a two-step solvent welding process consisting of primer conforming to ASTM F656 and cements conforming to ASTM D2564. Cement manufacturers make no distinction between these two different piping systems in regard to cutting, preparation, joining, cure times and set times. In above-ground installations, cellular core PVC has the same requirements for horizontal and vertical support. Additionally, both are combustible per ASTM E136 and, therefore, cannot be installed in a return air plenum without additional fire wrap. Below ground, there is a major installation requirement difference. While both products are suitable for burial, as mentioned earlier, solid wall pipe is somewhat more robust and stiffer, particularly in sizes 6 inches and smaller. However, ASTM F891, Appendix X3, Installation, paragraph X3.1, has the following limitation: Maximum aggregate size shall be limited to inch (13 mm) for angular and inch (19 mm) for rounded particles. This statement is significant, as ASTM D2321, Standard Practice for Underground Installation of Thermoplastic Pipe for Sewers and Other Gravity-Flow Applications , allows aggregate and stone that pass through a 1-inch sieve. In other words, foam core PVC pipe takes even more care than solid wall PVC pipe to be installed below ground. Mechanical maintenance: In facilities that require regular or frequent cleaning, the use of mechanical devices such as auger bits or snakeheads is commonplace, and special consideration should be used when selecting what material can or should be specified. Repeated passes with a mechanical cleaning device can eventually nick, chip or wear down the thin inner wall to the point where the cellular core is exposed to the effluent being conveyed. While there is no definitive study that determines how many passes with a mechanical device will lead to pipe degradation, many specifiers will avoid the issue altogether and require solid wall PVC. Jobsite damage: Because cellular core pipe consists of two thin skins, it is more susceptible to job site damage. Damage to the surface of cellular core pipe could lead to increased leaks in the pipe itself or an issue known as "weeping" should the pipe have to endure sustained static head pressure in the event of a blockage during operation of the system. Cellular core PVC does offer advantages, primarily lower cost, lighter weight and easier cutting. However, these advantages must be evaluated against the loss of strength and sound attenuation, additional care required in underground installations, and additional maintenance considerations that are inherent with cellular core products. l Paul Tully is a field technical representative for Charlotte Pipe and Foundry Co. Tully, a 30-year plumbing industry veteran, focuses educating engineers, designers, contractors and code officials on products and industry trends.
Decision Criteria Checklist
To determine whether cellular core PVC is acceptable or appropriate for a given commercial project, evaluate these decision criteria: Does the AHJ allow cellular core piping in commercial construction? What are the burial conditions? What are the backfill materials? Is sound attenuation important to the successful use of the facility? Will the lower price of using cellular core PVC outweigh the addi- tional cost of adding higher-performing sound-abatement materials? Will the facility require regular or frequent cleaning using mechani- cal devices? Will the savings generated by using cellular core PVC outweigh any potential future issues?
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