What SDR means

Standard Dimension Ratio or SDR describes a pipe's geometry, it is simply the ratio of outer diameter of the pipe to its wall thickness. As an example, a pipe with SDR11 has an outer diameter that is 11 times its wall thickness. Therefore, if the outer diameter or OD of the pressure pipe is known, then the wall thickness can be calculated for any given SDR number.

The Higher the SDR number the thinner the pipe wall is in relation to its Outer Diameter.

The Lower the SDR number the thicker the pipe wall will be in relation to its Outer Diameter.

SDR Calculations

The SDR number is the outer diameter of pipe (D) divided by its wall thickness (s) as demonstrated in the diagram. We can also make other calculations from this with any of the given data, for example we can determine the wall thickness from the outer diameter and SDR number.

Example Calculation

A 90mm outer diameter pipe has an SDR of 11, which means we would Divide 90mm by the SDR of 11 = Wall thickness of 8.18mm*

Furthermore; we can now make a calculation to determine the pipe's inner diameter by taking the OD of 90mm and subtracting 8.18mm twice.

* Plastic pressure pipe wall thicknesses are manufactured within a tolerance range usually fractions of millimetres, so actual thicknesses may vary slightly, but are still very accurate.

SDR & the relation to a pipes pressure rating

The wall thickness of any given pressure pipe will directly correlate to its pressure handling capabilities. SDR being a ratio of a pipe's Outer diameter to its wall thickness in this respect, should indicate its pressure handling capability.  However, SDR does not take into account pipe materials. Different materials have very different properties, so therefore SDR does not give accurate indications of pressure rating across the spectrum of different pipe materials. As such, SDR11 HDPE pipe would not have the same pressure rating as SDR11 PP Pipe.

Plastic pressure pipes of different outer diameters, but of the same material and the same SDR, will have the same pressure handling capabilities as each other, because the ratio of wall thickness to outer diameter and the material is constant across the different outer diameters of pipe.

For example, Our polypropylene pressure pipe that is SDR11 is rated as 10 Bar, and will have the same 10 bar pressure rating across all outer diameters.  All of our SDR 17.6 polypropylene pipe is rated for 6 Bar regardless of outer diameter. 

SDR is a dimensionless number based on the geometry of pipe.  Two pipes of the same outer diameter and wall thickness will have the same SDR as each other, but if they are manufactured from different materials they may not have the same pressure rating, as different materials inherently have different properties. So SDR cannot be used to identify pressure rating across materials, but it can within the constraints of a single material. Our SDR11 HDPE pipe are rated 15 bar & SDR11 PP pipe are pressure rated as 10 bar, which demonstrates this perfectly.

The impact of specifying an incorrect SDR

Incorrect specification of the SDR for your pipeline can result in various technical and economic impacts downstream from design. These include a lack of availability of fittings, leading to significant costs incurred through custom made/modified products, welding issues on site due to varying wall thicknesses between pipe and pipe fittings, and unwanted turbulence and friction loss caused by irregular SDRs between a pipe and the pipe fittings. To avoid these issues, it is recommended to match your SDR to your fittings. This ensures that the fittings are readily available and that welding issues on site are avoided. Additionally, it prevents unwanted turbulence in your pipe system, which can cause wear and tear on the pipe and fittings. Considering that the lifetime of a pipe installation may be expected to be 20 to 30 years, ongoing issues from mismatched or incorrect SDR would have a long term impact, which can be avoided at the outset.  If you require advice on which SDR and fittings are best for your next plastic pressure pipe project, please contact our friendly customer service team at pipewarehouseuk.com to discuss your requirements.

SDR rated pipe at Pipe Warehouse UK

All information provided in this article is intended as a preliminary guide only. The information contained herein does not constitute advice and some subjects briefly mentioned in this article may need further research to be fully understood. Pipe Warehouse UK is not responsible for any issues arising from the use or reliance on the information contained in this article.