What in the HDPE is going on?

National Fire Protection Association, Standard for Installation of Private Fire Service Mains and Their Appurtenances (NFPA 24), lists the requirements for underground piping serving fire sprinkler systems.  Typically, PVC DR-14, or ductile iron, pipe is used for fire service undergrounds.  However, it is becoming more common to see the use of HDPE pipe being utilized for fire service mains.

HDPE, high-density polyethylene, is polyethylene thermoplastic made from petroleum.  Per NFPA 24, Table 10.1.1 HDPE is permitted for use as a fire service main provided it meets AWWA C906 standard, and is specifically UL listed for use as fire service main/underground piping (this documentation should be submitted with the plans for review).

HDPE shall be tested the same as  all underground fire service mains per NFPA 24: -

All piping and attached appurtenances subjected to system working pressure shall be hydrostatically tested at 200 psi (13.8 bar) or 50 psi (3.5 bar) in excess of the system working pressure, whichever is greater, and shall maintain that pressure at ±5 psi (0.35 bar) for 2 hours.
An important observation when hydrostatically testing HDPE is that a standard 200 psi hydrostatic test will fail. If there are no leaks, the pipe stretches and will show a drop in pressure. Generally, the best way to test this pipe is to pressurize it and leave it pressurized and then come back later and bump the pressure back up to 200 psi after it has dropped to a point where the pressure holds steady. Then do the hydro. If there is a leak, the pipe pressure will continue to fall and never stabilize.

This is just a brief introduction to HDPE piping used for fire service mains.  What are some other challenges that you have found with this pipe? Where have you seen HDPE utilized successfully? How have you seen HDPE fail?