Addressing Bentonite Contamination, Gouges, Pipe Relaxation, and Alignment in Horizontal Directional Drilling (HDD)
One of the numerous advantages of high-density polyethylene pipe is the eco-friendly means of installation. Horizontal Directional Drilling (HDD) is a manner of installation that provides minimal surface soil disruption and allows gas distribution piping systems to travel under streams, roads, and other environmentally sensitive areas.
Traditional means of installation that rely on open trenches would be extremely disruptive, costly, and in many cases downright impossible. At the same time, HDD inherently involves fluctuating temperatures, wind, dust, dirt, rain, snow, and ground water – and these environmental contaminants pose potential threats to electrofusion joints.
Qualified operators and installers in the natural gas piping industry must have the patience and training to mitigate and work around these threats. Even on sunny, clear and calm days, special care must be taken in preparing HDD pipe for electrofusion.
Bentonite Contamination
When a directional drill is passing through the earth, a slurry of naturally occurring clay powder, called bentonite, and water provides lubrication, reduces heat, and helps keep the structure of the bore hole open throughout the boring and pulling process.
Inevitably, the process of pulling the pipe through the drilling path completely coats it in this slurry. If bentonite contaminates a fusion joint, it acts as a barrier to the polyethylene bond and can greatly weaken the joint. Bentonite is often measured in microns and may not be visible to the naked eye.
But don’t let the lack of size fool you – even unseen bentonite can negatively affect an electrofusion joint. It is extremely important to make sure qualified fusers are aware of potential bentonite contamination.
In preparing for electrofusion, many manufacturers and gas utilities have adopted new procedures to minimize bentonite contamination, including three easy steps:
1. A general wash zone far outside the footprint of the fitting
2. Peeling the pipe approximately 1 inch beyond the fitting footprint
3. Using 96% or greater isopropyl alcohol wipes to clean the peeled area
It is imperative that isopropyl alcohol wipes stay within the peeled area of the pipe. Trace amounts of Bentonite may remain on the un-peeled wash zone, which then transfer into the freshly peeled fusion area.
Gouges in HDPE
After the bore hole is complete, operators change the directional drill head, and pull the HDPE back through the bore hole. After the pull is complete, the exposed HDPE is inspected for gouges and other potential damage.
In cases of gouging and HDD, more pipe can be pulled through to determine if the gouge is isolated or continuous. In most cases, any gouge less than 10 percent of the pipe wall thickness is acceptable for service. If the gouge is 10 percent or greater, that section of pipe must be removed completely.
Most electrofusion manufacturer’s instructions state that the fusion zone must be free of all gouges and scratches. So, what are the dangers of gouges in the electrofusion zone? A gouge can hold contamination like small pebbles, sand, or bentonite that can contaminate the fusion.
If we assume you have a fusion zone that has a few small (well under 10% wall thickness) gouges, those gouges are meticulously cleaned with alcohol wipes and a small flat head screw driver. After peeling the pipe if there are still very clean gouges visible, they will be filled when the fusion takes place from the melt pool created by the electrofusion coupling.
That begs the question whether there is enough molten plastic created by the heat from the coupling to fill these voids? How does a person in the field measure the volume of the gouge to ensure there is enough material to compensate for these voids?
The current answer is that most manufacturers and Operator Qualification (OQ) programs do not have a process to allow for any size acceptable gouges in the fusion zone.
Pipe Relaxation
The pipe pulling process can put tremendous strain on the entire length of pipe. To protect against this tensile strain, a mechanical or manufactured weak link is used to protect the pipe. The weak link is designed to break before the pipe reaches a point where it could be permanently damaged. Even with this weak link, HDPE can stretch a great deal during the pull.
The standard practice is to let pipe relax for 24 hours after a pull so it can retract to its original length. Not following this relaxation practice can put a great deal of strain on an electrofusion joint. Pulling pipe results in elongation, and a slight reduction in Outside Diameter (OD). If an electrofusion coupling is installed before the pipe can relax there are two major potential failures:
1. As the pipe tries to relax the electrofusion coupling will impede the pipe’s ability to do so. This creates an incredible amount of tensile strain directly on the fusion zone, even though nothing is pulling on the pipe.
2. With the slight reduction in OD, the electrofusion coupling will struggle to transfer heat and achieve the proper melt penetration. It is important to remind installation crews of the proper relaxation time before tying in pulled pipe.
Pipe Alignment/Dig Backs
During an HDD operation the pipe generally enters and exits the ground on an angle through pre-dug pits. These intersections are where one pull ends and another one begins. After installation, installers must dig back the pipe to lay down at the level of the installation to connect an electrofusion coupling.
Depending on the depth and angle of the pipe, it may be necessary to excavate a great deal of earth around the exit point to allow the pipes to come together without stress. While it is a tempting to bend the pipe with shale bars or an excavator to achieve proper alignment, this is a mistake. Even when using an electrofusion alignment clamp, the pipe joint should not be under any stress.
Professionals should consider electrofusion clamps as restraints. Conversely, installers should not use them to align the pipe. Achieve alignment by digging back enough soil and/or blocking under the pipe so the two ends come together with no stress. This step is necessary for the integrity of the electrofusion joint and should not be avoided – even for pipe at a steep angle.
Conclusion
When properly prepared, an HDPE electrofusion joint remains one of the strongest methods of gas distribution pipe joining. In hydrostatic quick burst tests, properly prepared electrofusion joints have exceeded 800psi, with the pipe yielding before the actual fitting. These tests prove electrofusion is more than adequate for most natural gas piping Systems across the United States as they operate at 100psi or lower.
Construction sites are not controlled laboratory environments, but with the help of the above considerations, electrofusion can help you solve your challenges in natural gas piping projects and beyond.