The Plastics Pipe Institute Inc. (PPI) has published a new document to address ground heat exchanger piping longevity, a frequent topic of discussion in the industry, according to the association. PPI Statement AF Projected Design Life of Plastic Piping used in Geothermal Ground Heat Exchangers is available on PPI’s Building & Construction Division’s website www.plasticpipe.org/bcd.
For Ground Source Heat Pump (GSHP) systems, the piping material used for the ground heat exchanger (GHX) is critical to the overall success of the system. It must provide corrosion resistance, flexibility, chemical resistance, resistance to slow crack growth, impact resistance, long-term hydrostatic strength (pressure capability), and temperature resistance.
GSHP pioneers experimented with various types of piping materials starting in the 1970s and 1980s and ultimately found that high-density polyethylene (HDPE) provided the properties needed for the majority of GHX applications. Crosslinked polyethylene (PEX) was approved for use in geothermal systems in 2008, followed by polyethylene of raised temperature resistance (PE-RT) piping which was recently approved. Both PEX and PE-RT are based on HDPE materials, but can operate continuously at temperatures up to 180°F, when that capability is needed. The manufacturing of these piping materials and their associated fittings is tightly controlled through rigorous standards, third-party certification programs, and code enforcement. PPI Statement AF provides detailed information about each of these approved piping materials, as well as the test methods used to predict their long-term hydrostatic strength.
“While approved piping materials HDPE, PE-RT, and PEX have proven to be exceptionally reliable, the uniqueness of each GHX application makes it difficult to accurately predict an extended design life for these materials as a category.” stated Lance MacNevin, P. Eng., senior director of engineering for PPI's Building & Construction Division. “The design life for a piping material can be defined as the length of time that it is projected to operate reliably in a standard and approved application, while the service life is the length of time that a piping material actually operates while in service.”
According to the PPI Statement, while some HDPE PE4710 piping materials have a projected design life of 100 years or more in certain applications where internal water pressures, loads, and temperatures are relatively constant (e.g., municipal water systems), the piping used in GHX applications experiences several more variables in operation which can reduce its service life, including the following:
- Potential damage during installation in boreholes
- Wide range of fluid temperatures
- Changes in internal hydrostatic pressure which varies with fluid temperature and elevation
- Cyclic pressure changes when system pumps are activated and deactivated
Due to these operational variables, which are unique for each ground heat exchanger application, PPI conservatively projects that the design life of GHX plastic piping materials, when specified correctly and installed according to industry and manufacturers’ guidelines, is typically in excess of fifty years. In some ground source heat pump systems, the actual service life of the GHX piping may be far in excess of this value.
