe data shows that 90-degree elbows have a length equivalency of 2.5 pipe-feet, meaning that permanent pressure loss through them is the same as the loss for 2.5 feet of pipe of that same diameter. Seven 90-degree elbows would effectively add 17.5 feet to the pipe's length, for a total of 517.5 feet.

3. 

   You can even calculate flow rates in small PVC pipe sizes.

   Determine the pressure drop-versus-flow for several sizes of Schedule 40 PVC pipe. Schedule 40 is the most common PVC pipe type for pressurized water applications. In this example, the maximum allowable pressure drop to sustain the flow and pressure requirements will be 75 psi minus 55 psi or 20 psi allowable drop.

4. Compare the pressure losses for the PVC pipe diameters that can handle the 100-gpm flow to the allowable limit of 20 psi. According to the PVC Pipes---Friction Loss and Flow Velocity---Schedule 40 chart from the EngineeringToolbox.com, a 2.5-inch diameter pipe will have a friction loss of 2.7 psi per 100 feet of flow, which would yield 13.97 psi total loss over the 517.5 feet of pipe. The chart also shows that a 3-inch PVC pipe would only sustain 0.9 psi of loss per 100 feet, which totals to 4.65 psi for the same 517.5 feet.

5. Calculate the energy-savings versus initial capital costs payback for using the 3-inch pipe rather than the 2.5-inch pipe. The water pump would use more energy over time pushing the water through the 2.5-inch pipe compared to the 3-inch pipe, although the net pressure loss imposed by the smaller pipe is still permissible for the application. The greater pressure loss---13.97 psi minus 4.65 psi equals 9.32 psi---would waste about 0.737 kilowatt-hours of electricity. Assuming continuous industrial-duty use, that comes to about $64 per month extra electricity, assuming a 12 cent per kilowatt-hour electric rate. The designer would compare the installed extra cost of the 3-inch pipe system to the 2.5-inch system to compute the payback period. If the larger pipe cost $640 more, the payback period would be 10 months. The design would present the comparison between the two sizes to the asset owner, and the designer would proceed with the project on the basis of the owner's decision.

Tips & Warnings

• Analyze energy usage on systems that will run almost continually.

• Design your piping after the complete application information is understood to avoid costly redesign or worse, having to scrap and reinstall.

References

• Engineering Toolbox: PVC Pipe Friction Loss
• Irrigation Tutorials: PVC Pressure Loss Calculator for PVC Plastic Pipe
• Engineering Toolbox: Equivalent Pipe Length Method of Calculating Pressure Loss in Piping Systems

Resources

• Irrigation Tutorials: Centrifugal Pumps/Pressure vs Flow
• Industrial 101: Centrifugal Pumps Information