Polypropylene-Random (PP-R) Piping Systems Serve Multiple Applications
Standard Dimension Ratio
Standard dimension ratio (SDR) is the ratio of the outside pipe diameter to its wall thickness and is used as a method for rating a pipe's durability against pressure. A high SDR ratio indicates that the pipe wall is thinner compared to the pipe diameter, resulting in lower pressure rating. A lower SDR indicates that the pipe wall is heavier, resulting in a higher pressure rating.
In North America, fusible plastics used for pressure piping, including PP-R, are typically manufactured according to SDR dimensions as specified in ASTM International standards. All pipe sizes in a given SDR have the same pressure ratings. The ratings do not decrease with larger size SDR pipes as they do with schedule or type-based pipe commonly used in North America such as steel and copper.
In some cases, PP-R and other fusible plastics are made to schedule sizes. These are typically either non-pressure drainage products or specialty chemical process piping. In most of these cases, there are different ASTM designations to differentiate the products, such as ASTM F1412 for corrosive (acid) waste piping. The standard for PP-R pressure piping used in mechanical and plumbing system is ASTM F2389. Additionally, for potable water systems, the piping must comply with NSF 61 and the oxidative resistance testing of ASTM F2389. In North America, there are several manufacturers with listings per ASTM F2389 for mechanical applications, and one manufacturer has a listing for NSF 61 and oxidative resistance for potable water.
SDR is one of the major factors used in engineering pipe systems for a specific application, where each SDR has its own advantage. For instance, a heavy wall pipe with an SDR of 7.4 could be specified for high-stress applications such as hot water recirculation that requires increased pressure and temperature ratings, while a thinner wall pipe with an SDR of 17.6 would provide maximum flow rate while minimizing material weight, cost and fusion time. PP-R pipe manufacturers list SDRs and their appropriate application.
Images courtesy of Aquatherm
Examples of pipe SDRs.
Left: A thicker wall provides increased pressure and temperature ratings for high-stress applications such as hot water recirculation.
Middle: A medium wall thickness provides higher flow rates while maintaining high pressures and is suitable for most mechanical and industrial applications.
Right: A thinner wall allows maximum flow rate while minimizing material weight, cost and fusion times and is used for chilled, cooling and condenser applications.
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