For Australians that have tried working with the pipe sizing tool within Revit, you would know that Revit doesn’t quite size things right. In fact it over sizes the pipework and calculated pipe sizes can get out of hand.
I had the pleasure to speak about this topic at the 2016 Revit Technology Conference (RTC) back in May where I demonstrated a simple workflow that sizes domestic water pipework to AS3500 compliant sizes. The main goal of the demonstrated workflow was to keep things as simple as possible, which I guess ends up making it a workaround rather than a true workflow, but it something that everyone is able to get access to without knowledge of coding for the Revit API.
How do you size pipework?
Most Australians use a few tried and true methods. Those being AS3500.1 using the pipe sizing tables in Appendix C, the book Selection & Sizing of Copper Tubes for Water Systems otherwise known as Barrie Smith’s Blue Book. Some of you probably even just size off the top of your head with a confident “She’ll be right!” and you’re done.
Verifying all of that design data by hand is not that easy though, so why not use Revit? You can verify your design on the fly and be confident that your documents are compliant with Australian Standards.
How does Revit size Pipework?
If you know where to look, it doesn’t take long to figure out how Revit sizes domestic water pipework; it uses table E103.3(3) of the International Plumbing Code (IPC). Just like AS/NZS3500 and Barrie Smith, the IPC provides a table that can be referenced to convert loading units (fixture units in Revit) to a probable simultaneous flow.
Comparing extracts of both the IPC and AS3500, we can start to analyse the differences between the two standards. The conversion rates are 1 gallon = 3.785 litres, therefore 1 gallon per minute is 0.063 litres per second.
Based on the IPC sizing charts, 1 fixture unit is 3 gallons per minute, or 0.18l/s whereas in AS3500, 1 loading unit is 0.09l/s. To give a better understanding, this is how the figures compare visually.
In the image, the IPC is blue, AS35000 is orange and Barrie Smith is green. What interested me the most is that many hydraulic designers have told me “I don’t use Barrie Smith as his results oversize the pipework” but you can see in the comparison that Barrie Smith and AS3500 are almost identical, with AS3500 actually returning probable simultaneous flow rates higher than that of the Blue Book from around the 22 fixture unit mark.
So the IPC probable simultaneous flow rates far exceed that of the Australian Standards, but is there any correlation between the two? For the large part, there is; again it is easier to show visually.
By simply taking the IPC flow rates in litres per second and dividing the figure by AS3500 flow rate in litres per second to compare the two, we get a clear picture of how many times greater the flow rate the IPC is compared to our local standards. The IPC peaks at about 3.6 times the flow at around 10 fixture units and then tapers off towards 3 times the flow compared to Australian Standards.
The mean value across all the figures works out to a multiplier of around 3.2, however looking at the data I most frequently verify manually, it is generally branches of 10-40 loading units, taking the mean value of this range, the multiplier between the flow rates sits at 3.46.
Besides the incorrect use of a meme or two, what it all means is that we can make some simple tweaks to our family content to start using the Revit pipe sizing tool for Australian Standard compliant pipework.
Taking my plumbing fixture family, I added the parameters AS3500Multiplier, AS3500LoadingUnits and an optional yes/no parameter IPCtoAS3500 each of these parameters do the following:
AS3500Multiplier – the multiplier to convert between IPC and AS3500, I used 3.5
AS3500LoadingUnits – the actual loading units, in this case I used 2.
IPCtoAS3500 – when set to true, it adjusts the out of the box Revit parameter CWFU
In the CWFU parameter I used the formula if(IPCtoAS3500, AS3500LoadingUnits / AS3500Multiplier, AS3500LoadingUnits)
Basically, if the box is checked, we’ll divide 2 loading units by 3.5, if it’s not checked, the loading units will remain 2. There is no real reason to do this if you’re only sizing within Australia or New Zealand, but if you want to use the same set of families across both standards, this is how you would go about it.
Taking a simple example of 15 toilets on a single branch, using Revit to size the pipework, we end up with pipework that is somewhat oversized, the the main branch being sized up to 32mm diameter after 9 toilets.
I have a Excel sheet where I transcribed table 48 from the Blue Book, filling that sheet out with our fixture count, we can see that if we’re only taking into account velocity requirements when sizing our pipework, 32 diameter is quite oversized even for 15 toilets and that 25 or even 20 diameter would be a more acceptable size.
To verify my Excel figures, here is an extract I’ve scanned from the Blue Book, you can click on both images for full size.
Using our reduced fixture unit ratings however changes things quite a bit. With our fixture units divided by 3.5, we get more realistic results, the pipework being sized to 25mm diameter after 10 toilets and remaining 25 diameter for the rest of the run. Referring back to our Excel/Blue Book references above, we now have Australian Standard compliant pipe sizing!
You might be thinking though, this is a small scale example, does it work on a larger scale? In the example below, each box represents a group of fixtures totaling 100 fixture units each.
So each individual branch is providing 100 loading units and the total provided on this run is 500 loading units. We divide these figures within our plumbing fixture families by 3.5 to give our reduced rates for Revit sizing and once again our pipe sizing is on point with a size of 32mm generated by Revit. Referring back to the Blue Book, we can see that 32mm gives a velocity of 1.482m/s which falls within our AS3500 range.
Further reviewing our sizes with the Blue Book, we can see that 500 fixture units will flow through a 50 diameter pipe at a velocity of 1.774m/s.
It’s really that simple. You can use any multiplier to suit your needs, but after extensive testing, a multiplier of 3.5 gives consistent results across many different designs where I have used Revit to size AS3500 compliant pipework.
In the next article, I’ll show how to use Dynamo to perform further calculations for design verification purposes.