Domestic Water Systems – SDR’s, PN Ratings and Flow Rates

We are often asked about the differences between SDR’s, PN ratings and Flow Rates for Water Piping Systems. Here is a quick summary of these terms and their impact on day to day plumbing for domestic installations as defined in AS3500.

SDR is the abbreviation for Standard Dimension Ratio. It is the ratio of the maximum outside diameter of a length of pipe in comparison to the minimum wall thickness of that pipe. As an example, say for a length of 20mm water pipe where the maximum outside diameter is 20.3mm and the minimum wall thickness is 1.9mm then the SDR is 20.3/1.9 = 11. If the wall thickness was 2.8mm then the SDR would be 7.4 (20.3/2.8). So for a stated outer diameter of pipe, you can see that SDR decreases as wall thickness increases.

So what I hear you say! Well as pipe wall thickness increases and SDR decreases then you must be aware that the pipe with the LOWER SDR also has a LOWER flow rate for a given nominal diameter.

SDR’s for Couta Water Pex single layer product are 9 for 16mm pipe and 11 for 20, 25 and 32mm piping.

Don’t get caught under sizing a job because of confusion over SDR’s.There also seems to be confusion over the required Pressure Ratings (PN designation) of water piping systems.

AS3500 states that the maximum static pressure at any outlet within a building should not exceed 500kPa. This is important as pressures above 500 kPa can cause damage from water hammer, reduced life of appliances, taps and fittings, and cause excessive noise in the system.

A piping system with a PN12.5 rating has a minimum pressure rating of 1,250kPa – more than double the required 500kPa! And typically burst pressures for this type are pipe are in the order of 4,000kPa and more.

Systems with higher PN ratings typically achieve this with increased wall thickness at the expense of the flow rate of the system.

For more information, please visit WaterPex website