We all know PVC pipe, which comes in a variety of colors. These colors include white, blue, green, purple, gray, and tan; each color is generally associated with a specific use (potable water, sewer or drainage, reclaimed water, etc.). Missing from the list is black.
You have probably seen black pipe on job sites or at supply houses. We haven’t overlooked black pipe. The fact is most black pipe is not PVC but either ABS (Acrylonitrile-Butadiene-Styrene) or HDPE (High Density Polyethylene). ABS is generally limited to use in sanitary sewer applications for indoor plumbing applications. HDPE on the other hand has a much wider range of uses.
Note that I am not talking about the thin-wall, flexible polyethylene tubing that we all use for landscape or agricultural drip or orchard systems. Nor am I talking about the low-head single and dual-wall corrugated black pipe used for storm water management, though we know and use that as well. I am talking about thick-walled piping suitable for demanding applications like water mains, mining operations, landfill gas or leachate collection systems, gas mains, and sewer force mains.
In recent years there has been an up-tick in the use of HDPE for a broader range of irrigation applications. The increased use can be attributed to a few of this pipe’s unique characteristics:
- Increased Flexibility over PVC
- Greater Cyclic Resistance to Material Failure (It handles surges well)
- Impact Resistance
- Greater Freeze Resistance
- Resistance to Abrasion
- Resistant to Weather and UV Degradation
- Reduced Chemical Reactivity
- Stronger Joints
- Reduced Need for Thrust Blocks or Joint Restraints
- Reduced Harmful Chemical By-Products From its Manufacture or Installation
Irrigation Designers have recently discovered HDPE. More irrigation systems are switching to HDPE for use as mainline and even lateral line piping. The pipe is more flexible than PVC which allows for installation in a curvilinear landscaping without the need to install a preponderance of fractional elbows. The pipe’s thick walls and impact resistance reduce the likelihood of incidental damage during installation or planting or property maintenance or enhancement operations.
HDPE pipe is assembled using a heat fusion process in lieu of volatile chemical solvents. After all, the pipe is resistant to virtually all chemical solvents. Joining HDPE pipe requires heat fusion equipment or specially designed electro-fusion socket fittings to physically meld the pipe and fitting together. The result is that a properly constructed HDPE joint is stronger than the pipe itself.
HDPE mainline system costs are comparable to PVC when one considers all of the factors.
- The pipe comes in longer lengths requiring fewer couplings
- Joint strengths eliminates the need for thrust blocks or joint restraints
- No primers or chemical solvent cements are required
- Easily adapted to pipe pulling installation techniques
The joining process does require training and special equipment. Joint assembly times vary with the size of the pipe and the wall thickness (Dimensional Ratio [DR] Number). The joining sequence for “Butt Fusion” involves trimming and truing the ends of the pipe or fitting, heating the two ends to be joined, pressing the heated ends together to a size-specific pressure (verified by a rolled bead of melted plastic on either side of the joint), and cool down time before the joint can be released from the fusion welding apparatus.
“Socket Fusion” fittings are pre-wired with an electrical heating filament embedded in the plastic. The fitting is hooked to a special transformer that is equipped with a scanning device. The scanner is used to read a bar code on the fitting to automatically set the heat level and duration for the imbedded filament. The result is a fool proof joint.
The primary down side to HDPE is its coefficient of thermal expansion. The pipe expands and contracts with temperature changes far more than PVC. This is not a major problem once the pipe is in the ground where temperatures generally remain constant. It can be a problem in on-grade installations or where extreme temperature variation exists between assembly on grade and installation in a cool trench. Consideration should be given to expansion fittings or other techniques to accommodate length changes.
Lastly, and with reference back to color coding, HDPE pipe for domestic water applications comes with a blue stripe while pipe intended for reclaimed water comes with a purple stripe. Other applications (natural gas, television cable, etc.) come in solid colors such as yellow, orange, red, etc.).
Indeed, Black is Beautiful; look for increased use of this material for serious irrigation applications in the future.
K. F. Duke