Water under the bridge?

Brian Vinchesi

At GIS in San Antonio, I was surprised and disappointed to find the battle between high density polyethylene (HDPE) pipe versus polyvinyl chloride (PVC) pipe for irrigation systems (presented alphabetically so there is no bias) is still strong. So many people I talked to believed this issue was “water under the bridge.”

A little over four years ago, I wrote my first GCI column titled: “HDPE vs. PVC.” I reviewed the column to see what, if anything, had changed. Back then I started with “…many times the decision of which type of pipe to use is not based on science or engineering, but on trends or salesman recommendations. It is important to look at the technical aspects of the pipe (pressure rating and velocity) for each type of piping system and determine which is the best type of piping material for your golf course.” This still holds true, but there is more science and a lot more experience with HDPE piping systems. The issues in Florida with HDPE systems (Polygate) has pushed the science along, and designers, suppliers and installers now have a better understanding of what they can and can’t do.

For today’s systems, PVC pipe is usually 200 psi (SDR21). Ductile iron fittings are used for mainline and PVC gasketed fittings for lateral piping. Whereas Class 200 PVC is always manufactured with the same resin (1120, 1220) HDPE piping is available in two common resins, 4710 and 3408. In golf, 4710 is the predominate resin. With the two different DR’s, as the resin changes, so do the pressure ratings and characteristics of the pipe. Therefore, you need to know which resin is being provided. HDPE piping (4710, 3408) is available in a 200 psi (DR11 or DR9) as well as 160 psi (DR13.5, DR11), 125 psi (DR17, DR13.5) and 100 psi (DR21, DR17). You can see: the higher the DR, the weaker the pipe.

Piping standards require the plastic irrigation piping system working pressures be no more than 72 percent of the rated pressure of the pipe. For 200 PSI pipe, this is 156 psi. For 160 psi rated pipe, this is 125 psi. It doesn’t matter whether it is HDPE or PVC. The same standard also limits velocities to 5 feet per second in plastic pipes whether it is HDPE or PVC. Some like to quote AWWA standards which are less stringent when it comes to HDPE and PVC plastic pipe. But AWWA standards are for municipal water systems, not irrigation systems. The proper standard to adhere to is “ASABE 376.2; Design, Installation and Performance of Underground Thermoplastic Irrigation Pipelines.”

The cost of plastic pipe is based on the cost of the plastic resin used to make it. Because of their different chemical makeups, to obtain the same material strength (pressure rating) HDPE pipe has a thicker wall than PVC so it uses more plastic. As a result, HDPE will cost more unless you lower the pressure rating (DR) or raise the velocity. If your system includes a 4-inch pipe carrying 200 gpm (pretty common on a golf course) in a PVC Class 200 system (SDR 21), the velocity would be 4.92 fps. In an HDPE 200 psi (PE4710, DR11) pipe, the velocity would be 6.18 fps. If the pipe was HDPE 160 psi (PE4710, DR13.5), the velocity would be 5.67 fps. For proper design, HDPE pipe is usually one pipe size larger than the comparable PVC pipe. That keeps the velocities within the standard, but raises the cost.

HDPE fittings only come in DR 11 and sometimes DR 13.5. That makes the use of lower-pressure rated pipes more difficult as the pipe and fitting ends are of different thicknesses. Do you set the fusion machine for the thickness of the fitting or the thickness of the pipe? Consensus seems to be you need to have the thicknesses of the fitting the same as the pipe so the fusion joint will not fail over time.

While we know lots about PVC piping as it has been used for over 50 years, the industry is still studying HDPE to determine the best design and installation practices for golf course irrigation systems.Make “apple-to-apples” instead of the “apples-to-oranges” comparison for the different materials.

When doing a direct-cost and feature comparison, consider the scientific data and not just sales talk and you will have a long-lasting system that won’t give you or your successor issues.

Brian Vinchesi, the 2009 EPA WaterSense Irrigation Partner of the Year, is president of Irrigation Consulting Inc., a golf course irrigation design and consulting firm headquartered in Pepperell, Mass., that designs irrigation systems throughout the world. He can be reached at bvinchesi@irrigationconsulting.com or 978/433-8972.