Why it’s important to specify the correct SDR for your HDPE project

Specify the wrong Standard Dimensional Ratio (SDR) in your poly pipe system, and you could be up for “20 to 30 years of heartache and pain.”

It might sound dramatic, but according to Director of Advanced Piping Systems, Caleb Craig, “time and time again we witness the long-lasting consequences of poorly selected SDRs on mine sites across the country.”

Selecting the correct SDR is a vitally important step in ensuring the working pressure rating of your pipeline is covered. Keep reading to find out more about SDRs, and why it’s necessary to select the right type for your next pipe project.

Standard Dimensional Ratios and Pressure Nominals explained

SDR is used by pipe manufacturers to rate the working pressure capacity of a pipe. It is calculated by determining the relationship of a poly pipe’s outside diameter to its minimum wall thickness.

The higher the calculated SDR, the lower the amount of pressure the pipe can handle, and conversely, the lower the calculated SDR, the higher the amount of pressure the pipe can handle.

“SDR is a ratio that manufacturers have produced for the market to ensure that everyone is working off the same standard,” explains Caleb.

“SDRs can range from 7.4 right through to SDR 41, but the most common would be SDR 11.”

Knowing the outside diameter (OD) and SDR of a pipe allows you to calculate the wall thickness of the pipe, which then allows you to know what the pressure nominal (PN) will be.

“For example, if the OD is 180mm, and the SDR is 11, you can divide these two numbers which results in a 16.4mm wall thickness.

“A pipe with 16.4mm wall thickness will be able to cope with a PN 16 pressure, which is 1600 kPa.”

While it’s useful to know how SDR and PN is calculated, PIPA has produced a handy reference document outlining standard pipe pressure ratings, determined by SDR.

The importance of specifying the correct SDR

To ensure your pipe system can withstand its operational pressure over an extended period, specifying the correct SDR is “vitally important” to ensuring the working pressure rating of your pipeline is covered, Caleb explained.

“If we take the example above, where the pipeline is operating at 1600 kPa, you need a pipe that can withstand that pressure over a very long period of time — which is only possible with the correct SDR.”

The impact of specifying an incorrect SDR

Specifying the wrong SDR for your pipeline can lead to several technical and economic impacts downstream from design, including:

• A lack of availability of fittings when the project proceeds, resulting in significant costs incurred through custom made/modified products.
• Welding issues on site with varying wall thicknesses between pipe and pipe fittings; and
• Unwanted turbulence and friction loss caused by irregular SDR’s between a pipe and the pipe fittings.

1. Availability of fittings

Worldwide, polyethylene pipes and fittings come in a range of standard sizes.

“SDR 17 and SDR 11 are the most common pressure ratings in the market, meaning they’re readily available,” said Caleb.

“Because of this, moulded fittings are only ever generally manufactured, in SDR 17 or SDR 11.”

When budgets are at play and timelines tight, we know that finding efficiencies is a priority for many project managers. However, issues can occur when irregular SDRs are specified, often under the belief that it will save money.

“The problem with specifying irregular SDRs is that it can be difficult to source matching fittings,” Caleb said.

“While pipe can be made in any SDR required, fittings are injection moulded and generally imported from overseas, which can mean big lead times.

“We often hear of projects that have specified a PN 12.5 pipeline, with a four-week product turnaround and six-week project turnaround, and they get all the pipe manufactured ready to go before realising they don’t have any appropriate fittings to join the pipe together.

“They end up having to spend a huge amount of money developing custom fittings.”

To counter this problem, Caleb says it is always best to match your SDR to your fittings.

“If a customer comes to us requesting 13.6 SDR pipe, we can supply it to them, but we warn them they will have to use SDR 11 fittings, because there’s nothing else available.

“We suggest changing both the pipe and fittings to SDR 11, to save time and money down the track.”

2. Welding issues on site

Another reason for ensuring both your pipe and fittings have the same SDR is to avoid welding issues on site.

“Not only is welding pipes with two different wall thicknesses against best practice for butt welding, many mine sites don’t allow the practice anymore,” Caleb said.

“We’ve heard of instances where contractors have welded them together, and then quality assurance has undertaken the required checks, and requested that the whole pipe system needs to be redesigned and welded.

“It’s an absolute nightmare for anyone in that situation, plus it can cause pipe failure when under pressure, as it’s not a satisfactory weld.”

If you do need to weld pipes with different wall thicknesses, the only way to undertake this is by installing an SDR transition between the two.

“But this adds a huge amount of cost, again, to the whole process and to the weld,” Caleb said.

3. Unwanted and accentuated turbulence

The third reason to always specify the correct SDR is to avoid unwanted turbulence in your pipe system.

“Some turbulence in a pipeline is okay, particularly when you’re pumping slurries, to prevent jams,” Caleb explained.

“But when you weld two pipes with a different SDR, whatever’s being pumped through your pipe will hit a step and create additional, unwanted turbulence.

“Whatever material is being pumped through will spin out of control and wear out a big hole in the flange area.

“Then, as it’s coming out of the flange area and hits the next weld, the process starts again.

“It really wears out the pipe, particularly where there’s a fitting or a bend.”

If you’d like some advice about which SDR and fittings are best for your next HDPE project, contact our friendly customer service team to discuss your requirements today.