Did you know that the foam insulation is the most important component in pre-insulated pipe used for chilled water and hot water hydronic systems? The quality of the foam itself, as well as the manner in which it is applied, have the biggest impact on the life expectancy of an underground pre-insulated pipe system. Premature failure and loss of thermal efficiency are expensive problems, and they are very difficult to correct after installation. Surprisingly, these failures can be very easy to avoid, but it takes effort early on in the design and procurement stages of a project. This article will cover some common failures found in pre-insulated piping systems, and a few recommendations on how to avoid them.
First, let’s take a look at a typical cross-section for pre-insulated pipe used in underground chilled water applications.
This example shows a steel service pipe with polyurethane foam and high-density polyethylene (HDPE) jacket. A similar product might have a PVC service pipe (instead of steel) or perhaps a fiberglass jacket (instead of HDPE jacket). Regardless of what service pipe or jacket is utilized, the polyurethane foam is a constant feature for this type of pipe system. What is not constant, from manufacturer to manufacturer, is the quality of the foam, and how it is applied between the service pipe and the jacket.
Foam insulation is generally applied by one of three methods:
- Foam is poured between an HDPE, PVC or FRP commodity pipe (serving as the jacket) and the service pipe. This is normally done in 20-foot lengths of pipe, and the process can be as simple as setting the pipe assembly in a stand and literally pouring the foam mixture into the void left between the two pipes. The foam expands as it is released into the cavity and, in theory, fills the gap between the service pipe and the jacket.
- Another method sees the foam injected between the jacket (commodity pipe again) and the service pipe. Injection wands are inserted into the void between the two pipes, and the foam is released gradually as the wand is retracted out of the pipe.
- The last method relies on a spray-applied application. Here, the foam is sprayed as the service carrier rotates on a wheel-set and progresses through a spray chamber. The foam is fully exposed and designed to “rise” to the specified thickness. As the foam cures and exits the spray chamber, it is inspected for voids or defects in the application. The pipe assembly then continues to a section where the jacket, whether extruded HDPE or helically-wound FRP, is applied directly over the foam.
This article provides more details about each of these processes, but if you pause for a moment to think about each of them, you may realize that only one of them allows for full length, 360° visual inspection of the foam before the jacket is applied.
Having discussed the foam application, let’s turn to the quality of the foam itself. It is important that good quality chemicals and blowing agents be used in the manufacture of the foam. A quality manufacturer will have procedures in place to periodically test foam batches to ensure the specified density and thermal efficiency values. Ensuring that the foam adheres to both the service pipe and the jacket is an important criteria, and one that is not easily achieved.
Signs of trouble…
So what are the main problems, and how do they manifest themselves? Let’s look at some examples!
This first picture shows how poured or injected foam has left an air pocket between the jacket and service pipe. In a chilled water application, moisture condensation will soon occur, and premature corrosion will attack the service carrier. Because of the lack of foam, the compression strength of the jacket is compromised in these “soft spots”, which can lead to a failure of the jacket, and ingress of ground-water. None of this is good!
This example shows how the foam has shrunk and separated from the jacket pipe. Because the foam was poured or injected, it is unlikely that the jacket interior surface was pre-treated to adhere to the foam. With an air void between the jacket and the foam, the strength of the pipe is compromised, and moisture is also allowed to attack the foam.
Here you can compare the cell structure of injected foam vs. that of sprayed foam. The foam spray application allows for a more uniform cell structure. Having a consistent density across the foam application provides better thermal efficiency, and superior support for the exterior jacket.
Here is a common problem encountered with fabricators that do not have proper equipment or manufacturing methods. Can you identify the problem? If you noticed that the service pipe is not centered within the jacket, you are correct. This is a serious problem for installers that need to perform field joints on two pieces of straight pipe. If you weld the service pipe from one section of pipe to the next, how do you properly join the exterior jackets that are now out of alignment? Flexible shrink sleeves to make up for the misalignment? Not so great, is it?
How to avoid these pitfalls?
As the specifying engineer for the underground hydronic piping system, we recommend that you specify foam insulation that can be visually inspected, a full 360° around the pipe and for the full length of the pipe section, prior to the application of the jacket.
Some manufacturers may suggest that they rely on X-ray inspection for quality control, but they only perform these inspections on a small sample of a fabrication run. If you are satisfied with X-ray inspection as a method for identifying manufacturing defects in pipe, be sure to specify that 100% of the the job-specific pipe be inspected and that job-specific reports be furnished during the project’s submittal stage.
Once you have specified a satisfactory method for application and testing of the foam insulation, we recommend that you insist that all fittings (elbows, reducers, tees) be pre-fabricated by the pipe system manufacturer. Pre-fabricated fittings will benefit from superior insulation and will prevent water entry via the sub-standard “field-applied jackets”.
Lastly, ensure that the foam will retain its efficiency by specifying a diffusion barrier between the foam and the jacket. This is a very inexpensive feature that has a dramatic impact to the thermal performance of the foam. You can learn more about the use of a diffusion barrier here.
Looking out for your client!
It is a very expensive proposition, and unfortunately all too common, for relatively new piping systems to have to be pulled out of the ground due to premature failures. With a small effort at the design stage of the project, you can make a big difference in the life of an underground hydronic system. Plan ahead and you can save your client a substantial amount of money and grief.
If you need assistance with specifying an underground hydronic system, please contact our office. We will be glad to look at your specific project and offer recommendations for a solid and long-lasting design.