Victoria Johnson, PE, a Plumbing Engineer with Bass, Nixon & Kennedy Engineers in Raleigh, is designing a project in North Carolina that includes several freeze-resistant, boxed, in-ground hydrants and is wondering what material would be best for the buried water supply lines.
The hydrants will be fed from a secondary backflow preventer, and the total length of buried pipe is about 900 feet to the farthest hydrant. Water is sourced from aboveground water tower tanks, and the water temperature varies from about 40°F in the winter to close to 90°F in summer. Items of concern are minimizing fittings below grade and possible thermal expansion due to the long, straight runs.
Our experts weighed in on the Open Forum on ASPE Connect. Here’s what they say about the varying pipe materials for this application.
Suitable Pipe Materials for Buried Water Supply Lines
HDPE and PEX
HDPE and PEX are both flexible materials that come in long, continuous coils, with excellent toughness and freeze-beak resistance. Each of these materials and their approved joining techniques are described in AWWA standards for water service (up to 3-inch diameter): AWWA C901 for HDPE pipe and AWWA C904 for PEX water service pipe.
HDPE is fusion welded (requiring no fittings), and it can be installed by directional boring if going under existing pavement.
CPVC and PVC
Thermoplastics, such as CPVC and PVC, are good candidates for underground installations since they are immune to damage from naturally corrosive soil conditions. ASTM D2774: Standard Practice for Underground Installation of Thermoplastic Pressure Piping states: “Provisions for pipe expansion and contraction shall be made with all thrust transmitting joint systems. When assembled outside the trench, the pipe shall be allowed to cool in the trench before backfilling to minimize stress due to thermal contraction.”
CPVC and PVC manufacturers outline methods such as pipe snaking to compensate for expansion and contraction in underground installations. This is mostly important when the pipe has been joined outside on a hot summer day—the temperature underground is colder, so the pipe will contract.
PP-RCT is a polypropylene material with a co-extruded middle layer to limit expansion and contraction. This material is fusion welded (butt or socket fusion, similar to PE for natural gas lines) and is ideal for underground water services.
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