Pipe sizing - Quantified performance in NCC 2022 Volume Three

We’ve ‘quantified’ many of the Performance Requirements and developed new Verification Methods that you can use when developing Performance Solutions.

To explore these changes, we’ll use pipe sizing to show how using Performance Solutions has been simplified.

Some Performance Requirements are qualitative – meaning they are subjective and not easily assessed. This can make it difficult to determine if the requirement is met and can discourage practitioners from using a Performance Solution.

Quantifying these requirements (or ‘quantification’) provides a measurable level (like numbers and percentages) you need to meet. We call this a metric. The benefit of this, is that you have a clear bench mark to work to. This provides a clear performance level and helps reduce misinterpretation.

One of the key quantified Performance Requirements for pipe sizing is water service velocity. This is covered in these Performance Requirements in Section B of Volume Three:

• B1P2 Velocity (cold water services)
• B2P3 Velocity (heated water services)
• B3P3 Velocity (non-drinking water services)
• B6P2 Velocity (rainwater services)
   

These Performance requirements are quantified using the same methodology, with slightly different metrics, depending on the situation.

They provide practitioners with a maximum velocity for the pipe system, as well as outlining exceedance criteria.  For instance B1P2 states:

• …pipework water velocity does not exceed 3 m/s for more than 1% of the time that the water is required during the annual peak hour.

While B2P3(b) states:

• (b) 1.2 m/s for the flow and 1.0 m/s for the return of copper circulatory heated water service for more than 1 % of the time that water is required during the peark hour; and
  …

In all cases, the metrics provide an equivalent level of performance to the Deemed-to-Satisfy (DTS) Provisions.

This allows for different approaches for pipe sizing of water services, whilst still meeting a minimum performance level, all thanks to a quantified metric!

Another way we’re applying quantification in the NCC is by including new Verification Methods. Verification Methods are tests or calculations which prescribe a way to directly comply with the Performance Requirement.

In Volume Three, there are 6 new Verification Methods related to pipe sizing:

• C1V1 Determination of sanitary plumbing wastewater flowrates
• C1V2 System 1 common discharge design
• C1V3 System 2 Common discharge design
• C1V4 System 3 branch design
• C1V5 Stack design
• C2V3 Determination of sanitary drainage wastewater flowrates

Using C1V1 as an example, it provides an approach to comply with the requirements for pipe sizing of sanitary plumbing systems using a quantified metric.

The first part of this Verification Method states:

(1) Compliance with C1P3 for pipe sizing is verified for each sanitary plumbing pipework section when the discharge flowrate is not less than the greater of

• (a) the probable simultaneous wastewater flowrate calculated in accordance with (2); or
• (b) the Discharge Unit (DU) value of the highest fixture connected upstream of the pipework section as given in Table C1V1b, in litres per second.
  …

C1V1 includes the calculation for the probable simultaneous wastewater flowrate (flowrate), or the likely flowrate for pipework considering the expected use of the fixtures upstream of the section of the pipe in use.

It also considers a frequency factor. The frequency factor takes into account the likelihood of simultaneous usage, based on building classification and fixture type.

Clause (b) of C1V1(1) also sets a minimum limit, which is the highest volume of fixture discharge. This ensures that the pipe is appropriately sized for the expected discharge volume.

No, for example Section C contains other changes to the Performance Requirements for sanitary plumbing and drainage, like pressure and system ventilation (C1P5). This is quantified, as follows:

A sanitary plumbing system must ensure that ventilation is provided to avoid hydraulic load imbalance such that—

• (a) there is less than 1% likelihood during the annual peak hour that when any fixture discharges, air pressure at any trap seal exceeds ±375 Pa difference from atmospheric pressure; or
• (b) an equivalent level of safety to human health is achieved as a system complying to (a).

This quantification example provides the appropriate limits within the sanitary plumbing system, before the risk of sewer gasses escaping, by limiting the pressures within the system to not exceed a ±375 Pa air pressure limit. To use this approach to demonstrate compliance with these Performance Requirements detailed technical modelling and analysis is needed.

More information on the quantification metric used in these Performance Requirements is in our new quantified Performance Requirements article.