The Difference Between Plastic Pipe SDR and DR

Question: Regarding plastic pipe, what’s the difference between SDR and DR?


Answer: Standard Dimension Ratio (SDR) and Dimension Ratio (DR) both reference the same ratio (i.e., a dimensionless term established by dividing the average outside diameter of the polyethylene (PE) pipe by the minimum pipe wall thickness.
Here are SDR-11 pipe calculations (SDR = Average outside diameter divided by minimum wall thickness):

2-in.- diameter pipe…2.375 divided by .216 = SDR-11

4-in.- diameter pipe…4.500 divided by .409 = SDR-11

6-in.- diameter pipe…6.625 divided by .602 = SDR-11

8-in.- diameter pipe…8.625 divided by .784 = SDR-11
Many PE pipe manufacturers use the SDR method of rating pressure piping. An SDR-11 means that the outside diameter (OD) of the pipe is eleven times the thickness of the wall. With a high SDR ratio, the pipe wall is thin compared to the pipe diameter. With a low SDR ratio, the pipe wall is thick compared to the pipe diameter. So a high SDR pipe has a low-pressure rating, and low SDR pipe has a high- pressure rating.


Tip: Here are two examples that show why the SDR/DR dimensional ratio is important with PE pipe:


When performing the squeeze-off procedure on PE pipe, be sure to identify the diameter and SDR number of the PE pipe (e.g., 6-in. IPS, SDR-11 compared to 6-in. IPS, SDR-13.5) so that you use the correct gap stop required for the diameter and wall thickness of the pipe you are about to squeeze.
When performing the butt fusion procedure with a hydraulic fusion machine, the correct fusion pressure must be calculated based on the diameter and SDR of the PE pipe.

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